Howard E. Epstein

ANPP ESTIMATES FROM NDVI FOR THE CENTRAL GRASSLAND REGION OF THE UNITED STATES

Several studies have suggested the existence of a positive relationship be- tween the Normalized Difference Vegetation Index (NDVI) derived from AVHRR/NOAA satellite data and either biomass or annual aboveground net primary production (ANPP) for different geographic areas and ecosystems. We calibrated a 4-yr average of the ingegral of the NDVI (NDVI-J) using spatially aggregated values of ANPP. We also provided an estimate of the energy conversion efficiency coefficient (?) of Monteiths equation. This is the first attempt to calibrate a standard NDVI product for temperate perennial grasslands. We found a positive and statistically significant relationship between NDVI-I and ANPP for grassland areas with mean annual precipitation between 280 and 1150 mm, and mean annual temperature between 40 and 20C. Depending on the method used to estimate the fraction of photosynthetic active radiation, the energy conversion officency coefficient was constant (0.24 g C/MJ), or varied across the precipitation gradient, from 0.10 g C/MJ for the least productive to 0.20 g C/MJ for the most productive sites.

Plant-Soil interactions in temperate grasslands

We present a conceptual model in which plant-soil interactions in grasslands are characterized by the extent to which water is limiting. Plant-soil interactions in dry grasslands, those dominated by water limitation (‘belowground-dominance’), are fundamentally different from plant-soil interactions in subhumid grasslands, where resource limitations vary in time and space among water, nitrogen, and light (‘indeterminate dominance’). In the belowground-dominance grasslands, the strong limitation of soil water leads to complete (though uneven) occupation of the soil by roots, but insufficient resources to support continuous aboveground plant cover. Discontinuous aboveground plant cover leads to strong biological and physical forces that result in the accumulation of soil materials beneath individual plants in resource islands. The degree of accumulation in these resource islands is strongly influenced by plant functional type (lifespan, growth form, root:shoot ratio, photosynthetic pathway), with the largest resource islands accumulating under perennial bunchgrasses. Resource islands develop over decadal time scales, but may be reduced to the level of bare ground following death of an individual plant in as little as 3 years. These resource islands may have a great deal of significance as an index of recovery from disturbance, an indicator of ecosystem stability or harbinger of desertification, or may be significant because of possible feedbacks to plant establishment. In the grasslands in which the dominant resource limiting plant community dynamics is indeterminate, plant cover is relatively continuous, and thus the major force in plant-soil interactions is related to the feedbacks among plant biomass production, litter quality and nutrient availability. With increasing precipitation, the over-riding importance of water as a limiting factor diminishes, and four other factors become important in determining plant community and ecosystem dynamics: soil nitrogen, herbivory, fire, and light. Thus, several different strategies for competing for resources are present in this portion of the gradient. These strategies are represented by different plant traits, for example root:shoot allocation, height and photosynthetic pathway type (C3 vs. C4) and nitrogen fixation, each of which has a different influence on litter quality and thus nutrient availability. Recent work has indicated that there are strong feedbacks between plant community structure, diversity, and soil attributes including nitrogen availability and carbon storage. Across both types of grasslands, there is strong evidence that human forces that alter plant community structure, such as invasions by nonnative annual plants or changes in grazing or fire regime, alters the pattern, quantity, and quality of soil organic matter in grassland ecosystems. The reverse influence of soils on plant communities is also strong; in turn, alterations of soil nutrient supply in grasslands can have major influences on plant species composition, plant diversity, and primary productivity.

PRODUCTIVITY PATTERNS OF C3 AND C4 FUNCTIONAL TYPES IN THE U.S. GREAT PLAINS

We analyzed the productivity of C3 and C4 grasses throughout the Great Plains of the United States in relation to three environmental factors: mean annual tem- perature, mean annual precipitation, and soil texture. Productivity data were collected from Natural Resource Conservation Service (NRCS) rangeland survey data. Climate data were interpolated from weather stations throughout the region. Soil texture data were obtained from NRCS State Soil Geographic (STATSGO) databases. A geographic information system was used to integrate the three data sources. With a data set of spatially random points, we performed stepwise multiple regression analyses to derive models of the relative and absolute production of C3 and C4 grasses in terms of mean annual temperature (MAT), mean annual precipitation (MAP), percentage sand (SAND), and percentage clay (CLAY). MAT, MAP, and soil texture explained 67-81% of the variation in relative and absolute production of C3 and C4 grasses. Both measures of production of C3 grasses were negatively related to MAT and SAND, and positively related to CLAY. Relative production of C3 grasses decreased whereas absolute production of C3 grasses increased with MAP. Produc- tion of C4 grasses was positively related to MAT, MAP, and SAND, and negatively related to CLAY. MAP was the most explanatory variable in the model for C4 absolute production. MAT was the most explanatory variable in the three other models. Based on these regression models, C3 grasses dominate 35% of the Great Plains under current climatic conditions, mainly north of Colorado and Nebraska. Under a 20C increase in MAT, C3 grasses recede northward and retain dominance in only 19% of the region. MAT, MAP, and soil texture are important variables in explaining the abundance and dis- tribution of C3 and C4 grasses in the Great Plains. Accordingly, these variables will be important under changing CO2 and climatic forcings.

REGIONAL PATTERNS OF DECOMPOSITION AND PRIMARY PRODUCTION RATES IN THE U.S. GREAT PLAINS

Warmer regions generally exhibit greater rates of soil respiration and organic matter decomposition than colder regions. In the Great Plains of the United States, soil organic matter declines from the northern part of the region to the south, suggesting greater decomposition rates in areas with warmer temperatures. Our study used a regional data set of aboveground net primary production, soil organic carbon, soil texture, and climate to evaluate the environmental controls over areal patterns in decomposition rates, (k; expressed as grams per year per gram of initial mass), throughout the U.S. Great Plains. We conducted multiple regression analyses of steady-state k with respect to mean annual temperature, mean annual precipitation, and percentage soil clay content to examine both the combined and individual effects of these independent variables on regional decomposition rates. Our results indicated that precipitation contributes more than either temperature or soil texture to areal patterns of decomposition rates in the U.S. Great Plains, explaining >30% of the areal variability in k. Decomposition rates increased with increasing precipitation and with decreasing soil clay content. Temperature explained <8% of the regional variability in k. Ancillary analyses that related temperature and aboveground net primary production in the region indicated that plant productivity declines with increasing temperatures. This suggests that the reduction in soil organic matter to the south in the U.S. Great Plains may be due to reduced plant inputs rather than to increases in decomposition rates. The response of decomposition to temperature is probably constrained by moisture in this water-limited region. Therefore, changes in decomposition rates resulting from temperature dynamics are likely to be minimal unless they are accompanied by sufficient changes in precipitation.

Ecological responses of dominant grasses along two climatic gradients in the Great Plains of the United States

. Few empirical data exist to examine the influence of regional scale environmental gradients on productivity patterns of plant species. In this paper we analyzed the productivity of several dominant grass species along two climatic gradients, mean annual precipitation (MAP) and mean annual temperature (MAT), in the Great Plains of the United States. We used climatic data from 296 weather stations, species production data from Natural Resource Conservation Service rangeland surveys and a geographic information system to spatially integrate the data. n n n nBoth MAP and MAT were significantly related to annual above-ground net primary production (ANPP). MAP explained 54% to 89% of the variation in ANPP of two C4 short-grasses, Bouteloua gracilis and Buchloe dactyloides, and two C4 tall-grasses, Andropogon gerardii and Schizachyrium scoparium (= Andropogon scoparius). MAT explained 19% to 41% of the variation in ANPP of two C4 grasses, B. gracilis and B. dactyloides, and 41% to 66% of the variation in ANPP of two C3 grasses, Agropyron smithii and Stipa comata. ANPP patterns for species along both gradients were described by either linear, negative exponential, logistic, normal or skewed curves. Patterns of absolute ANPP (g/m2) for species differed from those of relative ANPP (%) along the MAP gradient. Responses were similar for species with common functional characteristics (e.g. short-grasses, tall-grasses, C3, C4). n n n nOur empirical results support asymmetric responses of species to environmental gradients. Results demonstrate the importance of species attributes, type of environmental gradient and measure of species importance (relative or absolute productivity) in evaluating ecological response patterns.

EFFECTS OF TEMPERATURE AND SOIL TEXTURE ON ANPP IN THE U.S. GREAT PLAINS

Aboveground net primary production (ANPP) in grassland ecosystems is positively related to mean annual precipitation (MAP). However, at any given level of precipitation, other factors may effect ANPP. Our objective was to determine the importance of temperature and soil texture in explaining ANPP in the Great Plains of the United States. We constructed a spatial database of ANPP, climate, and soil texture for the region using a geographic information system. Holding MAP constant at 5-cm intervals, we related ANPP to mean annual temperature (MAT), and soil sand and clay contents. Our findings indicate that MAT and soil texture are important variables for explaining patterns of ANPP, after accounting for the variability explained by MAP. There is a negative relationship between temperature and ANPP when MAP is held constant; this has important climate-change implications. Results revealed an MAP crossover point for the inverse texture effect at ∼80 cm of rainfall, much higher than previously reported. The c...

Regional productivities of plant species in the Great Plains of the United States

Few studies have analyzed the production of plant species at regional scales in grassland ecosystems, due in part to limited availability of data at large spatial scales. We used a dataset of rangeland surveys to examine the productivities of 22 plant species throughout the Great Plains of the United States with respect to three environmental factors: temperature, precipitation and soil texture. Productivity of plant species was obtained from Natural Resource Conservation Service (NRCS) range site descriptions. We interpolated climate data from 296 weather stations throughout the region and used soil texture data from NRCS State Soil Geographic (STATSGO) databases. We performed regression analyses to derive models of the relative and absolute production of each species in terms of mean annual temperature (MAT), mean annual precipitation (MAP), and percentage SAND, SILT and CLAY.MAT was the most important factor for 55% of species analyzed; MAP was most explanatory for 40% of the species, and a soil texture variable was most important for only one species. Production of C3 species tended to be negatively related to MAT, MAP and positively related to CLAY. Production of C4 shortgrasses, in general, was positively related to MAT and negatively related to MAP and SAND, whereas C4 tallgrass productivity tended to be positively associated with MAP and SAND, and was highest at intermediate values of MAT. Our results indicate the extent to which functional types can be used to represent individual species. The regression equations derived in this analysis can be important inclusions in models that assess the effects of climate change on plant communities throughout the region.

Regional climatic similarities in the temperate zones of North and South America

We performed an analysis of the climatic patterns of the temperate zones in North and South America using a global database of monthly precipitation and temperature. Three synthetic variables, identified by a principal component analysis (PCA) of the monthly data, were used: mean annual precipitation, mean annual temperature and the proportion of the precipitation falling during summer. We displayed the spatial gradient of the three variables by constructing a com- posite colour raster image. We used a parallelepiped classification algorithm to locate areas in both continents that are climatically similar to five North American Long Term Ecological Research sites and to two South American long- term ecological research sites. The same algorithm was used to identify areas in South America which are climatically similar to some of the major grassland and shrubland types of North America. There is substantial overlap between the climates of North and South America. Most of the climatic patterns found in South America are well represented in North America. How- ever, there are certain climates in North America that are not found in South America. An example is a climate with relatively low mean annual temperature and high summer precipitation. The climatic signatures of three North American LTER sites (Cedar Creek, CPER and Sevilleta) were not found in South America. The climatic signatures of two LTER sites (Konza and Jornada) had some representation in South America. Two South American research sites (Rio Mayo and Las Chilcas) were well represented climatically in North America. The climates of six out of seven selected North American grassland and shrubland types were represented in South America. The northern mixed prairie type was not represented climatically in South America. Our analysis sug- gests that comparisons of North and South America can provide a powerful test of climatic control over vegetation.

Plant functional type effects on trace gas fluxes in the shortgrass steppe

Plant community structure is expected to regulate the microbial processes of nitrification and denitrification by controlling the availability of inorganic N substrates. Thus it could also be a factor in the concomitant release of NO and N2O from soils as a result of these processes. C3 and C4 plants differ in several attributes related to the cycling of nitrogen and were hypothesized to yield differences in trace gas exchange between soil and atmosphere. In this study we estimated fluxes of NO, N2O and CH4 from soils of shortgrass steppe communities dominated by either C3 plants, C4 plants or mixtures of the two types. We collected gas samples weekly from two sites, a sandy clay loam and a clay, throughout the growing seasons of 1995 and 1996. Plant functional type effects on gas fluxes at the clay site were not apparent, however we found several differences among plant communities on the sandy clay loam. CH4 uptake from atmosphere to soil was significantly greater on C4 plots than C3 plots in both years. NO fluxes were significantly greater from C4 plots than from C3 plots in 1995. NO fluxes from C3 and mixed plots were not significantly different between 1995 and 1996, however fluxes from C4 plots were significantly greater in 1995 compared to 1996. Results indicate that under certain environmental conditions, particularly when factors such as moisture and temperature are not limiting, plant community composition can play an important role in regulating trace gas exchange.

Response of the Shortgrass Steppe to Changes in Rainfall Seasonality

ABSTRACT Studies in temperate grassland ecosystems have shown that differences in composition of C3 and C4 plant functional types can have important influences on ecosystem pools and processes. We used a plant community dynamics model (STEPPE) linked to a biogeochemical cycling model (CENTURY) to determine how ecosystem properties in shortgrass steppe are influenced by plant functional type composition. Because of phenological differences between C3 and C4 plants, we additionally simulated the effects of precipitation seasonality on plant communities and examined how C3 and C4 composition interacts with precipitation to affect ecosystems. The model output suggests that differences in C3 and C4 composition can lead to differences in soil organic carbon (C) and nitrogen (N) within 1000 simulation years. Soil organic C and N (g C and N m−2 to 0.2-m depth) were least in a 100% C4 community compared with a 100% C3 community and a mixed C3–C4 community. A change in the time of maximum precipitation from summer to spring in a simulated shortgrass steppe slightly favored C3 plants over C4 plants. The proportion of total net primary production accounted for by C3 plants increased from 21% to 25% after 200 years, when 90 mm of precipitation was switched from summer to spring. Soil organic matter (SOM) was relatively stable in the C4-dominated communities with respect to changes in precipitation seasonality, whereasSOM in the C3 community was sensitive to precipitation seasonality changes. These results suggest an important interaction between plant community composition and precipitation seasonality on SOM, with phenology playing a key role.