Terrestrial net primary production estimates for 0.5° grid cells from field observations—a contribution to global biogeochemical modeling

Net Primary Production (NPP) is an important component of the carbon cycle and, among the pools and fluxes that make up the cycle, it is one of the steps that are most accessible to field measurement. While easier than some other steps to measure, direct measurement of NPP is tedious and not practical for large areas and so models are generally used to study the carbon cycle at a global scale. Nevertheless these models require field measurements of NPP for parameterization, calibration and validation. Most NPP data are for relatively small field plots that cannot represent the 0.5° × 0.5° grid cells that are commonly used in global scale models. Furthermore, technical difficulties generally restrict NPP measurements to aboveground parts and sometimes do not even include all components of aboveground NPP. Thus direct inter‐comparison between field data obtained in different studies or comparison of these results with coarse resolution model outputs can be misleading. We summarize and present a series of methods that were used by original authors to estimate NPP and how and what we have done to prepare a consistent data set of NPP for 0.5 °grid cells for a range of biomes from these studies. The methods used for estimation of NPP include: (i) aggregation of fine‐scale (plot or stand‐level) vegetation inventory data to larger grid cells, (ii) mapping of grid cells and area weighting of field NPP observations in each mapped class, (iii) direct correlation of extensive data sets of ground measurements with remotely sensed spectral vegetation indices, (iv) local modeling of NPP using key independent variables, for which maps are available at the scale of the grid cell, and (v) regression analysis to link productivity with controlling environmental variables. For a few grid cells whose NPP were obtained for multiple years, temporal analysis was conducted. The grid cells are grouped to the biome level and are compared with existing compilations of field NPP and the results of the Miami potential NPP model. Mean NPP was similar to the well‐known compilation of Whittaker and Likens, except for temperate evergreen needle‐leaved forest, woodland, and shrubland. The grid cell datasets are a contribution to the International Geosphere‐Biosphere Programme (IGBP) Data and Information System (DIS) Global Primary Production Data Initiative (GPPDI). The full dataset currently contains 3654 cells (including replicate measurements) developed from 15 studies representing NPP in croplands, sparse vegetation, shrub lands, grasslands, and forests worldwide. An edited subset consists of 2335 cells in which outliers were removed and all replicate measurements were averaged for each unique geographical location. Most of the data incorporated into GPPDI were wholly or partly developed by participants in the GPPDI, in addition to the present authors. These studies are gathered together here to provide a consistent account of the grid cell component of GPPDI and an analysis of the entire data set. The datasets have been deposited in an IGBP‐DIS GPPDI database ( http://daacl.esd.ornl.gov/npq/GPPDI/Combined_GPPDI_des.html ).     

Carbon and nitrogen dynamics across a natural precipitation gradient in Patagonia,Argentina

Abstract. Both ecosystem carbon gain and nutrient availability are largely constrained by the magnitude and seasonality of precipitation in arid and semi‐arid ecosystems. We investigated the role of precipitation on ecosystem processes along an International Geosphere Biosphere Programme (IGBP) transect in temperate South America. The transect consists of a contiguous precipitation gradient in the southern region of Argentinean Patagonia (44–45° S), from 100 mm to 800 mm mean annual precipitation (MAP) and vegetation ranging from desert scrub to closed canopy forest. Gravimetric soil water content tracked changes in seasonal and annual precipitation, with a linear increase in soil water content with increasing MAP. Above‐ground net primary production (ANPP) increased linearly along the gradient of precipitation (ANPP =– 31.2 + 0.52 MAP, r²= 0.84, p= 0.028), supporting the relationship that carbon assimilation is largely controlled by available water in these sites, and was in general agreement with regional models of ANPP and rainfall. However, inorganic soil nitrogen was also highly linearly correlated with both MAP ([N] = 0.19 MAP – 32, r²= 0.96, p= 0.003) and ANPP (ANPP = 2.6 [N_inorganic]+59.4, r²= 0.79, p= 0.042), suggesting a direct control of precipitation on nitrogen turnover and an interaction with nitrogen availability in controlling carbon gain. The asynchrony of precipitation and changes in dominant vegetation may play important roles in determining the carbon‐nitrogen interactions along this rainfall gradient.     

Influence of climate variability on plant production and N‐mineralization in Central US grasslands

Abstract. We assessed the influence of annual and seasonal climate variability over soil organic matter (SOM), above‐ground net primary production (ANPP) and in situ net nitrogen (N) mineralization in a regional field study across the International Geosphere Biosphere Programme (IGBP) North American mid‐latitude transect (Koch et al. 1995). We hypothesized that while trends in SOM are strongly correlated with mean climatic parameters, ANPP and net N‐mineralization are more strongly influenced by annual and seasonal climate because they are dynamic processes sensitive to short‐term variation in temperature and water availability. Seasonal and monthly deviations from long‐term climatic means, particularly precipitation, were greatest at the semi‐arid end of the transect. ANPP is sensitive to this climatic variability, but is also strongly correlated with mean annual climate parameters. In situ net N‐mineralization and nitrification were weakly influenced by soil water content and temperature during the incubation and were less sensitive to seasonal climatic variables than ANPP, probably because microbial transformations of N in the soil are mediated over even finer temporal scales. We found no relationship between ANPP and in situ net N‐mineralization. These results suggests that methods used to estimate in situ net N‐mineralization are inadequate to represent N‐availability across gradients where microbial biomass, N‐immobilization or competition among plants and microbes vary.     

Terrestrial transects for global change research

The International Geosphere-Biosphere Program has proposed a set of large-scale terrestrial transects to study the effects of changes in climate, land use, and atmospheric composition (global change) on biogeochemistry, surface-atmosphere exchange, and vegetation dynamics of terrestrial ecosystems. The transects ( 1000 km) will be located along existing environmental and land use intensity gradients that span transitions between biomes in regions likely to be widely affected by forcing from components of global change or where the impacts of global change are likely to feed back to affect atmospheric, climatic, or hydrologic systems. Experimental studies on the transects will examine short-term changes in ecosystem function and biosphere-atmosphere interaction in response to variation in primary controlling variables. A hierarchy of modeling approaches will develop predictions of long-term changes in biome boundaries and vegetation distribution. The proposed initial set of IGBP terrestrial transects are located in four key regions: (1) humid tropical forests undergoing land use change, (2) high latitudes including the transition from boreal forest to tundra, (3) semi-arid tropical regions including transitions from dry forest to shrublands and savannas, and (4) mid latitude semi-arid regions encompassing transitions from shrubland or grassland to forests. We discuss here the rationale and general research design of transect studies proposed for each of these priority regions.GCTE Focus 1 Office     

Challenges of a changing Earth

The Challenges of a Changing Earth: Global Change Open Science Conference was held in Amsterdam, The Netherlands, from 10 to 13 July 2001     

Land surface phenology and temperature variation in the International Geosphere–Biosphere Program high-latitude transects

The International Geosphere–Biosphere Program has delineated five study areas that form a northern high‐latitude network for the analyses of vegetation and carbon dynamics. We examined the magnitude and significance of changes in the land surface phenologies of ecoregions within these transects using the NASA Pathfinder Advanced Very High‐Resolution Radiometer Land dataset. We applied the seasonal Mann–Kendall (SMK) trend test, a robust and nonparametric approach, to determine the significance of trends in the normalized difference vegetation index (NDVI) over the five transects. The SMK trend test provides an important alternative over the frequently used but unreliable trend analysis based on linear regression. In addition, we modeled the land surface phenology using quadratic or nonlinear spherical models to relate the NDVI data to accumulated growing degree‐days (base 0°C). Nonlinear spherical models parsimoniously describe the green‐up dynamics in taiga and tundra ecoregions. Models for each ecoregion within each transect were fitted separately for two time periods (1985–1988 and 1995–1999) and their parameter coefficient estimates were compared. In 10 of 24 ecoregions that comprise 72% of the land area in the transects, the date of the peak NDVI value was significantly earlier (range 2–18 days) in the second study period than in the first study period. This progression was more pronounced in North America than in Siberia (weighted average of 9.3 vs. 6.3 days earlier). Understanding of what constitutes significant change in land surface phenology amidst background variation is a critical component of global change science. A diversity of datasets, techniques, and study areas has led to a range of conclusions about boreal phenology. We discuss statistical pitfalls in standard analyses and offer a framework to conduct statistically reliable change assessments of land surface phenologies.