<figure><img src="image_51.png" /><figcaption>**Figure 2.** Age-depth model for Garba Guracha (GG) presenting the prior parameters into the Bayesian framework on the upper panels Modified from Bittner *et al.* (2020).</figcaption></figure>

axis (De’ath 1999). Charcoal and pollen plots and numerical analyses were done using several R packages (R Core Team 2012). Full code, data and further details can be downloaded from Gil-Romera’s GitHub repository shorturl.at/rxMW1 and from Neotoma database.

## 9.3 DATING

The 15.9 ka BP GGU17-1B age-depth model was built combining 48 dated samples using the Bayesian framework of the Bacon package in R (Blaauw and Christen 2011; R Core Team, 2017) (Figure 2). Dates used in this model can be found in supporting online material, and chronological methods are described in Bittner *et al.* (2020).

## 9.4 INTERPRETATION

We identified 98 taxa and counted an average of 456 pollen grains (SD = 120). We define six pollen zones (GGU1-GGU6) representing the last 13.7 ka BP of vegetation dynamics in GG area (Figure 3).

### 9.4.1 GGU1 Afroalpine vegetation (1170–1031 cm, 13.7–13 ka BP, 30 pollen samples)

The landscape was dominated by afroalpine vegetation reflecting cool, dry conditions (Figure 3). PAR values indicate a clear expansion of afroalpine and dry forest communities, with a minor development of the Ericaceous belt. At c. 13.1 ka BP we identify a large pulse of biomass burning that may be related to local fire activity. However, we cannot discard the possibility that charcoal particles accumulated on a frozen lake surface (Tiercelin *et al.* 2008) prior to the beginning of our record, which were deposited on the lake bottom after the ice melted.
<figure><img src="image_52.png" /><figcaption>**Figure 3.** Pollen diagram for lake Garba Guracha at 3950 m asl. Analyst: Graciela Gil-Romera. Vegetation belts are presented in percentages (filled silhouette) and PAR (in bars). Taxa included in each group can be found in the dictionary stored at shorturl.at/rxMW1.</figcaption></figure>
## 9.4.2 GGU2 Afroalpine-Ericaceous belt turnover (1030–637 cm, 13–8.5 ka BP, 80 pollen samples)

### 9.4.2.1 GGU 2.1 Amaranthacea-Poaceae dominating (1030–900 cm, 13–11.2 ka BP, 26 pollen samples)

Afroalpine herbs (e.g. Amaranthaceae) abruptly decrease at 12.5 ka BP coinciding with Younger Dryas-like conditions (YD). YD conditions have been described in Eastern Africa before (Talbot *et al.* 2007) broadly concurring in time (12.9–11.7 ka BP) with the YD in the Greenland ice cores (GS1, Rasmussen *et al.* 2006). While the YD may not have had a great impact on most lowland African records, afroalpine communities clearly declined, especially the PAR of these communities, except *Artemisia* which does not decrease in abundance. The lower forest seems to expand despite PAR suggesting a poor development. These vegetation dynamics may be pointing to a short and abrupt cold, dry pulse at these altitudes.

### 9.4.2.2 GGU 2.2 Expansion of Erica (899–637 cm, 11.2–8.5 ka BP, 54 pollen samples)

There is a notable expansion of most vegetation communities, as indicated by all PAR values and increased PrC scores. This expansion is especially important in the Ericaceous belt, potentially responding to increasing temperatures and moisture availability linked to the African Humid Period onset (AHP) (deMenocal *et al.* 2000). Under those conditions *Erica* thickets may have fostered biomass burning during the dry season, activating a fire feedback well known in the local modern-day fire ecology (Johansson and Granström 2014). Such a mechanism would explain the Ericaceous PAR variability – that does not always correspond to that of abundance – where expansion events are followed by burning that leads to new resprouting thickets and therefore new biomass to be burnt. In addition, dry forests keep a relatively constant and low presence despite an initial PAR increase, that together with the *Erica* dominance, may suggests the establishment of a wetter and warmer phase during this period. The relative expansion of hydrophytes such as Cyperaceae would also indicate higher water availability. From 10 to 8 ka BP Poaceae expansion coincides with reduced CHAR and *Erica* PAR indicating a progressive opening of the landscape.

## 9.4.3 GGU3 Full extension of the Ericacoeus belt (636–549 cm, 8.5–7.2 ka BP, 30 pollen samples)

*Erica* spreads as indicated by its PAR, with a consequential biomass burning and decline in the herbal community while the dry forest boundaries keep similar PAR and abundances. We infer relatively similar climate conditions at this stage, with no clear community transformations as supported by unchanging PrC scores.

## 9.4.4 GGU4 Ericaceous belt-dry montane forest turnover (548–291 cm 7.2–3.6 ka BP, 52 pollen samples)

### 9.4.4.1 GGU4.1 Progressive *Erica* decline (547–418 cm, 7.2–5.4 ka BP, 33 pollen samples)

The Ericaceous belt slightly contracts at 7.2 ka BP as the Upper dry forest boundary expands, concurring with increasing biomass burning. The Lower dry forest expands at the end of this zone, while the Ericaceous belt progressively contracts as illustrated by increased PrC scores. The GG landscape may have opened up further as *Erica* cover declines from 6 ka BP onwards, enabling a higher deposition of lowland dry forest pollen taxa, indicating progressively drying conditions.

### 9.4.4.2 GGU4.2: Lower dry Afromontane forest expansion (417–291 cm, 5.4–3.6 ka BP, 19 pollen samples)

The Ericaceous belt and the local biomass burning continue declining while the Lower dry forest boundary, dominated by *Podocarpus* and *Olea*, expands with varying pollen concentrations,
concurring with the spread of *Artemisia* with equivalent values to those of zone GGU2a. This
landscape succession suggests drying conditions concurring with the regional AHP termination
(de Menocal *et al.* 2000).

9.4.5 GGU5 Upper dry forest expansion, (290–109 cm, 3.6–1 ka BP, 50 pollen samples)

This zone features the constant presence of the dry forest communities, with *Juniperus* dominating
until c. 2 ka BP. After this *Juniperus* declines in conjunction with new biomass burning and the
scarce, consistent presence of ruderals (e.g. *Plantago*) or other herbs in the Afroalpine domain.
We hypothesize that this landscape change is likely due to a dry forest upwards shift under similar
climate conditions to GGU4b. Increasing human pressure from c. 2 ka BP may have triggered
more frequent or intense fires to obtain new pastures at the Upper dry forest-Ericaceous Belt
limit. The increase in PAR c. 2 ka BP in all forest components might be linked to brief climate
changes, as increasing temperature or short drought spells (Marshall *et al.* 2009).

9.4.6 GGU6 Upper dry forests and ruderal plants presence (108–60 cm, 1000–94 yr BP, 17 pollen samples)

The last millennium at GG presents a locally open landscape with abundantherbs, especially
those indicating human agency (e.g. *Plantago*), with scarce *Erica* presence and an upwards shift
of the Upper dry forest. While we cannot discount the possibility of persistent drying conditions
and relative temperature changes, we understand landscape changes in this zone to be mainly
human-driven.

ACKNOWLEDGEMENTS

This study was funded by the DFG research unit FOR 2358 ‘Mountain Exile Hypothesis’.

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<figure><img src="image_53.png" /></figure>

Taylor & Francis

Taylor & Francis Group

http://taylorandfrancis.com
CHAPTER 10

Lower to Mid-Pliocene pollen data from East
African hominid sites, a review

Raymonde Bonnefille

Honorary Director of Research, CNRS, CEREGE, Aix Marseille University, CNRS, IRD, INRAE, Coll. France, Technopole Arbois-Méditerranée, Aix en Provence, France

Benjamin Bourel

CEREGE, Aix Marseille University, CNRS, IRD, INRAE, Coll. France, Technopole Arbois-Méditerranée, Aix en Provence, France

ABSTRACT: This paper presents fossil pollen data (39 samples) available for the Lower Pliocene outcrops at five hominid sites in East Africa located within the intertropical region between 3° South to 11° North. They are dated from 4.2 to 2.95 millions years (Ma). Most of these data were obtained many years ago, except one additional pollen assemblage recently obtained in the Woranso-Mille area of the Lower Awash valley, Ethiopia. In East Africa, Plio-Pleistocene sedimentary sequences are internationally renowned for the continuous palaeo-anthropological researches pursued for decades, following the discoveries of preserved remains of fossil hominins associated to a rich, diversified and abundant mammalian fauna. As a result of these new discoveries, geological studies have continued, being connected to following field expeditions and exploration of new sites in the Ethiopian Rift. We present here the revised geological context, adding chronological precision to previous age constraints of the original pollen data. For each terrestrial Pliocene site, we also include some comments regarding the significance of pollen counts and briefly discuss their interpretation for reconstructing past vegetation, environment, and inferred climatic parameters at the studied hominin sites.

**10.1 INTRODUCTION**

Pollen data provide information dealing with plant biodiversity and composition of past vegeta-
tion. Together with other palaeoecological proxies, these results concern regional environmental
conditions, habitats, and food resources. All items are abundantly discussed to explain hominin
species niches and evolution. Pollen studies from outcrops at hominin sites – that will be pre-
sented hereafter, and from deep core sediments off eastern Africa (e.g. Feakins et al. 2013) stand
as critical contribution to the knowledge of hominin palaeoenvironments. Pollen data presented
here mostly concern (with one exception) Pliocene outcrops from northern Ethiopia and Tanzania
investigated a few decades ago.

Since the first publications of the original pollen data (Bonnefille 2010; Bonnefille and
Riollet 1987; Bonnefille *et al.* 1987), new dated tephra (or tuffs) have been identified in the same
outcrops (Table 1) and progress in the ⁴⁰Ar/³⁹Ar dating method have been made, notably for
<table><caption>**Table 1.** List of dated tuffs from the Pliocene Hadar Formation (Ethiopia) used to calculate interpolated age of Hadar pollen samples (Campisano 2007).</caption><thead><tr><td>Tuff and basalts</td><td>Abbreviation</td><td>Age (Ma)</td><td>Original references</td></tr></thead><tbody><tr><td>Bouroukie Tuff 2U</td><td>BKT-2U</td><td>2.96±0.006</td><td>Campisano 2007</td></tr><tr><td>Bouroukie Tuff 2L</td><td>BKT-2L</td><td>2.96±0.02</td><td>Semaw et al. 1997</td></tr><tr><td>Kada Hadar Tuff</td><td>KHT</td><td>3.20±0.01</td><td>Walter, 1994</td></tr><tr><td>Triple Tuff 4</td><td>TT-4</td><td>3.256±</td><td>Campisano 2007</td></tr><tr><td>Kada Damum Basalt</td><td>KMB</td><td>3.30±0.04</td><td>Renne et al. 1993</td></tr><tr><td>Kada Me'e Tuff</td><td>KMT</td><td>3.36±0.002</td><td>Campisano 2007</td></tr><tr><td>Sidi Hakoma Tuff</td><td>SHT</td><td>3.42±0.03</td><td>Walter and Aronson 1993</td></tr></tbody></table>

sites located in the Lower Awash Valley. Allowing previous stratigraphic placement of the pollen samples initially positioned versus tephra layers used as marker beds, it is now possible to revise ages of the pollen assemblages and place them in a more accurate chronological framework.

In this paper, we present the updated pollen data as supplementary data available for further research and interpretation (Supporting Online Material [SOM] Tables S1 to S4). We discuss them according to geographical location and chronological order. However, while addressing past vegetation changes through time it is necessary to consider the present and past altitudinal location at each site. Indeed, under the same global climatic conditions, past vegetation can differ between sites at different geographic locations. It is well known that relief modifies local climatic conditions. Altitudinal distribution of the vegetation inside zones (or belts) influences diversity in habitat patterns (Friis et al. 2010). A key feature of tectonic activity is the disruption of the river drainage. Geomorphological processes maintain habitats with variable topography through time (Reynolds et al. 2011). However, establishing past elevation at each fossil site for different time periods during the Lower Pliocene remains difficult. In this paper it is accepted that Ethiopian rifting and architecture of the Omo–Turkana basin was already attained since the eruption of the Gombe Group Basalt dated at c. 4.2 Ma (Erbello and Kidane 2018).

## 10.2 PRELIMINARY REMARKS FOR FURTHER USE OF POLLEN DATA

Pollen analysis of Plio-Pleistocene deposits from East Africa started in the 1970's with the first discoveries and field exploration in the search of hominin fossil remains documenting our ancestral origin. These initial pollen studies were strongly linked to the pioneer original stratigraphic, geological descriptions and mapping of the sedimentary terrestrial outcrops that yielded these remains. The universal and standardized framework, adopted at that time for collecting faunal remains and pollen samples at Laetoli, and at Hadar (Figure 1), was shared by all scientists involved, and still stands valid. Therefore, each pollen sample was given a label starting with the number corresponding to the fossil fauna locality, including hominin localities such as Afar Locality 333 (AL 333). Pollen sampling done along a section follows with underscore letters (a, b, c, etc.) indicating the stratigraphic succession.

Progress in absolute dating techniques provided further precision to date the fossil hominins remains, the abundant associated fauna and, therefore, pollen data. The initial stratigraphic position of pollen samples can now be integrated into the new geological studies recently carried out at each of the different sites (Campisano 2007; Campisano and Feibel 2008a, 2008b; Deino 2011; Harrison 2011). At the time of collecting pollen samples (except for sample MRD-210), GPS was not available, therefore approximate coordinates for the Hadar samples had to be
<figure><img src="image_54.png" /><figcaption>**Figure 1.** Location map of the Pliocene sites in Tanzania and Ethiopia (Middle and Lower Awash).</figcaption></figure>

calculated by importing the initial map previously established on aerial photographs (Bonnefille *et al.* 1987) into a Geographic Information System (QGIS v3.2.2) (Table 2).

In the early days of pioneer African palynology, fossil pollen studies at Olduvai, Laetoli and Hadar were also associated to preliminary investigation of modern pollen soil samples from various vegetation types and plant collecting in vegetation surrounding the Pliocene sites. Interpretation of pollen data in term of past vegetation changes through time should also consider additional information of the setting in which these data were obtained. Indeed, different environmental contexts such as the occurrences of small or big lakes, existing delta or meandering river and floodplains, locally modify the pattern of the regional vegetation reflected by the composition and percentages of pollen taxa in the assemblages (spectra).

In this paper, sedimentary conditions provided by detailed and new geological studies are briefly summarized while comparing the chronological succession of the pollen data between the different terrestrial sites. Pliocene East African pollen sites are located at various elevations, even in the African Rift. Researchers who will address the vegetation dynamic through time, such as comparing with isotopic studies, have to remember that composition of modern and past vegetation vary according to local climatic patterns, relief and occurrence of water bodies. Moreover, East Africa is an area of tectonic changes and active volcanism. But palaeo-topography and palaeo-geography are far from being established at sites older than 3 Ma.

Pollen data included in this paper inform on terrestrial past vegetation for snapshot windows through the Pliocene. Considering their accurate dating they can be considered as valuable benchmark in the expectation of continuous pollen record from terrestrial cores. Although thoroughly investigated, Pliocene outcrops of tropical East Africa, hardly preserved fossil pollen.
<table><caption>**Table 2.** List of Pliocene pollen samples from the Lower Awash, Ethiopia. Nb 0 (Bonnefille *et al.* 1987; Saylor *et al.* 2019), N (Bonnefille *et al.* 2004).</caption><thead><tr><th>Nb 0</th><th>N</th><th>Stratigraphic position</th><th>Locality</th><th>Geographical coordinates</th><th>Collector</th><th>Submember</th><th>Age (Ma)</th></tr></thead><tbody><tr><td>55Nj</td><td>(13)</td><td>Clay 2 m below tuff BKT-2</td><td>Hadar, Kada Hadar</td><td>11° 08'31"N; 40°34'49"E</td><td>RB, AV</td><td>KH-2</td><td>2.95</td></tr><tr><td>116f</td><td>(12)</td><td>Clay 10 m above tuff complex TT-4</td><td>Hadar, Kada Hadar</td><td>11° 08'25"N; 40°35'34"E</td><td>RB 1973</td><td>DD-1/DD-2</td><td>3.22</td></tr><tr><td>116e</td><td>(11)</td><td>Silt 2 m above tuff complex TT-4</td><td>Hadar, Kada Hadar</td><td>11° 08'25"N; 40°35'34"E</td><td>RB 1973</td><td>DD-1</td><td>3.23</td></tr><tr><td>333d</td><td>(10)</td><td>Clay Ostracod layer in tuff complex TT-4</td><td>Hadar, Baruteita</td><td>11° 07'39"N; 40°34'38"E</td><td>RB, AV</td><td>SH-4/DD-1</td><td>3.256±0.018</td></tr><tr><td>333c</td><td>(9)</td><td>Black clay, 50 cm below ostracod layer</td><td>Hadar, Baruteita</td><td>11° 07'39"N; 40°34'38"E</td><td>RB, AV</td><td>SH-4</td><td>3.24</td></tr><tr><td>333b</td><td>(8)</td><td>Sandy clay below tuff complex TT-4 and Pink Marl</td><td>Hadar, Baruteita</td><td>11° 07'39"N; 40°34'38"E</td><td>RB, AV</td><td>SH-4</td><td>3.25</td></tr><tr><td>134b</td><td>(7)</td><td>Green clay 20 cm below Tuff complex TT-4</td><td>Hadar</td><td>11° 07'45"N; 40°35'53"E</td><td>RB 1974</td><td>SH-4</td><td>3.25</td></tr><tr><td>266b</td><td>(6)</td><td>Green clay below tuff complex TT-4</td><td>Hadar, Kada Hadar</td><td>11° 07'40"N; 40°35'44"E</td><td>RB 1974</td><td>SH-4</td><td>3.25</td></tr><tr><td>228b</td><td>(5)</td><td>Silt lower contact of pink marl</td><td>Hadar, Sidi Hakoma</td><td>11° 06'19"N; 40°34'35"E</td><td>RB 1974</td><td>SH-3/SH-4</td><td>3.28</td></tr><tr><td>228a</td><td>(4)</td><td>Brown clay gastropod, 11 m above SH-3s</td><td>Hadar, Sidi Hakoma</td><td>11° 06'19"N; 40°34'35"E</td><td>RB 1974</td><td>SH-3</td><td>3.30</td></tr><tr><td>280 Nj</td><td>(3)</td><td>Silty clay 3 m above top SH-3s</td><td>Hadar, Kada Hadar</td><td>11° 07'15"N; 40°37'53"E</td><td>RB 1974</td><td>SH-3</td><td>3.30</td></tr><tr><td>KMm</td><td>(2)</td><td>Clay with plant debris, gasteropods</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.35</td></tr><tr><td>KMl2</td><td>(2)</td><td>Silty sand with plant remains</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.35</td></tr><tr><td>KMl1</td><td>(2)</td><td>Silty sand with plant remains</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.36</td></tr><tr><td>KMk</td><td>(2)</td><td>Laminated clay with intercalated tuff</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.36</td></tr><tr><td>KMj</td><td>(2)</td><td>Laminated clay</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.37</td></tr><tr><td>KMi</td><td>(2)</td><td>Grey clay with gastropods</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.37</td></tr><tr><td>KMh</td><td>(2)</td><td>Ostracods clay with gastropods 10 m above SHT</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.37</td></tr><tr><td>KMg</td><td>(2)</td><td>Silty sand 2 m above lignite layer</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.37</td></tr><tr><td>KMf</td><td>(2)</td><td>Clay gastropods and ostracods above lignite layer</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.37</td></tr><tr><td>KMe</td><td>(2)</td><td>Lignite thick layer within KMT</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.36±0.02</td></tr><tr><td>KMd2b</td><td>(2)</td><td>Clay with gastropods, fishes below lignite layer</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.37</td></tr><tr><td>KMd2a</td><td>(2)</td><td>Clay with gastropods, fishes below lignite layer</td><td>Hadar, Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.37</td></tr><tr><td>KMc</td><td>(2)</td><td>Silty clay 2.5m above sample b</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.37</td></tr><tr><td>KMb</td><td>(2)</td><td>Brown silt 1 m above sample a</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.38</td></tr><tr><td>KMa</td><td>(2)</td><td>Black clay 26 m above SHT</td><td>Hurda</td><td>11° 06'36"N; 40°39'03"E</td><td>RB, AV</td><td>SH-2</td><td>3.38</td></tr><tr><td>398b</td><td>(1)</td><td>Green clay interbedded between two SHT layers</td><td>Dikika Oudaleita</td><td>11° 06'12"N; 40°36'39"E</td><td>RB 1974</td><td>BM</td><td>3.42</td></tr><tr><td>MRD-210</td><td></td><td>Deltaic sands, below Mille Tuff</td><td>Worenso MRD-VP-1</td><td>11°32'55"N; 40°28'01"E</td><td>Levin 2018</td><td>BM</td><td>3.80</td></tr><tr><td>Meshellu</td><td></td><td>"Lignite/coal" 20 cm thick above green clay</td><td>Dikika Meshellu River</td><td>11°05'00"N; 40°40'37"E</td><td>RB, JK 1973</td><td>BM</td><td>4.00</td></tr></tbody></table>

Among the whole set of sediment samples collected from the different geological strata at the different sites, the degree of success in pollen extraction as result of preservation was <10% of the total processed samples, despite preferentially selecting lacustrine deposits (Bonnefille *et al.* 1987). It is hoped that further investigation and pollen studies carried out on sediment obtained from deep terrestrial drilling projects, such as the Hominin Sites and Palaeolakes Drilling Project (Cohen *et al.* 2016) can provide additional results. Following this line, two cores (NAW14-1A, NAO14-1B) (Figure 2) collected from the assumed position of the Hadar palaeo-lake indicate that the core sediments cover the period between 2.91 Ma and 3.33 Ma (Campisano 2012; Campisano *et al.* 2017; Garello 2019). These sediments have provided nine additional pollen assemblages corresponding to Pliocene strata (Bourel 2020).

During the early days of development of tropical palynology, fossil pollen identifications were based upon many botanical papers dealing with plant systematics. Descriptions of pollen
<figure><img src="image_55.png" /><figcaption>**Figure 2.** Location map of the lower Pliocene pollen samples from Aramis (dated 4.4 Ma) in the Middle Awash valley.</figcaption></figure>

from modern plants of tropical Africa illustrated in several pollen atlases (Bonnefille 1971a, 1971b; Bonnefille and Riotlet 1980) were the basis for fossil pollen identifications. It might also be noted that all pollen identifications and counting of the data presented herein were carried out in the same laboratory though its location and name have changed over the years (pre-1979: Laboratoire de Géologie du Quaternaire (LGQ) CNRS Bellevue-Meudon; 1979–1994 LGQ, Marseille-Luminy University; post-1994: CEREGE, Aix-en-Provence). Researchers and technical staff used the same criteria and nomenclature for their pollen diagnoses and compared fossil pollen to analogues included in the same modern pollen reference collection (n=7000 species) built by the first author, and now stored at CEREGE. This fact points to the consistency and taxonomical homogeneity of the East African Pliocene pollen data provided in this paper. Considering that our pollen studies were initiated before isotopic methods had been developed, the reference collection is focused on tree species more than on herbaceous plants. The homogenous pollen morphology of all genera and species among gramineae does not allow distinction between C₃ and C₄ grasses.

From the phytogeographical point of view, the African Plio-Pleistocene sites discussed here are located within the intertropical region. This zone belongs to the phytogeographical domain 'Somalia-Masai *Acacia-Commiphora* steppe', mostly defined upon species endemism and homogenous ecological climatic conditions. It is named after the abundance of more than hundred trees species for each of these two very common genera (White 1983). This region is now also called and mapped as 'deciduous bushland and thicket' (van Breugel et al. 2015). The landscape of the 'Somalia-Masai *Acacia-Commiphora* deciduous bushland and thicket' does not appear homogenous since a great diversity of local ecosystems exists. The density of forest cover varies near rivers, lakes, springs, high water tables, and on mountains. On a broader regional scale, the density of tree and shrub cover within this landscape depends on the monsoon rainfall distribution at the different sites. Moreover, the Pliocene sites investigated in this paper are located between 500 and 1500 m asl. Therefore, temperature, amount of rainfall, and patterns of rainfall distribution vary accordingly to the topographic context of the Rift.
Proximity of mountains and volcanoes allows various types of evergreen forests to be estab-
lished on their slopes. Within the large Somalia-Masai region, the vegetation belts on different
mountains have distinct floristic characteristics (such as different types of forests) and constitute
an independent phytogeographical domain called 'Afromontane', with specific plant taxa (White
1983). Pliocene fossil pollen assemblages from fluviatile sediment collected in the lowlands may
include pollen signal of different ecosystems mixed together. While reconstructing past vegeta-
tion around a studied site, spatial distribution of different ecosystems may be postulated using
knowledge of the palaeogeographical and geological contexts, both at the local and regional
scales. Such an approach relies on thorough studies of modern pollen deposition (taphonomy),
and appropriate assumptions based upon geological studies.

**10.3 LOWER PLIOCENE DATA (4.4 TO 3.8 MA)**

Palaeobotanical data from this time period come from Ethiopia (four sites) and Tanzania (one site).

**10.3.1 Middle Awash Valley, Aramis locality, 565 m asl, 10°30′N, 40°30′E, 4.3 Ma,**
**Ethiopia**

Extensive Pliocene floodplain deposits occur in the Middle Awash Valley of the Ethiopian Rift. At Aramis (Figure 2), field prospections provided fossil remains of *Ardipithecus ramidus*, an interesting hominin associated with an abundant fauna dated at 4.4 Ma (White *et al.* 2009). Pollen samples collected directly in the sediment that contained *Ardipithecus ramidus* did not provide consistent pollen results. We found a few grains of sedges (Cyperaceae), grasses and palm tree attributed to *Hyphaene/Borassus*. All these plants were later documented by abundant phytoliths found in the same dated sediment (Barboni *et al.* 2019; WoldeGabriel *et al.* 2009). Fossil wood studies identified trees such as *Ficus* (Jolly-Saad *et al.* 2010), *Syzygium, Cordia* and *Rothmania* (Jolly-Saad and Bonnefille 2012), together with *Celtis*, a tree identified by the abundant calcified stones of its fruits (WoldeGabriel *et al.* 2009). All these trees provide edible fruits. In agreement with the fossil fauna and preserved geological evidences (tufa) for groundwater discharge areas (springs), the palaeobotanical record is interpreted as a spring forest or palm woodland that allowed a forest habitat for this hominin adapted to climbing within a grass-dominated landscape (Barboni *et al.* 2019).

**10.3.2 Lower Awash valley, Meshellu/Dikika, 500 m asl, 11°05′N, 40°41′E, 4 – 3.8 Ma,**
**Ethiopia**

A pollen sample collected in 1973 by R. Bonnefille and J. Kalb from lacustrine organic sediment exposed along the Meshellu drainage, east of the Dikika area (Figure 3) has provided a pollen spectrum that documents a past vegetation that is fairly different from that of Aramis site (Bonnefille 2010, Table 1) (SOM Table S1). According to its stratigraphic position below the Sidi Hakoma Tuff (SHT), the pollen sample belongs to the Basal Member of the Hadar Formation. It is likely placed between the Ikini Tuff dated to 3.8 Ma (Wynn et al. 2006) and the Afar Straitoid Series Basalt dated to 4 Ma (Wynn et al. 2006). At Meshellu/Dikika, c. 4 Ma ago, a large extensive swamp with abundant aquatic *Typha* and sedges occurred among an herbaceous vegetation including herbs such as *Isoglossa*, *Apiaceae*, *Brassicaceae*, *Caryophyllaceae*, *Gentianaceae*, *Polygonaceae*, mostly abundant in current highland swamps or grasslands, such as observed by the first author in the Bale Mountains. All tree pollen taxa (*Alchornea*, *Macaranga*, *Hagenia*, *Rubiaceae*, *Hymenodictyon*, etc.) belong to highland humid forests in which the conifer *Podocarpus* is absent. This pollen assemblage is devoid of pollen taxa from dry evergreen bushland or
<figure><img src="image_56.png" /><figcaption>**Figure 3.** Location map of the Pliocene pollen samples from Woranso/ Mille (3.8 Ma), and Hadar site (3.4 to 2.9 Ma), in the Lower Awash valley.</figcaption></figure>

sub-desert steppe which prevail in present dry climatic conditions of the Ethiopian Rift. Humid and cool local conditions explain the occurrence of an extended swamp among a diversified tropical forest with great affinities with high elevation humid forests and grasslands of the Ethiopian plateau today.

### 10.3.3 Lower Awash Valley, Woranso-Mille, 660 m asl, 11°33′N, 40°28′E, 3.8 Ma, Ethiopia

The MRD-210 pollen spectrum was recently extracted from a sandy horizon directly providing remains of *Australopithecus anamensis* dated 3.8 Ma (Figure 3) (Saylor *et al.* 2019) (SOM Table S2). Within the geological context of deltaic deposits, abundant pollen of the creeping herb *Tribulus terrestris* and Nyctaginaceae with the forb *Aerva* occupied the alternatively inundated floodplain such as found today in the Okavango Delta. Pollen of *Acacia* (a botanical genus now separated into *Vachellia* and *Senegalia*) is exceptionally abundant. The low pollen representation of this taxon in samples of modern vegetation, and its low dispersal ability (Hamilton 1982) indicate that dense *Acacia* woodland occurred locally. Including *Euclea* and *Combretum*, such woodland seems to have affinities with modern deciduous woodland encountered today on escarpments at c. 1300–1500 m asl. *Boscia* and Capparaceae together with *Rhus*, *Acalypha* and *Trilepisium* could also be found in riparian forest. Other tree pollen, such as *Juniperus*,
<figure><img src="image_57.png" /><figcaption>**Figure 4.** Topographic map showing the Laetoli location on the slopes of several volcanos around the Ngorongoro caldera, East African Rift (Tanzania).</figcaption></figure>

*Podocarpus, Olea* and *Hagenia* attest to the occurrence of a drier conifer mountain forest, similar in composition to that present on Eastern Ethiopian highlands. Habitat scores based on plant functional types (PFTs) method are highest for steppe (25%) and woodland (34%) whereas riparian dry open *Acacia* woodland is documented, locally, in the palaeovegetation (Saylor *et al.* 2019).

#### 10.3.4 Laetoli site, 1800 m asl, 3°13'S, 35°13'E, 3.83 – 3.8 Ma, Tanzania

In the southern part of the Eastern Rift Valley, the Laetoli site is located at much higher elevation (1800 m asl) than other Middle and Lower Pliocene deposits from the Awash Valley in Ethiopia (Figure 1). Deposits consist of air-fall tuffs provided by the repeated activity of Ngorongoro volcanic highlands with no evidence of permanent water system other than possibly localized springs, and river drainage, south of our sampling area.

Pollen preservation was very poor in most terrestrial samples (n=40) collected in the volcanic fossil-rich deposits of the Upper Laetolil Beds yielding remains of *Australopithecus afarensis*, including tracks of its fossilized footprints (Leakey *et al.* 1987). The Upper Laetolil Beds revealed less productive than the other sites, a conclusion confirmed by further investigation (Rossouw and Scott 2011).

Fossilized termite mounds were so abundant that we decided to investigate their infill in order to obtain some information on past vegetation (Bonnefille and Riollet 1987). It is assumed that termites feeding upon local surrounding vegetation, the ventilation shaft must have been open to the air during the interval in which the termitary was occupied. The hives were sealed shortly after the demise of the colony. Atmospheric pollen trapped in the hives was preserved under anaerobic conditions and the termite structure buried and fossilized by the volcanic ash. Indeed, macroscopic observation during field collecting showed organic matter, vegetal fragment tissues
and pollen preserved in the hive chamber infill. Under such taphonomic conditions, possible bias of these pollen assemblages can be expected. In order to test such hypothesis, modern soil samples were collected in different ecosystems of the regional vegetation along a 30km altitudinal transect, together with plant species (appendix A in Leakey et al. 1987). Modern pollen assemblages from short and medium grasslands between 1500 to 1800 m asl are characterized by the dominance of pollen from herbaceous plants, pollen of grasses being less abundant than pollen of others herbs or forbs of the Asteraceae, Acanthaceae, Amaranthaceae, Fabaceae and Cyperaceae. Overgrazing and burning by pastoralists may have contributed to increasing forbs and ligneous plants typical of the current short or medium grasslands of Laetoli/Olduvai region. In modern pollen surface samples collected from the wooded grassland or woodland surrounding the Laetoli area today at 1700–1800 m elevation, tree pollen including *Acacia*, *Euclea* and other Sudano-Zambezian taxa account for 20% of the total counts (Bonnefille and Riotlet 1987). A homogenous signal of *c*. 2% pollen from forests, now covering the slopes of the 20 to 30 km distant volcanoes correspond to aeolian pollen transport. In the fossil pollen data, the distribution of the various taxa appears consistent with that in modern soil samples, an argument bringing to the fore that past vegetation at the site can be reconstructed from fossil pollen extracted from termite mounts infill.

Altogether, pollen data at Laetoli consist of ten pollen assemblages sampling only the Upper Laetolil Beds between 3.83 to 3.68 Ma. The oldest nine pollen samples collected in a 4 metres stratigraphic interval around Tuff 1 (Hay 1987) span an interval of *c*. 6 ka before 3.83 Ma, on the basis of sedimentation rate (Deino 2011). In between, there is a time span of 150 ka with no pollen information. The youngest pollen assemblage was obtained from a hive chamber collected 3 m below the footprint tuff (Tuff 7) dated 3.66 Ma (Deino 2011). It has an attributed age of 3.68 Ma (youngest sample ML-81-7).

The Laetoli pollen data (SOM Table S3) total 4354 identified and counted pollen grains. We note an important number of pollen taxa from trees and shrubs (26), most of them being regular components of evergreen bushland and mountain forests. Some pollen taxa *Boscia* and *Celtis* being also identified as seeds and fruit stones, macro-botanical remains recovered from the top part of the Laetolil Beds at 3.66 Ma (Bamford 2011). It is likely that deciduous woodland occupied the peneplain between Eyasi and Olduvai divides from 3.8 to 3.6 Ma ago. Percentages of pollen from Afromontane forests are three to five times greater than today indicating that such forests were more extensive or closer to the site during the Mid-Pliocene. The Laetoli herbaceous palynoflora is also remarkable by a great number of pollen taxa (*c*. 54) that belong to tropical families such as Acanthaceae, Amaranthaceae, Euphorbiaceae, Zygophyllaceae, with many Asteraceae, Fabaceae and Solanaceae, etc. The fossil pollen composition points to rich and diversified low and medium grasslands, together with grass pollen being more abundant than in the pollen samples from modern vegetation, a conclusion in agreement with the record of several grass phytoliths (Rossouw and Scott 2011).

At Laetoli, the fossil pollen data show that several different vegetation types, highland forests, woodland and grasslands, were available to the rich fauna and bipedal hominin *Australopithecus afarensis*. The local vegetation at Laetoli was woodland or wooded grassland, whereas highland forests were located on nearby volcanoes. The nine oldest Laetoli pollen spectra appear close in age to that recently obtained at Woranso/Mille, in the Lower Awash valley of the Ethiopian Rift (Saylor *et al.* 2019) at more than a thousand kilometres distance in the northern tropics. The occurrence of *Acacia*, *Boscia*, *Celtis*, Capparaceae, and several others at both sites despite strongly different sedimentological contexts, indicates a great expansion of this woodland vegetation that was widespread in the southern and northern tropics. Woodland and wooded grasslands close to forests were available both for *A. anamensis* in Ethiopia and *A. afarensis* in Tanzania. At Woranso-Mille, these hominins made use of mixed grassland–wooded vegetation more or less similar in composition to woodlands and grasslands existing in the Laetoli plains,
but in Ethiopia such vegetation was associated with a more densely wooded riparian forest along
an important river delta.

10.4 MID-PLIOCENE (3.42-2.95 MA), ETHIOPIA

Fossil pollen from this time period are only documented at Hadar site, 500 m asl, along the lower valley of the Awash River (11°06'N, 40°35'E), in the Afar depression of the Ethiopian Rift (Figure 2). The sediments and intercalated volcanic tuffs were deposited in a low topography of a rapidly subsiding sedimentary basin (Roman *et al.* 2008). Pollen data from 27 samples (totalling over 15,000 counted grains) were obtained during the first years of palaeontological exploration of the research area (1973–1977). They were collected from sediments outcropping in the type sections of the Hadar Formation dated from 3.42 to 2.95 Ma (Figure 5). In the original publication, these results were presented according to stratigraphic position and the conventional K-Ar dates available at that time (Bonnefille *et al.* 1987). We revisit here the chronological attribution of former pollen samples using the most recent tephro-chronostratigraphy, including additional <sup>40</sup>Ar/<sup>39</sup>Ar dates and newly identified and dated tephra (tuffs) (Campisano 2007; Campisano and Feibel 2008a, 2008b; Roman *et al.* 2008) (Table 2, Figure 5).

The oldest pollen sample (398b) was collected at Oudaleita, a locality on the right bank of the Awash River (Bonnefille et al. 1987), now included in the research area of the Dikika Project, although the outcrops still belongs to the Hadar geological Formation. It was extracted from a lacustrine diatom-rich green clay intercalated between two layers of the Sidi Hakoma Tuff (SHT) that separates the Sidi Hakoma Member from the the Basal Member. SHT is precisely dated 3.42±0.03 Ma at Hadar (Walter and Aronson 1993 revised by Campisano 2007) and correlates with two other tuffs from the Turkana basin (Brown 1982), notably with the Tulu Bor B at East Turkana also dated 3.42±0.03 Ma (McDougall et al. 2012). At the lowest level of the Sidi Hakoma Member of the Hadar Formation, the perfectly-dated pollen assemblage 398b shows a remarkable diversity of plants attested by 57 identified pollen taxa. This is partly explained by the high total pollen count (c. 1200 grains) providing highly reliable information. But such pollen assemblage fits well with deposition of fluvial deltaic channel, as shown by pollen content of a modern sample mud from the Awash River (Bonnefille 1969). Abundant grass pollen (36%) are associated with Afroalpine herbs such as *Alchemilla*, *Carduus*, *Laurembergia*, *Lythrum*, *Potamogeton* and *Hypericum*, which today in Ethiopia is largely represented by an abundant small tree (rather than herb species) at the upper limit of the the forest zone. Therefore, such pollen assemblage likely documents mountain grassland above the forest zone, while sedges and *Typha* were scarce. A total of 24% pollen from trees and shrubs indicates an important tree cover, whereas the high pollen taxa diversity attests of extended well-established woodlands. Significant percentages of *Celtis*, *Acacia*, *Dombeya*, *Ekebergia*, *Euclea*, *Brucea* and *Juniperus* indicate that components of the present-day evergreen bushlands and deciduous woodlands that occupy the mid-elevation vegetation zone (1500–1800 m asl) on slopes of the northern Ethiopian plateau, were particularly dense at that time. A high percentage of ferns spores (15%) in this oldest Hadar pollen sample is unusual and likely reflect high atmospheric humidity, or more humid conditions such as attested also by the occurrence of *Alangium* and *Garcinia*, these trees being components of more humid forests (Bonnefille *et al.* 1987).

The next samples (n = 14) were collected within a 19 m-thick section exposed at Hurda wadi (previously spelled Ourda) and located c. 26 m above the SHT Tuff. This section of dark clay deposits includes a thick coal/lignite layer (sample KMe) intercalated between sandy levels of SH-2 and SH-3 sub-members. Recent geological study found that this lignite likely correlates to the thin darker unit layer within the new Kada Me'e Tuff complex (KMT), newly identified below the SH3 sands and dated 3.36±0.02 Ma (Campisano 2007; Campisano and Feibel 2008b). This date was not available in former publications, although it is in fairly good agreement with
previous estimated chronological placement of the samples (Bonnefille et al. 2004, Figure 3). The chronology of the numerous pollen data from the lower SH-2 sub-member of the Sidi Hakoma Member is now well constrained. It spans a time interval of c. 30 kyr, from 3.38 to 3.35 Ma. Interpolated ages were calculated for sample KMa to m considering that all samples are located below the SH-3 sands. However, this unit defined in the Hadar type section is not clearly expressed in the Hurda section where clay deposits have a greater thickness than in the Hadar type sections (Campisano and Feibel 2008a). Pollen particularly well-preserved in the Hurda section are stratigraphically attributed to the SH-2 sub-member of the Hadar Formation. We attribute their preservation to anoxic conditions attested by the occurrence of *Cyprideis ostracods* indicating paludal environment (Peypouquet et al. 1983), and the presence of natrojarosite, a blue mineral attesting of acidic environment without clastic input (Tiercelin 1986).

The pollen assemblages are divided into two pollen zones showing different taxonomic composition and pollen percentages. The lower samples (KMa to i) surrounding the 3.36 Ma lignite and KMT Tuff, are characterized by abundant pollen of grasses associated to sedges and aquatic *Typha* (23% of the total pollen), a plant that requires freshwater. *Typha* pollen are recorded under such high percentages only in modern samples from periodically inundated delta setting, such as the Omo delta (Vincens 1982). Pollen of the aquatic *Polygonum* and abundant Asteraceae (former Compositae) confirm that it was similar to a high elevation swamp. The next five samples (KMj to m) show an important increase in arboreal pollen from humid forest such as *Ilex, Myrica, Prunus* and *Hagenia*, all trees characterizing the upper limit of the forest zone, such as in humid southwestern Ethiopian highlands (Bonnefille et al. 1993). These trees have low dispersal ability (Hamilton 1972). A total arboreal pollen percentage of 20%, and the great number of tree taxa, many from mountain forests, a few from evergreen woodland, indicate that a diversified well-established humid forest occurred at a few kilometres distance from the Hurda section located closer to the depocenter of the Hadar lake. At that time, the landscape in this area was densely wooded with grasslands occupying the lake shores, with *Typha* and rare sedges swamp. Hominin remains have not been found at Hurda, but there are many of their fossils extracted from the Sidi Hakoma section in SH-2 and SH-3 sub-members of the Hadar area located a few kilometres west of Hurda. At Hadar, vertisol mudstones are representative of floodplain deposits of a meandering fluvial system in which remains of hominin fossils are found (Campisano and Feibel 2008a).

Following the chronological succession of the Hadar Formation, four pollen samples collected higher up in the stratigraphic section at localities 228, 280, 266 (Figure 2), in sediment yielding gastropod beds and pink marl below the Triple Tuff, are dated between c. 3.30 to 3.25 Ma (Table 2). Although sediments deposited during this time period correspond to beach/nearshore and lacustrine environment (Campisano and Feibel 2008a), their pollen spectra strongly differ from those of the KM section (Bonnefille et al. 1987). Collected at sites all less than 5km apart (Bonnefille et al. 1987), these pollen spectra show consistent results characterized by the predominance of grass pollen (c. 80%), whereas *Typha* (5%) and sedges (10%) are recorded under much lower percentages. Pollen of mountain forest (0 to 5%) disappear or occur as one or two taxa only. The herbaceous component of the pollen assemblages varies as expected in open grasslands with rare trees occupying the flat plains surrounding a large fluctuating lake. The occurrence of several herbs such as *Alchemilla, Artemisia, Plantago*, now common in the Afroalpine grassland, together with more abundant Asteraceae, associated with the dry conifer *Juniperus, Phoenix* (palm) tree, and Ericaceae shrubs indicates cooler conditions. At the top of the SH-4 sub-member included in the Mammoth event, drier, colder, and more variable climatic conditions were inferred by a statistical process used to calculate humidity coefficient from pollen data (Bonnefille et al. 2004). Sample 228b collected at hominin locality 228 provided the lowest value of humidity coefficient.

Above in the stratigraphy, a group of six pollen samples was extracted from clay deposits in stratigraphic proximity to the Triple Tuff TT-4 initially dated at 3.26 Ma (Walter 1994), and
now considered at 3.256±0.018 Ma (Campisano 2007). Tuff TT-4 marks the limit between the Sidi Hakoma and the Denen Dora Members of the Hadar Formation. Abundance of grass pollen together with fluctuating percentages of the aquatic *Typha* and sedges, suggest vegetation typical of periodically flooded lake margins. The occurrence of the shrub *Dodonaea viscosa* observed as a pioneer on basaltic sub-modern lava flows (field observation by RB) and known as a fire-resistant species, appears in sediment contemporaneous with eruption of the basalt. In these six samples, arboreal taxa such as *Commiphora*, *Acacia*, *Hyphaene/Borassus*, and *Tamarix* are pollen markers of the arid sub-desert steppe like the one now occupying the Rift Valley. The co-occurrences of these trees indicate that warmer climatic conditions established in the region c. 3.25 Ma, during a period of higher global climatic variability attested by the marine isotopic record (Dolan et al. 2015). At Hadar, in the lower Awash valley, samples dated 3.25 Ma were collected at three distinct localities 134, 333 and 116, all found in the northern part of the Hadar area, upstream the Kada Hadar wadi (Bonnefille et al. 1987). A sample of green clay exposed at locality 134, below the TT-4 (top of SH-4 sub member) produced a total pollen count exceeding 2300 grains distributed among 34 distinct plant taxa. Grasses average 60% whereas tree pollen has increased with the addition of taxa commonly found in mid-elevation woodland such as *Croton*, *Acacia*, *Dodonaea*, *Commiphora*, *Hyphaene/Borassus* and *Tamarix*. All are indicators of woodland located at lower elevation, under warmer climatic conditions than mountain forest trees. Abundant pollen of herbs such as Amaranthaceae, Acanthaceae and *Tribulus* that grow on delta plains also indicate higher aridity and/or temperature or increased salinity of the lake waters and margins, a characteristic also indicated by ostracods assemblages from the same layer (Peypouquet et al. 1983).

The next three samples come from the hominin locality AL333, but are not contemporaneous with hominins remains since they were collected from sediment lower in the stratigraphy. A sample of sandy clay below TT-4 shows high grass percentage (66%), associated with abundant *Typha* (32%) suggesting the return of an extended marsh or a shallow freshwater fluctuating lake. Few tree pollen grains are found in the black clay that lies 50cm below the ostracod layer. In the sample from the ostracod layer corresponding to the boundary between Sidi Hakoma and Denen Dora Members, grasses (98%) dominate the pollen assemblage. We also note pollen from the aquatic herb of Onagraceae.

At locality 116, two kilometres towards the escarpment, grass pollen is still dominant (82%) and sedges increase. A subsequent percentage of olive tree pollen associated with *Combretum* and *Euclea* indicates that dry seasonal woodlands spread and came closer to the lake. The pollen sample 116e, upstream of the Kada Hadar wadi, may have consisted of a wooded grassland environment with olive trees on the northern slope towards the escarpment. In conclusion, the composition of pollen assemblages close in stratigraphy to the 3.25 Ma dated Triple Tuff is in good agreement with lacustrine and episodic sub-aerial exposures of depositional environment at the top of the Sidi Hakoma Member, and the base of the Denen Dora Member (Campisano and Feibel 2008a).

After a gap in the record of c. 300 ka with no pollen information for the Kada Hadar Member, the next spectrum come from the brown clay exposed just below the ash fall Bouroukié Tuff (BKT-2), at locality 55N where the first tuff sample was collected and provided the age of 2.88±0.08 Ma (Walter and Aronson 1982). In the upper KH-2 sub-member, the BKT-2 Tuff complex is now considered 2.95 Ma (2.94–2.96 Ma by Campisano 2007; DiMaggio et al. 2008). A rich pollen assemblage (n > 1000) extracted from this clay shows more than 30 taxa and 7.5% trees and shrubs. The arboreal taxa are exclusively found in current Afromontane forests (*Myrica*, *Podocarpus*, *Juniperus*, *Hagenia* and *Ericaceae*) and others are markers of evergreen drier woodland/bushland (*Euclea*, *Ekebergia*, *Dodonaea*, *Acacia* and *Rumex*). Sedges, *Typha* and grasses constitute the herbaceous cover on shores of a shallow lake. A certain degree of salinity can be inferred from the high proportion of Amaranthaceae/Chenopodiaceae herbs or shrubs and the low proportion of *Typha*.
<figure><img src="image_58.png" /><figcaption>**Figure 5.** Stratigraphic position of the original pollen samples (red color) from Bonnefille *et al.*, (1987), in the composite section of the Hadar Formation after Campisano and Feibel (2008).</figcaption></figure>

After the deposition of BKT-2 Tuff, the Hadar geological Formation ends with a well-marked unconformity (Figure 5). The Busidima Formation lies stratigraphically above it, after a gap of deposition of c. 0.3 Ma. Sediments of the Upper Pliocene (2.9 to 2.6 Ma) lack in the Hadar Formation. A few additional pollen spectra were extracted from archaeological horizons dated 2.35 Ma, i.e. in the Pleistocene Busidima Formation. They are not discussed in this paper restricted to the Pliocene.

## 10.5 DISCUSSION

### 10.5.1 Interpretation of pollen data

In the difficult challenge of interpreting pollen assemblages in terms of pattern of past vegetation, landscape or habitat we rely on our field experience and botanical knowledge of tropical vegetation improved by additional exploration of pollen signal from many other modern ecosystems in East Africa (Bonnefille 2011). In pollen studies from temperate vegetation where most pollen is dispersed by wind, taxa with the greatest percentages are assumed to reflect the local vegetation because the greatest amount of pollen is dispersed at proximity of the plant producing them. A similar pattern is suggested for tropical vegetation such as shown in the sub-desert vegetation
of the plains surrounding lake Turkana (Bonnefille 1977). For example, pollen assemblages of modern soil samples collected in different vegetation zones along the slopes of the escarpment of the Ethiopian Rift, north of Hadar, provided pollen indicators of the distinct vegetation belts along an altitudinal gradient, the succession comprising samples from sub-desert steppe to evergreen bushland and Afromontane forest (Bonnefille et al. 1987). Different vegetation types illustrate a similar gradient on a broader scale in Tanzania (Vincens et al. 2006), whereas other samples from southwestern Ethiopia document humid forests from mountains (Bonnefille et al. 1993). However, in the tropical region, pollen from grasses, dispersed by wind, is produced in significantly greater amounts than pollen from other tropical plants, notably most forest trees (Hamilton 1982). As a consequence of this discrepancy, pollen percentages will not directly reflect the proportion of plants in one given vegetation type. Moreover, the sedimentological context in which the fossil pollen grains are deposited, modify the distribution pattern of the percentages of different taxa. Fluvial transport over long distances, can mix distinct pollen grains from various vegetation types, that do not necessarily reflect their surrounding ecological conditions (Vincens 1982). Composition of the pollen assemblage varies according to the deposition mode of the sediment. At Hadar, pollen assemblages such as from the mud of shallow fresh water lake or swamps are characterized by the abundance of *Typha*. Pollen assemblages such as sample 134 which is from a river channel deposit (Campisano and Feibel 2008a), includes a great diversity of pollen taxa from various vegetation types covering a larger source area, thus potentially representing the entire river basin (Bonnefille et al. 1987).

### 10.5.2 Environment of hominins through the Mid and Upper Pliocene

Through the million-year time span between 3.8 and 2.9 Ma, the 39 pollen assemblages discussed in this paper clearly indicate that geographically distinct hominin sites are associated with different types of vegetation. Grasslands, wooded grasslands, woodlands and forests (riparian or mountain), all existed, including different species composition and tree cover density. In a same region, pollen studies show that past vegetation types were also changing through time. In the lower Awash valley of Ethiopia, the oldest sample (4–3.8 Ma) from Meshellu now located at c. 500 m elevation, indicate proximity of high elevation mountain forest. At Worenso–Mille, the pollen assemblage associated to *Australopithecus anamensis* which fossil remains were found in the same strata dated to 3.8 Ma at the same locality, documents a mid-elevation *Acacia* woodland with abundant *Acacia*, *Cordia*, *Croton*, *Rhus*, *Tamarix* and various shrub taxa from the Cappariaceae and also Amaranthaceae that indicate a warm climate. Closer to the northern Ethiopian escarpment, where deciduous and woodland dominated the local landscape some riparian forest and grasslands occupied the delta that also received a pollen signal of distal mountain forest. There are few highland dry conifer forests taxa such as *Olea* and *Juniperus*. A closely related mid-elevation woodland is documented at Laetoli in Tanzania, south of the equator, at about the time when the oldest *A. afarensis* appeared in the East African fossil record. Such drier past vegetation contrasts with that of more humid conditions found in the Lower Awash valley, a few hundred thousand years later when *A. afarensis* prevailed at Hadar.

It is at Hadar that well-preserved, numerous pollen data best document past vegetation. At Hadar from 3.4 to 3.2 Ma, the remarkable feature is the floristic affinities of the fossil taxa to components of the present Afromontane vegetation domain. These include taxonomic attribution of fossil pollen to living plants common in high elevation evergreen bushlands, forests and Afroalpine grasslands. These similarities clearly indicate much cooler and wetter vegetation and a past climate that contrasts sharply with the extremely arid current climate of the Afar region. During the Mid-Pliocene, atmospheric CO₂ concentration was likely similar to today (400 ppm) (Ning Tan et al. 2020), which may partly explain forests expansion at that time. From 3.4 Ma to 2.9 Ma, global decrease in CO₂ concentration is postulated to have enhanced polar glaciation. During the Pliocene, oceanic and atmospheric circulations were totally different from the present
situation. Some modelling experiments indicate more precipitation over East Africa at 3.3 Ma (Dolan et al. 2015).

At 3.4 Ma, in the oldest deposits at Hadar, all the trees belong to woodlands and forests now encountered only in Ethiopian highlands (Friis et al. 2010). The fossil pollen data set lacks indicators of the common sub-desert trees (such as Capparidaceae, Salvadoraceae, *Ziziphus*, *Combretaceae*). Indeed, Mid-Pliocene vegetation at Hadar was markedly different from that of today. *Australopithecus afarensis* and *A. anamensis* were sharing a vegetated landscape that is difficult to reconstruct. Elevation and climatic conditions of the region differed markedly from the current palaeogeographical setting. In a subsiding flat spreading basin, the Hadar Lake could have been closer to the Rift escarpment and its relief. At that time, it could be postulated that the large lake was located at elevation much higher than 500 m. Today forests are established above 1800 m elevation, under a minimum amount of over 1000 mm/yr precipitation with no short dry season, and an average annual temperature of 18°C. These requirements significantly contrast with the low rainfall, high temperature and long dry season of today's climatic conditions in the Rift. Regular characteristics of the sedimentology during the time interval 3.4 to 3.2 Ma, presume of stabilized forest soil and agree with pollen indicating forests occurrences. From 3.32 to 3.22 Ma, the climatically stable and humid lacustrine phase at Hada corresponds to a global period of low variability attested by the astronomically calculated lower insolation and precession amplitude (Campisano and Feibel 2007, Figure 3). In contrast, during the deposition of the KH-2 Member (3.12 to 2.96 Ma), drier conditions were inferred from faunal studies, notably the increase in bovid tribes (wildebeests and gazelles) adapted to grazing arid lands after 3.15 Ma (Campisano and Reed 2007). The drying trend supported by faunal analysis cannot be confirmed by pollen since no data exist for this time range. DD and KH Members were not initially densely sampled for pollen studies. However, at the top of the KH-2 sub-member of the Hadar Formation, the pollen spectrum BK-2 documents open woodland indicating long dry seasons unsuitable for forest growth. Although derived from a single pollen assemblage, this interpretation is in good agreement with sedimentological and faunal interpretation of local environment and global climate patterns around 3 Ma.

### 10.5.3 Pollen reconstructed climatic conditions

An attempt to reconstruct climatic parameters from Hadar pollen data was performed (Bonnefille et al. 2004). Such method uses references of selected taxa frequencies in pollen assemblages obtained from 966 samples collected in different vegetation types at East African sites where corresponding meteorological parameters were interpolated. In 2001, R. Bonnefille collected at Hadar 13 new modern pollen assemblages from soil and river mud. Thirty years had passed since the collection of modern pollen data had been obtained and used as a calibration data set. We processed the new modern samples as if they were fossil ones and apply to them previous methodological approaches developed to calculate climatic parameters from upper Pleistocene and Holocene pollen data (Bonnefille et al. 1992; Jolly et al. 1998). The results could be compared to measured mean annual rainfall and temperature values provided by a meteorological station located nearby Hadar. Such procedure constituted a control test intended to validate our method. Indeed pollen-reconstructed values obtained for both modern surface soil and river samples collected in 2001 (these likely analogues to fossil pollen deposition) correspond closely to those measured at the nearby meteorological station (Bonnefille et al. 2004). Our statistical method was therefore validated. We could apply such likelihood procedure to infer mean annual precipitation and temperature for the Pliocene. Of course, the fact that climatic requirements from plant taxa during the Pliocene remained the same as today has to be an accepted assumption. For Hadar, the results indicate that from 3.42 to 3 Ma, mean annual terrestrial temperature was c. 10°C lower than the 25°C current value. Mean annual rainfall values average 1000 mm/yr, which is two to three times the amount registered today (Bonnefille et al. 2004). Moreover, a noticeable cold shift
towards 15°C associated with a minor rainfall increase is registered at c. 3.36–3.35 Ma (samples KMk to KMm section, Figure 3, Table 2) after deposition of the anoxic black organic layer ‘lignite’ (KMe) dated by the KMT Tuff at 3.36±0.02 Ma (Campisano 2007), and occurring in a stable deposition of lacustrine clay. Below the lower limit of the Mammoth sub-chron (3.33 Ma), the cool and humid event registered at Hadar appears in good contemporaneity with the oceanic first isotopic cooling event (Shackleton et al. 1995), therefore connecting a local environmental event to a global climatic event (Campisano and Feibel 2007). Pollen results and inferred climatic parameters clearly show that global climatic variations had a strong impact upon climate and vegetation in East Africa. However, the vegetation changes documented here for a few thousand years concern changing species of trees in the forest, but the forest was maintained in the region throughout this period. At Hadar, fossil remains of *A. afarensis* are found in many localities both before and after the cooling spell (below and above the SH-3 sands). The palaeo-geographical context indicates a permanent but fluctuating large lake, surrounded by periodically flooded flats covered by swampy sedges and cattail tall herbs. Located near a forest with great diversity of tree species, such environmental conditions offered various resources for food subsistence and shelter for this hominin which maintained abundant groups for more than half a million years.

Later, at about 2.95 Ma, when arctic glaciation extended, a single pollen assemblage documents past vegetation at Hadar. At that time a strong contrast was calculated in reconstructed values between mean annual temperature (MAT) inferred by ‘best analogue’ statistics and mean temperature of the coldest month (MTCO) inferred by the Plant Functional Type (PFT) method (Jolly et al. 1998). Mean annual precipitation remained high (>1000 mm), and the humidity coefficient showed its greatest value (Bonnefille et al. 2004). These values indicate stronger seasonal contrast in the local terrestrial climate, in good timing correspondence with increased variability of dust content registered in marine cores of the Arabian sea shown by a noticeable peak at 2.9 Ma (deMenocal 1995). In the marine core DSDP 231 from the Gulf of Aden, plant wax biomarkers show increased proportion of C4 plants, while pollen from the halophytes Chenopodiaceae/Amaranthaceae) are in greater percentages (Feakins et al. 2013) synchronously with the lowest percentages of total arboreal pollen (Bonnefille 2010). Several marine indicators point to strong changes over the whole East African region, and more contrasted climatic conditions at Hadar. However, in the Lower Awash Valley, trends towards more aridity did not reach a threshold that enable forest growth from the Hadar region. Many species of forest and woodland trees (e.g. Olea, Podocarpus, Hagenia, Myrica, Macaranga, Acacia, Ekebergia, Nuxia, Euclea, Juniperus, Dodonaea) persisted in the basin, until 2.9 Ma, still offering food and shelter resources. The variety of tree species indicates a diversified forest that does not resemble riparian forests, impoverished in species such as those found nowadays in arid region along the Awash or the Omo Rivers. Riparian forests, although providing a locally wooded environment among arid grassland floodplains, are less diversified in trees and herbs species (Carr 1998) and they are geographically localized. They could not provide food and continuous long-term living conditions for early hominins populations such those found in the Lower Awash Valley for a million years. Composition of the riparian vegetation is known to change very quickly. However, when hominins seem to have disappeared from the region (at 2.9 Ma), greater seasonal temperature (and rainfall) contrasts possibly made living conditions more difficult than those experienced earlier by occupants in the Hadar/Dikika region.

During the middle Pliocene, from 3.8 to 3.2 Ma, multiple, roughly contemporaneous sites, all containing hominins, are present across East Africa, spanning regular occurrence of *Australopithecus afarensis*. At Hadar (Ethiopia), Laetoli (Tanzania), East and West Turkana (Kenya), hominins were rare members of large mammalian fossil communities. Significant discrepancies in hominin abundance versus other mammalian fossils could have been caused by local ecology rather than by taphonomic biases (Villaseñor et al. 2020). Indeed, at Hadar, local ecological conditions with diversified woodlands and forests, and local grasslands (strongly differing from
modern savannah/steppe) provided favourable conditions for persistence of *A. afarensis* in the region. Stronger climatic changes after 2.9 Ma could have caused their extinction. But the Last Appearance Datum (LAD) of the species is difficult to determine due to a gap in the deposition of the sediments at Hadar after 2.9 Ma.

## 10.6 CONCLUSIONS

Pollen data from the Pliocene in East tropical Africa document many vegetation types that experienced strong changes in their taxonomic composition. Some of these changes occurred over a few thousand years. However, the resolution of the record is not sufficient to attribute them to fluctuations in precipitation following the monsoon precession cycle, although it is likely. Past vegetation for deposits c. 3.8 Ma is reconstructed as dense woodland or wooded grassland both in Tanzania south of the equator and Ethiopia, north of the equator. Such vegetation implies a seasonal distribution of rainfall over a broad region in East Africa. At 3.42 Ma many indicators of cool mountain forests and high elevation grasslands prevailed under humid conditions of higher rainfall, with short dry seasons that maintained forests at Hadar for more than 200 ka. A cold event documented slightly before 3.33 Ma, close to the lower limit of the Mammoth event, occurred in good timing connexion with the known marine isotope stage MIS M2 oceanic cooling and rainfall increase simulated by some model experiments (Dolan et al. 2015). At Hadar vegetation changes are indicated by modifications in the tree species composition of the forests. It is remarkable that such noticeable changes (both in vegetation and local climate) had no effect on *A. afarensis* local occurrences. Regarding hominins habitat, existing forests offered abundant and diverse food resources and shelters for *A. afarensis*. At Hadar, hominin fossils are found within sediments deposited before and after the cold spell. From 3.1 to 2.96 Ma, analyses of the fossil fauna, notably the abundance of different bovid tribes, are interpreted as indicating a drying trend in the palaeoenvironmental conditions. Increased body sizes of *A. afarensis* are documented synchronously. A single pollen assemblage confirms the drying trend post 2.9 Ma at a time of greater variability of the global climate following increasing arctic glaciations.

## ACKNOWLEDGEMENTS

We are grateful to two anonymous reviewers for their comments on an earlier version of the manuscript. We thank Doris Barboni for stimulating and encouraging this paper, her help to provide the data in digital format and final editing.

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<figure><img src="image_59.png" /></figure>

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CHAPTER 11

# Ecosystem change and human-environment interactions of Arabia

Sarah J. Ivory

*Department of Geosciences and the Earth and Environmental Systems Institute (EESI), Penn State University, University Park, USA*

Michèle Dinies<fnref n="1" />

*Institute of Geographical Sciences, Freie Universität, Berlin, Germany*

Anne-Marie Lézine

*Laboratoire d'Océanographie et du Climat, Expérimentation et Approche numérique/IPSL, Sorbonne University, CNRS-IRD-MNHN, Paris, France*

**ABSTRACT:** The Arabian Peninsula hosts some of the most extreme environments in the world. Arabian ecosystems are largely semi-arid to hyper-arid and yet are often highly biodiverse and unique owing to a location at a biogeographic nexus. There is an urgent need to better understand ecosystem resilience to biotic and abiotic disturbance; however, the modern vegetation across much of the sub-continent is still poorly understood. Palaeoecological records in Arabia have the potential to fill in gaps in our fundamental understanding of vegetation responses to climate, disturbance, and human modification; however, to do this, data must be accessible and easy to find. To this end, within the framework of the relaunch of the African Pollen Database, here we review existing palaeoecological datasets from the Arabian Peninsula, inventory those which are available, and synthesize results from these records. Due to the dearth of pre-Holocene information, this synthesis focuses on reconstructing vegetation from the Holocene Humid Period to today with emphasis on the impact of aridification and changing human livelihoods and culture on landscapes. Finally, as Arabia is perhaps the least well-studied region within the African Pollen Database, we offer some suggestions about fruitful directions for future palaeoecological research in this area.

## 11.1 INTRODUCTION

The Arabian Peninsula hosts some of the world's most extreme environments, including landscape gradients from montane woodland to cloud forest to active dune fields over small spatial scales, vast gravel deserts, and the world's largest sand sea, the Rub al'Khali. Its flora is complex biogeographically as well as ecologically very valuable (Kuerschner 1998). Due to its location

<footnote n="1">¹Other affiliation: German Archaeological Institute (DAI), Scientific Department of the Head Office, Berlin, Germany</footnote>
near Asia, Europe, and Africa, the flora of the Arabian Peninsula includes a high percentage of plants with affinities to neighboring regions co-occurring in unique formations, with many plants that only grow in these harsh climates (Ghazanfar 1992). Further, the deserts of Arabia are quite diverse, with the southern half situated within the Horn of Africa Biodiversity hotspot (Mittermeier et al. 2011).

The impact of climate change in Arabia is likely to put these fragile ecosystems at greater risk (Huang et al. 2017). Projections of climate that suggest that the “dry get drier” could further reduce moisture in a landscape already with a strong moisture deficit (Dahinden et al. 2017; Pachauri et al. 2014; Pausata et al. 2020). Further, the addition of increased rainfall variability could also intensify drought periods. Although many plants in the region are already adapted to arid and hyper-arid conditions, limited areas of grassland and woodland could undergo significant changes.

Further, despite climate being the main determinant of vegetation, land cover throughout the region is rapidly changing due to human activities (Alquarashi et al. 2016; Galletti et al. 2016). Rapid development following recent events, like the discovery of oil reserves in the Gulf States, has resulted in increased pressure on ecosystems as nations industrialize (Albalawi et al. 2018). Additionally, in much of Arabia, millions of people still live pastoral or agropastoral lifestyles and depend on ecosystem services for food, medicine, and fodder for animals (Ball et al. 2020). Freshwater availability is another important issue that can also be directly tied to vegetation. Ecosystem change associated with tree cover loss in wadis and southern Arabia is known to result in decreased surface water, declining water tables, and less spring discharge (Friesen et al. 2018; Hildebrandt et al. 2007). Thus, between the dual threats of climate and land-use change, it is important to better understand ecosystem changes in order to help develop sustainable management practices and validate models for an understudied region.

To this end, information is needed to better understand the relationship of Arabian vegetation to climate change on centennial timescales. Palaeoecological records, primarily from fossil pollen, provide key insights into the natural history and origins of the modern ecosystems in Arabia, feedbacks between climate and vegetation, ecosystem resilience, anthropogenic influences, and disturbance. In other regions, dense networks of these palynological records even allow for spatially explicit mapping of past vegetation (e.g. Dawson et al. 2016; Huntley 1990; Webb and McAndrews 1976; Williams and Jackson 2007). In contrast, Arabia is very data poor. This results form a few linked problems: 1) due to aridity, there is a lack of traditional palaeoenvironmental archives like lakes that have existed continuously through multiple climatic cycles, 2) access to regions that have potential for palaeoenvironmental work has become increasingly difficult due to evolving global politics, 3) existing pollen data from the region were housed within the African Pollen Database (APD), which lapsed in 2007 due to lack of funding, making accessibility of data also difficult. While these first two issues are a challenge to address, this paper, in conjunction with others in this special issue, emerge from an effort to renew and relaunch the APD (Ivory et al. 2020). This comes at a very critical time for using renewed, accessible data to synthesize the state of the art of palaeoecology in Arabia. Therefore, in this paper, we seek to: 1) review the state of current APD data holdings and those which have been generated since 2007, 2) synthesize these results from within the region in order to evaluate changes in vegetation since the Late Pleistocene, 3) provide recommendations for future work.

## 11.2 MODERN SETTING

The Arabian Peninsula (Figure 1) is situated in a complex geographical and geological setting between Africa, Asia, and the Mediterranean. Covering some 2.7 million km², this sub-continent spans approximately 15° latitude from the tropics to the subtropics (Kuerschner 1998). Because
of active tectonism, Arabia is bordered on most sides by mountains, including the Zagros Mountains to the north, Hajar Mountains to the east, the Hijaz, Asir and Yemen Highlands to the west, and Dhofar Mountains and Hadramawt escarpments to the south bordering the Arabian Sea. In the continental interior, due to significant rain shadows from these highlands, Arabia hosts extensive arid and hyper-arid environments including three sand seas (Rub al’Khali, Nafud, Wahiba Sands; Figure 1).

The climate of Arabia involves both northern and tropical atmospheric circulation systems. Rainfall is generally low (Figure 2), with most areas receiving less than 100mm/yr. This includes the hyper-arid interior and northwest coast (<50mm/yr), a large area from the Rub al’Khali (with 50–50mmyr), and c. 100mm/yr along the southern coasts. Notable exceptions to this are semi-arid regions in northern and southern Omani/Yemen mountains receiving c. 250mm/yr, and the southwestern highlands in Yemen receiving <400mm/yr (Almazroui et al. 2012; Fisher and Membery 1998). In the south, the summer rainfall is related to the yearly migration of the Intertropical Convergence Zone (ITCZ), which is located just south of the Oman/Yemen coasts in the summer months in its most northerly current position. However, despite the proximity of tropical convergence, southwesterly winds in June-July-August result in upwelling of cold waters off the southern Yemen/Oman coast which limits evaporation of moisture into air masses. In this region, cool, damp winds encounter steep, coastal escarpments and form dense seasonal fogs in summertime (Fisher and Membery 1998; Hildebrandt et al. 2007). Thus, instead of rainfall, a portion of annual moisture received in the coastal mountains of Yemen and Oman occurs as dew or horizontal precipitation (Hildebrandt et al. 2007). Further, as a result of the trapping of moisture along the coasts, there is a strong rainshadow and many inland areas receive no reliable moisture at all. In the north, beyond the limit of summer moisture, northwesterly winds occur in the winter months. These winds are associated with mid-latitude westerly circulation. Rainfall typically occurs due to cyclonic systems that develop over the Mediterranean Sea. These systems bring gentle rainfall to the Near East and may also be funneled up along the Zagros Mountains to the Persian Gulf delivering moisture as far south as the United Arab Emirates (UAE) and northern Oman.

The vegetation in Arabia owes its character to tectonics, geology, relief, biogeographic histories of different adjacent regions, climate change, and human presence and cultural evolution (Figure 1; Kuerschner 1998). Large parts of the peninsula maintained strong floral affinities with Africa before separation of the continents due to rifting around 15 myrs ago (Kuerschner 1998). Since that time, evolutionary isolation from the tropical vegetation of Africa coupled with new biogeographic connections with Europe and Asia and drying during the later Cenozoic have strongly shaped vegetation structure and community composition (Patzelt 2015). Three main phytogeographical units are found: the Saharo-Sindian/Arabian regional zone, the Somali-Masai regional center of endemism, and an Afromontane phytogeographic unit similar to the high mountains of East Africa. These occur along with intruders in the north and southeast of Mediterranean and Irano-Turanian origin.

Beyond the coasts, the modern continental interior of the Arabian Peninsula is dominated by hyper-arid to semi-arid ecosystems dominated by plants with Saharo-Arabian affinities (Figure 1). These regions have sparse vegetation with extremely low ground cover in diffuse or contracted patterns on slopes or in depressions. A few scattered trees with tropical affinities such as *Maeura crassifolia*, *Boscia arabica*, and *Acacia tortilis* also occur. To the east, species composition changes to reflect the proximity of Asia in the Gulf of Oman desert and semi-desert, where tree communities commonly include *Prosopis cineraria*.

In the inland gravel and rock deserts, shrublands with *Haloxylon salicornicum* cover most of North Arabia, while shrublands with *Rhanterium epapposum* are most common in the eastern part of the Arabian Peninsula. Both formations constitute highly appreciated grazing resources. Shrublands with *Calligonum comosum*, *Artemisia jordanica*, and numerous annuals characterizes the Great Nafud and Dahna sands. In the hyper-arid Rub’al Khali, annuals are uncommon, and
<figure><img src="image_60.png" /></figure>

**Figure 1.** Map of terrestrial ecoregions of Arabia and adjacent areas (Olson *et al.* 2001). Dots indicate sites with datasets relevant to the African Pollen Database (APD), including those already in the APD data holdings (green), those available after 2007 (yellow), and those not within the APD (red). Black dots indicate datasets in the European Pollen Database (EPD). Dotted line indicates the boundary between dominant summer and winter rainfall.

<figure><img src="image_61.png" /></figure>

**Figure 2.** Map of rainfall throughout Arabia during December (left) and July (right). Average monthly precipitation at 2.5' resolution comes from Worldclim2 (Fick and Hijmans 2017). Arrows indicate direction of prevailing seasonal winds.

very open shrublands cover thousands of square kilometers, dominated by the endemic species *Cornulaca arabica* associated with *Calligonum crinitum*, *Cyperus conglomeratus*, and *Tribulus arabicus*.

Trees are mainly restricted to vegetation on mountain slopes, wadis, and coastal plains. Within the wadis, different *Acacia* species are accompanied by Mediterranean and Irano-Turanian taxa in the northern, northwestern, and southeastern regions, such as *Nerium oleander*. Sudanian taxa such as the toothbrush tree (*Salvadora persica*) are common in wadi communities in the southern part of Arabia.
In the northernmost mountains, Mediterranean taxa play an important role. Steppic forests with pistachio (*Pistacia khinjuk*, *P. atlantica*) and juniper (*Juniperus phoenicea*) trees are found in the Hijaz Mountains. In the summit zone, oak-woodlands (with *Quercus calliprinos*) replace the juniper-pistachio woodlands.

In contrast, the southern and eastern mountains (Asir mountains, Yemeni highlands, the Hadramaut Plateau and Dhofar Mountains) show a strong Afromontane affinity. Typical representatives are olive trees (*Olea europaea* subsp. *cuspidata*) in the western and southern Arabian mountains and juniper (*Juniperus excelsa*) at highest altitudes (Kuerschner 1998). Tree heathers (*Erica arborea*) in Yemen, the southwestern Asir mountains, and the southernmost Hijaz mountains are another example of the close floristic connections between East-Africa (Baierle 1993; Deil and al Gifri 1998; Ghazanfar 1992; Gil-Lopez *et al.* 2017; Kuerschner 1998; Mandaville 1998).

Other tropical African elements are representatives of the Somali-Masai *Acacia-Commiphora* phytogeographic zone along the Oman and Yemen coasts (Ghazanfar 1992; Mittermeier *et al.* 2011; Patzelt 2015). These include dense stands of seasonal woodlands and forests dominated by *Terminalia* (formerly *Anogeissus*; including the endemics *T. dhofarica* and *T. bentii*; Ghazanfar 1992). Some of these restricted patches owe their existence to seasonal summer fogs and increased rainfall along the south facing seaward slopes where the rainfall may exceed 250 mm/yr. However, due to several factors, including human modification from over-grazing and wood harvesting, expansive coastal deserts including savanna grasslands and rocky xeromorphic scrubland replace tropical deciduous woodland and forest today even at elevation in many locations (Ball *et al.* 2020).

Mangroves are also found at stream outlets leading to coastal estuaries (kwars or khors) in some locations along the southern coasts of Oman and Yemen (Deil 1998). In contrast with the freshwater dominated mangroves common in southwestern Asia, with species like *Rhizophora mucronata*, these Arabian mangroves are dominated by halophytic trees such as *Avicennia marina*. These environments have also become relatively degraded by human land-use and are intensively grazed by animals such as camels during the dry season.

## 11.3 STATE OF RECORDS IN THE AFRICAN POLLEN DATABASE

While Arabian palaeoecological records are quite few, their representation as a percentage of those in existence in data repositories such as the APD is also exceptionally low in comparison to other regions of Africa (Table 1). For this review, we consider only Arabian records south of the Levant for consideration, as Levantine and Near Eastern records have been contributed commonly to the European Pollen Database due to their Mediterranean affinity. The original data holdings of the APD dating to 2007 includes only 7 datasets. Within the framework of the relaunch of the APD, work has begun to inventory all Arabian records and encourage new data contributions. As a result, Table 1 shows that, of the 40 publications including pollen datasets, 9 new datasets have been contributed to date (total 16 Arabian records in APD). Several observations can be made of the emerging datasets produced within the last 15 years, including new archives previously unrepresented in Arabia (*hyrax* middens, archeological contexts, estuary sediment cores) which have resulted in an influx of new information to help answer ecological questions.

## 11.4 VEGETATION HISTORY

### 11.4.1 Pre-Holocene

Little palaeoecological information exists about the Cenozoic flora of Arabia prior to the Late Pleistocene, and none which falls under the convention of the African Pollen Database. The little
<table><caption>**Table 1.** Inventory of all palynological records relevant to the African Pollen Database (APD).</caption><thead><tr><th>Type</th><th>Site Name</th><th>Latitude (°N)</th><th>Longitude (°E)</th><th>Country/ Ocean</th><th>Reference</th></tr></thead><tbody><tr><td>APD</td><td>Suwayh [Drill 2]</td><td>22.09</td><td>59.67</td><td>Oman</td><td>Lézine et al. (2002)</td></tr><tr><td>APD</td><td>Suwayh [Drill 1]</td><td>22.09</td><td>59.67</td><td>Oman</td><td>Lézine et al. (2002)</td></tr><tr><td>APD</td><td>Khor [F.B.]</td><td>25.50</td><td>51.42</td><td>Qatar</td><td>Bonnefille and Riollet (1988)</td></tr><tr><td>APD</td><td>Khor M</td><td>25.50</td><td>51.42</td><td>Qatar</td><td>Bonnefille and Riollet (1988)</td></tr><tr><td>APD</td><td>Site 36</td><td>25.53</td><td>51.50</td><td>Qatar</td><td>Bonnefille and Riollet (1988)</td></tr><tr><td>APD</td><td>Mundafan</td><td>18.53</td><td>45.38</td><td>Saudi Arabia</td><td>El-Moslimany (1983)</td></tr><tr><td>APD</td><td>al-Hawa</td><td>15.87</td><td>46.88</td><td>Yemen</td><td>Lézine et al. (1998)</td></tr><tr><td>APD</td><td>MD92-1002</td><td>12.03</td><td>44.32</td><td>Indian Ocean</td><td>Fersi et al., (2016).</td></tr><tr><td>available</td><td>SO90-56KA</td><td>24.83</td><td>65.92</td><td>Indian Ocean</td><td>Ivory and Lézine (2009)</td></tr><tr><td>available</td><td>Filim</td><td>20.60</td><td>58.19</td><td>Oman</td><td>Lézine et al. (2017)</td></tr><tr><td>available</td><td>Khawr Al Balid 1</td><td>17.00</td><td>54.34</td><td>Oman</td><td>Hoorn and Cremaschi (2004)</td></tr><tr><td>available</td><td>Khawr Al Balid 3</td><td>17.00</td><td>54.33</td><td>Oman</td><td>Hoorn and Cremaschi (2004)</td></tr><tr><td>available</td><td>Khawr Rawri 2</td><td>17.03</td><td>54.40</td><td>Oman</td><td>Hoorn and Cremaschi (2004)</td></tr><tr><td>available</td><td>Khawr Rawri 4</td><td>17.03</td><td>54.41</td><td>Oman</td><td>Hoorn and Cremaschi (2004)</td></tr><tr><td>available</td><td>Kwar al Jaramah</td><td>22.50</td><td>59.77</td><td>Oman</td><td>Lézine (2009)</td></tr><tr><td>available</td><td>Sumharam</td><td>17.04</td><td>54.43</td><td>Oman</td><td>Lippi et al. (2011)</td></tr><tr><td>available</td><td>Tayma</td><td>27.63</td><td>38.55</td><td>Saudi Arabia</td><td>Dinies et al. (2015)</td></tr><tr><td>Not in APD</td><td>MD 77 202</td><td>19.22</td><td>60.67</td><td>Indian Ocean</td><td>Van Campo (1983)</td></tr><tr><td>Not in APD</td><td>RC27-14</td><td>15.42</td><td>58.10</td><td>Indian Ocean</td><td>Overpeck et al. (1996)</td></tr><tr><td>Not in APD</td><td>RC27-28</td><td>18.50</td><td>57.98</td><td>Indian Ocean</td><td>Overpeck et al. (1996)</td></tr><tr><td>Not in APD</td><td>Hole 721B</td><td>16.68</td><td>59.86</td><td>Indian Ocean</td><td>Van Campo (1991)</td></tr><tr><td>Not in APD</td><td>Hole 723 A/B</td><td>18.05</td><td>58.44</td><td>Indian Ocean</td><td>Van Campo (1991)</td></tr></tbody></table>

Continued
<table><caption>**Table 1. Continued**</caption><thead><tr><th>Type</th><th>Site Name</th><th>Latitude (°N)</th><th>Longitude (°E)</th><th>Country/ Ocean</th><th>Reference</th></tr></thead><tbody><tr><td>Not in APD</td><td>MD 76 135</td><td>14.45</td><td>50.53</td><td>Indian Ocean</td><td>Van Campo (1983)</td></tr><tr><td>Not in APD</td><td>MD 76 136</td><td>12.88</td><td>46.80</td><td>Indian Ocean</td><td>Van Campo (1983)</td></tr><tr><td>Not in APD</td><td>Maqta oasis</td><td>22.77</td><td>58.96</td><td>Oman</td><td>Urban and Buerkert (2009)</td></tr><tr><td>Not in APD</td><td>an Nafud [profil 2]</td><td>25.85</td><td>41.43</td><td>Saudi Arabia</td><td>Schulz and Whitney (1986)</td></tr><tr><td>Not in APD</td><td>an Nafud [profil 3]</td><td>27.90</td><td>41.32</td><td>Saudi Arabia</td><td>Schulz and Whitney (1986)</td></tr><tr><td>Not in APD</td><td>an Nafud [profil 4]</td><td>28.02</td><td>41.15</td><td>Saudi Arabia</td><td>Schulz and Whitney (1986)</td></tr><tr><td>Not in APD</td><td>an Nafud [profil 5]</td><td>28.10</td><td>41.37</td><td>Saudi Arabia</td><td>Schulz and Whitney (1986)</td></tr><tr><td>Not in APD</td><td>an Nafud [profil 6]</td><td>28.45</td><td>41.22</td><td>Saudi Arabia</td><td>Schulz and Whitney (1986)</td></tr><tr><td>Not in APD</td><td>an Nafud [profil 7]</td><td>28.45</td><td>41.22</td><td>Saudi Arabia</td><td>Schulz and Whitney (1986)</td></tr><tr><td>Not in APD</td><td>As-Sirr</td><td>26.18</td><td>44.33</td><td>Saudi Arabia</td><td>Schulz and Whitney (1986)</td></tr><tr><td>Not in APD</td><td>Djebel Asmar</td><td>25.80</td><td>43.02</td><td>Saudi Arabia</td><td>Schulz and Whitney (1986)</td></tr><tr><td>Not in APD</td><td>Jebel Ghiran</td><td>20.66</td><td>46.73</td><td>Saudi Arabia</td><td>El-Moslimany (1983)</td></tr><tr><td>Not in APD</td><td>Rub'al-Khali</td><td>19.43</td><td>46.69</td><td>Saudi Arabia</td><td>El-Moslimany (1983)</td></tr><tr><td>Not in APD</td><td>Rub'al-Khali [AS-3-1]</td><td>21.88</td><td>49.72</td><td>Saudi Arabia</td><td>El-Moslimany (1983)</td></tr><tr><td>Not in APD</td><td>Rub'al-Khali [XX-B-5]</td><td>17.59</td><td>46.30</td><td>Saudi Arabia</td><td>El-Moslimany (1983)</td></tr><tr><td>Not in APD</td><td>Awafi</td><td>25.72</td><td>56.10</td><td>UAE</td><td>Parker et al. (2004)</td></tr><tr><td>Not in APD</td><td>Mleiha</td><td>25.13</td><td>55.86</td><td>UAE</td><td>Garcia Anton and Sainz Ollero (1999)</td></tr><tr><td>Not in APD</td><td>OS73</td><td>25.14</td><td>60.83</td><td>Gulf of Oman</td><td>Miller et al. (2016)</td></tr><tr><td>Not in APD</td><td>Sumharam</td><td>17.04</td><td>54.43</td><td>Oman</td><td>Bellini et al. (2020)</td></tr></tbody></table>
that can be inferred is based on the ancient biogeographic connection of Arabia with Eastern Africa. Prior to the inception of the East Africa Rift system, the Arabian and African plates were joined, forming a continuous stretch of land from what is now Ethiopia into southwestern Arabia (Yemen, Oman, and Saudi Arabia; Guba and Glennie 1998). This connection with Africa in the Eocene and Oligocene that sets the legacy of the modern floristic affinities with the African continent (Kuerschner 1998). By c. 30 Ma ago, the Arabian plate separated from Africa which likely had important consequences for the climate and flora of the region. Due to the relative biogeographic isolation of Arabia, endemic vascular plants emerged (Kuerschner 1998). During the last 10 myrs, global cooling led to increased aridity and the rise of C₄ tropical savanna and open desert ecosystems. Pollen and carbon isotopes on leaf waxes from DSDP Site 231 in the Gulf of Aden suggest a transition to aridity by 10Ma and from C₃ woodland to C₄ savanna by c. 3.5Ma (Bonnefille 2010; Feakins 2013). Aridification progressed at the beginning of the Quaternary. Decreases in sea levels related to glacial-interglacial fluctuations in the Red Sea and Persian Gulf led to decreased biogeographic barriers with Africa and Asia, facilitating plant migrations when sea levels were low (Kuerschner 1998). This likely resulted in the inclusion of many elements of Irano-Sindian elements in plant communities of western and northern Arabia.

Over the late Quaternary, marine sediment cores from the Arabian Sea and Gulf of Aden along with very sparse terrestrial records provide information about climate and vegetation. Geo-morphological studies of palaeolakes and dune mobilization coupled with observation speleothem growth phases suggest repeated oscillations of wetting and drying over at least the last 400kyrs (Figure 3; Burns et al. 2001; McClure 1984; Preusser et al. 2002; Rosenberg et al. 2011; Stokes and Bray 2005). This includes the development of inland lakes and aquifer recharge, such as have been observed during MIS5e in the Near East and Saudi Arabia (Issar and Bruins 1983; Petit-Maire et al. 2010; Rosenberg et al. 2011). δ¹⁸O measured on benthic and planktic foraminifera from marine sediment cores MD77 202 and MD76 136 suggest that climate at least over the last 160kyrs was strongly related to glacial-interglacial cycles (Van Campo et al. 1982). In tropical Africa, palaeoclimate and palaeoecology are also mechanistically connected to eccentricity-modulated insolation as well as teleconnections with northern latitude glaciation; however, the record in Arabia is not long enough to resolve the impact of changes in tropical insolation on climate.

Regardless, pollen analysis on marine sediment cores MD77 202 and MD76 136 highlight enhanced aridity during glacial periods such as MIS 6 and MIS2 characterized by high abundances of xerophytic herbs like Amaranthaceae (Figure 3). Despite relative wetness during interglacial periods, pollen assemblages appear to still contain many indicators that flora on the sub-continent has an important arid component. This includes high abundances of Asteraceae and *Tribulus*, but includes some more tropical elements, such as *Acacia* and higher abundances of grasses. This implies that coastal forests and mangroves may have expanded during interglacial periods (Fersi et al. 2016; Van Campo et al. 1982). In northern Arabia, drill cores from the Dead Sea provide insight into the environments from c. 147-89 ka. These show pronounced aridity through much of the penultimate glacial-interglacial transition with an abrupt transition to grass and woodland during the maximum interglacial from 124-115 ka (Chen and Litt 2018).

### 11.4.2 Holocene

Throughout the northern hemisphere tropics, the Pleistocene-Holocene transition was accompanied by an increase in summer insolation (Figure 4). This resulted in enhanced land-sea temperature contrasts which intensified northern tropical monsoon systems and is often referred to as the Holocene Humid Period (Mayewski et al. 2004). In North Africa, palaeoclimate and paleoecological research has focused on evidence of extensive closed-basin lake systems like Lake Mega-Chad and the resultant northward migration of savanna and tropical woodland ecosystems to form the “Green Sahara” (Hély and Lézine 2014; Hoelzmann et al. 1998; Watrin et al.
<figure><img src="image_62.png" /></figure>

**Figure 3.** Late Quaternary climate and vegetation of Arabia, including (from top to bottom) indication of periods of speleothem growth (Burns *et al.* 2001), dune mobilization (Stokes and Bray 2005), closed basin lake deposits in Rub al-Khali (McClure 1984), geochemical and pollen data from marine sediment core MD77 202 including δ¹⁸O*benthic*, humid-arid index, and pollen abundances (Van Campo 1983).

2009). In Arabia, however, although changes in atmospheric circulation and teleconnections with the high latitudes resulted in increased rainfall during this time, the pattern is quite complex and varies regionally, particularly the response of the vegetation to enhanced rainfall.

### 11.4.2.1 Palaeohydrology

Palaeoclimatic studies of cave and lacustrine deposits demonstrate changes hydrology consistent with altered atmospheric circulation involving both a more northerly ITCZ and intensified Mediterranean moisture source during the early and middle Holocene from at least 9-6 ka across much of the Arabian Peninsula (Dinies *et al.* 2015; Enzel *et al.* 2015; Fleitmann *et al.* 2007, 2011; Lézine *et al.* 1998, 2007, 2009, 2014; Parker *et al.* 2004, 2006; Schulz and Whitney 1987). In southern Arabia, δ¹⁸O from speleothems collected across a latitudinal gradient from northern Oman (Hoti Cave, 22°N) to southern Oman (Qunf Cave, 17°N) show a depletion in
oxygen isotopes attributed to the “amount effect” as well as indications of a southerly moisture source (Figure 4; Fleitmann *et al.* 2007, 2011). This implies increased tropical summer moisture associated with a northward migration of the ITCZ between 10-9 ka (Fleitmann *et al.* 2007, 2011;

<figure><img src="image_63.png" /></figure>

**Figure 4.** Holocene Humid Period climate and vegetation from Arabia including (from top to bottom) July solar insolation at 15N (Berger and Loutre 1991), speleothem δ¹⁸O from Hoti Cave (northern Oman) and Qunf Cave (southern Oman; Fleitmann *et al.* 2007), aridity index and percent Mediterranean trees (*Quercus/Olea/Juniperus*) from Ein Gedi (Israel; Litt *et al.* 2012), vegetation at Awafi (UAE; Parker *et al.* 2004), aridity index and freshwater mangrove pollen (*Rhizophora*) from Kwar al Jaramah and Filim (Oman; Lézine *et al.* 2017), aridity index and percent tropical trees (*Acacia, Dodonaea viscosa, Podocarpus, Ziziphus, Salvadora persica*) from Al-Hawa (Yemen; Lézine *et al.* 1998), and frequency diagram of numbers of waterlain deposits (Lézine *et al.* 2014).
Neff et al. 2001). Numerous studies link monsoon intensification to high summer insolation in the northern hemisphere at the early to mid-Holocene.

However, monsoon intensity based on marine core records responded non-linearly to radiative forcing with significantly centennial and millennial-scale variability (Gupta et al. 2003; Sirocko et al. 1993). This work suggests both direct and indirect forcing of the monsoon with variability mechanistically linked to teleconnections with both temperatures in the North Atlantic as well as changes in albedo over the Asian landmass. Although no quantitative estimates of rainfall increase are associated with those studies, palaeoclimate model simulations by Lézine et al. (2017) imply that large rainfall anomalies occurred in the south between 14-18°N at least as late as 6 ka with increased summer moisture limited mostly to southern Oman and Yemen. Only marginal rainfall increase occurred as far north as 20-24°N.

In addition to changes in tropical circulation, winter Mediterranean rainfall systems varied over the Holocene. Speleothem $\delta^{18}$O from caves in the Eastern Mediterranean, including Soreq Cave in Israel, lake level fluctuations, and the timing of sapropel deposition suggest that the winter rainfall zone underwent several transitions. Two major wetter phases were observed: an early Holocene Humid period (c. 10-7 ka) and a wetter phase at the end of the Middle Holocene, from c. 65-3 ka (Bar-Matthews et al. 1999; Bar-Matthews and Ayalon 2011; Litt et al. 2012; Migowski et al. 2004; Roberts et al. 2011; Robinson et al. 2006). This suggests an early phase of higher moisture associated with Mediterranean cyclonic systems coeval with early Holocene monsoon intensification in southern Arabia. However, unlike tropical Arabia, where indications of moisture persisted along the coast until 6-5 ka, the northern winter rainfall zone underwent a period of aridity during the mid-Holocene followed by renewed wetting after southern Arabia was already dry.

Higher rainfall across southern Arabia along coastal escarpments and highlands during the early Holocene resulted in increased streamflow into closed basins that comprise the continental interior. Waterlain deposits like lakes, ponds, and marshes are found that date to between 10-6 ka in southern Arabia (Figure 4; Enzel et al. 2015; Lézine et al. 2010). This includes palustrine deposits and extensive shallow endorheic lake systems in the Ramlat as-Sabatayn, Dhamar highlands in Yemen, and eastern fringes of the Rub al'Khali due to increased drainage from the landward sides of coastal escarpments (Davies 2006; Lézine et al. 1998; Parker et al. 2004). Wadis, or dry river valleys, which today rarely have flowing water, show evidence of fluvial activity during the early and mid-Holocene, including fine-grained deposits indicative of the development of permanent meandering streams in places like Wadi Sana, Yemen and Dhofar, Oman (Anderson 2007; Berger et al. 2012; Enzel et al. 2015; Sander 2006). Further, other geological evidence exists of groundwater recharge and higher water table from tufa and travertine deposits across Oman and Yemen which also date to the early Holocene (Cremaschi et al. 2015; Sander 2006). Carbonate-rich facies indicated by lithostratigraphic studies of the palaeolake deposits suggests that their development is due to enhanced runoff from adjacent highlands which trapped moisture rather than from direct rainfall onto lake surfaces (Lézine et al. 2007; Parker et al. 2004).

### 11.4.2.2 Southern Arabian palaeoecology

In southern Arabia, pollen, phytoliths, and macrobotanical remains show that the early Holocene was a period of major landscape transformation resulting in a reduction of arid and hyper-arid environments and expansion of diverse, savanna and woodland ecosystems due to increased moisture. As far north as 27°N, which today is largely outside of the summer southwest monsoon zone, phytolith, carbon isotopes, and pollen analysis from a palaeolake deposit at Awafi demonstrate the development of a $C_3$ Pooid grassland with higher abundances of trees like *Acacia*, *Tamarix*, *Prosopis*, and palms occurred from 9-6 ka (Parker et al. 2004). At Suwayh, along the northern Omani coast, increased abundances of grasses and some tropical trees is in agreement with denser vegetation with many taxa with Somali-Masai affinities (Lézine et al. 2002). To the southwest, in Wadi Sana in Yemen, pollen and macrobotanical evidence from hyrax middens and alluvial sediments demonstrates the development of woodland with a dense grassy understory along a permanent stream until at least 6-5 ka (McCorriston et al. 2002). This woodland included
a keystone tree taxon from coastal seasonal woodlands (*Terminalia dhofarica*), which during the mid-Holocene, appears to have had a more continuous range across coastal highlands, but also may have extended further inland along waterways (Oberprieler *et al.* 2009).

However, despite the expansion of tropical savanna and woodland in some locations in southern Arabia during the early and mid-Holocene, there are also many indications of persistent aridity. This suggests strong heterogeneity in local moisture and therefore strong local controls on vegetation. For example, pollen analysis also demonstrates the stability of many xerophytic taxa throughout this interval. Although pollen analysis on the palaeolake sediments suggest that the lake margin vegetation was dominated by local aquatic plants (*Typha*) due to the presence of water, the terrestrial flora around the lake was characterized by Arabian desert plants like Cyperaceae, *Calligonum*, and Amaranthaceae, common in the modern flora (Lézine *et al.* 1998, 2007). This is true as well in pollen records from coastal estuaries, which integrate pollen from a large regional watershed. For example, at Filim and Kwar al Jaramah along the Oman coast, desert taxa (*Artemisia*, Cyperaceae) increase in abundances after 6-5 ka but are a dominant component of the pollen spectra even prior to this (Lézine *et al.* 2017). These regionally integrating records do however support the idea of localized changes in vegetation structure and composition in distant headwaters. For example, at Al-Hawa, long distant transport of pollen from Afromontane and tropical woodland trees (*Podocarpus*, *Erica*, *Juniperus*) that are not observed today in modern pollen rain occurred in small abundances until the desiccation of the palaeolake around 7.2 ka (Lézine *et al.* 1998, 2007). This supports that idea that tropical African elements occurred more abundantly in the mountains that drain into the Ramlat as-Sabatayn and that these mountains likely acted as strong topographic barriers to rainfall even during times of enhanced monsoon intensity. This suggests that in contrast to widespread expansion of wetter biomes in Northern Africa, Arabian ecosystems even during the Holocene Humid Period were always on the margins of desert, perhaps due in part to the complex topography.

An additional impact of increased freshwater flux from heavier rainfall in southern Arabia during the Holocene Humid Period was the presence of *Rhizophora* mangrove vegetation throughout coastal estuaries. Today, mangroves are found only in very limited patches along the southern Arabian coast (Deil 1998). Further, species composition of modern mangroves is closely related to water chemistry, particularly salinity, within an estuary. For example, most mangroves remaining in arid and semi-arid regions are typically dominated by the halophytic taxon *Avicennia marina*. While this tree is also present in the early to mid-Holocene, increased abundances of mangrove pollen taxa that do not tolerate high salinity, such as *Rhizophora*, can only occur in the presence of increased freshwater flux from the land. *Rhizophora* pollen occurs in high abundances in sediments before 4 ka along the southern Arabian and east Asian coasts at Kwar al Jaramah, Suwayh, and Filim as well as being present in Arabian Sea marine sediment cores (Lézine et al. 2017). This suggests that increased freshwater runoff into coastal estuaries altered their water chemistry, allowing taxa like *Rhizophora* which do not tolerate very saline conditions to thrive during the mid-Holocene.

### 11.4.2.3 Northern Arabian palaeoecology

In northern Arabia, the record of early to mid-Holocene vegetation is quite different from southern Arabia. In particular, there is a marked difference in the timing of the maximum in Holocene moisture latitudinally from subtropical northern to tropical southern Arabia. Based on compilations of hydrological records from southern Arabia, Lézine et al. (2014) demonstrated that the Holocene Humid Period maximum was coeval at tropical sites and occurred from 10-6.5 ka. In contrast, in the subtropics, $\delta^{18}$O measurements on lacustrine sediments suggest increased winter moisture from 9-8 ka, with a marked arid interval from 8-6 ka despite continued winter rain influence (Bar-Matthews et al. 1999; Litt et al. 2012; Roberts et al. 2011; Stevens et al. 2006). Lithostratigraphic observations and pollen analysis from Lake Mirabad in Iran suggest warm and dry conditions at this time favored a more arid, open vegetation characterized by low abundances
of tree pollen in comparison to today (Van Zeist and Bottema 1977). Further south along the Dead Sea, lake level fluctuations indicate an early Holocene Humid Period (10-8.6 ka), increased aridity during 8-5.6 ka, and another phase of raised lake level indicating increased humidity during 5.6-3.5 ka (Migowski et al. 2006). The early Holocene vegetation transitions based on the core from the shoreline at Ein Gedi demonstrates low proportions of Mediterranean tree pollen such as deciduous oak, and scattered pistachio and olive pollen probably originating from the adjacent mountains. Amaranthaceae, together with Poaceae, *Artemisia*, *Ephedra*, and Asteraceae are the main constituents, associated to the Saharo-Arabian and Irano-Turanian biomes, indicating increased winter temperatures (Litt et al. 2012). A milder winter during early Holocene is in line with the increase of pistachio during this period in the Mediterranean (Rossignol-Strick 1999). Further south into Saudi Arabia, pollen from the Tayma record show regionally wetter conditions associated with increased Mediterranean woodlands (*Quercus*, *Pistachia*) in the Hijaz mountains in the west and expanded grasslands during the early Holocene only until 8 ka (Dinies et al. 2015, 2016). Other shallow lake sediments from the early Holocene corroborate an increase in humidity in this region (Crassard et al. 2013, Guagnin et al. 2017, Petraglia et al. 2020). At about 8 ka, a distinct vegetation shift at Tayma occurred, characterized by the retreat of grasslands and the re-expansion of desert dwarf-shrubs (i.e. *Haloxylon* type). This indicates the onset of arid conditions lasting at least until 4000 yrs BP, when the shallow-water lake dried out (Engel et al. 2012). Four pollen records (2-4 samples each) from diatomites in interdune depressions in the Nafud sand desert date to 9.5-5.8 ka and are similar to modern pollen spectra, pointing to persistent arid conditions throughout the Holocene (Schulz and Whitney 1987).

In the northern subtropics and into the Near East, winter moisture associated with enhanced westerly cyclonic activity increased at 6.5 ka. This is in agreement with palaeoecological work from Israel and Northern Jordan showing the expansion of Mediterranean forests with deciduous and evergreen oaks and olives (wild and/or cultivated) to lower elevations than present. At this same period, high lake levels at Dead Sea occurred between 6.3-3.3 ka (Litt et al. 2012). Indications of enhanced winter cyclonic moisture at this time extend into tropical Arabia, where Parker et al. (2004) observed a relatively stable period of mixed $C_3-C_4$ grassland and steppe at Awafi long after summer moisture had retreated from this region. However, there are no indications for wetter conditions in the more southern part of northwestern Arabia, where dwarf shrub steppe dominated until 4.2 ka.

#### 11.4.2.4 Characteristics of the Arabian Holocene Humid Period

Modeling and empirical studies have focused on determining if the termination of the Holocene Humid Period in the tropics and the retreat of the ITCZ was abrupt or gradual. Within Arabia, in particular, a contrast is observed in the apparent pace of hydrological and ecological change recorded in marine versus terrestrial settings (Lézine et al. 2014). Marine sediment cores from the Indian Ocean and Arabian Sea capture dust sourced from deserts of Arabia and East Africa during periods of dry or windy climate (Sirocko et al. 1993). Throughout the early Holocene, abrupt transitions associated with centennial-millennial scale variability in monsoon intensity are observed (Gupta et al. 2003). Further, at the end of the Holocene Humid Period, dust influxes rose in several of these cores, interpreted as abrupt reduction of moisture in the region and increased dust transport (Sirocko et al. 1993). However, few terrestrial archives of both climate and palaeoenvironmental information capture this abrupt variability or termination. Speleothem $\delta^{18}$O from southern Arabia exhibit a time transgressive reduction in rainfall (Fleitmann et al. 2007). At Hoti Cave in Northern Oman, a relatively rapid enrichment in $\delta^{18}$O occurs at c. 6 ka leading to a hiatus marking the initial southward retreat of the ITCZ and a decrease in summertime moisture around 23°N. Further south at Qunf Cave (17°N) along the Arabian Sea, $\delta^{18}$O instead followed a gradual enrichment beginning just after 6 ka and continuing until a depositional hiatus at around 2.5 ka. This gradual decline in rainfall is in agreement with palaeoclimate simulations across similar latitudinal transect, which suggest more northerly sites saw a more abrupt, earlier
decline in rainfall than southern sites (Lézine et al. 2017). Further evidence of a progressive
decline in moisture comes from Lézine et al. (2014) who synthesized ages of waterlain deposits
as indications of changes in rainfall. This analysis also suggests a gradual disappearance of surface
water until c. 6 ka, when most areas dried up. Most palaeoecological records from southern Arabia
also agree that ecosystem change followed a similar gradual transition toward semi-arid and arid
ecosystems which resemble modern biomes, particularly after 6 ka. At Awafi, this time marked
the transition back to from C₃ to mixed C₃–C₄ savanna (Parker et al. 2004). Macrofossils and
pollen from hyrax middens from Wadi Sana, Yemen, show a transition from gallery woodland
to semi-desert with an increase in Artemisia and Amaranthaceae (McCorriston et al. 2002).
Along the coasts in estuaries, declining freshwater inputs result in a relatively gradual decline in
freshwater mangrove taxon, Rhizophora, in favor of saltwater mangroves (Lézine et al. 2017).

Increases in rainfall and decreased rainfall seasonality throughout the Arabian Peninsula
during the early to Mid-Holocene resulted in the development of ecosystems whose composition
and dynamics were not analogous to those found today. In fact, most pollen spectra prior to 6 ka are
difficult to compare directly with pollen samples representing modern vegetation (El-Moslimany
1990; Lézine et al. 2010). We illustrate some examples of these in Figure 5 which shows affinities
of pollen taxa to modern phytogeographic regions. In this figure, most palaeoecological sites
presented here have affinities based on pollen assemblages to multiple modern vegetation types.
Here we will demonstrate those primary and secondary affinities to modern vegetation for no
analog pollen spectra.

For example, in the Near East at Wadi Faynan, increases in arboreal taxa from at least
8-6 ka suggest that SW Jordan hosted woodlands along waterways and expanded montane forest
communities (Hunt et al. 2004). Although many of these still occur either at the site or at least
within the region (Juniperus, Quercus) today, Ulmus is not recorded in modern samples in that
region. This is true for southern Arabia as well. For example, in Wadi Sana, in the modern
stony desert, occasional trees such as Maerua and Acacia are observed. In the mid-Holocene, in
addition to higher pollen abundances of these common modern arboreal taxa, higher abundances
of trees with tropical affinities only were found at the southernmost coast, such as Terminalia,
Olea, and Dodonea viscosa were also an important part of the arboreal community (McCorriston
et al. 2002; Ivory et al., 2021). However, it is important to note, that as discussed above, much of
southern as well as northern Arabia remained arid or semi-arid even at the peak in the monsoon,
thus we suggest that these changes in vegetation composition are likely linked to the presence of
increased surface water amidst persistent climatic aridity.

Finally, the increase in biomass during the early to Mid-Holocene also appears to have
resulted in a change in the dynamics of wildfires in some location in Arabia. Today through-
out Arabia, wildfire is extremely uncommon due to lack of fuel and continuity of fuel on the
landscape because of extreme aridity (Cao et al. 2015). Charcoal and microcharcoal datasets are
rarer even than other palaeoecological information; however, increased charcoal deposition in
palaeolake deposits, hyrax middens, and even the appearance of burnt surfaces in fine-grained
stream deposits is a testament to the increase in fuel locally, linked to higher rainfall seasonality
(Kimiaie and McCorriston 2014; McCorriston et al. 2002). Further, there are indications of
increased inputs of charred plant material during this time coeval with increased land use and
oasis cultivation, suggesting a relationship between fire and human presence (Dinies et al. 2016).

11.5 COUPLED HUMAN AND NATURAL SYSTEMS

Modern humans may have been present in Arabia for much of the Late Pleistocene. Arabian geog-
raphy in fact likely played a strong role in early human migration (Armitage et al. 2011; Petraglia
2011). Environmental change, mainly hydrological variations related to glacial-interglacial cycles
and monsoon intensity, is thought to have contributed to the movement of early humans out of
<figure><img src="image_64.png" /><figcaption>**Figure 5.** Holocene vegetation affinities in relation to ecoregion species composition as presented in Figure 1. For each site, there are two rows which represent the primary vegetation affinity (lower row) as represented by the pollen assemblages and the secondary affinity (upper row) in the case that pollen assemblage composition differed substantially from modern assemblages for numerous reasons (long distance transport, no modern analog, presence of significant aquatic vegetation).</figcaption></figure>

Africa and continuous human presence (deMenocal and Stringer 2016; Tierney et al. 2017). Increasing evidence exists of earlier lacustrine phases in central Arabia which could have supported a more resource-rich landscape into which early hunter-gathers may have migrated (Parker 2010). Further, low sea levels during glacials, particularly along the Red Sea as well as the gulfs to the east, would have facilitated migration of early humans as they may have for plants (Lambeck 1996; Lokier et al. 2015). Evidence from mitochondrial DNA implies populations present in Arabia at least as early as 60 ka (Fernandes et al. 2015); however, direct archeological evidence of this time is scarce. For example, lithic piles are commonly observed throughout Arabia, but limitations in dating complicates an assessment beyond morphological attribution of assemblages to at least Early and Middle Palaeolithic (Groucutt and Petraglia 2012; Rose and Usik 2010). Little palaeoecological evidence exists beyond marine sediment cores to provide an environmental context of these migrations or potential footprint of these early hunters on the landscape. Evidence was preserved only from the Neolithic and the onset of the Holocene Humid Period to suggest continuous human presence throughout much of the sub-continent.

During Late Pleistocene and Holocene, human-environment interactions in Arabia began to shift through accelerated technological advancements. Notably, the cultivation and domestication processes of annual plants started in the Fertile Crescent at this time. Numerous studies point to a long-lasting, multi-phased, and multi-centered domestication process of cereals and pulses, connected to multiple triggers including climatic changes as well as cultural and demographic
developments (e.g. Fuller *et al.* 2012; Hillman *et al.* 2001; Willcox 2012; Zeder 2011). These innovations are at the origin of far-reaching landscape transformations resulting in highly anthropogenic ecosystems such as gardens and fields. Perennial fruit trees such as olive and grapes were also domesticated and propagated from 9-6 ka (e.g. Zohary 1982). However, a much earlier start of these processes cannot be excluded for fruit trees, as indicated by the supposed early beginnings of domestication of figs near 11.4 ka (Kislev *et al.* 2006).

The spread of these Near Eastern domesticates into the Arabian Peninsula, the introduction of cultivars from other domestication centers such as African crops in Southern Arabia (e.g. millet; Tengberg 2003), and autochthonous agricultural developments on the Arabian Peninsula are not well known. With the exception of the southern mountains, modern precipitation is too low to practice durable dry-land farming which requires at least 250 mm/yr rainfall (e.g. Thalen 1979). Buried early-to-mid Holocene soils in Yemen show the potential for local agriculture during this wetter period, however these conditions have been lacking since Bronze Age (Pietsch *et al.* 2010). Arable farming thus was restricted to hydrologically favorable sites in most parts of the Arabian Peninsula, such as natural oases and wadis or depending on the development of irrigation. Terracing slopes or wadi beds are other basic features connected to gardening and oasis cultivation, (e.g. Avni *et al.* 2012; Buerkert and Schlecht 2010), often combined with run-off water harvesting installations at least in the past. The complex nature of sedimentary deposits from terraces and wadi beds is challenging for palaeoecological investigations. For example, these terrace features have fluvial accumulation resulting in discontinuous sedimentation, re-deposition, and often missing preservation of organic material. However, interdisciplinary research approaches indicate the potential in deciphering the fluctuating multiple uses, plant cultivation, and grazing during the past (Beckers *et al.* 2013; Charbonnier *et al.* 2017; Meister *et al.* 2016; Moraetis *et al.* 2020; Purdue *et al.* 2019) These basic landscape features necessitated adapted agricultural systems such as oasis agriculture, characterized today by its a combination of cultivation of perennial fruit trees and annual crops.

Neolithic archeological assemblages in the Nafud, the northern sand desert of Saudi Arabia, show a strong Levantine influence (Crassard *et al.* 2013), while southern Arabian archeological findings point to the persistence of regional agricultural developments. Archeo-zoological records of domesticated sheep, goats, and cattle since the 8ka point to connections to the Near East as well. Herding (and hunting) is thought to have been a major subsistence strategy since at least the Neolithic (c. 7-6 ka). Somewhat more favorable environmental conditions during the early to mid-Holocene, i.e. denser and more diverse vegetation, facilitated the spread of this new livelihood (Crassard and Drechsler 2013; Uerpmann *et al.* 2010). Its impact on natural ecosystems on a regional level is quite difficult to estimate. In southern Jordan, vegetation disturbance due to grazing is supposed to be small as compared to the impacts in later times by industrial activities such as ore production (Hunt *et al.* 2007). In southern Arabia in particular, decades of research have focused on the observation that southern Arabian populations seem to have adopted agriculture relatively late in comparison with the Gulf region (within the last 3-2 ka; Harrower *et al.* 2010; McCorriston 2013; McCorriston and Johnson 1998). Instead, Late Stone Age and Bronze Age people in southern Arabia retained pastoral livelihoods through much of the Holocene. Throughout Yemen and Oman, evidence of ephemeral camps exist in the highlands as well as in the inland draining wadis and around palaeolake shorelines, suggesting mobile lifestyles centred around animal-husbandry of cattle and goats by 8 ka, and eventually camels by 3.5ka (Lézine *et al.* 2010; Martin *et al.* 2009; McCorriston 2013).

However, despite the adoption of agriculture in southern Arabia, there is still some equivocal palaeoecological evidence for human environmental modification of the early to mid-Holocene landscape by nomadic pastoralists. In Wadi Sana, Yemen, evidence of burned surfaces and hearths contained in terrace sediments are coeval with evidence of dated occupation by pastoralists (Kimiae and McCorriston 2014; McCorriston *et al.* 2002). This suggests that the practice of controlled burning of the landscape to deter woody encroachment and promote tender grassy
shoot production for animal fodder, a practice common in modern pastoralist groups, may have been used at this time (Kimiae and McCorriston 2014). Hearths and settlement excavations from southern Oman and Yemen attest to wood harvesting, particularly taxa known for use as fodder, construction materials, or cooking fuel including *Acacia*, *Terminalia*, and *Ziziphus*. This evidence of fire and landscape modification is also visible in pollen and macrobotanical remains preserved in hyrax middens near archeological sites in Yemen (McCorriston *et al.* 2002; Ivory *et al.*, 2021). Further, following the initial decline in monsoon moisture around 6 ka, construction of simple check dams and irrigation features is observed along wadis in Oman and Yemen (Harrower 2008). This is also contemporaneous with the increase in construction of falaj systems in northern Oman to provide an additional source of freshwater in the lowlands. These water management structures are thought perhaps to mitigate increased moisture variability and decreased surface runoff associated with declining monsoon intensity prior to abandonment of the region around 5 ka (Harrower 2008).

The cultivation of crops however is unambiguous evidence of anthropogenic changes. Although pollen of *Cerealia* is present in moderate abundances as early as 10 ka in sediment cores (Hunt *et al.* 2004; Litt *et al.* 2012), these very few early Holocene pollen records do not offer clear clues due to the challenge in reliably distinguishing wild grasses from domesticated cereals on the Arabian Peninsula. However, since the 5 ka, the cultivation of cereals and other annuals is attested by seeds and fruits for different regions on Arabian Peninsula, indicating connectivity with the Near East (Boivin and Fuller 2009; Bouchaud *et al.* 2016). In the Gulf region, records of *Phoenix dactylifera* predate the cereals, and so far, the oldest date palm kernels are found on the Dalma islands dating to c. 8 ka (Beech and Shepherd 1994), with otherwise marine food remains dominating. Since 5 ka, the date palm seemed to have played an important role in the arable agriculture of the gulf region, while in other regions of the Arabian Peninsula, this now dominant tree was still missing (e.g. Schiettecatte *et al.* 2013) and oasis agriculture relied on other fruit trees such as grapes (Dinies *et al.* 2016).

The crop evidence since c. 5-4 ka agree with the emergence of large oasis settlements and increased connectivity, including maritime trade (e.g. Boivin and Fuller 2009; Hausleiter and Eichmann 2018). The storied “incense road” may be seen as part of this connectivity. Ongoing residue analyses in the oasis of Tayma, NW Arabia, point to likely local resin sources (*Pistacia*, *Commiphora*) during late mid-Holocene, while *Boswellia* became important during the last 3000-2000 years. The commercial harvest and trade of Boswellia resin (luban, frankincense) resulted in the construction of seaports such as Sumharam and Al Baleed in southern Oman to facilitate trade of frankincense along Arabian Sea trade routes (Hoorn and Cremaschi 2004; Lippi *et al.* 2011). Here too, the impact of oasis agriculture on the landscape on a regional scale is hard to estimate due to the very few palaeoecological records and probably varied depending on the local practices. Detailed archeobotanical analyses of a NW Arabian oasis indicate the persistence of desert vegetation without major shifts, pointing to exploitation of available natural resources, but no over-use (Bouchaud 2011). The modern heavily overused landscape situation seems to differ from that prevailing even just a few centuries ago, when grazing (and wood/shrub cutting) was restricted because of a missing permanent water supply. This today has become obsolete because of deep wells and mobile water tanks guaranteeing accessibility everywhere (Hunt *et al.* 2007; Thalen 1979).

## 11.6 CONCLUSIONS

The vegetation of Arabia is strongly imprinted by biogeographic and palaeoecological processes that unfolded over millions of years beginning with the separation from Africa during the Cenozoic. Although little empirical palaeoecological data exist, new efforts to relaunch the African Pollen Database including Arabia will galvanize new research efforts as well as syntheses by
conforming to FAIR data standards that make existing data easier to find and work with (Ivory
et al. 2020; Wilkinson et al. 2016). This review attempts to summarize some of this work and
inventory data holdings currently within the APD.

Palaeoecological information from Arabia make it clear that while ecologically diverse and endemic, Arabian ecosystems have existed on the edge of extreme aridity for at least hundreds of thousands of years. We recognize the strong need for more palaeoecological data which repre- sent pre-Holocene time periods in order to better disentangle the influence of climate events on the vegetation. Although deep, long-lived lakes are rare in the region, analysis of pre-Holocene palaeolake deposits as well as drilling campaigns in estuarine sites could help fill in some of this gap. Further, despite the highest data density within the early Holocene, these data are still very sparse. Improving spatial and temporal resolution of this period should also be considered a priority. This may be achieved by continued work on classical palaeoecological archives (palaeo- lakes, oases), complementing archeobotanical research, and the development of novel palaeoecological archives adapted to arid regions such as hyrax middens, termite mounds, and fine-grained fluvial deposits.

The antiquity of the human legacy on the environment is not well known. The human imprint on the landscape in Arabia locally is strong through crop cultivation in oases or wadis but seems to be reversible and without indication of over-use in vast desert and semi-desert regions used as rangelands. The density of palaeoecological information associated with archeological investigations do not lend themselves to a very clear picture of human modification of landscapes through many interventions. This region displays evidence of the some of the earliest plant cultivation, many of which leave telltale signals in pollen and macrobotanical records. Further, early evidence of water management and burning connected with pastoralism have acted as a filter on species composition of local vegetation and ecosystems for thousands of years. Deeper connections between the palaeoecological record, archeological data, and anthropological theory through continued empirical data generation and combined empirical and modeling approaches could prove extremely helpful for bettering understanding feedbacks between changing human demography, culture, and the natural environment.

Finally, we observe that Arabian landscapes, particularly semi-desert and desert ecosystems today are extremely fragile; however, they show an overall strong resilience throughout the Holocene as observed through the lens of the palaeoecological record. However, while some land-use practices have been observed by pollen data to degrade resource-rich landscapes such as intensive agriculture, other forms of traditional management alternately enriched landscapes, such as water management for agriculture and pastoralism. Palaeoecology in Arabia provides an important perspective and framework for lessons in sustainable management of dryland ecosystems under changing climate in the future.

ACKNOWLEDGMENTS

This work contributes to the ACCEDE ANR-NSF Belmont Forum project (18 BELM 0001 05 and NSF-1929563) as well as NSF-CNH 1617185. Thanks are due to the African Pollen Database for data access. SJI is funded by Penn State University, AML is funded by CNRS, MD is funded by the DFG priority program SPP Entangled Africa.

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CHAPTER 12

# The challenge of pollen-based quantitative reconstruction of Holocene plant cover in tropical regions: A pilot study in Cameroon

Marie-José Gaillard<fnref n="1" /> & Esther Githumbi<fnref n="1" />

*Department of Biology and Environmental Science, Linnaeus University, Kalmar, Sweden*

Gaston Achoundong

*National Herbarium, IRAD, Yaoundé, Cameroon*

Anne-Marie Lézine

*Laboratoire d'Océanographie et du Climat, Expérimentation et Approche numérique/IPSL, Sorbonne University, CNRS-IRD-MNHN, Paris, France*

Christelle Hély

*Institut des Sciences de l'Evolution de Montpellier, Université de Montpellier and Ecole Pratique des Hautes Etudes, Université PSL, Montpellier, France*

Judicaël Lebamba

*Département de Biologie, Université des Sciences et Techniques de Masuku, Franceville, Gabon*

Laurent Marquer<fnref n="2" />

*Institute of Botany, University of Innsbruck, Innsbruck, Austria*

Florence Mazier

*Environmental Geography Laboratory, GEODE UMR-CNRS 5602, Université de Toulouse Jean Jaurès, Toulouse, France*

<footnote n="1">¹Other affiliation: Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden</footnote>

<footnote n="2">²Other affiliation: Research Group for Terrestrial Palaeoclimates, Max Planck Institute for Chemistry, Mainz, Germany</footnote>
Furong Li

School of Ecology, Sun Yat-sen University, Guangzhou, China

Shinya Sugita

Institute of Ecology, Tallinn University, Estonia

**ABSTRACT:** Quantitative pollen-based reconstruction of Holocene plant abundance has not been attempted in the tropics so far. Tropical vegetation is characterized by a large number of entomophilous species and pollen-vegetation modelling for such plants is widely considered too problematic. However, there is a pressing need for quantitative reconstructions of Holocene plant cover in tropical regions to better assess climate- and human-induced environmental crises over time. Of particular note is the environmental crisis at the end of the 'African Humid Period' (c. 3 thousand years before present [ka BP]) which remains unquantified. Here, we present results from a pilot study in the Cameroon highlands including estimates of relative pollen productivity (RPP) for major taxa of the Afromontane forests and grasslands using the Extended R-Value (ERV) model, and the first reconstructions of past regional and local Holocene plant cover using the Landscape Reconstruction Algorithm (LRA). RPP estimates were obtained for 12 taxa. We found that *Celtis*, *Alchornea*, *Syzygium*, *Macaranga/Mallotus*, *Nuxia* type, and *Schefflera* have RPPs larger than Poaceae, and Moraceae, Combretaceae/Melastomataceae, *Prunus africana/Rubus pinnatus*, Cyperaceae, and *Podocarpus* lower RPPs than Poaceae. The LRA was applied to three Holocene pollen records from Cameroon (Bambili, Mbi, Mbalang) and one from Nigeria (Tilla). The results confirm the abruptness of the forest collapse in Cameroon at 3 ka BP, and suggest that it was faster and of larger magnitude than earlier anticipated. The LRA reconstruction also indicates that the regional landscape in the mountain region was more open than uncorrected pollen percentages alone suggest. Moreover, it supports the earlier interpretation of a dense forest at Bambili 11–3.5 ka BP and the local occurrence of Afromontane forest taxa at Mbalang in the Mid-Holocene.

## 12.1 INTRODUCTION: THE CHALLENGE

A way to anticipate the impact of current and future anthropogenic climate change on the environment, in particular in relation to the ongoing '6th extinction' (Barnosky et al. 2011), is to study past crises as accurately as possible. Tropical Africa experienced a major climate-driven environmental crisis at the end of the Holocene (Lézine et al., this volume, and references therein). This crisis affected all natural tropical African ecosystems. Near the equator this resulted in the collapse of Afromontane forests and the expansion of grasslands and woodland grasslands. While in the Sahara and Sahel tropical species disappeared. This crisis is well documented by numerous pollen records acquired over the last decades in western Africa (e.g. Hély et al. 2014; Vincens et al. 1999). However, the resulting land-cover change remains difficult to quantify because of the wide variety of pollen productivity and dispersal mechanisms of tropical plant species (Lézine et al. this volume; Bouimethraan et al., this volume); which implies unexpected and variable representation of the surrounding vegetation by pollen in studies of modern pollen-vegetation relationships (e.g. Gajewski et al. 2002; Julier et al. 2019). The Intergovernmental Panel on Climate Change (IPCC) pointed out that proxy-based climate reconstructions in Africa were too limited to support regional climate change assessments (Masson-Delmotte et al. 2013). In this
context pollen-based quantification of past land-cover change is essential for climate modelling and the understanding of past land cover-climate interactions (e.g. Gaillard et al. 2010) as they can be used to evaluate Dynamic Vegetation Model simulations (e.g. Lu et al. 2018) and Anthropogenic Land Cover Change scenarios (Kaplan et al. 2017; Li et al. 2020), and help to better evaluate carbon storage over time and its impact on climate.

Quantitative pollen-based reconstruction of Holocene plant abundance have not been attempted in the tropics so far. However, some studies have estimated Relative Pollen Productivity (RPP) of major taxa in tropical and subtropical regions of the world (e.g. Jiang et al. 2020; Wan et al. 2020), including two studies in South Africa (Duffin and Bunting 2008; Hill et al., this volume) and one study in Namibia (Tabares et al. 2021). Here we present results from a pilot study in Cameroon testing methods and models previously used and validated in the northern Hemisphere, i.e. the Extended R-Value (ERV) model (Parsons and Prentice 1981; Prentice and Parsons 1983) and the Landscape Reconstruction Algorithm (LRA; Sugita et al. 2007a and 2007b). The aim was to obtain first estimates of RPPs for major plant taxa and reconstruct their cover over the Holocene at both regional and local spatial scale on the Adamawa Plateau and in the montane/sub-montane regions of Cameroon (Figure 1). Reconstruction of the relationship between the cover of woodland and open land over the 3 ka BP Holocene environmental crisis in Cameroon (e.g. Lézine et al. 2013) is of particular interest and represents a first attempt to quantify the crisis effect on the vegetation cover. We present major results of the study to illustrate the potential of using the ERV model and the LRA in tropical areas, and to highlight major methodological issues and possible improvements.

## 12.2 MATERIALS AND METHODS

### 12.2.1 Study Region And Study Sites

The 30 sites selected for collection of modern soil pollen and related vegetation surveys for the RPP study are distributed at altitudes between 432 and 2990 m asl. in the montane/submontane and Guineo-Congolian sectors, except one sample at the southern border of the Sudano-Guinean sector (Figure 1, Table S1). A stratified random distribution of sites was chosen to ensure a good representation of the major taxa occurring in the Holocene vegetation history of the study region. However, site distribution was also influenced by logistical issues related to the number and season of the project's field expeditions and other limitations due to local circumstances.

The Holocene pollen sites used for the LRA reconstructions fall within the geographical range of the modern sites and are located at different elevations in the volcanic mountain range of Cameroon, at 2000–2200 m asl. in the Afromontane forest sector (Bambili, Lézine et al. 2013; and Mbi, Lézine et al. 2021), and at 1110–1260 m asl. in the Guineo-Sudanian wooded grasslands (Mbalang, Vincens et al. 2010) (Figures 1, 2; Table S3). An additional pollen site (Tilla, Salzmann et al. 2002) from the Sudanian wooded grassland at 690 m asl. in Nigeria was selected for the REVEALS reconstruction of regional vegetation needed to apply the LOVE model for reconstruction of the local vegetation around Mbalang (Figure 1; Table S3). For further details on the choice of sites, please see Supporting Online Material (SOM) section 12.2.4. Henceforth, all dates are given in calibrated ¹⁴C kyrs before present (CE 1950) (abbreviated ka BP).

### 12.2.2 Pollen And Vegetation Data

The collection of soil samples and vegetation surveys for the RPP study were performed in February 2010, October 2012, and April 2015. The pollen-vegetation dataset includes data from 4 sites in semi-deciduous forests, 10 sites in mountain forests, and 16 sites in savannahs (Figure 1;
<figure><img src="image_65.png" /><figcaption>**Figure 1.** Location of sites for the study of relative pollen productivities (surface soil samples and vegetation surveys) and fossil pollen records used for the application of the Landscape Reconstruction Algorithm. Simplified vegetation map of Cameroon (from Letouzey (1985) and White (1983), modified). Small map, lower right corner: I. Guineo-Congolian centre of endemism; XI. Guineo-Congolian/Sudanian transition zone; III. Sudanian centre of endemism; XVI. Sahelian transition zone. XVII. Saharan transition zone. Abbreviations: modern samples: MF montane forest; SA savannah; SDF semi-deciduous forest; fossil pollen sites: Bb Bambili; Mb Mbi; Mg Mbalang; Ti Tilla.</figcaption></figure>

Table S1). For further details on the methods used for collection of the pollen samples and vegetation surveys, pollen analysis, vegetation data handling, and on data access please see Supplementary Online Material, SOM-12.1.
<figure><img src="image_66.png" /></figure>

**Figure 2.** Altitudinal distribution of plant taxa discussed in the text (from Verlhac *et al.* 2018; modified). The altitudinal distribution of the main vegetation types (from Letouzey 1985) is shown on the left, while the altitudinal occurrence of Poaceae as a dominant taxon in wooded and open grasslands is indicated on the right. The colours refer to those in Figure 1 (except for Poaceae) and Figures 3, 5 and 6. The altitudinal location of pollen sites used for the LRA reconstructions is shown on the right, and the Nigerian site (Tilla) on the left. * indicates that the site Tilla belongs to another grassland system than the Cameroon sites (Figure 1).

### 12.2.3 The Erv Model and Erv Analysis

Detailed overviews of the underlying theory and assumptions of the ERV model and its developments can be found in Gaillard *et al.* (2008) and Bunting *et al.* (2013). Details on models and methods used for calculation of RPPs, and the computer programme for the ERV analysis are provided in SOM-12.1.4.

In this study, we used the three ERV sub-models developed by Parsons and Prentice (1981), Prentice and Parsons (1983), and Sugita (1994). The application of the ERV sub-models requires the following inputs:

* Pollen data: pollen % of taxon i at site k. In this study the maximum number of taxa analysed is 26 and k is a soil sample from a 1 m² area, for 30 sites (SOM-12.1, Table S2)

* Radius of the sedimentary basin: distance in m. In this study this is the radius of the area from which soil was collected, i.e. 0.5 m.

* Reference taxon: the RPP of the reference taxon is set to 1, and the RPP of all other taxa included in the analysis are related to that reference taxon. In this study Poaceae is used as the reference taxon.
* Vegetation data: Non-distance-weighted vegetation abundance of taxon i around site k. In this study percentage cover is used, from 0.5 to 3000 m in 5 m-concentric rings working out from the sampling location.

* Zmax: distance within which most pollen comes from. In this study estimated to be 200 km (see SOM-12.1.4).

The ERV model also requires that vegetation data is distance-weighted. We used three alternative models, 1/d (Prentice and Webb 1986), 'Prentice's model' for bogs (uses a Gaussian plume diffusion model (P-GPM) and assumes pollen deposition at one point; Prentice (1985)), and the Lagrangian Stochastic Model (LSM) (Kuparinen 2006; Kuparinen et al. 2007). P-GPM and LSM describe dispersion and deposition of pollen in different ways (e.g. Theuerkauf et al. 2013). For the application of these two models, additional inputs are needed:

* Fall speed of pollen (FSP) (m/s) of each taxon used in the analysis; FSP was calculated for the 26 taxa that were used in the ERV 'start run' (Table S2; see SOM-12.1 for details on the ERV analysis strategy and methods to calculate fall speed of pollen).

* Wind speed (m/s), in this study 3 m/s.; this wind speed is widely used in the RPP studies performed in the Northern Hemisphere; it is also close to the modern average 10-m wind speed over land in Europe and China (e.g. Grassi et al. 2015), and the global average 10-m wind speed over land (3.28 m/s (class 1), Archer and Jacobson 2005).

The ERV analysis estimates the RPPs and their standard errors (SEs) using the maximum likelihood method (Stuart and Ord 1994) and quantifies the best 'linear fit' between pollen percentages and distance-weighted plant abundances using the likelihood function score (LFS, Prentice and Parsons 1983) or log likelihood (log L; e.g. Li et al 2017) over distance (see SOM-12.1 for details). Beyond the distance at which the best linear fit is achieved, the curve of LFSs and log Ls reach an asymptote, i.e. do not decrease, respectively increase with distance.

The main outputs of one ERV analysis using the three ERV sub-models and the three distance-weighting models, i.e. nine runs of ERV analysis in total are:

* Vegetation dispersion with distance (a measure of vegetation stationarity, see SOM-12.1.3) (Figure 3A),

* Mean plant composition with distance for each taxon used in the analysis (Figure 3A),

* Adjusted pollen proportions (submodels 1 and 2) or relative pollen loading (submodel 3) and vegetation proportions (1, 2) or absolute vegetation abundance (3) as calculated by the maximum likelihood method; empirical modern pollen data are expressed in percentages, which implies a non-linear pollen-vegetation relationship that would not have any algebraic solution for the RPPs and the 'background pollen' in the ERV model equation. The pollen-vegetation relationship needs therefore to be linearized (Figure 4) (see SOM-12.1.4 for details),

* LFS and log L with distance (Figure 3A),

* RSAP (*sensu* Sugita 1994) calculated with the moving-window regression approach developed by S. Sugita (in Gaillard *et al.* 2008); RSAP is Z<sub>RSAP</sub> in the ERV model equation, i.e. the distance beyond which the 'background pollen' (second term in the ERV model's equation, *ω*<sub>i</sub>) is constant between sites within a region characterized by stationary vegetation; it is also the distance at which the linearity of the pollen-vegetation relationship is best and does not improve with distance (see SOM-12.1.4 for details); stationary vegetation implies that any area (e.g. 10 km × 10 km) has the same species composition as any other such area within the study region,

* RPP and their standard errors (SEs) with distance for each taxon used in the analysis (Figure 3B).

Details on the ERV analysis strategy are provided in SOM-12.1.4. The first ERV analysis run used the 26 taxa with both pollen and vegetation data in ≥ 5 sites (Table S2). Based on plots
<figure><img src="image_67.png" /><figcaption>**Figure 3.** Examples of results from the ERV analysis of pollen and vegetation data from 30 sites in Cameroon (see Figure 1 for site distribution). This analysis used 12 taxa, ERV submodel 2 and the Prentice's Gaussian Plume Model for distance weighting of vegetation (see Methods and SOM-12.1.4 for details). (A) Log likelihood, vegetation dispersion, and mean taxa composition (in proportion) over distance from the pollen sample points; note that the scale for Poaceae is on the right of the diagram. (B) Relative Pollen Productivities (RPPs) with distance from the pollen sample points for seven taxa. The colours relate to those in Figures 1, 2, 5 and 6; the five shades of red (dark brownish red to light pink) relate to the red colour in Figures 1 and 2 and to the five shades of red in Figures 5 and 6; the three shades of green relate to Figures 1 and 2 two shades of green together and to the three shade of green in Figures 5 and 6.</figcaption></figure>

of adjusted pollen percentages and vegetation proportions (Figure 4), the taxa with the best linear relationships were selected for the next runs of ERV analysis. In this paper, we present the results of the ERV analysis that produced the curve of log L closest to the theoretically correct curve (increases and reaches an asymptote at a certain distance; SOM-12.1.4), which was obtained with 12 taxa, ERV sub-model 2, and P-GPM for distance weighting of vegetation (Figure 3A; Table 1A).

### 12.2.4 Landscape Reconstruction Algorithm (LRA)

Details on the LRA and its assumptions are found in SOM-12.2.3. LRA comprises two models, REVEALS (Regional Vegetation Abundance from Large Sites) (Sugita 2007a) and LOVE (Local Vegetation Estimates) (Sugita 2007b). Both models reduce biases due to inter-taxonomic differences in pollen productivity and basin size. REVEALS estimates the mean regional vegetation composition of plant taxa in proportion (dimensionless) for a large region (minimum 100 km × 100 km; Hellman et al. 2008) using pollen records from large lakes. REVEALS assumes that no plants are growing on the deposition basin (Sugita 2007a). Therefore, model