Evidence that plant diversity and site productivity confer stability to forest floor microbial biomass

The ability of soil microbial communities to withstand punctual disturbance or chronic stress is important for the stability of ecosystem processes. Factors controlling microbial community composition or soil resource availability should be regarded as potential determinants of this stability. Here, we explored the effects of three stand types (jack pine, aspen and mixed-wood) and two geologic parent materials (clay and till), on the stability of the microbial biomass in the forest floor. We hypothesised that microbial communities in mixed-wood stands or on the clay soil would show greater resistance to, and resilience from, a dry wet disturbance, and a higher tolerance to incremental additions of HCI or Cu, than microbial communities in mono-specific stands or on the till soil. We also surveyed the understory vegetation, and measured chemical properties and microbial phospholipid fatty acid profiles in the forest floor, so as to gain insights into the factors regulating microbial stability. Microbial resistance to disturbance was found to be higher in mixed-wood than in mono-specific stands. Microbial communities from mixed-wood stands also showed a high tolerance to HCI and Cu stress over both geologic parent materials, as opposed to those in mono-specific stands that showed a high tolerance to stress on only one type of parent material. Some forest floor properties in mixed-wood stands (e.g. Ca on clay, mineralisable N and C/N ratio on till) were more similar to the more productive aspen, than to jack pine stands. Other properties (understory plant communities, pH, actinomycete and arbuscular mycorrhizae) of mixed-wood stands were transitional between those in aspen and jack pine stands, suggesting that both tree species contribute in structuring the forest floor microbial pool in mixed-wood stands. We put forward that this may provide a more diverse capability to resist disturbance and tolerate stress than in mono-specific stands. We found no effect of stand type on microbial resilience to disturbance, but resilience was higher on clay than on till plots. This could be due to a higher fungal/bacterial ratio on till plots, as slower fungal growth rates may hinder resilience, or to lower carbon and nutrient availability limiting the growth rate of resistant microbial cells. We conclude that plant diversity and site productivity are important drivers of forest floor microbial stability in the southern boreal forest of eastern Canada. (C) 2010 Elsevier Ltd. All rights reserved.