DOI: 10.5061/dryad.51c59zw4x
title: More widespread alien tree species do not have larger impacts on regeneration of native tree species in a tropical forest reserve

description: There is insufficient information regarding the factors affecting the
 environmental impacts of alien species. In particular, little is known
 about whether there is any relationship between the invasiveness
 (establishment and spread) of an introduced species and its per-capita
 impact. We experimentally assessed the relationship between the extent of
 spread of up to 29 alien plant species and their impact on recruitment of
 native tree species in Amani Botanical Garden, Tanzania. We also studied
 the effects of allelochemicals of selected alien on native plant species
 to assess potential mechanisms of impact. We found no relationship between
 the extent of spread of an alien tree species and their impact on seed
 germination, seedling survival and seedling communities of native trees in
 their understory, and no indication that allelochemicals consistently
 explain their effects on recruitment of the studied species. These results
 suggest that extent of spread cannot be used as a proxy for impact. Hence,
 managers should continue assessing both the spread and the impact of alien
 species when prioritising alien species for management.
description: Study 1 - Petri dish
 experiment Soil for Study 1
 and for analysis of the soil characteristics was taken from the top 20 cm
 at one, randomly selected location under four replicate trees of each
 species, at a distance of ca. 1 m from the trunk. Soil for analysis was
 stored in a fridge until use. Analysis of soil texture and chemical
 composition was carried out by the Mlingano laboratory in Muheza (Tanga
 District) according to standard laboratory protocols (Ministry of
 Agriculture and Fisheries, 2016). The mean values for the analysed soil
 variables are given in Appendix 2 of Kiswaga et al. The texture of the
 soil in one of the 60 samples was not analysed. Petri dishes (10cm
 diameter) were filled with soil taken from underneath individual trees.
 There were two Petri dishes with soil from each individual tree, resulting
 in eight replicates per soil origin. Four ml of activated carbon (Sigma
 Aldrich, St. Louis, MO) were added at a concentration of 20 ml/L to half
 of the Petri dishes to absorb organic compounds (Murrell et al., 2011).
 Petri dishes were watered and left for 24 hours prior to sowing of seeds
 to allow activated carbon to absorb soil allelochemicals. Petri dishes
 were arranged randomly on a laboratory table at room temperature at the
 Lushoto Silviculture Research Center (4°47’24“S, 38°17’43”E, ca. 1350
 m.a.s.l.). Three seeds each of <i>F. africana</i>, <i>M.
 capensis</i> or one seed of <i>I. scheffleri</i> were
 sown per Petri dish, taking care that the seeds were covered by soil in
 order to avoid drying of the seeds and maximising contact with the soil.
 There were 360 Petri dishes (15 species x 3 seed treatments x 8
 replicates). Petri dishes were watered as needed and the number of
 germinated seeds in each Petri dish was counted and recorded for 90 days
 from the date the experiment was set. The only species that germinated was
 <i>F. africana</i> and the other two species were therefore
 excluded from the analyses. Study 2 - Germination and survival
 in the field Seeds of
 <i>F. africana</i>, <i>M. capensis</i> and
 <i>I. scheffleri</i> were sown under 29 alien and three native
 species in ABG in March 2017. Under one individual tree of each of the 32
 species, ten seeds of each native species were sown in a randomly placed
 area (ca. 0.5 x 0.5 m per species) 1 m away from the trunk of the tree.
 Any leaf litter that was present was removed prior to sowing and put back
 afterwards to mimic the original conditions under the alien species. The
 locations where seeds were sown were marked and the number of seedlings
 was counted after two and eight months. Study 3 - Recruitment under alien
 plant species in ABG In March 2017,
 the natural recruitment of woody species was recorded under alien and
 native plant species in ABG. Four 2 x 2 m quadrats were laid in the
 cardinal directions at ca. 1 m from the stem base of one, randomly
 selected individual of each of 29 alien and three native tree species. In
 four cases (<i>C. robusta</i>, <i>L. camara</i>,
 <i>N. dullooa</i> and <i>S. occidentalis</i>)
 seedling occurrence was assessed in fewer than four quadrats; these
 species were excluded from the analysis. In each quadrat, all seedlings of
 woody plant species with a height less than 1 m were identified to the
 species level and counted. The seedlings of native species were assigned
 to successional stages (pioneer, secondary and climax) according to their
 growth characteristics and reproductive habit, based on Hamilton and
 Bensted-Smith ( 1989) and de Moraes et al. ( 2002). The litter depth was
 measured using a ruler and litter cover estimated visually.
description: Soil
 chemistry Chemical characteristics of soils under
 twelve alien and three native plant species. The first six columns provide
 details of the plant species, including name (first letters of species and
 genus names), plant family, origin of the species, the replicate sample
 number, altitude of sample collection and the extent of spread. The next
 fifteen columns indicate physical and chemical characteristics of the
 samples, including granulometric parameters, nutrient concentrations, pH,
 electric conductivity and cation exchange capacity. “NA” indicates that no
 value was provided. Study 1
 Germination of <i>Funtumia
 africana</i> in Petri dishes. The first eight columns provide
 details of the plant species, including name (first letters of species and
 genus names), plant family, origin of the species, plant life form, the
 replicate tree where soil was collected, the extent of spread, whether
 activated carbon was added and the altitude of sample collection. The
 ninth column indicates the number of germinated seeds per Petri dish and
 the remaining eleven columns indicate chemical parameters of the soils as
 described for the previous dataset. “NA” indicates that no value was
 provided. Study 2
 Germination of three native species
 under various plant species in the field. The first five columns indicate
 details of the plant species under which seeds were sown, including name,
 plant family, origin of the species, plant life form and the extent of
 spread. The next three columns provide longitude, latitude and altitude of
 the planting sites. The ninth column indicates which native species was
 sown, followed by two columns indicating the number of emerged and
 surviving seedlings. “NA” indicates that no value was provided.
 Study 3
 The number of saplings under various
 plant species in the field. The first six columns indicate details of the
 plant species under which saplings were counted, including name, plant
 family, origin of the species, plant life form, the extent of spread of
 the species and the number of the quadrat in which saplings were counted.
 The following five columns indicate longitude, latitude and altitude of
 the sites, as well as litter depth and cover. The last four columns
 indicate the name, status (alien/native) and successional status, as well
 as the number of saplings of each identified species per
 quadrat.