Effects of Soil pH, Competition, and Seed Predation on the Distributions of Two Tree Species

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Abstract
Field experiments were conducted to evaluate the effects of soil pH, competition, and seed predation on seedling establishment of two tree species in the Sierra Madre of northwestern Mexico. Quercus albocincta and Lysiloma divaricata occur on the same mountain slopes but have almost no overlap in distribution on a more local scale: Quercus dominates in distinct patches of open woodland found only on acid, nutrient—poor soil derived from hydrothermally altered volcanic rock while Lysiloma dominates in adjacent patches of drought—deciduous forest found only on less acid and more fertile soil derived from unaltered volcanic rock. The experiments were designed to test the hypotheses that Lysiloma is excluded from the oak woodland by the low soil pH and that Quercus is excluded from the deciduous forest by competition and/or seed predation. In the oak woodland, Lysiloma seedlings survived on the acid soil only if soil pH was increased, while Quercus seedlings showed no significant response to increase in soil pH. In the deciduous forest, Quercus seedlings required the exclusion of seed predators as well as removal of surrounding vegetation to survive. Lysiloma seedlings also required removal of some of the surrounding vegetation to establish in the deciduous forest, but less reduction in competition intensity was required than for Quercus seedlings.