Response of Individual Bouteloua gracilis (Gramineae) Plants and Tillers to Small Disturbances

Abstract

We evaluated effects of small disturbances that kill parts of individual plants on plant survival by measuring tiller survival for the perennial bunchgrass, Bouteloua gracilis (H.B.K.) Lag ex Griffiths (blue grama). The importance of soil texture, grazing by cattle, disturbance type and severity were evaluated. Two disturbance types (covering or removing tillers) and three disturbance severities (50, 75 and 90% tiller mortality) were used to represent effects of natural disturbances in shortgrass communities (cattle fecal pats, nest sites of Western harvester ants, burrows of small animals).Tiller survival was not affected by soil texture or grazing intensity, but was affected by disturbance type and severity. Plants that were covered showed a 33% increase in tiller survival for all levels of disturbance severity from August (1991) to June (1992). No net change in tiller number was observed for removed or reference plants. Different responses between disturbance types were likely due to increases in root:shoot ratios of covered plants that increased tiller production as a result of increased soil water acquisition. The number of tillers produced was small, but statistically significant (average = 20 tillers/plant), which shows that B. gracilis plants do not produce independent tillers, but consist of integrated physiological units (IPUs). The lack of plant mortality, even with 90% tiller mortality, indicates that small disturbances must kill entire plants before gaps in resource space are produced to initiate gap dynamics that result in the recovery of an individual B. gracilis plant. Because recovery through seedling establishment by B. gracilis occurs infrequently, the ability of this species to survive after partial plant mortality is important to its continued dominance of shortgrass steppe communities in the presence of these small but frequent disturbances.