Data from: 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 cover (-0.11).
The model for total understorey species richness showed an indirect effect
of 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 effect via 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 plants followed similar trends to those for total
understorey cover and species richness; however, there was an opposite
indirect effect of light heterogeneity for both cover and species richness
of non-vascular plants. Shrub cover had positive direct and negative
direct and indirect effects on both vascular and non-vascular cover and
species richness. Our findings indicate that understorey cover and species
diversity are driven by time since disturbance, light availability as
influenced by overstorey and shrub layers, 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
boreal forest understorey vegetation.