Carbon-water interactions in response to drought events in Monsoon Asia

1. Regional Climate and Atmosphere Chemistry temperature, precipitation, radiation, wind, pressure, humidity; CO2, Ozone, NOy, NHx Radiation, Humidity, Pressure, Wind, Precipitation, Temperature CO2, O3, NOy, NHx GHG Ecosystems Biophysics Radiation Reflectance/Transmission, Evaporation, Sensible Heat Flux, Water Balance GHG Water Temperature Radiation Soil moisture Temperature LAI Canopy conductance Transpiration LCLUC Urbanization, Agricultural practices, Deforestation/reforestation Bio-fuel plantation, etc. Boundary Land management Soil Biogeochemistry Mineralization, Nitrification/Denitrification (N2O, NO), Decomposition (CO2), Fermentation (CH4) Plant Physiology Photosynthesis, Respiration, Allocation, Nitrogen Uptake, ET, Turnover, Phenology Litter Nutrient Nutrient PFT Biomass Growth Efficiency PFT Abbreviations: LCLUC: Land cover and land use change GHG: Green House Gas PFT: Plant Function Type Dynamic Vegetation Succession, Biogeography Carbon, Water, Nutrient Water Transport Soil erosion, Soil water discharge, River discharge, Nitrogen leaching Water GHG Carbon, Water, Nutrient Water, Energy, CO2 Water Reservoir Lake, Stream, Ocean Carbon-water interactions in response to drought events in Monsoon Asia Hanqin Tian1, Chaoqun Lu1, Mingliang Liu1, Xiaofeng Xu1, Bo Tao1, Wei Ren1, Guangsheng Chen1 and Jia Yang 1,2 (1) Ecosystem Dynamics and Global Ecology Laboratory, School of Forestry and Wildlife Sciences, Auburn University, (2) Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China INTRODUCTION 2 Responses of NPP, ET and WUE to drought between different periods Our model simulation indicates that NPP anomaly relative to the average level of 1961-1990 shows significant fluctuation year by year (Figure 5) with exclusive change in climate (including temperature, precipitation, radiation and humidity). The intensive increases in NPP and ET centered in the 1960s when drought was less dominant. In the following first dry decade (1978-1987), both NPP and ET present similar reduction pattern in response to severe dry years when overall PDSI in Monsoon Asia was close to -1, but they abruptly return to or above the long-term mean in the years when soil moisture is relatively higher. However, in the recent dry decade (1996-2005), changes in NPP and ET turned out to be inconsistent. NPP decreased in dry years while ET increased. The fact that extreme drought events occurred more frequently in this time period and higher temperature was usually found along with severe drought might be partly responsible for the increased ET. Much concern has been raised recently over to what extent human activities have modulated the Asian monsoon, an important climate feature that affects water-ecosystem services in Monsoon Asia and, therefore, the lives of more than 60% of world’s population (Tian et al., 2003). Large human population and their huge demands for food, fuel and water in this region might have unexpectedly significant response to even little deviation from normal monsoon pattern. In recent years, the probably less summer precipitation, delayed monsoon season and longer breaks between rainy seasons have attracted much attention to drought events and their impacts on the terrestrial productivity, the carbon cycle and availability of water resources in Monsoon Asia. According to the IPCC (2007), extreme drought events will become more frequent, more widespread and more intense during the 21st century. In this study, we applied a highly integrated, process-based ecosystem model, the Dynamic Land Ecosystem Model (DLEM), in conjunction with newly developed high spatial resolution data of climate, atmospheric chemistry composition, and land use/land cover change, to explore the impacts of drought severity and duration on net primary production, water evapotranspiration and plant water use efficiency in the context of climate changes during 1961-2005. Figure 5 Palmer drought severity index (PDSI), temperature anomaly and the simulated NPP/ET anomaly by DLEM in Monsoon Asia during 1961-2005 3 Responses of NPP, ET and WUE to drought at spatial scales During both of the dry decades, precipitation significantly decreased in the northern China, southwestern India and mid-mainland of Monson Asia. However, in some area of Southeast Asia, drought intensively occurred in several months although the total rainfall amount didn’t reduce, or even increased (Figure 6). The spatial patterns of decreased precipitation and drought duration agree fairly well with the simulated reduction in NPP, ET and WUE driven by climate change alone, indicating that the combination of drought severity and duration can largely account for the climate-induced carbon, water dynamics and water use efficiency in dry years (Figure 6 and 7). Furthermore, the drier condition and longer drought in the recent decade lead to obvious reduction in water use efficiency over large area of Monsoon Asia in contrast to that during 1978-1987. Especially for the tropical and sub-tropical regions, drought led to drop of plant production while water discharge (as evapotranspiration) was accelerated by higher temperature in dry period (Figure 7 c, d). DATA AND METHODS We used Palmer Drought Severity Index (PDSI) produced by Dai et al. (2004) to identify the spatial distribution of drought events in Monsoon Asia during 1961-2005. Meanwhile, the monthly average drought severity and annual drought duration were adopted to account for the contribution of soil moisture status to terrestrial C and water dynamics. The DLEM is designed to simulate the variations in biogeography, hydrological cycle, plant physiological processes and soil biogeochemical cycles in land ecosystems driven by natural and anthropogenic forces such as climate variability and change, atmospheric CO2, tropospheric ozone, land-use change, nitrogen deposition, and disturbances (e.g., fire, harvest, hurricanes) on terrestrial carbon, water and nitrogen cycles (Figure 1). The DLEM has been well evaluated and extensively used in studying the responses of terrestrial ecosystems to natural and human disturbances in China, Asia, the United States and the whole globe (e.g. Tian et al., 2009 focusing on WUE issue). In order to investigate the drought impacts, we set up one simulation experiment where climate conditions changed from 1900 to 2005 while other environment factors kept the level of 1900. The period of 1961-2005 was emphasized in this study. The half-degree daily climate data we used to feed the DLEM (including average air temperature, maximum temperature, minimum temperature, total precipitation, relative humidity and shortwave radiation) during 1948-2005 were developed based on dataset of NCEP/NCAR reanalysis 1 (http://www.cdc.noaa.gov/cdc/data.ncep.reanalysis.html). For the period 1901-1947, we used de-trended climate data from 1948 to 1994. Figure 7 ET anomaly (a, b) and WUE anomaly (c, d) simulated by DLEM across Monsoon Asia during 1978-1987 (left lanes) and 1996-2005 (right lanes). Figure 6 Precipitation anomaly (a, b), average drought duration (c, d) and the simulated NPP anomaly (e, f) during 1978-1987 (upper lane) and 1996-2005 (lower lane). 4 Sensitivity of various biomes in response to drought stress In the recent decade (1996-2005), drought took place in more than half of the land area in each biome we studied (Table 1). The magnitude of drought is raised in contrast to that in the former dry decade (1978-1987) as well. NPP and WUE were reduced in all biomes during dry year relative to the long-term mean (1961-1990). Among them, grassland ranked the most sensitive biome in production decrease in response to drought stress, followed by forest and wetland. The comparison of two dry decades indicated that more severe or extended drought deteriorated the shrink in production and WUE. However, for wetland and desert, drier condition couldn’t further impact the C assimilation. But in terms of ET, all biomes showed declining trend in less dry period while it was increased or slowed down the decrease rate in most biomes except grassland over the drier decade. Table 1 The simulated changes in net primary productivity (NPP), evapotranspiration (ET) and water use efficiency (WUE) in response to drought among different biomes a * T1 is the period of 1978-1987. T2 is the period of 1996-2005. 5 sensitivity of different moisture zone to drought stress We divided Monsoon Asia into 5 moisture zones according to the ratio of long-term annual mean precipitation and evapotranspiration (Figure 8). Compared to the two dry decades, dry area increased most in arid zone, followed by transition zone and semi-arid zone. The least increase occurred in sub-humid zone. The average drought severity was raised most in transition and semi-arid zones. For all zones except semi-arid area, NPP further declined in response to more severe or extended drought. The higher temperature in dry years might be able to partly explain the less decrease in b Figure 2 Anomaly in annual mean precipitation (a) and temperature (b) during 1961-2005 relative to the average level of 1961-1990 NPP in semi-arid area most of which is located in high latitude region. Given the less drought magnitude, transition and sub-humid area are the most sensitive regions in terms of plant production to drought events. Likewise, both water content and temperature together determined ET changes. WUE decreased the most in sub-humid area where NPP reduction and ET increase occurred concurrently. Figure 1 Key components of Dynamic Land Ecosystem Model (DLEM) Table 2 The simulated changes in net primary productivity (NPP), evapotranspiration (ET) and water use efficiency (WUE) in response to drought among different moisture zones KEY RESULTS 1 Drought conditions During 1961-2005, larger area has been experiencing drought with longer duration period over the whole Monsoon Asia (Figure 3). Besides, the area suffered from severe and extreme drought has substantially increased throughout the past 45 years (Figure 4). The periods of 1978-1987 and 1996-2005 are identified as severe dry years when drought has taken place in more than 30% of land area within 10 consecutive years. Apparently, the latter decade showed drier condition with more occurrences of extreme drought events than the former one (Figure 4). Therefore, in this study we emphasize the responses of terrestrial carbon and water dynamics to drought between these two contrasting dry periods. Figure8 Moisture zones classified by the ratio of mean annual precipitation and simulated ET across Monsoon Asia during 1961-1990 * T1 is the period of 1978-1987. T2 is the period of 1996-2005. CONCLUSIONS 1 Land area experiencing drought substantially increased and length of drought extended as well during the past 45 years. Meanwhile, the proportion of extreme drought events sharply increased in the recent decade. 2 the spatial pattern of drought severity and duration should be combined together to better account for the dynamics in terrestrial C, water fluxes and water use efficiency in the drought-dominated period. 3 there are contrasting sensitivity of C and water fluxes in response to drought stress among various biomes and moisture zones. However, temperature effect is hard to separate from drought which is a comprehensive manifestation of balance in supply/demand of water and energy. REFERENCE Figure 3 Percentage of dry area and drought duration over Monsoon Asia during 1961-2005 Figure 4 Area experiencing different drought levels over Monsoon Asia from 1961 through 2005 Dai, A., K. E. Trenberth, and T. Qian, 2004: A global data set of Palmer Drought Severity Index for 1870-2002: Relationship with soil moisture and effects of surface warming. J. Hydrometeorology, 5, 1117-1130. Tian, HQ, G. Chen, M. Liu, C. Zhang, G. Sun, C. Lu, X. Xu, W. Ren, S. Pan and A. Chappelka. 2009. Model estimates of net primary productivity, evapotranspiration, and water use efficiency in the terrestrial ecosystems of the southern United States during 1895–2007. Forest Ecology and Management, doi:10.1016/j.foreco.2009.10.009 Tian, H.Q., J.M. Melillo, D.W. Kicklighter, S. Pan, J. Liu, A.D. McGuire and B. Moore III.  2003. Regional carbon dynamics in monsoon Asia and its implications to the global carbon cycle.  Global and Planetary Change 37:201-217. Acknowledgement:This study has been supported by NASA IDS Program (NNG04GM39C) and NASA LCLUC Program (NNX08AL73G_S01). We also thank Dr. A. Dai for providing PDSI data.