Changes in the abundance and distribution of woody plants related to dispersal mechanisms along a forest cover gradient.

摘要

Seed dispersal direction and distance depend on the interaction between landscape structure and seed dispersal mechanism. I hypothesized that a reduction of forest cover in the landscape will affect the dispersal of animal-dispersed species differently than wind dispersed species. Landscapes with less forest cover should have relatively lower densities of animal-dispersed plants (i.e., decreased landscape connectivity for animal dispersers) and relatively higher densities of wind-dispersed plants (i.e., fewer barriers to wind movement and increased connectivity) than landscapes with more forest cover.; In 1997 I sampled woody plant stem densities in 29 1-km2 sample landscapes, along a gradient of decreasing forest cover. The mean stem density of animal-dispersed stems increased with decreasing forest cover in the landscape (r2 = 0.44) with no change in the mean stem density of wind-dispersed species. Additional analyses using seed and stem densities in the landscape suggested that a reduction of forest cover in the landscape results in: (i) a decreased dispersal of the seeds of both wind- and animal-dispersed forest-habitat species within forest habitats, (ii) an increased dispersal of the seeds of wind- and animal-dispersed open-habitat species into open habitats, and (iii) an increased dispersal of animal-dispersed open-habitat species into remaining forest habitat.; Based on these results, I predicted an increase in the emergence of seedlings in forest habitats following an ice storm in 1998. I also predicted a greater increase in the emergence of seedlings of open-habitat, animal-dispersed species in forest habitats since landscapes with less forest cover will have more of these kinds of seeds in remaining forest patches. After resurveying the landscapes in 1998 and 1999 I found that the results supported my predictions.; My results suggest that landscape-scale forest patterns can affect vegetation composition within remaining forest patches. I speculate that the removal of forest from a landscape will increase the movement of animal-dispersed seeds from open habitats into remaining forest habitats in the landscape. Additional disturbances that damage or remove canopy crowns from forest patches would also result in the increased emergence of the seedlings of these species in remaining forest patches.