The forest between the trees: The herbaceous layer of Lucy’s Forest

Background/Question/Methods

Despite its small relative contribution to aboveground biomass in forest ecosystems (which is typically less than 0.1%) the herbaceous layer plays a disproportionately important role in maintaining ecosystem structure and function.  This has been particularly demonstrated in the Eastern Deciduous Forest of North America, a region made well-known to plant ecologists by E. Lucy Braun in her classic book, Deciduous Forests of Eastern North America.  Although virtually all of her book’s quantitative data were of overstory tree species, she consistently devoted considerable effort in her very even handling of each of the nine forest regions of the Deciduous Forest Formation to species comprising the herb layer.  Clearly, Dr. Braun was keenly aware of the ecological significance of this diminutive vegetation stratum of the forests which were her passion.  The purpose of this review is to underscore the many ways in which the herb layer of eastern deciduous forests—and of forests in general—represents an essential component of these ecosystems.  In addition, I will discuss the variety of threats to the herb layer—and, by extension, these forests—by anthropogenic activity.

Results/Conclusions

Among forest vegetation strata, species diversity is clearly highest in the herb layer (up to 90%).  Thus, forest plant diversity is a function of herb-layer richness. In forests recovering from canopy disturbances (e.g., wind throw, harvesting), competitive interactions within the herb layer can determine the reproductive success of dominant overstory species. Indeed, forest strata (herb layer and overstory) can form linkages via parallel responses to environmental gradients. Because the herb layer is sensitive to a variety of natural and anthropogenic disturbances, its study can provide important information regarding the site characteristics of forests, especially patterns of historical land-use practices.

Currently, the herbaceous layer of the eastern deciduous forest has been altered in many ways.  We can consider changes following discrete disturbance events, such as clearcut harvesting or severe damage from wind (e.g., tornados), as acute responses, often transient in nature.  Changes following alteration of the environment on longer time scales, such as conversion of forest to agricultural land and then back to forest, are legacy responses.  In addition, many anthropogenic changes in the environment represent chronic disturbances that may result in novel responses of herb layer species, including increases in atmospheric CO₂, invasions of forests by exotic species, increased atmospheric deposition of N, and global warming.  Lucy’s Forest has undergone—and continues to undergo—significant, often unprecedented change.