INFLUENCE OF GAP DISTURBANCES AND TYPES OF MICROSITES ON SEEDLING ESTABLISHMENT IN BOUTELOUA-GRACILIS

1 We examined the effects of patch disturbance leading to removal of conspecific neighbours, and of microsite type on the emergence, survival, and performance of seedlings of Bouteloua gracilis at a shortgrass steppe site. Our hypothesis was that the death and/or removal of a full-size B. gracilis plant, would result in increased availability of below-ground resources and thus promote the establishment of conspecific seedlings. 2 In 1990 and 1991, experimental plots were treated to mimic the effects of some common patch disturbance types. Caryopses were then planted into both plant microsites (i.e. located in the hummock formed by adult plants of B. gracilis) and bare soil. Seedlings were watered until they were past the stage of adventitious root development. Soil water contents were recorded during and after the cessation of watering. 3 In 1991, results clearly showed that the effect of removal of established neighbours was significant in favouring seedling establishment. Differences among disturbance types were of lesser magnitude. Seedling responses were associated with soil water relations. Following cessation of watering, control plots with intact canopies showed more rapid depletion of soil water than disturbed plots. 4 The positive effect of bare soil microsites during emergence and early survival was associated with changes in superficial (0-5 cm) soil-water relations. Type of microsite was important for plant responses only during the early stages of seedling development, although soil water differences remained significant after the cessation of watering. 5 These results support the idea that gap-disturbances must occur to achieve the regeneration of the dominant species. The detrimental effect of microsites assocaited with the plant hummock may affect the pattern of emergence, resulting in a tendency for seedlings to establish in areas that were not occupied by their predecessors. Interference of established plants via below-ground competition and microsite favourability plays a key role in the regeneration of B. gracilis in this shortgrass community.