Ecological Indicators (Oct 2023)
Revisiting the model system for forest succession: Eighty years of resampling Piedmont forests reveals need for an improved suite of indicators of successional change
Abstract
Understanding of secondary succession, long-used as a unifying theme in ecology, continues to be informed by early-20th century chronosequence studies of old fields in the southeastern United States. However, growing evidence suggests classical, site-based indicators of successional change alone are not robust enough to capture realized compositional variation in eastern North American forests a century later. We illustrate how long-term data can provide deeper insight into forest dynamics and help identify additional indicators for predicting successional change.Using 80 years of permanent plot data from 36 forest stands in the Piedmont of North Carolina (USA) as a case study within the model system examined by foundational authors, we examine long-term trends in tree species abundance in both old-field pine forests transitioning to hardwood dominance and long-standing hardwood stands representing a range of historical, topographic, and edaphic conditions. We use a suite of descriptive and multivariate analyses to examine these long-term data and to assess them within the context of site conditions and novel drivers of change (e.g., removal of chronic fire, hurricane damage, increase in herbivore populations, and introduction of non-native plants and pathogens).Results indicate that these southeastern forests have undergone various perturbations that have collectively resulted in forests that are developing differently than predicted by classical models. Of particular note is the low recruitment of putative climax species such as oaks (Quercus spp.) and hickories (Carya spp.) and their replacement by novel understory communities, dramatic loss of dominant species (e.g., Cornus florida) due to nonnative pathogens, overcrowding by invasive exotic species, shifts in stem size distributions due to deer herbivory, and overall accelerated shifts in successional trajectory due to hurricane damage.We propose that potential shifts in predator abundance, nonnative species dispersal risk, pest and pathogen potential, changes in disturbance regimes, and frequency and timing of high-intensity disturbance can interact in various ways leading to variable and sometimes stochastic successional outcomes, and so these variables should be considered repeatedly through time as indicators of successional change in addition to classically-utilized indicators such as underlying site and abundance conditions. Using such a broad (and even dynamic) suite of indicators of forest change can more comprehensively enable accurate, long-term successional floristics modeling and resulting ecological and silvicultural management of temperate forests.
