Abstract. Although the effects of cultivation on soil organic matter and nutrient supply capacity are well understood, relatively little work has been done on the long-term recovery of soils from cultivation. We sampled soils from 12 locations within the Pawnee National Grasslands of northeastern Colorado, each having native fields and fields that were historically cultivated but abandoned 50 yr ago. We also sampled fields that had been cultivated for at least 50 yr at 5 of these locations.Our results demonstrated that soil organic matter, silt content, microbial biomass, potentially mineralizable N, and potentially respirable C were significantly lower on cultivated fields than on native fields. Both cultivated and abandoned fields also had significantly lower soil organic matter and silt contents than native fields. Abandoned fields, however, were not significantly different from native fields with respect to microbial biomass, potentially mineralizable N, or respirable C. In addition, we found that the characteristic small-scale heterogeneity of the shortgrass steppe associated with individuals of the dominant plant, Bouteloua gracilis, had recovered on abandoned fields. Soil beneath plant canopies had an average of 200 g/m 2 more C than between-plant locations.We suggest that 50 yr is an adequate time for recovery of active soil organic matter . and nutrient availability, but recovery of total soil organic matter pools is a much slower process. Plant population dynamics may play an important role in the recovery of shortgrass steppe ecosystems from disturbance, such that establishment of perennial grasses determines the rate of organic matter recovery.

In the shortgrass steppe region of North America there is a controversy about the ability of the dominant species to recruit from seedlings. The prevailing view is that Bouteloua gracilis is incapable of recruitment from seedlings in areas receiving <380 mm of annual precipitation. A common explanation for this situation is that environmental conditions permitting seedling establishment are infrequent. To assess the frequency of environmental conditions appropriate for the recruitment of B. gracilis we used a soil water simulation model and long—term climatic data in conjunction with detailed information about the ecophysiological requirements for seed germination and growth of seminal and adventitious roots. We found that recruitment events occur as frequently as every 30—50 yr on silty clay, silty clay loam, and silty loam soils, but less than once in 5000 yr on sandy soils. Simulated frequencies of recruitment were sufficient to account for the observed abundance of B. gracilis in 7 of 11 soil textures evaluated. The differences in silt content and available water holding capacity accounted for the difference among soil textures in the probability of occurrence of recruitment events. Therefore, soil texture variability may explain the spatial pattern of recruitment and of population recovery after disturbance that occur at the soil type and microsite scales. Annual precipitation explained a large fraction of the temporal variability in recruitment. On average, recruitment occurred in years when precipitation was above the mean. The occurrence of recruitment events in some dry years (precipitation < mean), and their absence during some wet years (precipitation > mean), emphasizes the importance of the intraseasonal distribution of precipitation. The sensitivity of recruitment to soil water availability suggests that climate change, particularly changes that increase or decrease the amount or the effectiveness of soil water, could have important effects on the future of populations of B. gracilis.

Abstract. 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. Both 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 Buchloë 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). Our 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.

Plowing and subsequent abandonment of semiarid grasslands in the shortgrass steppe region of North America results in both short‐ and long‐term changes in plant community structure. The traditional Clementsian model of succession in which shortgrasses rapidly dominate the vegetation was modified for these grasslands in the 1970s so that it predicted a prolonged stage characterized by the dominance of the bunchgrass Aristida purpurea, followed by a very slow recovery of shortgrasses after large‐scale disturbances. Because neither the Clementsian nor the modified model was supported by results of recent scale‐dependent field experiments and simulation analyses, we designed a study to evaluate recovery of shortgrasses communities on old fields abandoned for 53 yr in northeastern Colorado, USA. Our objectives were: (1) to compare species composition on abandoned fields with that of adjacent, unplowed areas, (2) to compare vegetation on these fields with predictions from the prevailing conceptual models, and (3) to evaluate the relationship between recovery patterns within fields and distance from the source of propagules at the edge of a field. We reached different conclusions based upon the choice of indicator of recovery. For most cases (9 of 13 fields), relative shortgrass cover did not fit predictions of either the Clementsian model or the modified model. High shortgrass cover on two of the remaining fields was similar to that expected by the Clementsian model, and low shortgrass cover on the remaining two fields was similar to that expected by the modified model. Two fields with high shortgrass cover were dominated by Buchloe dactyloides, a species less resistant to drought and grazing than is Bouteloua gracilis, the dominant species in undisturbed communities. Uniformity in cover of other perennial graminoids and density of perennial forbs and annuals on and off fields indicated that these groups had recovered on most fields. However, differences in similarity of species composition on and off fields indicated that none of the fields had recovered. High variability in recovery of vegetation among fields with similar annual climatic variables and soil textures may be attributed to differences in initial conditions management practices through time, fine‐scale climate, and/or other site characteristics that were not measured in this study. We found the perennial bunchgrass Bouteloua gracilis on all fields sampled, and it dominated basal cover on two fields. Four groups of fields were distinguishable based on the relationship between Bouteloua gracilis cover and distance from the edge with unplowed vegetation: (1) fields with uniformly high cover of Bouteloua gracilis; (2) fields with a decrease in cover with distance, and cover dominated by Bouteloua gracilis; (3) fields with a decrease in cover with distance, and cover dominated by Buchloe dactyloides; and (4) fields with uniformly low cover of Bouteloua gracilis and Buchloe dactyloides, and dominated by other perennial graminoids, indicating that a mid‐ to late‐...

We examined the effects of small, patch—producing disturbances on a shortgrass plant community dominated by the perennial grass blue grama (Bouteloua gracilis [H.B.K.] Lag. ex Griffiths). Size distributions and rates of occurrence were determined from field data and from the literature for B. gracilis plants and for three disturbance types: cattle fecal pats, western harvester ant mounds, and small animal burrows. A Spatially explicit simulation model was then used to determine the effects of each disturbance. The average turnover rates (percent affected per year) of the basal cover and density of B. gracilis plants in the simulated plots were calculated for three topographic positions and three grazing intensities. Disturbance size relative to the size of B. gracilis plants distinguished two classes of disturbances. Ant mounds and animal burrows were significantly larger than B. gracilis plants, while fecal pats were comparable in size to B. gracilis plants. The frequency of occurrence of a disturbance was found to be a good predictor of the amount of basal cover and the number of pants killed each year by each disturbance type. The fastest turnover rates for basal cover were associated with the smallest and most frequent disturbance type (fecal pats). The effects of these disturbances were found to be dependent on topographic position and grazing intensity, especially the former. Slopes and lowlands had faster turnover rates than uplands. Our results suggest that the recovery of B. gracilis is dependent on the spatial scale of the disturbance.