The Formation of Dense Understory Layers in Forests Worldwide: Consequences and Implications for Forest Dynamics, Biodiversity, and Succession
Previous Advances in Threat Assessment and Their Application to Forest and Rangeland Management The Formation of Dense Understory Layers in Forests Worldwide: Consequences and Implications for Forest Dynamics, Biodiversity, and Succession Alejandro A. Royo and Walter P. Carson by land ownership and administrative boundaries. In many cases, the risk to forest understories was particularly acute Alejandro A. Royo, research ecologist, Forestry Sciences if the effects of multiple stressors occurred in a stand, either Laboratory, USDA Forest Service, Northern Research in tandem or within a short period of time. Specifically, the Station, Irvine, PA 16329; and Walter P. Carson, associate synergy between overstory disturbance and uncharacteristic professor, Department of Biological Sciences, University of fire regimes or increased herbivore strongly controls species Pittsburgh, Pittsburgh, PA 15260. richness and leads to depauperate understories dominated Abstract by one or a few species. We suggest that aggressive expansion by native Alterations to natural herbivore and disturbance regimes understory plant species can be explained by considering often allow a select suite of forest understory plant species their ecological requirements in addition to their environ- to dramatically spread and form persistent, mono-dominant mental context. Some plant species are particularly invasive thickets. Following their expansion, this newly established by virtue of having life-history attributes that match one or understory canopy can alter tree seedling recruitment rates more of the opportunities afforded by multiple disturbances. and exert considerable control over the rate and direction Increased overstory disturbance selects for shade-intolerant of secondary forest succession. No matter where these species with rapid rates of vegetative spread over slower native plant invasions occur, they are characterized by one growing, shade-tolerant herbs and shrubs.
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