Understanding impacts of altered disturbance regimes on community structure and function is a key goal for community ecology. Functional traits link species composition to ecosystem functioning. However, there is evidence that changes in the distribution of functional traits at community scales in response to disturbance can be driven not only by shifts in species composition, but also by shifts in intraspecific trait values. Understanding the relative importance of these two processes can have important implications for predicting the time scale of responses to altered disturbance regimes. We experimentally manipulated fire return interval (control 3 yr return intervals, fire suppression, and annual burn treatments) in replicated blocks of a fire-adapted, long leaf pine (Pinus palustris) ecosystem in North Carolina, USA and measured plant trait and community responses along lowland-wet to upland-dry gradient over a four year period. We addressed the following questions: Does altering the fire return interval affect community composition and diversity? Does altering the fire return interval alter the position of the shrubby ecotone along the soil moisture gradient? Does altering the fire regime shift community level trait distributions, and if so to what extent is this driven by changes in species composition relative to intraspecific trait shifts.
We found strong evidence that altered fire return interval impacted understory plant communities. 1) Annual burn sites lost species and suppression sites gained species over the course of the experiment, so that by the end of the period, there were significant differences in richness between these two treatments (p = 0.02). We also observed shifts in community composition, but these shifts were only significant in the upland plots along the gradient. 2) We also found that the number of burns a site had experienced significantly predicted the shift in the ecotone boundary (p = 0.001), with suppression sites experiencing an upward shift in the ecotone and annual burn sites a downward shift. 3) Both Functional Richness and Functional Evenness were significantly altered by the experimental treatments with annual burn sites experiencing a decline in Functional Richness (p<0.001) and an increase in Functional Evenness (p=0.06) relative to suppression sites. The relative importance of intraspecific trait shifts and compositional shift in contributing to community level changes in SLA varied depending on landscape position. These results suggest that while changes in disturbance regime can have consistent results on communities and traits, the mechanisms underlying these shifts can be variable across landscapes.