An appraisal of the classic forest succession paradigm with the shade tolerance index

Background/Question/Methods

Forest succession is a continuous stochastic process that occurs at the level of individual trees and results in the replacement of one tree species by another at the level of forest stands. Understanding the mechanisms underlying forest succession has remained a challenging scientific problem for more than a century. Existing succession theories are mostly qualitative and represented as conceptual models having substantial limitations in their appraisal and validation. We revisited the classic theory of forest succession relating shade tolerance and species replacement, and assessed its validity to understand patch-mosaic patterns of forests across the US ecoregions. The classic succession paradigm has been formulated based on observations of temperate forest patterns in northern and central parts of the Eastern hemisphere, where the gap dynamics and shade tolerance driven succession are most noticeable and easy to observe. In a broad range of plant ecology literature shade tolerance is considered as a primary factor underlying forest successional dynamics. We employ the original forest characteristic called the "shade tolerance index" to analyze forest succession patterns in US ecoregions using the USDA FIA forest inventory data, individual-based forest simulations and Markov chain models.

Results/Conclusions

The shade tolerance axis defined according to the classic succession paradigm demonstrated amazing similarities with other successional models such as the empirically defined continuum index which captures successional patterns in southern Wisconsin, and the successional dynamics of the two-species system of White Pine-Eastern Hemlock in the northeastern US. We further studied the spatiotemporal correlations between shade tolerance and other stand level characteristics across the US ecoregions using a data-intensive approach, and found that the shade tolerance index is uncorrelated with biomass, basal area, stand age, and biodiversity. The shade tolerance index allowed us to test the validity of shade tolerance driven succession in particular ecoregions. Based on the distribution of shade tolerance across stands as well as their convergence rate toward equilibrium, we derived a classification of US ecoregions. We have discovered that shade tolerance driven succession is strictly connected with climatic variables. The importance of this successional mechanism decreases along the transition from North to South within the eastern part of the Humid Temperate Domain, and, evidently, shade tolerance is not related to successional patterns in arid ecosystems such as the southern part of the Dry Domain.