Predicting Ecosystem Resilience and Drought-Tolerance Thresholds in the face of Global Climate Change and Drought

Global temperatures during the years of 2000–2009 were some of the driest and warmest on record. These changing climatic conditions reflect the importance of ecosystem resilience in the face of altered temperature and precipitation patterns. Ecosystem resilience is the ability of ecosystems to function with the same feedback loops and sensitivity to changed surroundings during periods of disturbance as they would under undisturbed conditions. This function has implications for vegetation productivity, and therefore, carbon balance and food security as well. Ponce Camps et al., (2012) measured ecosystem resilience as the capacity of ecosystems to absorb disturbances from early twenty first century drought, while still maintaining late twentieth century above ground net primary production (ANPP) when there was high annual water availability. The authors found that ecosystem water use efficiency (WUEe) was similar across biomes, suggesting that ecosystems are able to cope with variations in hydroclimatic conditions by adjusting to both droughts and periods with high levels of precipitation, but only up to a point. Future climate change and predicted increases in drought frequency and severity may well overwhelm his ability. The authors compared data from both hemispheres over the years 2000–2009 to data from 1975–1988, in order to predict future ecosystem resilience and the threshold for WUEe. —Hilary Haskell
            Ponce Camps, G., Moran, S., Huete, A., Zhang, Y.,  Bresloff, C., Huxman, T., Eamus, D., Bosch, D.D.,  Buda, A.R., Hearstill Scalley, T., Kitchen, S.G., McClaran, M.P., McNab, W.H., Montoya, D.S., Morgan, J.A., Peters, D.P.C., Sadler, E.J., Seyfried, M.S., Starks, P.J., 2012. Ecosystem Resilience despite Large–scale Altered Hydroclimatic Conditions. Nature.

Ponce Campos et al. used a data set for this study that consisted of twelve U.S. Department of Agriculture sites and seventeen sites in Australia, including a variety of rainfall levels from both the Northern and Southern hemispheres. This data set from the years 2000–2009 was compared to data from 1975–2008 as a baseline to demonstrate the change in hydroclimatic conditions over recent years. The 1998 data came from long term ecological research sites. Precipitation and temperature were measured in a homogeneous vegetated area with no major disturbances in 2000–2009.
To quantify ecosystem resilience, the authors used the ecosystem’s functional response to disturbances characterized by rain–use efficiency (RUE) and water–use efficiency (WUEe). RUE is calculated by dividing ANPP by precipitation over a period of time. Similarly, ecosystem water–use efficiency was calculated by dividing ANPP by evapotranspiration. Evapotranspiration is defined as precipitation minus the water lost from plant leaves. The drought events of the early 2000s are recognized as a departure from typical climate variability. There was significant (P < 0.002) decrease in the Palmer Drought Severity Index (PDSI) over 1980–1999 and 2000–2009 for the U.S. and Australia, indicating increased aridity. Also, warm season temperatures over the 2001–2009 time period were significantly higher, (P
The Moderate Resolution Imaging Spectroradiometer (MODIS) produced the Enhanced Vegetation Index (EVI) through satellite observations to estimate collective plant behavior. This study quantified the relationship for the biomes and precipitation patterns between the EVI data and ANPP field methods for 2009–2009, using data from ten sites in the United States, and developed the following relationship: ANPPs = (51.42) EVI1.15. From this equation, there is evidence that the RUE data from the late twentieth century are consistent with plant production responses to precipitation. Low mean RUE for biomes with high precipitation suggest that some water from precipitation is not consumed by plants, but rather, abiotic aspects of the water cycle. This finding is further confirmed by comparing the positive relationship between evapotranspiration and plant production. Ponce Campos et al. concluded that the mean ecosystem water use efficiency was constant across the entire precipitation gradient. Furthermore, there were no significant differences among WUEe for the three data sets, suggesting that biomes remain sensitive to water availability during warmer drought conditions.
The most severe drought years coincided with maximum ecosystem WUEe in all biomes studied, indicating that ecosystems remain productive in extremely dry years by increasing their WUEe. However, Ponce Campos et al. also considered ecosystem resilience during the wetter mid-to-late drought (2003–2009) years, compared to hydroclimatic conditions in 1975–1998, and found that WUEe across all biomes and hydroclimate periods was at a minimum value. WUEe did not significantly (P > 0.05) vary in different hydroclimate periods, suggesting that biomes are able to respond to high annual precipitation, even during drought, by using available water more efficiently.
WUEe can increase or remain constant during changing hydroclimate patterns. During drought, the authors found that biomes’ WUEe increased as drought severity increased. Therefore, plant productivity remained near constant at late–twentieth century levels despite decreased precipitation. However, during wetter years, the biomes demonstrated similar consistency through absorbing and adapting to the disturbance of drought, and remained sensitive to water availability based on ANPP.
Ponce Campos et al. recognize that there are additional factors that may account for the plants’ and biomes’ responses to precipitation disturbances, such as vegetation structure and function and plant-soil feedbacks that the RUE or WUEe do not take into account. These factors are also important in considering ecosystem’s vulnerability and tolerance to changing hydroclimatic patterns.
During dry years, the sites with high plant productivity also had higher WUEe. These data indicate that vegetation is able to remain sensitive to water availability, coping with the stress of drought or respond to high annual precipitation with extra growth. This adaptability therefore demonstrates an ecosystem’s resilience to more severe and frequent changes in hydroclimatic conditions, from global climate change. During drought, ecosystems with high plant productivity during normal hydroclimatic had WUEes that were similar to the plants adapted to less productive, arid ecosystems. However, the authors warn that if all ecosystems are subjected to limited water variability, there will be a cross-biome maximum WUEe that will not be sustainable if drought continues and hydroclimatic conditions continue to become drier and hotter.
The ANPP/evapotranspiration model from cross–biome WUEe will not be sustainable if more arid, hot hydroclimatic conditions persist. These changing hydroclimate conditions will surpass a threshold that biomes are able to endure, without resulting in drought–induced mortality. Die off and decreased resilience would occur for ecosystems with the most variability in precipitation (grasslands). For grasslands, increasing aridity during prolonged warm drought led to a decrease in WUEe and resilience.
Ecosystems are able to respond and adapt to droughts, by an increase in WUEe during dry years and through resilience during wet years, in order to maintain vegetation productivity. This study combines data gathered from several biomes over different time periods on two continents, with changing hydroclimatic conditions. Ponce Campos et al. were able to find that in the face of global temperature increases and the increased frequency and severity of droughts, there will be increased vegetation mortality that could threaten ecosystems’ resilience across biomes. This decrease in resilience would result from species die off and thus changes in ecosystem structures. Thus, species die off could negatively affect crop yields and soil carbon balance feedback loops in the future. 

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