Ecosystem effects of community assembly history: A test with wood-decay fungi

Background/Question/Methods

Carbon dynamics and other ecosystem processes have proven difficult to explain as they often appear idiosyncratic. Recent studies have identified community structure such as species diversity and composition as a major determinant of ecosystem processes. However, community structure itself often appears idiosyncratic, and it has become clear that an overlooked source of this idiosyncrasy is immigration history, the sequence and timing in which species join the community. What remains poorly understood is whether and how immigration history affects ecosystem processes indirectly via community structure. We provide experimental evidence that historically derived variation in community structure influences carbon dynamics and that the direction and magnitude of this effect depend on nutrient availability. In laboratory microcosms, we used wood-decay fungi from New Zealand beech forests as a model system to determine how immigration history and nitrogen availability interact to affect carbon release from decomposing wood. 
Results/Conclusions

Our molecular data revealed complex interactions between fungal species that depend on immigration history. For example, one of the ten species introduced was highly competitive and dominated the community in most cases, but when a particular second species was established first, it prevented the otherwise competitive species from becoming abundant. Further, only when this second species was introduced first did a third species become abundant. Yet, this third species reduced the abundance of the facilitator, indicating an intransitive competitive hierarchy. These compositional differences in turn altered carbon release from wood and other ecosystem processes. Depending on which species arrived first, there were marked differences in respiration rate, wood mass loss, and the carbon to nitrogen ratio of remaining wood. In one case, where one species was introduced first, carbon to nitrogen ratio was nearly three times higher than when other fungi were introduced first. Moreover, the effect of history on fungal composition and wood decomposition varied greatly with the amount of nitrogen added to soil and wood. These results suggest that historical information on species immigration may be essential, though difficult to obtain, for explaining seemingly idiosyncratic variation in carbon dynamics and other ecosystem processes.