Linking resource availability and heterogeneity to understorey species diversity through succession in boreal forest of Canada

Understorey vegetation hosts the majority of species diversity and contributes greatly to ecosystem functioning in natural systems. Although patterns of understorey abundance, species diversity and composition associated with forest stand development are well researched, mechanisms driving these patterns remain largely speculative. We sampled fire origin stands of varying stand ages and overstorey compositions on mesic sites of the boreal forest of Canada and used structural equation modeling (SEM) to link time since fire (stand age), light availability and heterogeneity, substrate heterogeneity and soil nitrogen to understorey vegetation cover and species diversity. The most parsimonious model for total understorey cover showed a positive direct effect of stand age (r = 0.43) and an indirect effect via mean light level (0.18) and shrub cover (-0.11), with a positive total effect (0.50); the percent broadleaf canopy had a direct negative effect (-0.22) and an indirect effect via shrub c...