Microbial priority effects on decomposing leaves in contrasting northern temperate forests
The identity of early colonists of a habitat patch may affect subsequent community composition and ecosystem processes, known as “priority effects”. This phenomenon can amplify the effect of stochastic events over time, but what determines the importance of priority effects relative to stabilizing niche processes is not well understood. Decomposition of plant tissue is performed by diverse communities of saprotrophic fungi. These communities are made up of groups of fungi that have evolved to be most competitive when decomposing different compounds. To test the importance of priority effects we conducted an experimental manipulation of leaf colonization using three fungal isolates differing in functional traits (extracellular enzyme production, growth rates and spore production). We hypothesized that, during decomposition of more labile substrates, priority effects will be particularly important due to high nutrient availability allowing many organisms to colonize and competitively exclude subsequent colonists. In contrast to this, on more recalcitrant material, communities should be more specialized and thus converge despite different initial colonists.
Sugar maple and red oak leaves (representing labile and recalcitrant leaves, respectively) were sterilized and then pre-colonized with one of three functionally distinct fungal isolates. The colonized leaf material was placed into litter bags and into the field in late May 2012 in Manistee National Forest. The bags were placed in nine forest stands, representing three replicate stands of each of three ecosystems. After one or five months in the field, leaf mass loss was calculated and material was subsampled for further analyses. Mass loss data showed that there were significant effects of both isolate and ecosystem on decomposition rates. The significant effect of isolate indicates the importance of priority effects on microbial communities and the ecosystem processes they mediate. In contrast to our hypothesis, priority effects appear equally strong for labile and recalcitrant leaves and in different ecosystems, although the specific isolate effects are reversed depending on leaf type. The ecosystem effect on decomposition was consistent with “home field advantage”, as leaves tended to decompose at higher rates in their native ecosystems. Enzymatic profiles and terminal restriction fragment length polymorphism (TRFLP) will also be performed to examine priority effects in microbial community composition and functional traits.