A study by Woody Weeds MSc student Samson Aman Samson Kiswaga, published in Ecology and Evolution, reveals no relationship between spread of a range of alien plants and the seed germination, seedling survival or seedling communities of native trees in the Amani Botanical Garden (ABG) in Tanzania.
Samson, a student at Sokoine University of Agriculture, assessed the effects of up to 29 alien plant species that differed in the extent of spread on germination, abundance, and diversity of native tree species. He specifically sought to determine if the spread of planted alien species – such as the Mexican Rubber Tree (Castilla elastica Cerv), the African Oil Palm (Elaeis guineensis Jacq) and the coffee plant (Coffea canephora Pierre ex A.Froehner) – related to their per capita impact on germination, establishment, and survival of three native tree species and the abundance and species richness of seedlings growing in the understory of these alien species. He also studied possible mechanisms underlying the impact on native tree species, in particular the effect of allelochemistry.
Samson Aman Samson Kiswaga, lead author of the paper in Ecology and Evolution, said, “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 prioritizing alien species for management.”
The ABG was established in 1902 and is one of the oldest botanic gardens in Africa, with more than 1,000 species of plants from all over the world, many of which were planted within two decades following the establishment of ABG. The native tree species which formed part of the study were Funtumia africana (Benth.) Stapf, Macaranga capensis (Baill.) Sim and Isoberlinia scheffleri (Harms) Greenway.
In these experimental studies, we worked with tree species belonging to different stages of succession, but the germination rate of M. capensis and I. scheffleri was very low, which, in addition to the small number of species sown, makes it impossible to generalize the results.
It is unclear why the germination rate of the untreated seeds was low, but M. capensis seeds are eaten by birds and passage of the gut may stimulate germination, while I. scheffleri seeds swell and produce mucus when soaked in water, which results in very rapid germination, and the experimental conditions in the Petri dish study may have been suboptimal for these species. However, the seedlings of 50 native species recorded under alien trees in the ABG allowed us to make this comparison and the results indicate that there was no difference in the effect of the extent of spread of alien plant species on seedling establishment among native tree species of different successional stages.
One of the most striking findings of the study, the scientists say, was the difference in the relationships between seedling abundance and species richness of alien and native species. The positive relationship between native seedling species richness and abundance appears typical for “normal” ecological systems: With increasing sample size, the number of detected species increases too.
Samson Aman Samson Kiswaga concluded that more multispecies studies that assess patterns in impacts are needed, because management should target alien species, such as Psidium guajava, that both spread rapidly and cause major impact on native species, biodiversity, and ecosystem functioning.