Climate but not disturbance history controls grassland plant community structure

Background/Question/Methods

Changes in climate and disturbance regimes are thought to be key controls of grassland plant community structure. We examined the relative importance of changing climate and fire history in controlling species richness, evenness and composition in remnant rough fescue grasslands in Prince Albert National Park, Saskatchewan, Canada. We used a 35-year (1975-2010) dataset where plant community composition was measured every 8-15 years in 24 plots, each containing 25 1-m² permanent quadrats. Prescribed burn experiments were conducted on the site between 1975 and 1982 and in 2009. We measured the change in both the average plant community structure in each plot and the heterogeneity among the quadrats in each plot between sampling periods. We used structural equation modeling (SEM) to examine the relative importance of climate, fire history, and plant community processes in controlling plant community structure and heterogeneity. Exogenous variables included the previous year growing season precipitation, 5-year mean precipitation, time since last burn, and number of burns. Endogenous variables that also impacted plant community structure included changes in plant cover and litter cover.

Results/Conclusions

The structural equation models explained between 36 and 91% of changes in mean plant community composition, species richness and evenness, and between 45 and 76% of the changes in small-scale heterogeneity in composition, richness, and evenness. Changes in climate were the primary drivers of changes in plant community structure and heterogeneity with precipitation in the previous growing season having much stronger impacts than 5-year average precipitation. The only significant effect of disturbance regime was a drop in species richness with increasing time since fire. The overall effects of previous growing season precipitation were complex with both direct effects on community structure and indirect effects mediated through cover and litter. Increasing precipitation increased both plant community cover and litter which in turn caused declines in evenness and richness. Increasing precipitation, however, had much stronger positive direct effects on evenness and richness. Plant community heterogeneity strongly declined with increasing precipitation. Overall these results demonstrate a strong control of fine-scale community structure by climate in these grasslands, with disturbance regime playing only a minor role. Fire has been thought to be one of the key mechanisms in these grasslands, however these results point to an overriding influence of climate on community structure.