The effect of introduced canopy tree species in a tropical, wet, montane forest on the taxonomic diversity and abundance of microbial communities

Background/Question/Methods

Introduced species are one of the biggest threats to biodiversity worldwide, and the replacement of native plant species with exotic invaders is increasingly common. Given the importance of soil microbes in biogeochemical cycling and ecosystem functioning, it is important to understand how introduced plants may alter soil microbial communities.  Microbial composition differs in soils that are under different species of plants; differences may be due to differences in trees’ canopy cover, rooting depth, litter quality, and litter quantity, or to secondary effects on soil properties including pH, moisture, and nutrient levels. These previous findings suggest invasive plants differing in their ecological traits from native plants have the potential to alter soil microbial communities, but little empirical evidence is currently available. This question is of unique importance in Hawaii since island flora are especially susceptible to invasion. To investigate the effect of introduced plant species on the soil microbial community, a study was conducted in the Kohala Forest Reserve, a tropical, forest on the island of Hawaii. Soil and litter samples were taken from 10 tree stands and 2 pasture sites; canopy tree species investigated were the native Metrosideros polymorpha and the introduced Cryptomeria japonica, Eucalyptus spp., Araucaria spp., and Casuarina spp. The microbial community was characterized by T-RFLP. Soil and litter were analyzed for cations, carbon, nitrogen, phosphorus, pH, and other properties that could influence the microbial community. 

Results/Conclusions

Terminal restriction fragment length polymorphism (T-RFLP) analysis indicated that the soil microbial community differed significantly under Eucalyptus, but not under the other three introduced species (NMDS ordination, p<0.001). The Eucalyptus microbial community was more similar to an adjacent deforested pasture site than to other forested stands. Differences were also seen in community richness and diversity; the native M. polymorpha had the highest richness and greatest number of terminal restriction fragments unique to the species, while Eucalyptus and pasture sites had the lowest. Of the soil and litter properties measured, soil pH was the best correlated to the variation in soil microbial community composition between species (r=0.802, p<0.001). My findings provide evidence that in cases where introduced tree species mediate an environmental factor, such as soil pH, they can affect the soil microbial community by significantly changing conditions. However, this is not always the case and the affect of an introduced tree species on the soil microbial community depends heavily on the characteristics of the specific species.