Debra P. Coffin

SOIL ORGANIC MATTER RECOVERY IN SEMIARID GRASSLANDS: IMPLICATIONS FOR THE CONSERVATION RESERVE PROGRAM'

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 his- torically 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, po- tentially 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, po- tentially mineralizable N, or respirable C. In addition, we found that the characteristic small-scale heterogeneity of the shortgrass steppe associated with individuals of the dom- inant plant, Bouteloua gracilis, had recovered on abandoned fields. Soil beneath plant canopies had an average of 200 g/m2 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.

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.

The Importance of Soil Water in the Recruitment of Bouteloua Gracilis in the Shortgrass Steppe

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...

A gap dynamics simulation model of succession in a semiarid grassland

Abstract We applied a gap dynamics conceptualization of succession to a semiarid grassland by focusing on gaps produced in the belowground resource space by death of an individual of the dominant species, the perennial grass blue grama ( Bouteloua gracilis (H.B.K.) Lag. ex Griffiths). We developed a gap dynamics simulation model to examine long-term successional dynamics on gaps, time required for B. gracilis to recolonize gaps, and spatial variability of gap dynamics at the scale of the landscape. The model is similar to gap models used in forest, but is based on belowground resource use and life history characteristics of plants in shortgrass communities, and incorporates effects of small-scale disturbances and stochastic environmental factors. Average time required for B. gracilis to dominate biomass on a simulated plot was 20 years, a more rapid recovery time than has been reported from experimental studies of large-scale disturbances. Our initial simulations assumed the only control on establishment of B. gracilis seedlings was the occurrence of a restrictive set of microenvironmental conditions. A second set of simulations incorporating effects of seed availability resulted in an average recovery time of 65 years. The scale dependence of successional processes important to recruitment of B. gracilis plants is indicated since this is still a faster recovery time than was observed experimentally. Aboveground biomass on the simulated plots was dominated by B. gracilis through time and space. Relative proportion of aboveground biomass and average biomass values for each species were comparable to the composition of shortgrass plant communities at the Central Plains Experimental Range study site in northcentral Colorado. This initial analysis suggests that gap processes are important to shortgrass communities, and that a gap dynamics conceptualization based on the importance of small, frequently occurring disturbances is a promising alternative to the previous conceptual model of shortgrass communities based on effects of large-scale disturbances.

The advantage of long-distance clonal spreading in highly disturbed habitats

SummaryClassical theory states that cover of annual plants should increase relative to perennials as disturbance frequency increases. However, it has been suggested that long-distance clonal spreading can allow some perennial plants to survive in highly disturbed areas by quickly spreading into disturbed patches. To evaluate these hypotheses, we analysed data of plant distributions in two different ecosystems, a barrier island and a short-grass steppe. The disturbances studied were sand deposition during storms (overwash) on the barrier island and grazing by cattle in the short-grass steppe. In each case the disturbance frequency varied over the ecosystem; we categorized different areas in terms of their disturbance frequencies. All plant species in each area were categorized as one of four plant life forms (1) annual or biennial, (2) herbaceous perennial without long-distance clonal spreading (3) herbaceous perennial with long-distance clonal spreading (i.e guerilla form) and (4) woody plant. Percentage cover of each plant life form in each disturbance frequency category was calculated. In both ecosystems, (1) there was an increase in the relative cover of annuals as one moved from areas of low to moderate disturbance frequencies, but then a decrease in cover of annuals as one moved into the areas of highest disturbance frequency and (2) the guerilla forms showed the greatest relative increase in cover from moderately to highly disturbed areas. The combination of two factors can explain this pattern: (1) long-distance clonal spreading effectively reduces the time to colonization of recently disturbed sites and (2) effects of the disturbances in these two systems are probably more severe for seeds than for stems. We illustrate these effects using a spatially explicit simulation model of the population dynamics of plants in a disturbed landscape.

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. 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 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). 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.

Recovery of Vegetation in a Semiarid Grassland 53 Years after Disturbance

Plowing and subsequent abandonment of semiarid grasslands in the short- grass 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 shortgrass 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 dom- inated 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 had been reached. Our results contrast with the conventional view of shortgrass community response to disturbances, and suggest an alternative view of the recovery process that focuses on inter- actions 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.

Effects of soil texture and precipitation on above‐ground net primary productivity and vegetation structure across the Central Grassland region of the United States

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 pre- dicted 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 tempera- ture 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- com- pared 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 re-

Multiple landscape scales: An intersite comparison

Vegetation transect data from three locations were analyzed to determine if multiple scales of pattern could be detected. The sites included a semiarid grassland in New Mexico, a series of calcareous openings in a deciduous forest in Tennessee, and a shrub-steppe system in Washington. The data were explored with four statistical techniques. A scale of pattern was accepted if detected by more than one analytical method or located by a single method in multiple taxa. The analyses indicated 3–5 scales of pattern on all three sites, as predicted by Hierarchy Theory.

Modeling vegetation structure-ecosystem process interactions across sites and ecosystems

Abstract We describe an approach to investigating and understanding the interactions between vegetation structure and ecosystem processes that uses simulation models as a framework for comparison and synthesis across ecosystems arrayed along environmental gradients. The models are individual-based vegetation simulators and compartment models of nutrient cycling and soil water relations. Applications focus on interactions and feedbacks between vegetation structure (species composition, size structure) and ecosystem processes (water balance, nutrient cycling), and how these relationships vary across environmental gradients. Preliminary results indicate that life-history traits of plants have a profound influence on system-level behaviors, and that differences between grasslands and forests can be attributed largely to contrasting traits of grasses and trees. Experiments with linked vegetation-ecosystem process models diverge from simulations with either model run independently, suggesting the importance of feedbacks between details of vegetation pattern and ecosystem processes. The development of a fully coupled vegetation-ecosystem process model that is sufficiently general to simulate systems dominated by multiple lifeforms presents several conceptual, logistical, and scaling challenges, but also provides for new opportunities in ecosystem theory.