[1] Field-chamber measurements of soil respiration from 17 different forest and shrubland sites in Europe and North America were summarized and analyzed with the goal to develop a model describing seasonal, interannual and spatial variability of soil respiration as affected by water availability, temperature, and site properties. The analysis was performed at a daily and at a monthly time step. With the daily time step, the relative soil water content in the upper soil layer expressed as a fraction of field capacity was a good predictor of soil respiration at all sites. Among the site variables tested, those related to site productivity (e.g., leaf area index) correlated significantly with soil respiration, while carbon pool variables like standing biomass or the litter and soil carbon stocks did not show a clear relationship with soil respiration. Furthermore, it was evidenced that the effect of precipitation on soil respiration stretched beyond its direct effect via soil moisture. A general statistical nonlinear regression model was developed to describe soil respiration as dependent on soil temperature, soil water content, and site-specific maximum leaf area index. The model explained nearly two thirds of the temporal and intersite variability of soil respiration with a mean absolute error of 0.82 mmol m À2 s À1. The parameterized model exhibits the following principal properties: (1) At a relative amount of upper-layer soil water of 16% of field capacity, half-maximal soil respiration rates are reached. (2) The apparent temperature sensitivity of soil respiration measured as Q 10 varies between 1 and 5 depending on soil temperature and water content. (3) Soil respiration under reference moisture and temperature conditions is linearly related to maximum site leaf area index. At a monthly timescale, we employed the approach by Raich et al. [2002] that used monthly precipitation and air temperature to globally predict soil respiration (T&P model). While this model was able to explain some of the month-to-month variability of soil respiration, it failed to capture the intersite variability, regardless of whether the original or a new optimized model parameterization was used. In both cases, the residuals were strongly related to maximum site leaf area GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 17, NO. 4, 1104, doi:10.1029/2003GB002035, 2003 15 -1 index. Thus, for a monthly timescale, we developed a simple T&P&LAI model that includes leaf area index as an additional predictor of soil respiration. This extended but still simple model performed nearly as well as the more detailed time step model and explained 50% of the overall and 65% of the site-to-site variability. Consequently, better estimates of globally distributed soil respiration should be obtained with the new model driven by satellite estimates of leaf area index. Before application at the continental or global scale, this approach should be further tested in boreal, cold-temperate, and tropical biomes as well as for non-woody vegetation.INDEX TERMS: 1615 Global...
The prediction of soil C stocks across the landscape has been increasingly studied in many areas of the world. Soil organic C storage in mountain areas is highly heterogeneous, mainly as a result of local-scale variability in the soil environment (topography, stoniness, parent material) and microclimate. The aims of the present study are to estimate soil organic C stocks (SOCS) in mineral soils of high-altitude grasslands of the Pyrenees and determine whether climatic and topographic variables can be used as predictors of SOCS and organic C content in the surface soil horizons of these ecosystems. For that purpose we sampled 35 soil profiles in subalpine and alpine grasslands including a range of altitudes, slopes and aspects. We analysed the soils for stoniness, bulk density, total C, texture, and C-to-N ratio and determined topographical variables. We used georeferenced climatic information for climatic descriptions of the sites. SOCS were highly correlated with soil depth. However, we were not able to predict soil depth by using environmental and topographic variables. In spite of this fact, altitude and aspect explained 41.2% of the SOCS variability while summer temperature and precipitation combined with aspect explained 56.9% of the variability of the organic C content of the surface layer (OC). The SOCS were low at high altitudes, probably as a result of an overall temperature limitation of net primary productivity. Under these conditions, the effect of aspect was small. The highest SOCS occurred at the lowest altitudes for ENE or WNW aspects, showing sharper decreases J. Garcia-Pausas (&) Á P. Casals Á M.-
17Mechanistic water balance models can be used to predict soil moisture dynamics and 18 drought stress in individual forest stands. Predicting current and future levels of plant 19 drought stress is important not only at the local scale, but also at larger, landscape to 20 regional, scales, because these are the management scales at which adaptation and 21 mitigation strategies are implemented. To obtain reliable predictions of soil moisture 22 and plant drought stress over large extents, water balance models need to be 23 complemented with detailed information about the spatial variation of vegetation and 24 soil attributes. We designed, calibrated and validated a water balance model that 25 produces annual estimates of drought intensity and duration for all plant cohorts in a 26 forest stand. Taking Catalonia (NE Spain) as a case study, we coupled this model with 27 plot records from two Spanish forest inventories in which species identity, diameter 28 and height of plant cohorts were available. Leaf area index of each plant cohort was 29 estimated from basal area using species-specific relationships. Vertical root 30 distribution for each species in each forest plot was estimated by determining the 31 distribution that maximized transpiration in the model, given average climatic 32 conditions, soil attributes and stand density. We determined recent trends (period 33 1980-2010) in drought stress for the main tree species in Catalonia; where forest 34 growth and densification occurs in many areas as a result of rural abandonment and 35 decrease of forest management. Regional increases in drought stress were detected for 36 most tree species, although we found high variation in stress changes among 37 individual forest plots. Moreover, predicted trends in tree drought stress were mainly 38 due to changes in leaf area occurred between the two forest inventories rather than to 39 climatic trends. We conclude that forest structure needs to be explicitly considered in 40 assessments of plant drought stress patterns and trends over large geographic areas, 41 A c c e p t e d m a n u s c r i p t Abbreviations: DDS -Daily drought stress; DI -Drought intensity; NDD -Number 50 of drought days; LAI -Leaf area index; PET -Potential evapotranspiration; SFI -51 Spanish forest inventory. 52A c c e p t e d m a n u s c r i p t 4
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