Understanding how biotic processes and abiotic variables interactively determine community structure and function is particularly challenging among wood decay fungi due to the opaque nature of the system. At the individual level, abiotic conditions are known to exert strong controls on fungal activity, yet these individual relationships often fail to correlate with decomposition rates and community composition across broad geographic extents. A potential explanation for this disconnect is that species interactions serve as the primary drivers of community composition and functioning in these systems, with abiotic controls emerging only after these community patterns are taken into account. To test this hypothesis, we conducted a field experiment at the long-term Soil Warming and Nitrogen plots at the Harvard Forest LTER. A total of 144 wood blocks were placed in the field, with subsequent decomposition and fungal community composition assayed after two years. Species co-occurrence patterns were partitioned into environmental (niche-based) associations and residual species associations. Along with basic community diversity indices, these species associations were used to investigate the relative importance of fungal associations versus microsite conditions (soil temperature, moisture, pH, %N, %C) and plot-level treatments (+5°C and N addition) as drivers of fungal community structure and function.
Functional outcomes and community outcomes were largely unrelated to abiotic conditions, with microsite and plot-level abiotic variables explaining <15% of the total variability in decomposition and fungal activity, and with none of the environmental variables showing statistically significant effects. In contrast, taxonomic richness, evenness, and species co-occurrence patterns all exhibited significant main effects or significant pairwise interactions as predictors of structure and function, accounting for 52% of the variation in decomposition rates and 65% of the variation in fungal activity. A greater proportion of positive vs. negative species associations in a community (as might be indicative of competitive interactions) was linked to strong declines in decomposition rates and lower overall diversity. In addition, the relationship between diversity and function was mediated by community evenness, with highly-even communities exhibiting a positive diversity-function relationship and uneven communities exhibiting a negative response. Combined, these results suggest that species interactions, rather than abiotic conditions, are the primary drivers of structure and function in wood-decay fungal communities.