What maintains plant species diversity has been the subject of much debate with no general consensus. In forest ecosystems in which understory plants account for the majority of floristic diversity, a crucial question is whether understory plant diversity is driven by resource quantity or resource heterogeneity. This study sought to reconcile the two hypotheses in relation to their effects on understory plant diversity in forest ecosystems. A database of studies that investigated the effects of resources on understory plant diversity was compiled and analyzed using log-linear models. Whether resource quantity or resource heterogeneity is the determinant of understory plant diversity in individual studies was dependent on stand successional stage(s), presence or absence of intermediate disturbance, and forest biome within which the studies were conducted. Resource quantity was found to govern species diversity in both young and mature stands, whereas resource heterogeneity dominated in old-growth stands. Resource quantity remained the important driver in both disturbed and undisturbed forests, but resource heterogeneity played an important role in disturbed forests. We argue that neither resource quantity nor heterogeneity alone structures species diversity in forest ecosystems, but rather their influences on understory plant diversity vary with stand development and disturbances in forest ecosystems.
Questions What is the nature of the interactions between higher vegetation strata (overstorey) and lower strata (understorey vegetation) and among understorey vegetation layers in closed canopy forests? How does the abundance or richness of one vegetation layer affect the abundance or richness of the other? Location Boreal mixed‐wood forests of Ontario and Quebec, Canada. Methods We sampled fire‐origin stands of varying overstorey composition from broad‐leaf‐dominated to mixed to conifer‐dominated stands on mesic sites in Ontario and subhydric sites in Quebec. Overstorey tree species composition and understorey shrub, herb, bryophyte and lichen species cover and richness were estimated within 400‐m2 circular plots. In addition, soil nutrients, coarse woody debris and light conditions were measured. Overstorey composition was expressed as the percentage basal area of broad‐leaf tree species. Path analysis was used to examine interactions among the forest layers. Results Overstorey broad‐leaf composition had positive effects on shrub and herb layer cover and herb layer richness, and negative effects on bryophyte and lichen species cover and richness. Shrub layer cover had no effect on herb layer cover, but shrub layer richness had a positive effect on herb layer richness. Herb layer cover and richness had negative effects on bryophyte species cover and richness. Bryophyte cover had no effect on lichen cover, but its richness was positively related to lichen richness. In both regions, soil pH, total nitrogen, total phosphorus and cation exchange capacity of the forest floor were positively correlated with overstorey broad‐leaf composition and with shrub layer cover and herb layer cover. Conclusions Increasing overstorey broad‐leaf composition, through its influence on soil nutrients, promotes shrub and herb layer species, but limits bryophyte and lichen species. In the overstorey‐controlled understorey resource environment, cover and richness of shrub and herb layers increase with resource availability, whereas bryophytes and lichens show positive associations with abundance of coarse woody debris.
Understorey vegetation accounts for the majority of plant diversity in boreal forest ecosystems and contributes to ecosystem functioning. In restoration of degraded forested ecosystems, however, understorey vegetation is often restored passively, contrasting to clear strategies such as informed species choice and site improvement intervention for overstorey vegetation. The choice of overstorey‐centred restoration strategy may have important consequences for understorey vegetation. We examined the effects of substrate material, overstorey type and time since reclamation (age) on understorey vegetation following reclamation of oil sands mining in Alberta, Canada. We sampled cover, richness, evenness and composition of understorey vegetation at 94 sites of conifer, mixedwood and broadleaf overstorey types on three reclamation substrates (overburden, secondary overburden and tailings sand), with age ranging from 4 to 30 years. Total, woody and non‐woody understorey cover and species richness were the highest on secondary overburden and the lowest on tailings sand, and total cover also decreased with age. Woody cover and richness were the highest under broadleaf overstorey, while non‐woody cover and richness were the lowest under conifer overstorey. Overall species evenness was not significantly affected by substrate type, overstorey type or age, but woody evenness was the highest on secondary overburden and the lowest on tailings sand, and non‐woody evenness showed overstorey‐dependent responses to age. Species composition varied with substrate type, overstorey type and age. Indicator species analysis revealed that tailings sand with conifer overstorey favoured grasses, while overburden and secondary overburden supported a mix of grasses, forbs and shrubs. Synthesis and applications. Our study demonstrates that overstorey‐centred reclamation strategies impact the abundance, diversity and composition of understorey plant communities following oil sands mining. Landforms constructed with secondary overburden substrates and revegetated with mixedwood or broadleaf tree species provide the most favourable habitats for understorey vegetation, while tailings sand provide a poor substrate for understorey species diversity and composition. We therefore recommend utilizing secondary overburden and overburden substrate material during landform construction, and employing revegetation prescriptions that target mixedwood and broadleaf overstorey types to promote productive and diverse understorey plant communities on the reclaimed landscape.
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