The Ecological Significance of the Herbaceous Layer in Temperate Forest Ecosystems

摘要

Despite a growing awareness that the herbaceous layer serves a special role in maintaining the structure and function of forests, this stratum remains an underappreciated aspect of forest ecosystems. In this article I review and synthesize information concerning the herb layer's structure, composition, and dynamics to emphasize its role as an integral component of forest ecosystems. Because species diversity is highest in the herb layer among all forest strata, forest biodiversity is largely a function of the herb-layer community. Competitive interactions within the herb layer can determine the initial success of plants occupying higher strata, including the regeneration of dominant overstory tree species. Furthermore, the herb layer and the overstory can become linked through parallel responses to similar environmental gradients. These relationships between strata vary both spatially and temporally. Because the herb layer responds sensitively to disturbance across broad spatial and temporal scales, its dynamics can provide important information regarding the site characteristics of forests, including patterns of past land-use practices. Thus, the herb layer has a significance that belies its diminutive stature.nnAmemorable moment in my graduate training occurred during my first course at Duke University in 1978. Our class was in the field, learning the ecology and taxonomy of tree species of the North Carolina Piedmont, when I had the temerity to inquire about the identification of a particular forest herb. “Oh, that's just a step-over,” the professor replied, with a bit of humor, suggesting that herbaceous plants on the forest floor were of little importance to the forest and thus merited “stepping over” in the pursuit of studying trees.nnI do not share this anecdote to suggest that most people with an interest in forests hold the herbaceous layer in low esteem. On the contrary, the ecology of the herbaceous layer has been the focus of numerous studies, including such recent syntheses as a book (Gilliam and Roberts 2003a) and extensive reviews (Whigham 2004, Roberts 2004, Gilliam 2006). Rather, this story shows how far vegetation scientists have come in the past few decades toward helping forest managers, conservation biologists, and other ecologists appreciate the importance of the herbaceous layer, and setting the stage for enhancing this appreciation among biologists in a wide variety of disciplines.nnStudies of the ecology of the herbaceous layer of forests have been carried out over nearly half a century. Some of these earlier studies focused on the response of herb communities to environmental gradients within forests (Struik and Curtis 1962, Anderson et al. 1969), whereas others emphasized structural aspects of the herbaceous layer, such as biomass (Zavitkovski 1976). Still other studies characterized ecosystem processes associated with the herbaceous layer, such as productivity (Siccama et al. 1970). In this article, I review the recent literature to highlight the ecological significance of the herbaceous layer to the structure and function of forest ecosystems. There is a natural tendency to overemphasize the dominant vegetation of forests—trees—which is understandable, considering that a forest is delineated from other vegetation types by the prevalence of trees. This overemphasis is unfortunate, however, because it ignores a component—the herbaceous layer—whose ecological importance to the forest ecosystem is quite disproportionate to its minimal biomass and limited visibility in the landscape.