Nitrogen Effects in Grasslands

Human-derived nitrogen (N) flows into natural systems via agriculture, fossil fuel combustion, and other actions associated with industrialization, are recognized among the most significant and widespread threats to plant community diversity and stability. Global transfer of N from atmospheric to terrestrial pools is estimated to have more than doubled within the past century. Fertilization experiments in California grasslands have suggested that N addition has a positive effect on the growth and abundance of exotic annual grasses, often at the expense of drought-resilient forb and legume species. However, these effects are somewhat variable, depending on local environmental conditions. In California grasslands, long-term resilience to climate variation may be contingent on maintaining the abundance of these drought-adapted species. Unfortunately, studies that have assessed the impacts of N addition on plant diversity are often limited to small plots over a few years, which impair translation of their outcomes to the ecosystem as a whole.

Because all plant ecosystems exhibit spatial heterogeneity (the formation of distinct vegetation patches), approaches that account for this variation, rather than averaging effects over the entire sampling area, may generate stronger predictions. For example, rare and drought-tolerant plant species may depend on isolated patches to act as reservoirs from which outward colonization is possible when environmental conditions favor their growth. While patch formation is viewed as an important factor in the maintenance of plant diversity, it is rarely quantified, and even less so related to the effects of biological stressors, such as N deposition. Researchers out of the Eviner Lab sought to address this knowledge gap through long term experiments at HREC.

 By examining the groups of plant species that constitute discrete patches within the landscape, the researchers worked to infer what species associations appear to be preferable, and which species rely on patch formation for long-term persistence. To evaluate how N deposition interacts with the underlying pattern of vegetation, their study aimed to establish the baseline rates of change in vegetation over space and time, and assessed how N deposition alters these patterns, from those driven by highly local plant-plant interactions to larger-scale changes in key environmental variables, such as soil depth. They then evaluated the interaction between fertilizer enrichment and turnover in vegetation to identify how N deposition can alter community response to environmental variation. These two analyses presented a refined picture of N deposition effects on plant diversity, providing land managers and restoration practitioners with a framework to replicate natural vegetation patterns and enhance resilience.

Land managers may seek to generate spatial and temporal patchiness as a restoration practice to help comparatively weaker species establish without being overburdened by competitive stress. Targeted management strategies to reduce the impacts of rapid change driven by N deposition may lead to improved outcomes and greater efficiency of resource use—this may involve treatments designed to reduce the abundance of unfavorable species in conditions where their populations are expected to grow, or seed additions that maintain distinct patches of favorable species until unfavorable conditions improve.

This research built on several existing long-term datasets present at Hopland REC, Sierra Foothill REC, and McLaughlin Natural Reserve, and the 50+ year UC Cooperative Extension Forage Monitoring dataset. HREC also contains existing nutrient addition experiments as part of the Nutrient Network sampling program, which provided data on the long-term effects of nutrient deposition on temporal turnover in species composition. Inclusion of this long-term data in analysis permitted comparison between nutrient-driven changes to spatial and temporal patterns of community heterogeneity across California grasslands.