Citation:

Nash, M S., E. Jackson, AND W. G. Whitford. EFFECTS OF INTENSE, SHORT DURATION GRAZING ON MICROTOPOGRAPHY IN A CHIHUAHUAN DESERT GRASSLAND. JOURNAL OF ARID ENVIRONMENTS 56(3):383-393, (2004).

Impact/Purpose:

The primary objectives of this research are to:

Develop methodologies so that landscape indicator values generated from different sensors on different dates (but in the same areas) are comparable; differences in metric values result from landscape changes and not differences in the sensors;

Quantify relationships between landscape metrics generated from wall-to-wall spatial data and (1) specific parameters related to water resource conditions in different environmental settings across the US, including but not limited to nutrients, sediment, and benthic communities, and (2) multi-species habitat suitability;

Develop and validate multivariate models based on quantification studies;

Develop GIS/model assessment protocols and tools to characterize risk of nutrient and sediment TMDL exceedence;

Complete an initial draft (potentially web based) of a national landscape condition assessment.

This research directly supports long-term goals established in ORDs multiyear plans related to GPRA Goal 2 (Water) and GPRA Goal 4 (Healthy Communities and Ecosystems), although funding for this task comes from Goal 4. Relative to the GRPA Goal 2 multiyear plan, this research is intended to "provide tools to assess and diagnose impairment in aquatic systems and the sources of associated stressors." Relative to the Goal 4 Multiyear Plan this research is intended to (1) provide states and tribes with an ability to assess the condition of waterbodies in a scientifically defensible and representative way, while allowing for aggregation and assessment of trends at multiple scales, (2) assist Federal, State and Local managers in diagnosing the probable cause and forecasting future conditions in a scientifically defensible manner to protect and restore ecosystems, and (3) provide Federal, State and Local managers with a scientifically defensible way to assess current and future ecological conditions, and probable causes of impairments, and a way to evaluate alternative future management scenarios.

Description:

Microtopography describes variations in soil surface elevation (nim or cm) for a scale of a few meters of horizontal distance, Small-scale (few centimeters) changes in vegetation communities synchronized with the elevation differences were observed in drained marsh (Zedler & Zedler, 1969). Vivian-Smith (1997) reported that species richness and evenness were significantly higher in areas characterized by small-scale heterogeneous microtopography The amount of water available to vegetation is related to elevation, therefore, germination and seedling establishments are effected by the soil surface microtopography (Smith and Capelle, 1992). Desert grasslands are characterized by microtopography that appears to be the result of long-term accumulation of soil around grass tussocks. Desertification or degradation of Chihuahuan Desert grasslands has been hypothesized to result from changes in spatial distribution of soil resources (Schlesinger et al., 1990). Desert grasslands are characterized by fine scale patchiness of soil nutrients and water (Schlesinger & Pilmanis, 1998). Soil nutrient rich patches in desert grassland are small mounds that are occupied by bunch grasses, The unvegetated depressions between grass tussocks act as microcatchments for water and are responsible for retaining water and nutrients in-situ. Factors that affect this fine scale patchiness are hypothesized to be the precursor of larger scale changes leading to desertification. Livestock grazing has been shown to change the spatial distribution of grasses in the short-grass steppe (Alder & Lauenroth, 2000). They attributed changes in the spatial heterogeneity of the dominant grass, Bouteloua gracilis, primarily to grazing effects. A study of changes in microtopography on grazing gradients in Chihuahuan Desert grassland documented
reduction in the abundance, height and depth of mounds and depressions respectively as a
result of chronic, long-term grazing by domestic livestock (Nash et al., 2002). The combination of biomass removal by grazers and the compaction and breakdown of mounds by hoof action are the mechanisms by which the fine scale microtopography is lost. Intense, short-duration grazing is one of the grazing management systems that have been proposed for maintaining productivity of and rangelands (Savory, 1978, Volesky et al., 1994). Short-duration grazing is hypothesized to increase production of grasses and intense, short-duration hoof action is generally thought to increase water infiltration and incorporation of litter into the soil (Savory, 1978), We hypothesized that short duration grazing would have little effect on the microtopography that is characteristic of desert grassland. We also hypothesized that short-duration grazing during the non-growing season (winter) would have less impact on desert grassland microtopography than intense, short-duration grazing during the growing season
(summer)

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