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

Summary 1 Understorey vegetation comprises the majority of species diversityand contributes greatly to ecosystem functioning in borealforests. Although patterns of understorey abundance, species diversityand composition associated with forest stand developmentare well researched, mechanisms driving these patterns remain largely speculative. 2 We sampled fire origin stands of varying stand ages and overstorey compositions on mesic sites of the borealforest of Canada and used structural equation modeling (SEM) to link timesince fire (stand age), light availability and heterogeneity, substrate heterogeneityand soil nitrogen to understorey vegetation cover and species diversity. 3 The most parsimonious model for total understorey cover showed a positive direct effect of stand age (r = 0.43) and an indirect effectvia mean light level (0.18) and shrubcover (-0.11), with a positive total effect (0.50); the percent broadleaf canopy had a direct negative effect (-0.22) and an indirect effectvia shrubcover (-0.11). The model for total understorey species richnessshowed an indirect effectof stand age via mean light (0.24), light heterogeneity(0.10), and substrate heterogeneity(0.07), with a positive total effect (0.52); percent broadleaf canopy had an indirect effectvia light heterogeneity(0.09), and substrate heterogeneity(-0.10). Soil nitrogen did not significantly influence either understorey cover or species richness. The models for vascular plantsfollowed similar trends to those for total understorey cover and species richness; however, there was an opposite indirect effectof light heterogeneityfor both cover and species richnessof non-vascular plants. Shrubcover had positive direct and negative direct and indirect effectson both vascular and non-vascular cover and species richness. 4. Synthesis. Our findings indicate that understorey cover and species diversityare driven by time since disturbance, light availability as influenced by overstorey and shrublayers, but with important additional effects mediated by light and substrate heterogeneity. Non-vascular understorey vegetation is more strongly determined by time since disturbance than vascular vegetation, and negatively affected by broadleaf tree abundance. The overall results highlight the importance of colonization, light availability and heterogeneity, substrate specialization and growth dynamics in determining successional patterns of borealforest understorey vegetation. This article is protected by copyright. All rights reserved.