A MODEL OF PATCH DYNAMICS, SEED DISPERSAL, AND SEX-RATIO IN THE DIOECIOUS SHRUB LINDERA BENZOIN (LAURACEAE)

1 We used a combined model of forest canopy dynamics and patch-specific demography to examine the response of a forest understorey shrub, Lindera benzoin, to changes in rates of forest disturbance, rates of long distance seed dispersal, and relative effects of canopy closure on males and females. Because L. benzoin is dioecious, our analysis allowed an examination of sexual dimorphism and its effect upon habitat-specific patterns of sex ratio. 2 The matrix modelling approach followed that of Horvitz and Schemske (1986), in which demographic parameters of a population experiencing new treefall gap conditions are modified using functions describing changes in growth, survivorship and reproduction as gaps close. 3 In our analysis, effects of full canopy closure were estimated by comparing growth of adults in new treefall gaps with those in fully closed understorey, and by comparing mortality of seedlings and juveniles within subplots classified according to mean light level. Growth, survival and reproduction in intermediate successional patches leading to fully closed canopy followed a response pattern expected for shade-tolerant species, where declines in fitness components only become important as full canopy closure approaches. 4 Our results suggest positive effects of an increased rate of canopy disturbance and increased long-distance seed dispersal on mean population growth rate (fitness) in L. benzoin. 5 Our analyses predict only slightly male-biased adult sex ratios that result from relatively lower growth and survivorship for adult females (assumed to be associated with greater reproductive costs). 6 We discuss our results with respect to observed patterns of sexual dimorphism, seed dispersal, and adult sex ratio in L. benzoin, and in terms of the overall potential influence of forest gap dynamics on the demography of understorey shrubs.