Stochasitc vs. niche-based processes: What drives lichen community assembly following fire disturbance?

Background/Question/Methods

According to Chase (2007), communities subjected to disturbance should follow niche-based assemble rules, have lower diversity, greater abundances of tolerant species, and be more similar.  Conversely, undisturbed communities will be more subject to stochastic processes, encompass a greater variety of species, and be less similar.  To test this, we examined the response of boreal lichen communities in Boundary Waters Canoe Area Wilderness, USA, to fire disturbance.  Specifically, we hypothesized that burned areas would contain communities dominated by a few resilient species, and therefore, be more similar than those in non-burned communities.  Lichen communities were sampled within and outside regions burned by the Cavity Lake Fire of 2006.  Ten sites were selected, 5 within and 5 outside the burn.  Each site contained 4 sample areas comprised of three diverging 3-m transects spaced 10-m apart, giving 20 areas for each treatment and 40 samples total.  Sampling techniques followed Johannson et al. (2006) and species abundance, substrate type, canopy cover and dominant vegetation were recorded.  Data were analyzed using ANOVA and PCA ordination.  Jaccard’s distance was used to compare communities and the betadisper function in the vegan R package was used to compare the degree of community similarity.

Results/Conclusions

Burned communities were more similar to each other than unburned communities (P<0.001).  The mean distance from the centroid of community composition was 80% greater in the unburned plots than in the burned plots.  Burned areas showed lower species abundances and diversity and were dominated by just two species, Cladonia fimbriata and Peltigera apthosa, which comprised nearly 70% of samples.  By contrast, the three most commonly encountered species in non-burned communities, C. fimbriata, C. coniocrae and Lepraria lobificans, made up only 10 to 15% of samples.  These data support Chase (2007) and suggest that fire disturbance enhanced niche-based processes in lichen assemblages, whereas stochastic processes were more important in unburned areas, and this allowed varied communities to develop and beta-diversity to be greater.