Landscape and Microsite Influences on Shrub Recruitment in a Disturbed Semi-Arid Quercus-Juniperus Woodland

Abstract

Seedling demography of the leguminous shrubs Leucaena pulverulenta and Acacia berlandieri was monitored to determine the functional role of microsites in the restoration of disturbed semiarid woodlands. Seeds were sown on replicated landscapes during 1989 and again in 1990 into four microsites after brush clearing: 1) Rocky, with shallow soils ( 80%), intermediate on Rocky (65%), and lowest on Herbaceous (36%) microsites. Desiccation appeared to be the primary cause of first-season mortality. Differences between the microsites in seedling height and number of leaves were significant the first planting but not the second. Overwinter mortality of Leucaena and Acacia seedlings was lowest on Cleared microsites (55 and 7%, respectively) and greatest on microsites dominated by Herbaceous vegetation (100 and 87%, respectively). Survival of established seedlings during the second growing season exceeded 80% for both species on all microsites. A. belandieri had the highest establishment potential and the greatest ecological amplitude, with recruitment after two years being comparable on Cleared, Rocky and Tree microsites (72-78 seedlings). L. pulverulenta showed marked preferences for Cleared microsites (47 seedlings established) relative to Rocky (13 seedlings), Tree (12 seedlings) and Herbaceous microsites (0 seedlings). Spatial variability in emergence between replicated landscapes (range 35 to 78%) was greater than the variability between microsites within a landscape (48 to 62%). Differences in seed disappearance associated with surface runoff from high intensity storms and losses to granivory on Tree microsites were the primary causes of differences in recruitment between landscapes. Inter-landscape variability in granivory and precipitation runoff therefore overshadowed the effects of within-landscape seed placement among first year seedlings. Generalizations of seedling safe site characteristics based solely on short-term (one year), pseudoreplicated, within-landscape studies may therefore be misleading. Failure to account for variability in important processes at greater spatial scales may significantly influence the robustness of experimental microsite study results derived from small-scale research.