Elsevier

Forest Ecology and Management

Volume 259, Issue 7, 20 March 2010, Pages 1311-1327
Forest Ecology and Management

Model estimates of net primary productivity, evapotranspiration, and water use efficiency in the terrestrial ecosystems of the southern United States during 1895–2007

https://doi.org/10.1016/j.foreco.2009.10.009Get rights and content

Abstract

The effects of global change on ecosystem productivity and water resources in the southern United States (SUS), a traditionally ‘water-rich’ region and the ‘timber basket’ of the country, are not well quantified. We carried out several simulation experiments to quantify ecosystem net primary productivity (NPP), evapotranspiration (ET) and water use efficiency (WUE) (i.e., NPP/ET) in the SUS by employing an integrated process-based ecosystem model (Dynamic Land Ecosystem Model, DLEM). The results indicated that the average ET in the SUS was 710 mm during 1895–2007. As a whole, the annual ET increased and decreased slightly during the first and second half of the study period, respectively. The mean regional total NPP was 1.18 Pg C/yr (525.2 g C/m2/yr) during 1895–2007. NPP increased consistently from 1895 to 2007 with a rate of 2.5 Tg C/yr or 1.10 g C/m2/yr, representing a 27% increase. The average WUE was about 0.71 g C/kg H2O and increased about 25% from 1895 to 2007. The rather stable ET might explain the resulting increase in WUE. The average WUE of different biomes followed an order of: forest (0.93 g C/kg H2O) > wetland (0.75 g C/kg H2O) > grassland (0.58 g C/kg H2O) > cropland (0.54 g C/kg H2O) > shrubland (0.45 g C/kg H2O). WUE of cropland increased the fastest (by 30%), followed by shrubland (17%) and grassland (9%), while WUE of forest and wetland changed little from the period of 1895–1950 to the period of 1951–2007. NPP, ET and WUE showed substantial inter-annual and spatial variability, which was induced by the non-uniform distribution patterns and change rates of environmental factors across the SUS. We concluded that an accurate projection of the regional impact of climate change on carbon and water resources must consider the spatial variability of ecosystem water use efficiency across biomes as well as the interactions among all stresses, especially land-use and land-cover change and climate.

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).

References (79)

  • C. Zhang et al.

    Impacts of climatic and atmospheric changes on carbon dynamics in the Great Smoky Mountains

    Environmental Pollution

    (2007)
  • N. Agam et al.

    Soil water evaporation during the dry season in an arid zone

    Journal of Geophysical Research

    (2004)
  • Alexander, R.B., Smith, R.A., 1990. County-level estimates of nitrogen and phosphorus fertilizer use in the United...
  • Y.F. Bai et al.

    Primary production and rain use efficiency across a precipitation gradient on the Mongolia Plateau

    Ecology

    (2008)
  • Birdsey, R.A., Heath, L.S., 1995. Carbon changes in U.S. forests. In: Joyce, L.A. (Ed.), Productivity of America's...
  • R.A. Birdsey et al.

    Current and historical trends in use, management, and disturbance of U.S. forestlands

  • R. Birdsey et al.

    Forest carbon management in the United States: 1600–2100

    Journal of Environmental Quality

    (2006)
  • Bonan, G., 1996. The NCAR land surface model (LSM version 1.0) coupled to the NCAR community climate model. Technical...
  • F.S. Chapin et al.

    Principles of Terrestrial Ecosystem Ecology

    (2002)
  • A.H. Chappelka et al.

    Ambient ozone effects on forest trees of the eastern United States: a review

    New Phytologist

    (1998)
  • H. Chen et al.

    Effect of land-cover change on terrestrial carbon dynamics in the southern USA

    Journal of Environmental Quality

    (2006)
  • G. Chen et al.

    Climate impacts on China's terrestrial carbon cycle: an assessment with the dynamic land ecosystem model

  • G.J. Collatz et al.

    A coupled photosynthesis- stomatal conductance model for leaves of C4 plants

    Australian Journal of Plant Physiology

    (1992)
  • Dentener, F.J., 2006. Global Maps of Atmospheric Nitrogen Deposition, 1860, 1993, and 2050. Data set. Available on-line...
  • de Wit, C.T., 1958. Transpiration and crop yields. Agricultural Research Reports 64.6. Wageningen, Netherlands, Pudoc....
  • Dickinson, R.E., Henderson-Sellers, A., Kennedy, P.J., Wilson, M.F., 1993: Biosphere–atmosphere transfer scheme (BATS)...
  • L.A. Donovan et al.

    Ecophysiological differences among pre-reproductive and reproductive classes of several woody species

    Oecologia

    (1991)
  • P.S. Eagleson

    Climate, soil and vegetation, 2. The distribution of annual precipitation derived from observed storm sequences

    Water Resources Research

    (1978)
  • J.R. Ehleringer et al.

    Carbon isotope ratios in belowground carbon cycle processes

    Ecological Applications

    (2000)
  • D. Entekhabi et al.

    Land surface hydrology parameterization for atmospheric general circulation models including subgrid scale variability

    Journal of Climate

    (1989)
  • Enting, I.G., Wigley, T.M.L., Heimann, M., 1994. Future emissions and concentrations of carbon dioxide, key...
  • G.D. Farquhar et al.

    A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species

    Planta

    (1980)
  • G. Farquhar et al.

    Stomatal conductance and photosynthesis

    Annual Review of Plant Physiology

    (1982)
  • B. Felzer et al.

    Effects of ozone on net primary production and carbon sequestration in the conterminous United States using a biogeochemistry model

    Tellus

    (2004)
  • B. Felzer et al.

    Future effects of ozone on carbon sequestration and climate change policy using a global biochemistry model

    Climatic Change

    (2005)
  • Holland, E.A., Braswell, B.H., Sulzman, J.M., Lamarque, J.F., 2005. Nitrogen Deposition onto the United States and...
  • C. Homer et al.

    Development of a 2001 National Landcover Database for the United States

    Photogrammetric Engineering and Remote Sensing

    (2004)
  • C. Homer et al.

    Completion of the 2001 National Land Cover Database for the conterminous United States

    Photogrammetric Engineering and Remote Sensing

    (2007)
  • T.A. Howell

    Enhancing water use efficiency in irrigated agriculture

    Agronomy Journal

    (2001)
  • Cited by (316)

    View all citing articles on Scopus
    View full text