Academic journal article The American Midland Naturalist

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

Academic journal article The American Midland Naturalist

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

Article excerpt

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.

INTRODUCTION

Grassland communities have recently been conceptualized as dynamic mosaics of patches, each undergoing its own successional dynamics through time (Loucks et al., 1985; Coffin and Lauenroth, 1988, 1990a). Gaps created by the death of an individual plant of the dominant species are one type of patch that initiate successional processes called gap dynamics (Watt, 1947; Shugart, 1984). Growth, senescence, death and regeneration of individual plants govern succession on gaps and, consequently, determine community composition through time on larger spatial scales (Watt, 1947; Bormann and Likens, 1979). The basic premise of the gap dynamics concept is that an individual of a dominant species must die before a gap is produced and successional dynamics are initiated (Watt, 1947; Shugart, 1984; Coffin and Lauenroth, 1990a). In grasslands dominated by long-lived perennials, natural mortality of dominant grasses occurs infrequently (Wright and Van Dyne, 1976; Loucks et al., 1985). Other factors, such as disturbances, that cause mortality of individual plants or parts of plants may be essential to gap formation.

Small-scale disturbances are important in maintaining spatial heterogeneity and species richness in grasslands (i.e., Baxter and Hole, 1967; Platt, 1975; Hobbs and Mooney, 1985; Rabinowitz and Rapp, 1985; Collins, 1989; Gibson, 1989). Studies have focused on plant recolonization of patches (i.e., Rabinowitz and Rapp, 1985; McConnaughay and Bazazz, 1987; Coffin and Lauenroth, 1989a, 1990b; Coffin et aL, 1998), attributes of disturbances such as size and frequency (Miller, 1982; Coffin and Lauenroth, 1988) and activities of insects that cause small disturbances (Rottman and Capinera, 1983). Little is known about how the type and severity of small disturbances affect mortality of dominant individual plants to create gaps in different environments within grasslands. Furthermore, disturbances similar in size to individual plants may kill all or part of a plant, and, thus may, or may not, produce a resource gap that allows replacement or recovery of an individual of the dominant species. …

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