The ongoing changes in the global climate expose the world's ecosystems not only to increasing CO 2 concentrations and temperatures but also to altered precipitation (P) regimes. Using four well-established process-based ecosystem models (LPJ, DayCent, ORCHIDEE, TECO), we explored effects of potential P changes on water limitation and net primary production (NPP) in seven terrestrial ecosystems with distinctive vegetation types in different hydroclimatic zones. We found that NPP responses to P changes differed not only among sites but also within a year at a given site. The magnitudes of NPP change were basically determined by the degree of ecosystem water limitation, which was quantified here using the ratio between atmospheric transpirational demand and soil water supply. Humid sites and/or periods were least responsive to any change in P as compared with moderately humid or dry sites/periods. We also found that NPP responded more strongly to doubling or halving of P amount and a seasonal shift in P occurrence than that to altered P frequency and intensity at constant annual amounts. The findings were highly robust across the four models especially in terms of the direction of changes and largely consistent with earlier P manipulation experiments and modelling results. Overall, this study underscores the widespread importance of P as a driver of change in ecosystems, although the ultimate response of a particular site will depend on the detailed nature and seasonal timing of P change.
Understanding how cool-season C3 and warm-season C4 grasses will respond to climate change is critical for predicting future grassland functioning. With warming, C4 grasses are expected to increase relative to C3 grasses. But, alterations in the seasonal availability of water may also influence C3/C4 dynamics because of their distinct seasons of growth. To better understand how shifts in the seasonal availability of water can affect ecosystem function in a northern mixed grass prairie in southeastern Wyoming, we reduced early season rainfall (April – June) using rainout shelters and added the amount of excluded precipitation during the latter half of the growing season (July-September), effectively shifting spring rainfall to summer rainfall. As expected, this shift in precipitation seasonality influenced patterns of soil water availability, leading to increased soil respiration in the summer months and sustained canopy greenness throughout the growing season. Despite these responses, there were no significant differences in C3 aboveground net primary production (ANPP) between the seasonally shifted treatment (SEAS) and the plots that received ambient (AMB) precipitation. This was likely due to the high levels of spring soil moisture present before rainout shelters were deployed that sustained C3 grass growth. However, in plots with high C4 grass cover, C4 ANPP increased significantly in response to increased summer rainfall. Overall, we provide the first experimental evidence that shifts in the seasonality of precipitation, with no change in temperature, will differentially impact C3 vs. C4 species, altering the dynamics of carbon cycling and canopy albedo in this extensive semi-arid grassland.
<p>Terrestrial ecosystems play a major role in the interannual variability of the global carbon budget representing a substantial sink equivalent to about one-third of current anthropogenic CO<sub>2</sub> emissions (Le Qu&#233;r&#233; et al., 2018). Therefore, it is vital to understand how plants and vegetation respond to the impacts of climate extremes as this impacts the productivity terrestrial ecosystems. The conterminous United States (CONUS) represent a diverse range of climate conditions and ecosystems where productivity and its interannual variability are controlled regionally by rainfall and/or temperature. The responses of ecosystem productivity to wet and dry years have been previously investigated over the CONUS using annual aboveground net primary productivity (ANPP) data from multi-site observations (Knapp and Smith, 2001). From this previous study, a positive asymmetry of ANPP in response to rainfall anomalies was found at individual sites (i.e. an increase of ANPP in wet years was greater than a decline in dry years). Here, we evaluate the asymmetry of ecosystem productivity to rainfall over the entire CONUS from 2010 to 2018 using multiple data streams including: the Global Unified Gauge-Based precipitation data, the GRIDMET surface meteorological data (maximum temperature, minimum temperature, precipitation accumulation, and Palmer Drought Severity Index), the SMOS satellite L-Vegetation optical depth product, CO<sub>2</sub> fluxes (net ecosystem exchange (NEE) & gross primary productivity (GPP)) derived from eddy covariance measurements, MODIS ANPP product, Fluxnet GPP at site scale, and three different GPP products from observation-driven models. We address the following two questions: (1) How does ecosystem productivity across the CONUS respond to rainfall anomalies during the period 2010-2018? (2) Does the evidence for positive asymmetry previously observed using site studies hold true across the entire CONUS? For this, we define an asymmetry index (AI) where positive AI indicate a greater increase of productivity in wet years compared to the decline in dry years, and negative AI indicate a greater decline of productivity in dry years compared to the increases in wet years. We find that the spatial patterns of AI across the CONUS are similar amongst the different products and exhibit more pronounced negative asymmetries over the Great Plains and the west north central region whilst positive asymmetries are observed over the southwestern USA during the 2010-2018 period. While the &#8220;shrubland&#8221; biome shows a persistent positive asymmetry during the period, the &#8220;grasslands&#8221; biome appears to have switched from positive (observed by Knapp and Smith, 2001) to negative anomalies during the last decade. The observed asymmetry of the different GPP products is reflected by the negative asymmetry of the precipitation anomalies (skewness of precipitation annual anomalies), which we conclude is the primary driver of negative asymmetry across the US continental surface.</p><p><strong>References</strong></p><p>Knapp, A.K., Smith, M.D., 2001. Variation Among Biomes in Temporal Dynamics of Aboveground Primary Production. Science (80-. ). 291, 481. https://doi.org/10.1126/science.291.5503.481</p><p>Le Qu&#233;r&#233;, C., Andrew, R.M., Friedlingstein, P., Sitch, S., et al., 2018. Global Carbon Budget 2018. Earth Syst. Sci. Data 10, 2141&#8211;2194. https://doi.org/10.5194/essd-10-2141-2018</p>
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