title: Pollen-based reconstruction of the vegetation and climate of the eastern Mediterranean region during the Holocene
Creators: name: Cruz Silva, Esmeralda
Publisher: name: University of Reading
Year: 2024

description: This thesis presents reconstructions of the vegetation and climate history of the Eastern
Mediterranean region during the Holocene, based on a comprehensive pollen data set, the
development and application of new reconstruction techniques, and rigorous statistical analyses.
The Eastern Mediterranean is of interest because it experienced one of the earliest transitions to
permanent agriculture, facilitating the expansion of human populations in the region.
Nevertheless, there were significant changes in population across the region during the Holocene,
as witnessed by rise and often abrupt decline of specific ancient civilizations. There is an ongoing
debate about whether climate changes influenced vegetation, which in turn affected the
availability of resources for ancient societies, or if human activities altered vegetation and
subsequently those changes impacted the climate. The current uncertainty arises from a focus on
records from individual sites and the lack of a comprehensive regional understanding of
vegetation and climate history. Understanding historical events in the Eastern Mediterranean
region necessitates robust reconstructions of the vegetation and climate in the region during the
last 12000 years in order to disentangle the causal factors underpinning observed changes in
vegetation and societal events in the region.
The thesis leverages the Eastern Mediterranean-Black Sea-Caspian Corridor (EMBSeCBIO) project,
which collected modern and fossil pollen data for the Eastern Mediterranean region, to develop or
apply statistically based predictive models for vegetation and climate reconstruction.
Standardized data syntheses, when analysed using statistical techniques, are an effective tool for
generating objective regional environmental reconstructions, addressing uncertainties and
reducing analytical biases.
In this thesis, a new technique for reconstructing vegetation is developed and calibrated by using
a modern pollen training dataset to characterize the composition and variability in the abundance
of pollen taxa within different biomes. The new technique performs better than existing
vegetation reconstruction methods, is more robust when applied to pollen time series, and allows
the explicit recognition of assemblagesthat have no analogue in the modern vegetation. This new
method was then applied to fossil pollen data from the Eastern Mediterranean region to predict
changes in vegetation types through the Holocene. These reconstructions were used to address
several controversies about the regional vegetation history. These analyses showed that the
greatest extent of non-analogue vegetation occurred during the early Holocene, the timing of
re-afforestation was broadly synchronous across the region, that the maximum expansion of
temperate deciduous forest occurred between 5.5 and 5 ka, and that the increase in
drought-adapted vegetation in the late Holocene was regionally heterogeneous.
The thesis addresses climate changes during the Holocene by applying a recently developed
method, frequency-weighted (fx) Tolerance-weighted Weighted Averaging Partial Least Squares
(fxTWA-PLS) to the EMBSeCBIO database in order to reconstruct four climate variables crucial to
vegetation growth and relevant to human well-being. Reconstructions were obtained for the mean temperature of the coldest month (MTCO), mean temperature of thewarmest month
(MTWA), growing degree days above a threshold of 0° C (GDD0
) and plant-available moisture,
represented by the ratio of actual to equilibrium evapotranspiration (α), for 71 individual pollen
records. The reconstructions were compared with independent evidence for climate changes and
with climate model simulations in order to identify the drivers of observed changes. The
glacial-Holocene transition and the early part of the Holocene were characterised by conditions
colder and drier than present. Rapid increases in temperature and moisture occurred between ca
10.3 and 9.3 ka,which is considerably after the end of the Younger Dryas. MTCO showed a gradual
increase from 9 ka to the present, indicating that winter temperatures were forced by orbitally
induced increases in insolation during the Holocene. MTWA also showed an increasing trend
from 9 ka to 5 ka, followed by a gradual decline towards present-day conditions. The initial
increase in summer temperature is not consistent with a response to orbital forcing, but
comparison with climate model simulations suggests that a delayed response to summer
insolation changes is likely a reflection of the persistence of the Laurentide and Fennoscandian ice
sheets; summercooling post-5 ka is consistent with the expected response to insolation changes.
After the initial rapid increase in plant-available moisture between 11 and 9.3 ka, subsequent
changes were small.
This thesis represents the first effort to obtain a comprehensive and quantitative reconstruction of
regional vegetation and climate changes in the Eastern Mediterranean. In addition to providing a
secure foundation for documenting the environmental history of the region through the
Holocene, it also provides the necessary basis for understanding historical events and their
potential impact on the regional vegetation. It has resolved a number of persistent controversies
about the nature of regional vegetation changes and provides a resource to explore the remaining
controversies about the interaction between climate, vegetation and human activities in a region
that is of great interest to multiple scientific communities.