Introduction

Major anthropogenic changes in the frequency and severity of natural disturbance regimes can radically alter understory species composition and threaten the long-term sustainability and biodiversity of plant ecosystems (Alpert and others 2000, Roberts 2004, Rooney and others 2004). These changed disturbance regimes often trigger rapid expansion of native plant species that previously occupied a relatively minor portion of the understory flora (de la Cretaz and Kelty 1999, Mallik 2003, Vandermast and Van Lear 2002). Following their release, these herbs, shrubs, trees, and vines aggressively colonize and overtake disturbed patches forming persistent, nearly monospecific, and seemingly impenetrable thickets. This layer, identified in the literature as competing vegetation, interfering plants, low canopy, non-crop vegetation, native invasives, recalcitrant understory layer, or weeds, creates conditions below its canopy that reduce tree seedling establishment and survival, inhibit seedling growth into the sapling-size class, and alter species composition (Bashant and others 2005, Horsley 1993a, Messier and others 1989, Nilsen and others 2001, Tappeiner and others 1991). The impacts of this interfering layer alter the rate, direction, and composition of tree regeneration so profoundly that forest recovery following disturbance may contrast sharply with the predicted patterns of vegetation development for a particular forest type. Thus, these dominant understory layers often are the crucial factor determining success or failure of tree regeneration following harvest, thus threatening sustainable forest management (Ehrenfeld 1980, Gill and Marks 1991, Huenneke 1983).

First, the processes that cause the formation of recalcitrant understory layers are reviewed. Second, how these layers alter the rate and direction of forest succession is described. Third, published work is reviewed to identify how these layers control tree recruitment, growth, and survivorship and, thus, patterns of tree regeneration and succession. Fourth, the most prominent causal mechanisms for the formation of these layers are identified and the consequences of their formation on successional dynamics and forest regeneration are outlined. Finally, how recalcitrant understory layers may reduce floristic diversity is discussed, arguing for their incorporation into forest successional models, and management options for mitigation of their impacts are explored.