Survival of tree seedlings across space and time: estimates from long-term count data

1 Tree diversity in forests may be maintained by variability in seedling recruitment. Although forest ecologists have emphasized the importance of canopy gaps in generating spatial variability that might promote tree regeneration, the effects of canopy gaps on seedling recruitment may be offset by dense forest understories. 2 Large annual fluctuations in recruitment processes, coupled with the long life of forest trees, can provide an alternative mechanism for maintaining tree diversity. Evaluating the relative importance of spatial vs. temporal variability in recruitment, however, requires estimates of seedling survival that are extensive on both scales. 3 Estimates of survival derived from conventional statistical methods require that individual seedlings are monitored through time, and are therefore often limited in extent. We demonstrate a relatively fast and easy alternative approach to measuring survival that uses repeated counts of individuals in quadrats. 4 Annual seedling counts were used to quantify both the effects of small canopy gaps and understorey shrubs on the survival of Acer rubrum (red maple) seedlings and the magnitude of residual spatial and temporal variability in seedling survival. 5 We found that a dense understorey had a greater effect on seedling survival than did canopy gaps. Our canopy gaps had only a slight positive effect on seedling survival, and their benefit was offset by a large negative effect of the understorey shrub Rhododendron maximum. 6 Annual fluctuations in recruitment processes were seven times greater than spatial variability across transects and the effect of individual years on seedling survival was larger than even that of understorey shrubs. 7 The long life span of trees coupled with large annual variability in recruitment success across species may maintain the diversity of forest trees through a storage effect.