Ingrid C. Burke

The Importance of Land-Use Legacies to Ecology and Conservation

Abstract Recognition of the importance of land-use history and its legacies in most ecological systems has been a major factor driving the recent focus on human activity as a legitimate and essential subject of environmental science. Ecologists, conservationists, and natural resource policymakers now recognize that the legacies of land-use activities continue to influence ecosystem structure and function for decades or centuries—or even longer—after those activities have ceased. Consequently, recognition of these historical legacies adds explanatory power to our understanding of modern conditions at scales from organisms to the globe and reduces missteps in anticipating or managing for future conditions. As a result, environmental history emerges as an integral part of ecological science and conservation planning. By considering diverse ecological phenomena, ranging from biodiversity and biogeochemical cycles to ecosystem resilience to anthropogenic stress, and by examining terrestrial and aquatic ecosystems in temperate to tropical biomes, this article demonstrates the ubiquity and importance of land-use legacies to environmental science and management.

INTERACTIONS BETWEEN INDIVIDUAL PLANT SPECIES AND SOIL NUTRIENT STATUS IN SHORTGRASS STEPPE

The effect of plant community structure on nutrient cycling is fundamental to our understanding of ecosystem function. We examined the importance of plant species and plant cover (i.e., plant covered microsites vs. bare soil) on nutrient cycling in shortgrass steppe of northeastern Colorado. We tested the effects of both plant species and cover on soils in an area of undisturbed shortgrass steppe and an area that had undergone nitrogen and water additions from 1971 to 1974, resulting in significant shifts in plant species composition. Soils under plants had consistently higher C and N mineralization rates and, in some cases, higher total and microbial C and N levels than soils without plant cover. Four native grasses, one sedge, and one shrub differed from one another in the quantity and quality of above- and belowground biomass but differences among the six species in soil nutrient cycling under their canopies were slight. However, soils under bunchgrasses tended to have higher C mineralization and microbial biomass C than soil under the rhizomatous grass, Agropyron smithii. Also, the one introduced annual in the study, Kochia scoparia, had soils with less plant-induced heterogeneity and higher rates of C and N mineralization as well as higher levels of microbial biomass C than soils associated with the other species. This species was abundant only on plots that had received water and nitrogen for a 4-yr period that ended 20 yr ago, where it has persisted in the absence of resource additions for 20 yr, suggesting a positive feedback between plant persistence and soil nutrient status. Plant cover patterns had larger effects on ecosystem scale estimates of soil properties than the attributes of a particular plant species. This result may be due to the semiarid nature of this grassland in which plant cover is discontinuous and decomposition and nutrient availability are primarily limited by water, not by plant species-mediated characteristics such as litter quality. That local plant-induced patterns in soil properties significantly af- fected ecosystem scale estimates of these properties indicates that consideration of structural attributes, particularly plant cover patterns, is critical to estimates of ecosystem function in shortgrass steppe.

Heterogeneity of soil and plant N and C associated with individual plants and openings in North American shortgrass steppe

Small-scale spatial heterogeneity of soil organic matter (SOM) associated with patterns of plant cover can strongly influence population and ecosystem dynamics in dry regions but is not well characterized for semiarid grasslands. We evaluated differences in plant and soil N and C between soil from under individual grass plants and from small openings in shortgrass steppe. In samples from 0 to 5 cm depth, root biomass, root N, total and mineralizable soil N, total and respirable organic C, C:N ratio, fraction of organic C respired, and ratio of respiration to N mineralization were significantly greater for soil under plants than soil from openings. These differences, which were consistent for two sites with contrasting soil textures, indicate strong differentiation of surface soil at the scale of individual plants, with relative enrichment of soil under plants in total and active SOM. Between-microsite differences were substantial relative to previously reported differences associated with landscape position and grazing intensity in shortgrass steppe. We conclude that microscale heterogeneity in shortgrass steppe deserves attention in investigation of controls on ecosystem and population processes and when sampling to estimate properties at plot or site scales.

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.

REGIONAL AND TEMPORAL VARIATION IN NET PRIMARY PRODUCTION AND NITROGEN MINERALIZATION IN GRASSLANDS

Spatial variability that occurs at large scales has long been used by ecologists as a tool to examine the controls over ecosystem structure and function. Correlations of control variables such as climatic factors and response variables such as vegetation and soil carbon storage across broad regions have played a crucial role in predicting the response of ecosystems to global climate change. Despite the importance of these large-scale space-for-time substitutions, there are substantial limitations. One of these limitations is that many of the possible control factors covary with one another, and only some of the important control factors actually exist in large-scale databases. Thus, the true proximal controls may be difficult to identify. A second limitation is that models of spatial variability may not be appropriately applied to temporal variability. In this paper, we utilize a new approach to determine the extent to which N availability may constrain aboveground primary productivity in the Central Grassland region of the U.S. The strong relationship between average annual primary production and average annual precipitation found in spatial patterns in ecosystems globally has often been interpreted as evidence of a fundamental water limitation. However, temporal variation in annual aboveground net primary production (ANPP) indicates that other factors constrain production. We generated a spatial and temporal database for annual aboveground net primary production and annual net N mineralization by linking a database of input variables (precipitation, temperature, and soils) with predictive models. We generated independent data sets of aboveground net primary production and net N mineralization by using regression models to predict aboveground net primary production, and the Century model to simulate net N mineralization. Our analyses indicate that net primary production and net N mineralization both increase with mean annual precipitation; thus, it is not possible to separate the extent to which ANPP is controlled by water or N availability. Nitrogen use efficiency (NUE) increased with increasing precipitation across the region. Aboveground net primary production decreased with increasing temperature across the region, while N mineralization increased slightly, leading to decreasing (NUE) with increasing temperature. At high precipitation levels, aboveground net primary production increased and N mineralization decreased slightly with increasing soil fineness. Nitrogen use efficiency generally increased with increasing pools of soil organic matter, likely because in grasslands, the proportion of recalcitrant organic matter increases with the total organic matter pools. A comparison of interannual variation in net N mineralization with average spatial variation indicated a high degree of inertia in the response of N availability to precipitation levels. Our simulation results as well as field results of Lauenroth and Sala (1992) raise important questions about the applicability of space-for-time substitutions when dealing with ecosystem function. The structure of the systems appears to provide important constraints on the temporal variability that are not evident in an analysis of spatial variability.

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.

Regional Analysis of the Central Great Plains

lobal-scale impacts of human activities are changing the way many ecologists define research problems. The new definitions entail a shift of focus from sites and site-specific experiments to regions and regional analyses. It is at the regional scale that interactions and impacts of large-scale processes, such as global warming, can be assessed and understood (Pastor and Post 1986, Rosswall et al. 1988). Furthermore, regions represent socioeconomic and political units whose behavior will both influence, and in turn be influenced by, global change. This shift in focus to the regional scale is accompanied by a new set of challenges that will require new research questions and methods. Most current knowledge about ecosystems has been generated from studies in

Grassland Precipitation-Use Efficiency Varies Across a Resource Gradient

ABSTRACT Aboveground net primary production (ANPP) is positively related to mean annual precipitation, an estimate of water availability. This relationship is fundamental to our understanding and management of grassland ecosystems. However, the slope of the relationship between ANPP and precipitation (precipitation-use efficiency, PUE) has been shown to be different for temporal compared with spatial precipitation series. When ANPP and precipitation are averaged over a number of years for different sites, PUE is similar for grasslands all over the world. Studies for two US Long Term Ecological Research Sites have shown that PUE derived from a long-term dataset (temporal model) has a significantly lower slope than the value derived for sites distributed across the US central grassland region (spatial model). PUE differences between the temporal model and the spatial model may be associated with both vegetational and biogeochemical constraints. Here we use two independent datasets, one derived from field estimates of ANPP and the other from remote sensing, to show that the PUE is low at both the dry end and the wet end of the annual precipitation gradient typical of grassland areas (200–1200 mm), and peaks around 475 mm. The intermediate peak may be related to relatively low levels of both vegetational and biogeochemical constraints at this level of resource availability.

BIOGEOCHEMISTRY IN A SHORTGRASS LANDSCAPE: CONTROL BY TOPOGRAPHY, SOIL TEXTURE, AND MICROCLIMATE

Biogeochemistry of terrestrial ecosystems is controlled by interactions among factors operating at several spatial and temporal scales. The purpose of this study was to evaluate the relative importance and interaction of relatively static landscape factors and more dynamic factors in a shortgrass steppe landscape. The landscape factors examined were topographic position, and soil texture. The dynamic factors studied were seasonal climate and the localized effects of individual plants on soils. Patterns were evaluated by sampling soil between and under individual Bouteloua gracilis plants in paired upland (erosional) and lowland (depositional) plots at eight locations at the Central Plains Experimental Range (CPER), Colorado. We quantified five organic C and N pools (total, fine and coarse particulate organic matter [POM], mineral-associated organic matter [MAOM], and potentially mineralizable C and N), and we estimated seasonal patterns of in situ N dynamics with three methods (extractable inorganic N, ne...

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.