Model estimates of net primary productivity, evapotranspiration, and water use efficiency in the terrestrial ecosystems of the southern United States during 1895–2007
Introduction
Ecosystem productivity and water use (i.e., evapotranspiration, ET) are tightly coupled at multiple scales (Chapin et al., 2002, Waring and Running, 2007). In terrestrial ecosystems, for example, water availability is the primary limiting factor for plant growth in over 40% of vegetated areas, while another 33% of the area is limited by cold temperatures and frozen water, where the water is not available for plant growth (Nemani et al., 2003). Vegetation can respond to water stress in several ways: increasing water uptake from soil, increasing water use efficiency (WUE) and reducing water losses, etc. (Hsiao, 1973, Waring and Running, 2007). Determining the functional responses of plants to water stress is still one of the most complex issues in plant stress biology (Bray, 1997). In order to respond to global environmental change with sound land management practices, there must be a clear understanding of how multiple stresses affect all ecosystem processes.
It is likely that global climate changes will continue to influence the Northern Hemisphere's precipitation distributions, with increased frequency, duration and intensity of drought and other extreme climate events (Saxe et al., 2001, IPCC, 2001, IPCC, 2007, Salinger et al., 2005). These changes imply that in the future, water distribution among different terrestrial ecosystems will be more variable. In the mean time, changes in other factors such as land-use and land-cover types and atmospheric composition (tropospheric ozone, atmospheric CO2 and nitrogen deposition) interact with global climate change to influence the water budget, plant water use strategy, and the global carbon cycle. Understanding the interactions between the carbon and water cycles has been recognized as one of the gaps in global change research (Jackson et al., 2005, Ehleringer et al., 2000, Pereira et al., 2004).
There are many methods of addressing the interactions between the water and carbon cycles. Of these, WUE, the ratio of carbon gain during plant photosynthesis to water loss during evapotranspiration, is an essential concept for studying these interactive effects because it couples the water and carbon cycles very well. WUE can also be defined in various ways at different spatial scales or for different study objectives (Hsiao, 1973, de Wit, 1958, Farquhar and Sharkey, 1982, Donovan and Ehleringer, 1991, Jones, 1992, Steduto, 1996). Field measurements of WUE for few ecosystems have become available (e.g., Law et al., 2002, Yu et al., 2008, Sun et al., 2002) in recent decades thanks to the development of eddy covariance systems. However, due to complex interactions between water and carbon and uncertainty in the interactive influences of multiple environmental factors on WUE in a large-scale ecosystem, the long-term dynamics of WUE on a large scale have rarely been quantified. The emergence and application of mechanism-based models have made it possible to scale up WUE from stand or field level to ecosystem level and to better understand the impact of individual and multiple environmental factors on WUE.
The southern United States (SUS) has experienced significant changes in climate, atmospheric composition, and land-use and land-cover types during the 20th century (IPCC, 2007, Schimel et al., 2000, Norby et al., 2005, Chappelka and Samuelson, 1998, Felzer et al., 2004, Holland et al., 2005, Dentener, 2006, Chen et al., 2006a, Woodbury et al., 2007, Birdsey et al., 2006). The SUS has relatively higher forest coverage, is one of the major suppliers of wood products and has some of the greatest potential for carbon sequestration in the country (Birdsey and Heath, 1995, Birdsey and Lewis, 2003, Woodbury et al., 2007). However, there are increasing concerns about rapid urbanization, wetland losses, extreme climatic conditions (such as severe droughts and floods), and forest plantation expansion and how these are impacting regional water and carbon resources (Sun et al., 2008, McNulty et al., 1997, Jackson et al., 2005). We are not aware of much work that has been done to quantify the long-term dynamics of ecosystem water use and its interactions with ecosystem productivity in the SUS.
This study used a well-evaluated integrated ecosystem model (Dynamic Land Ecosystem Model, DLEM) and constructed long-term data of environmental factors to simulate the spatial and temporal changes of water, nitrogen and carbon cycles. Our objectives were to quantify: (1) Ecosystem NPP, ET and WUE in the southern United States; (2) Impacts of changing environmental factors (combined changes in climate, land-use and land-cover types, atmospheric CO2 concentration, nitrogen deposition, and tropospheric ozone concentration) on the NPP, ET and WUE of different ecosystems.
Section snippets
Ecosystem water use efficiency calculation
Water use efficiency can be defined in many ways. On an ecosystem scale, three major definitions are generally used: (1) Gross primary production (GPP) based WUE: GPP/ET; (2) Net primary productivity (NPP) based WUE: NPP/ET; (3) Net ecosystem carbon production (NEP) based WUE: NEP/ET. Annual rainfall was also used to replace ET to calculate rainfall use efficiency (RUE, e.g., Huxman et al., 2004, Bai et al., 2008). In this study, we primarily used the second definition (NPP-based WUE) to
Temporal and spatial variability of net primary productivity
In this study, changes in five environmental factors (climate, atmospheric ozone concentration and nitrogen deposition, CO2 concentration and land-use types) were all combined (COMB) in our model simulation, which revealed the overall response of terrestrial ecosystems to environmental changes. During 1895–2007, we found that annual mean NPP of the terrestrial ecosystems in the SUS was 1.18 Pg C/yr and ranged from 0.92 Pg C/yr in 1925 to 1.45 Pg C/yr in 2001 (Fig. 6A). Although it varied
Water use efficiency of different biomes
Different water use efficiencies have been found among different plant species (Huxman et al., 2004, Ponton et al., 2006, Emmerich, 2007, Yu et al., 2008). In this study, we found that the WUE of different biomes showed a decreasing order of: Forest > Wetland > Grassland > Cropland > Shrubland in the southern United States. Using a different WUE definition from this study, Emmerich (2007) also found that ecosystem WUE (i.e., net ecosystem production/ET) of grassland is higher than that of shrubland in
Conclusions
This study represents the first attempt to fully assess the long-term changes of NPP, ET, and WUE with the goal of understanding the interactions between carbon and water resources under multiple stresses at the regional scale in the southern U.S. Our results show that the NPP and WUE of terrestrial ecosystems have increased about 27%, and 25%, respectively, during 1895–2007 with substantial inter-annual variation, while ET has had little change as a whole. The increase in WUE was primarily
Acknowledgements
This study has been supported by the U.S. Department of Energy (DOE) NICCR Program, the USDA CSREES program and the Southern Forest Research Partnership (SFRP).
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