Positive and negative interactions between environmental conditions affecting Cercocarpus ledifolius seedling survival.

We evaluated the balance between positive and negative effects of environmental conditions on first-year seedling survival of the tree Cercocarpus ledifolius during two summers, 1996 and 1997. The experimental design was fully crossed with two levels of water, with and without supplementation, two levels of herbivory, with and without protection, and three major microhabitats, open interspaces, under the canopy of Artemisia tridentata shrubs, and under the canopy of mature C. ledifolius trees. Effects of drought and herbivory on seedling survival depended on the year. Water supplementation and herbivory protection during the dry summer of 1996 (27.7 mm) generally increased seedling survival. Additionally, survival tended to be greatest beneath C. ledifolius canopies. More important ecologically were the significant interactions. In 1996, water supplementation increased survival more with than without herbivory protection. The three-way interaction, treatment-microhabitat combination, was most important; by far the greatest survival was in the water supplementation and herbivory protection in the tree microhabitat. During the wet summer of 1997 (158.5 mm), neither water supplementation, herbivory protection, nor microhabitat were significant as main effects. The water-supplemented and herbivory-protected treatment again combined to yield highest survival, but this time in open interspaces rather than beneath trees. Our study shows how the importance of individual limiting factors and the relative favorableness of particular microhabitats appear to change across years depending on environmental conditions.