Floral sex ratios in scrub oak (Quercus ilicifolia) vary with microtopography and stem height

Abstract

We measured floral sex ratios (number of male inflorescences:number of female flowers) and height of stems (= ramets) of monoecious scrub oak (Quercus ilicifolia) growing along a topographic gradient on the slope of a 20 m deep depression. Stems lower in the gradient experienced increasingly severe conditions in terms of a shorter growing season and a higher incidence of killing frosts. At the top of the gradient, floral sex ratios of tall stems were male biased; however, sex allocation patterns at the bottom showed no such size-dependent relationship. With decreasing elevation (greater stress), the production of male flowers declined more rapidly than that of female flowers. Tall stems reduced overall resource allocation to flower production proportionately more with decreasing elevation than did short stems, but this reduction was again more marked in the male flowering function than in the female. These differential patterns of sex investment explain, at least in part, the variation in size-related gender relationships along this gradient. The more stressful environmental conditions prevailing at the bottom of the depression and the relative costs of the male and prezygotic female function may combine to produce these flowering patterns.