How dynamic are understory communities and the processes structuring them in mature conifer forests?

Abstract

Even though understory plant communities play important roles in northern forest ecosystems, our understanding of their long‐term successional dynamics and the associated driving processes is still limited. This study used spatially explicit data from five, long‐term, permanent plots in mature lodgepole pine (Pinus contorta) stands in the Canadian Rocky Mountains to: (1) assess whether understory communities showed homogenization over time either within or between plots and if compositional changes observed were supportive of either the initial or relay floristics model; and (2) evaluate the relative importance of the environment vs. dispersal limitations in structuring these understory communities and how this changes over time. We found between‐plot convergence and within‐plot homogenization over time, largely due to increasing cover of mosses and later successional species. These changes were mainly in relative abundance, thus supporting the initial floristic model. However, the appearance of late successional species and the disappearance of early successional species over the study period suggested that the relay floristic model was also important. Most of the variation in understory species composition and richness was explained by spatial variables that were independent of environmental factors, suggesting that dispersal limitation was important in these communities. However, environmental variables explained an important fraction of the variation in species richness and composition, especially in more xeric environments; from this we infer that deterministic processes still played an important role in structuring these communities. In most plots, our results also showed increasing importance of dispersal limitations following canopy closure and we attribute this to both the homogenisation of the understory environment and increased abundance of later successional species which were more dispersal limited.