Mineral stress (nutrient deficiency and (or) ion toxicity) is a widespread phenomenon in forests around the world. However, with the exception of N limitation, its significance is often under appreciated. On weathered, acidic soils that support many of the world’s forests, P, Ca, and Mg deficiencies and toxicities of Al and Mn are important constraints to forest productivity. Nutrient resources are a primary controller of forest function and structure and have important trophic implications, because foliar nutrient status is an important determinant of leaf palatability and consumer fitness. Nutrient acquisition and utilization in forest ecosystems is strongly influenced by environmental factors, which are changing at unprecedented rates with regional and global climate shifts. Here we examine nutrient limitations common to temperate, sugar maple ( Acer saccharum Marsh.) dominated forests as a model for understanding how climatic factors influence the acquisition and utilization of nutrient resources in forest ecosystems. In general, foliar nutrient imbalances created by soil weathering and acidification impair sugar maple physiology and correlate with health decline symptoms. Extremes in light environment, temperature, precipitation, pathogen attack, and herbivory tend to induce and (or) negatively interact with nutrient imbalances in sugar maple. A conceptual model is presented that characterizes abiotic and biotic interactions influencing sugar maple health and fitness in the context of nutrient limitation.

Biotic resistance and disturbance are fundamental processes influencing plant invasion outcomes; however, the role of consumers in regulating the establishment and spread of plant invaders and how disturbance modifies biotic resistance by consumers is unclear. We document that fire in combination with experimental exclusion of rodent consumers shifted a native desert shrubland to a low‐diversity, invasive annual grassland dominated by Bromus tectorum (cheatgrass). In contrast, burned plots with rodents present suppressed invasion by cheatgrass and developed into a more diverse forb community. Rodents created strong biotic resistance to the establishment of aggressive plant invaders likely through seed and seedling predation, which had cascading effects on plant competition and plant community diversity. Fire mediated its positive effects on plant invaders through native plant removal and by decreasing the abundance and diversity of the rodent community. The experimental disruption of plant and consumer‐mediated biotic resistance of plant invaders using fire and rodent exclusion treatments provides strong evidence that native plants and rodents are important regulators of plant invasion dynamics and plant biodiversity in our study system. While rodents conferred strong resistance to invasion in our study system, fluctuations in rodent populations due to disturbance and climatic events may provide windows of opportunity for exotic plant species to escape biotic resistance by rodent consumers and initiate invasions.

Disturbance patterns strongly influence plant community structure. What remains less clear, particularly at a mechanistic level, is how changes in disturbance cycles alter successional outcomes in plant communities. There is evidence that fire suppression is resulting in longer fire return intervals in subalpine forests and that these lengthened intervals increase competitive interactions between aspen and conifer species. We conducted a field and greenhouse study to compare photosynthesis, growth and defense responses of quaking aspen and subalpine fir regeneration under light reductions and shifts in soil chemistry that occur as conifers increase in dominance. The studies demonstrated that aspen regeneration was substantially more sensitive to light and soil resource limitations than that of subalpine fir. For aspen, light reductions and/or shifts in soil chemistry limited height growth, biomass gain, photosynthesis and the production of defense compounds (phenolic glycosides and condensed tannins). Biomass gain and phenolic glycoside concentrations were co-limited by light reduction and changes in soil chemistry. In contrast, subalpine fir seedlings tended to be more tolerant of low light conditions and showed no sensitivity to changes in soil chemistry. Unlike aspen, subalpine fir increased its root to shoot ratio on conifer soils, which may partially explain its maintenance of growth and defense. The results suggest that increasing dominance of conifers in subalpine forests alters light conditions and soil chemistry in a way that places greater physiological and growth constraints on aspen than subalpine fir, with a likely outcome being more successful recruitment of conifers and losses in aspen cover.