Submitted to: Ecological Society of America Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: July 1, 2002
Publication Date: August 4, 2002
Citation: BESTELMEYER, B.T., BROWN, J.R., HAVSTAD, K.M. WHAT DOES AN ECOLOGICAL THRESHOLD LOOK LIKE? SCALE-DEPENDENT CAUSES OF A GRASSLAND-SHRUBLAND TRANSITION. 87TH ANNUAL MEETING, ECOLOGICAL SOCIETY OF AMERICA. 2002. ABSTRACT P. 319. Technical Abstract: Recent concepts in rangeland health and management emphasize that ecosystem structure may shift abruptly in time once a critical threshold is crossed. Transitions between grasslands and desert shrublands, for example, are thought to be triggered by a combination of high grazing pressure and drought. Once thresholds are crossed, novel constraints prevent reestablishment of historic ecosystem states. Thus, land management policy is being oriented towards identifying and avoiding these thresholds. One problem with implementing such policy is that land managers and ecologists are not sure what critical thresholds are; this stems in part from a lack of conceptual clarity. We argue that thresholds in rangeland structure are not singular phenomena but exist at several levels along a chain of causation. We illustrate this conceptual approach with a study of a grassland-shrubland transition in south-central New Mexico. At the broadest scale (25 km2), the shift from a grass-dominated area to a shrub-dominated area is coincident with a slight change in topographic position, a change in soil texture, and increased channelization. At intermediate scales (100-1000 m2), the maintenance of grass cover is promoted by the presence of patches oriented perpendicularly to the direction of slope, whereas shrubs dominate where patches are elongated in the direction of the slope. At the finest scales (1-10m2), grasses are associated with patches having sandier surface textures, low calcium carbonate at the surface, and higher infiltration rates than shrub-dominated patches. Thus, the causes of the shift from grassland to shrubland are scale-dependent and thresholds in several processes can be identified.