Effects of recurring fire disturbance on plant functional diversity may promote soil carbon accumulation in temperate deciduous forests

Background/Question/Methods

Disturbances such as fire can alter the functional composition of plant communities by favoring acquisitive species with fast-growing, low C:N tissues. This collective change in the functional characteristics of plant communities (functional diversity, FD) after disturbance should be a major driver of carbon dynamics. But the utility of FD in linking disturbance to carbon cycling, particularly soil organic carbon (SOC), remains uncertain. Two components of FD may be important drivers of SOC storage: (1) the most abundant functional trait value (community-weighted mean) or (2) the variety of functional trait values (functional divergence) in the plant community. We tested for relationships between these two FD components and SOC content in Midwestern deciduous forest stands with or without a history of fire and arrayed across a moisture gradient to maximize differences in FD.  We measured six plant functional traits for the 24 most common understory species and weighted them by species abundance to calculate community-weighted means for each trait and functional divergence. We also fractionated soil carbon into pools with different residence times: particulate (fast-cycling) and mineral-associated (slow-cycling) carbon pools. Using mixed effects models, we assessed which FD component best explained variation in the SOC pools. 

Results/Conclusions

Fire disturbance shifted the functional composition of understory communities from primarily woody-dominated to fast-growing, low tissue C:N grass and forb-dominated plant communities. Fire also increased functional divergence by promoting communities with more contrasting traits. Effects of fire disturbance on SOC depended on pool type; the fast-cycling SOC pool increased 25% (± 10% SE) while the slow-cycling SOC pool decreased 21% (± 8% SE) with fire disturbance. Functional divergence explained a significant portion of variation in the fast-cycling SOC pool (ΔAIC_c = 3.8, partial R² = 0.14), such that plant communities with a greater range of functional trait values had larger pools of fast-cycling SOC than communities with similar traits. In contrast, the slow-cycling SOC pool was best explained by tree seedling density (ΔAIC_c = 26.0, partial R² = 0.71). These findings suggest that fire disturbance may increase SOC inputs by enhancing the functional divergence and thereby niche complementarity of forest understory communities. This is one of only a few studies in forests that have found empirical support for fire disturbance impacting soil functioning via its influence on FD.