Samuel F. Bartels

Is understory plant species diversity driven by resource quantity or resource heterogeneity

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.

Mechanisms Regulating Epiphytic Plant Diversity

Epiphytic plants are important components of the forest ecosystem, but little is known about their ecology due to logistical constraints and the lack of a conceptual framework to guide epiphyte community studies. Given the present state of knowledge and renewed interests in epiphyte community studies, a mechanistic framework is needed to guide investigations of epiphyte assemblages. This article identifies six putative mechanisms of epiphyte species diversity, and highlights possible direct and indirect linkages among the mechanisms. The mechanisms include constrained dispersal, slow growth rate, substrate availability, host tree mortality, disturbance, and global climate change. They are identified as inherent, local- and stand-level, and landscape-level mechanisms. Evidence supporting the proposed mechanisms and their linkages is typically inductive due to inadequate observational and manipulative studies. Sufficient testing and experimental manipulations over broad spatial and temporal scales are necessary for future predictions of epiphytic plant diversity patterns.

Dynamics of epiphytic macrolichen abundance, diversity and composition in boreal forest

Summary 1. Epiphytic macrolichen lichens are important components of forest ecosystems, but their responses to stand-replacing fire and multiple successional pathways of the canopy tree layer are poorly understood. 2. We examined the dynamics of epiphytic macrolichens following wildfire and tested the independent and interactive effects of time since fire and overstorey composition on epiphytic macrolichen abundance, diversity and composition in boreal forests of Canada. Epiphytic macrolichens were sampled in 51 stands of conifer, mixed-wood and broadleaf overstorey types ranging from 7 to 209 years since fire. 3. Macrolichen abundance estimated as percentage cover continuously increased with stand age with highest cover in 209-year-old stands for all overstorey types. Mixed-wood and conifer stands had higher macrolichen abundance than broadleaf stands in all age classes except similarly low abundances in stands ≤15 years old for all overstorey types. Species richness of epiphytic macrolichens reached peaks in 98- or 146-year-old stands. Mixed-wood stands had higher macrolichen species richness than broadleaf and conifer stands at 98 years old, but not at other age classes. Multivariate analysis indicated that the macrolichen communities were compositionally distinct for all age classes and overstorey types. 4. Synthesis and applications. Epiphytic macrolichen abundance continuously increases with time since fire. Our results also show that macrolichen diversity peaks at intermediate stand ages with highest diversity in mixed-wood stands. Our findings of distinct macrolichen communities supported by each age class and overstorey compositional type suggest that conservation of epiphytic macrolichen diversity would require forest managers to maintain a diverse age structure and overstorey composition in forest landscapes.

Water supply changes N and P conservation in a perennial grass Leymus chinensis.

Changes in precipitation can influence soil water and nutrient availability, and thus affect plant nutrient conservation strategies. Better understanding of how nutrient conservation changes with variations in water availability is crucial for predicting the potential influence of global climate change on plant nutrient-use strategy. Here, green-leaf nitrogen (N) and phosphorus (P) concentrations, N- and P-resorption proficiency (the terminal N and P concentration in senescent leaves, NRP and PRP, respectively), and N- and P-resorption efficiency (the proportional N and P withdrawn from senescent leaves prior to abscission, NRE and PRE, respectively) of Leymus chinensis (Trin.) Tzvel., a typical perennial grass species in northern China, were examined along a water supply gradient to explore how plant nutrient conservation responds to water change. Increasing water supply at low levels (< 9000 mL/year) increased NRP, PRP and PRE, but decreased green-leaf N concentration. It did not significantly affect green-leaf P concentration or NRE. By contrast, all N and P conservation indicators were not significantly influenced at high water supply levels (> 9000 mL/year). These results indicated that changes in water availability at low levels could affect leaf-level nutrient characteristics, especially for the species in semiarid ecosystems. Therefore, global changes in precipitation may pose effects on plant nutrient economy, and thus on nutrient cycling in the plant-soil systems.

Reclamation strategies for mined forest soils and overstorey drive understorey vegetation

1.Understorey vegetation accounts for the majority of plant diversity in 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. 2.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. 3.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. 4.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 to be 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. This article is protected by copyright. All rights reserved.