Demographic hallmarks of an overbrowsed population state in American ginseng

摘要

Effects of high deer herbivory in North America on populations of favored plant browse species have been well-documented, however since less palatable plants now dominate the understory, we asked whether these species could be vulnerable as well, and if so, what symptoms might signal that this was occurring? Using American ginseng ( Panax quinquefolius L.) as our representative less palatable understory plant, we compared two subpopulations within a single natural population that were differentially exposed to browse; one isolated from deer by growing atop a large, flat-topped boulder, and a browse-exposed subpopulation in the surrounding low-lying area. We tested the hypothesis that deer effects would be manifested in all parts of the life history; through reduced growth, survival and reproduction. In turn, we hypothesized that browse would reduce population growth rates, and that differences in stage structure of the population would be produced. Taking advantage of a 20 year record of formal demographic censusing, we showed that browse effects were manifested primarily in reduced size-specific growth, while size-specific fertility and survival were relatively unaffected by exposure to browse. Demographically, these differences in growth were sufficient to drive population size reductions of 4.5%/y in the off rock subpopulation while the on rock plants slowly increased in number. High browse off the rock resulted in high proportions of plants in a stunted juvenile state in the off rock population relative to the on rock plants. A high proportion of juveniles is therefore a clear symptom of an understory subjected to chronic overbrowsing, providing land managers a rapid way to assess whether deer could be impacting understory biodiversity. The sharp demographic contrasts we observed between browsed and unbrowsed subpopulations also implies that promotion of refugia within managed lands will likely become increasingly important management tools for biodiversity preservation as long as unchecked deer populations persist.