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.

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 F. africana, M. capensis or one seed of I. scheffleri 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 F. africana and the other two species were therefore excluded from the analyses.

Study 2 - Germination and survival in the field

Seeds of F. africana, M. capensis and I. scheffleri 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 (C. robusta, L. camara, N. dullooa and S. occidentalis) 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.

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 Funtumia africana 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.