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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.

The arid Southwestern region of the US has seen its native grasslands transformed into tracts of land overrun by shrubs over 120 years. An ecologically-based remediation approach is proposed to rehabilitate these desertified landscapes. This method has proven to be cheaper, to minimize dependence on agromonic practices and to protect the features of the natural landscape. However, this technique requires a longer time for changes to occur and a deeper understanding of arid land processes.

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 temperature, 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. Production 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 2⚬ C 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 distribution of C3 and C4 grasses in the Great Plains. Accordingly, these variables will be important under changing CO2 and climatic forcings.

A potentially important organizing principle in arid and semi-arid systems is the inverse-texture hypothesis which predicts that plant communities on coarse-textured soils should have higher above-ground net primary productivity (ANPP) than communities on fine-textured soils; the reverse is predicted to occur in humid regions. Our objectives were: (1) to test predictions from the inverse-texture hypothesis across a regional precipitation gradient, and (2) to evaluate changes in community composition and basal cover on coarse- and fine-textured soils across this gradient to determine how these structural parameters may affect ANPP. Sites were located along a precipitation gradient through the Central Grassland region of the United States: mean annual precipitation ranges from 311 mm/y to 711 mm/y, whereas mean annual temperature ranges from 9 °C to 11 °C. For both coarse- and fine-textured sites in 1993 and 1994, August - July precipitation in the year of the study explained greater than 92% of the variability in ANPP. Soil texture did not explain a significant proportion of the variability in ANPP. However, soil texture did affect the proportion of ANPP contributed by different functional types. Forbs and shrubs made up a larger proportion of total ANPP on coarse- compared to fine-textured sites. Shrubs contributed more to ANPP at the drier end of the gradient. Basal cover of live vegetation was not significantly related to precipitation and was similar for both soil textures. Our results revealed that across a regional precipitation gradient, soil texture may play a larger role in determining community composition than in determining total ANPP.

We evaluated the effects of western harvester ants (Pogonomyrex occidentalis Cresson) on short-term revegetation on their nest sites in a semiarid grassland in northeastern Colorado. Storage of germinable seeds in the soil of nest sites, biomass of plants around nest sites, longevity of nest sites and effects of date of nest abandonment were studied. Perennial plants dominated the nest sites 1 yr after abandonment, whereas annuals dominated the seed bank on the nest sites. Significant differences in plant recovery and seed storage occurred on the cone-shaped mound and the surrounding circular disk. Significantly more annuals occurred on the mound than on the disk reflecting the significantly greater number of seeds of annuals stored in the mound compared to the disk. The perennial grass Bouteloua gracilis (H.B.K.) Lag. ex Griffiths, which dominates most shortgrass communities, was not found on abandoned nest sites nor were its seeds stored in the soil of the mound or disk. A colony of Pogonomyrex occidentalis may inhabit a nest site for 29-58 yr.

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.

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-successional stage had been reached. Our results contrast with the conventional view of shortgrasses community response to disturbances, and suggest an alternative view of the recovery process that focuses on interactions between individual plants and their environment to explain recovery patterns that vary in time or space. Accounting for this variability in recovery is critical to the management of these systems, especially under conditions of changing climate and land use.

Our objective was to evaluate the effects of burrowing activities by banner-tail kangaroo rats (Dipodomys spectabilis Merriam) on plant community structure and species dominance for two patch types at the ecotone between shortgrass steppe and desert grassland in New Mexico, USA. 10 mounds produced by kangaroo rats were selected in patches dominated by Bouteloua gracilis (the dominant in shortgrass steppe communities) and 10 mounds were selected in patches dominated by B. eriopoda (the dominant in Chihuahuan desert grasslands). Plant cover and density by species were sampled from three locations associated with each mound: the mound proper, the edge of the mound in the transition area, and the off-mound vegetation. Similar cover of B. eriopoda for the edges of mounds in both patch types indicates the ability of this species to respond to animal disturbances regardless of the amount of cover in the surrounding undisturbed vegetation. By contrast, cover of B. gracilis was low for all mounds and mound edges in patches dominated by this species. Much higher cover of B. eriopoda on mound edges compared to the undisturbed vegetation in B. gracilis-dominated patches indicates that kangaroo rats have important positive effects on this species. Lower cover of perennial grasses and higher cover of forbs, shrubs, and succulents on the edges of mounds in B. eriopoda-dominated patches compared to patches dominated by B. gracilis indicate the importance of surrounding vegetation to plant responses on disturbed areas. Our results show that kangaroo rats have important effects on both species dominance and composition for different patch types, and may provide a mechanism for small-scale dominance patterns at an ecotone; thus providing further support for their role as keystone species in desert grasslands.

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.

Our overall objective was to use a soil water model to predict spatial patterns in germination and establishment of two important perennial C4-bunchgrasses across the North American shortgrass steppe and desert grassland regions. We also predicted changes in establishment patterns under climate change scenarios. Bouteloua gracilis dominates the shortgrass steppe from northeastern Colorado to southeastern New Mexico. Bouteloua eriopoda dominates desert grasslands in central and southern New Mexico. Germination and establishment for each species were predicted at 16 sites along the gradient using a daily time step, multi-layer soil water model (SOILWAT) to determine the percentage of years that temperature and soil water criteria for germination and establishment were met. Percentage of years with predicted establishment decreased from north to south for B. gracilis, but increased from north to south for B. eriopoda, comparable to observed dominance patterns. The 95 % confidence interval around the point at which simulated establishment were equal for the two species was near the location of the shortgrass steppe-desert grassland ecotone where both species are abundant. The intersection in percentage of years with establishment for the two species was predicted to move further north when climate was scaled using three Global Circulation Models (GCMs), indicating a possible northward expansion of B. eriopoda. Our results suggest that recruitment by seed may be an important process in determining, at least in part, the geographic distribution of these two species. Changes in climate that affect establishment constraints could result in shifts of species dominance that may or may not be accompanied by changes in species composition.