Logging legacies in a plant biodiversity hotspot: Altered distribution and abundance patterns of the shrub layer in the southern Appalachians

Altered disturbance regimes in forest ecosystems can lead to the formation of dense, persistent understory vegetation composed of a small number of dominant species. This understory layer often limits tree seedling recruitment, decreases biodiversity, and can alter forest ecosystem structure and functioning. In the southern Appalachians - one of the most biodiverse regions in North America - pervasive logging in the 19th and early 20th centuries may have contributed to the formation of a dense understory shrub layer that suppresses plant diversity and tree recruitment, but the evidence for this idea has been mixed. To help clarify this issue, we tested whether (a) the two dominant understory shrub taxa (Kalmia latifolia and evergreen Rhododendron species) were more common or more abundant in areas with a history of intense logging than in areas undisturbed by logging, and (b) the relationships between the understory shrub layer and environmental gradients that drive regional biodiversity and plant distributions in the Southern Appalachians (e.g., climate, soils) differ in logged versus undisturbed areas. We used publicly available, extensive, and spatially explicit data on vegetation, climate, topography, soils, solar radiation, and logging history in Great Smoky Mountains National Park to build beta and binomial regression models of shrub frequency and cover as a function of logging history and environmental variables. Contrary to our expectation and speculations in the literature, we found that the dominant shrub taxa were more common (frequent) in areas with no logging history. However, we also found that logging history effects on shrub abundance (cover) varied along environmental gradients. For example, in topographically shaded landscape positions evergreen Rhododendron thickets exhibited greater cover in logged areas than in areas with no logging history. On the other hand, Rhododendron thickets exhibited lower cover in logged areas than in areas with no logging history at low elevations. Thus, conservation planning and forest management in topographically complex regions need to consider both landscape context (e.g., elevation, landscape position) and past logging as important interacting drivers that either facilitate or suppress local formation of dense shrub understories that can ultimately impact tree regeneration patterns, biodiversity, and ecosystem functioning.