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Description
Climate change will result not only in changes in the mean state of climate but also on changes in variability. However, most studies of the impact of climate change on ecosystems have focused on the effect of changes in the central tendency. The broadest objective of this thesis was to assess the effects of increased interannual precipitation variation on ecosystem functioning in grasslands. In order to address this objective, I used a combination of field experimentation and data synthesis. Precipitation manipulations on the field experiments were carried out using an automated rainfall manipulation system developed as part of this dissertation. Aboveground net primary production responses were monitored during five years. Increased precipitation coefficient of variation decreased primary production regardless of the effect of precipitation amount. Perennial-grass productivity significantly decreased while shrub productivity increased as a result of enhanced precipitation variance. Most interesting is that the effect of precipitation variability increased through time highlighting the existence of temporal lags in ecosystem response.
Further, I investigated the effect of precipitation variation on functional diversity on the same experiment and found a positive response of diversity to increased interannual precipitation variance. Functional evenness showed a similar response resulting from large changes in plant-functional type relative abundance including decreased grass and increased shrub cover while functional richness showed non-significant response. Increased functional diversity ameliorated the direct negative effects of precipitation variation on ecosystem ANPP but did not control ecosystem stability where indirect effects through the dominant plant-functional type determined ecosystem stability.
Analyses of 80 long-term data sets, where I aggregated annual productivity and precipitation data into five-year temporal windows, showed that precipitation variance had a significant effect on aboveground net primary production that is modulated by mean precipitation. Productivity increased with precipitation variation at sites where mean annual precipitation is less than 339 mm but decreased at sites where precipitation is higher than 339 mm. Mechanisms proposed to explain patterns include: differential ANPP response to precipitation among sites, contrasting legacy effects and soil water distribution.
Finally, increased precipitation variance may impact global grasslands affecting plant-functional types in different ways that may lead to state changes, increased erosion and decreased stability that can in turn limit the services provided by these valuable ecosystems.
Further, I investigated the effect of precipitation variation on functional diversity on the same experiment and found a positive response of diversity to increased interannual precipitation variance. Functional evenness showed a similar response resulting from large changes in plant-functional type relative abundance including decreased grass and increased shrub cover while functional richness showed non-significant response. Increased functional diversity ameliorated the direct negative effects of precipitation variation on ecosystem ANPP but did not control ecosystem stability where indirect effects through the dominant plant-functional type determined ecosystem stability.
Analyses of 80 long-term data sets, where I aggregated annual productivity and precipitation data into five-year temporal windows, showed that precipitation variance had a significant effect on aboveground net primary production that is modulated by mean precipitation. Productivity increased with precipitation variation at sites where mean annual precipitation is less than 339 mm but decreased at sites where precipitation is higher than 339 mm. Mechanisms proposed to explain patterns include: differential ANPP response to precipitation among sites, contrasting legacy effects and soil water distribution.
Finally, increased precipitation variance may impact global grasslands affecting plant-functional types in different ways that may lead to state changes, increased erosion and decreased stability that can in turn limit the services provided by these valuable ecosystems.
ContributorsGherardi Arbizu, Laureano (Author) / Sala, Osvaldo E. (Thesis advisor) / Childers, Daniel (Committee member) / Grimm, Nancy (Committee member) / Hall, Sharon (Committee member) / Wu, Jingle (Committee member) / Arizona State University (Publisher)
Created2014
Description
The Basin and Range province of southwestern USA are composed of different grassland and shrubland ecosystems. Particularly understudied ecosystems in this region are playas, which are ephemerally-flooded wetlands located in topographic low areas of hydrologically-closed dryland catchments. There is not much known about the ecological functioning of playas and the role of playas within desert basins. Even less is known about how global change drivers may affect playas in the future. The main objective of this thesis was to better understand the ecological functioning and the impact of climate change on desert playa wetlands. I collected new data, used existing long-term data, and used simulation modelling techniques to address this objective. I compared playa soils to upland soils and found that playas were hotspots of soil organic carbon and nutrient storage within a desert basin. I also used existing data to analyze the response of above-ground net primary productivity (ANPP) to annual precipitation in playas and upland ecosystems. I found that playa ANPP responded in a non-linear concave-down relationship with annual precipitation amount. Playa ANPP peaked in moderately wet years and declined in very wet years, which was most likely due to flooding; whereas, upland ANPP increased linearly with precipitation. I measured soil organic carbon and nitrogen concentrations in a representative subset of playas and measured the biophysical characteristics of the upland catchments associated with each playa. I found that both catchment geomorphology and vegetation cover were correlated to differences in soil organic carbon and nitrogen among playas. These results showed the importance external soil-inputs delivered via surface runon to playas. Finally, I empirically measured groundwater recharge beneath playas and combined these empirical data with modelling data to forecast how playa groundwater recharge may change in the future. I concluded that playas contribute to groundwater recharge in desert aquifers, playa runon is a strong predictor of playa groundwater recharge, and climate change will have a net-positive impact on groundwater recharge beneath playas. Overall, my thesis research increased the understanding of the role of desert playas on the functioning of dryland ecosystems.
ContributorsMcKenna, Philip (Author) / Sala, Osvaldo E. (Thesis advisor) / Monger, H. Curtis (Committee member) / Hall, Sharon J (Committee member) / Childers, Daniel L. (Committee member) / Arizona State University (Publisher)
Created2016