Ph.D. Student, University of California Riverside
This research looks at the stress response of urban vegetation to temperature and drought anomalies
Urban vegetation provides many benefits, or ecosystem services, to cities. However, the continued provisioning of these services is contingent on how vegetation responds during times of stress. Urban plants are stressed by the unique characteristics of the urban environment; stressors such as temperature are expected to increase over the course of this century due to impacts from climate change. Despite the importance vegetation plays in urban ecosystems, the dynamics of how urban vegetation responds to stress is unknown.
The stress response of vegetation can be understood as its stability, which is composed of resistance and resilience to stress. Resistance refers to the capacity for vegetation to maintain similar levels of productivity even when placed under stressful conditions. Resilience refers to the capacity for vegetation to quickly recover pre-stress levels of productivity following the removal of a stressor. Both resistance and resilience may operate independently; more stable vegetation will have higher resistance and/or resilience.
This research looks at the stability of urban vegetation in Los Angeles, California to drought and temperature stress. Stability was determined for the summer months of June through August in a 17-year time series from 2000 through 2016. In order to calculate stability we determined land surface temperature, a drought index, and plant productivity (plant greenness) for each day; these were determined using multispectral satellite imagery. The stress response of plants was determined by looking at the change in plant greenness over time.
We sought to understand the factors influencing the stress response of urban vegetation. To do so we tested three hypotheses: microclimate, income, and development age. First, we tested microclimate on the coast to inland gradient. We expected increasing distance from the coast to increase the variability in climatic conditions and to decrease stability. Second, we tested the luxury effect, which says that higher income areas will have greater species richness. Greater species richness is related to higher stability, so we would expect greater stability in more affluent areas. Finally, we tested the legacy effect, which says that older neighborhoods will have more species richness; we would expect greater stability in these neighborhoods. This research will highlight the unique stress dynamics of urban vegetation and will lay the framework for a multi-city analysis.
Abstract: Human-driven environmental changes may simultaneously affect the biodiversity, productivity, and stability of Earth's ecosystems, but there is no consensus on the causal relationships linking these variables. Data from 12 multiyear experiments that manipulate important anthropogenic drivers, including plant diversity, nitrogen, carbon dioxide, fire, herbivory, and water, show that each driver influences ecosystem productivity. However, the stability of ecosystem productivity is only changed by those drivers that alter biodiversity, with a given decrease in plant species numbers leading to a quantitatively similar decrease in ecosystem stability regardless of which driver caused the biodiversity loss. These results suggest that changes in biodiversity caused by drivers of environmental change may be a major factor determining how global environmental changes affect ecosystem stability.
Pub.: 18 Apr '15, Pinned: 30 Jul '17
Abstract: Environmental stress changes the relationship between biodiversity and ecosystem functions, but the underlying mechanisms are poorly understood. Because species interactions shape biodiversity-ecosystem functioning relationships, changes in per capita interactions under stress (as predicted by the stress gradient hypothesis) can be an important driver of stress-induced changes in these relationships. To test this hypothesis, we measure productivity in microalgae communities along a diversity and herbicide gradient. On the basis of additive partitioning and a mechanistic community model, we demonstrate that changes in per capita interactions do not explain effects of herbicide stress on the biodiversity-productivity relationship. Instead, assuming that the per capita interactions remain unaffected by stress, causing species densities to only change through differences in stress tolerance, suffices to predict the stress-induced changes in the biodiversity-productivity relationship and community composition. We discuss how our findings set the stage for developing theory on how environmental stress changes biodiversity effects on ecosystem functions.
Pub.: 19 Aug '16, Pinned: 30 Jul '17
Abstract: Anthropogenic drivers of environmental change often have multiple effects, including changes in biodiversity, species composition, and ecosystem functioning. It remains unknown whether such shifts in biodiversity and species composition may, themselves, be major contributors to the total, long-term impacts of anthropogenic drivers on ecosystem functioning. Moreover, although numerous experiments have shown that random losses of species impact the functioning of ecosystems, human-caused losses of biodiversity are rarely random. Here we use results from long-term grassland field experiments to test for direct effects of chronic nutrient enrichment on ecosystem productivity, and for indirect effects of enrichment on productivity mediated by resultant species losses. We found that ecosystem productivity decreased through time most in plots that lost the most species. Chronic nitrogen addition also led to the nonrandom loss of initially dominant native perennial C4 grasses. This loss of dominant plant species was associated with twice as great a loss of productivity per lost species than occurred with random species loss in a nearby biodiversity experiment. Thus, although chronic nitrogen enrichment initially increased productivity, it also led to loss of plant species, including initially dominant species, which then caused substantial diminishing returns from nitrogen fertilization. In contrast, elevated CO2 did not decrease grassland plant diversity, and it consistently promoted productivity over time. Our results support the hypothesis that the long-term impacts of anthropogenic drivers of environmental change on ecosystem functioning can strongly depend on how such drivers gradually decrease biodiversity and restructure communities.
Pub.: 03 Jul '13, Pinned: 30 Jul '17
Abstract: Biotic indicators have been widely used to monitor wetland health. However, few studies have explicitly evaluated if plant diversity could serve as a useful community-level indicator of wetland stability, especially when wetlands are confronted with anthropogenic perturbations. Based on three-year record of wetland plant species abundance in Napahai plateau wetland, Shangri-la under the influence of varying anthropogenic perturbation types, our study tests the impact of such perturbations on plant richness and the relationship between ecosystem temporal stability and plant richness, and further assesses the effectiveness of using plant diversity indicator to probe ecosystem temporal stability of Napahai plateau wetland and the potential mechanisms. The results showed that anthropogenic perturbations could have contributed significantly to realistic variation in plant diversity, and further demonstrated that ecosystem temporal stability was positively related to realistic variation in plant diversity. In particular, communities with high levels of diversity might have better capacity to dampen perturbation impacts than communities with low levels of diversity, and statistical averaging could have played an important role in causing greater stability in more diverse communities. Also, asynchrony might have a stabilizing effect on community stability, and diversity could have stabilized communities through both species asynchrony and population stability propagation. Therefore, our results suggest that plant diversity could be used as a useful indicator of the stability conditions of plateau wetland ecosystems confronted with anthropogenic perturbations, and the preservation of plant communities at sufficient abundance and diversity is necessary for maintaining healthy plateau wetlands and for sustaining their essential ecosystem functions and services.
Pub.: 13 Mar '14, Pinned: 30 Jul '17
Abstract: The relationship between species diversity and ecosystem stability is controversial. Tilman et al. analyse biomass patterns over a decade in a grassland experiment with artificial plant communities, and provide evidence for a positive relationship between the number of plant species and the temporal stability of the ecosystem. Here we use data from a long-term biodiversity experiment with plant communities that were not controlled by weeding in order to show that diverse systems can be both stable and unstable.
Pub.: 30 Mar '07, Pinned: 30 Jul '17