OOS 62-2 - Variability in tree seed production at the core and margin of species ranges.

Masting, the synchronous boom-and-bust seed production of long-lived plants, is an important component of forest demography and has wide-ranging effects on animal populations and ecosystem processes. Considerable work has been done to document the differences in long-term patterns of seed production among plant species and to explore the geographic scale at which masting occurs. However, there is substantial unexplained variation in the long-term patterns of seed production among populations of a particular plant species. Here, we develop the environmental stress hypothesis to explain intraspecific variation in long-term seed production. We show linkages between that hypothesis and the resource budget model of masting. We present case studies from three groups of species (pines, oak, and spruce), and we use these to explore how environmental conditions and geographic placement within range affect long-term seed production of individuals and populations. Finally, we assess whether there are commonalities in geographic and environmental trends in long-term seed production. We found that in a well-studied oak species and a dryland pine, there were strong environmental correlates of both the variability and mean of long-term seed production. However, this pattern was not universal among all species studied. The environmental gradient hypothesis accounts for a substantial portion of intraspecific variation in long-term seed production in some tree species, and other factors likely contribute to geographic differences in seed production in other tree species.