Assessing intraspecific demographic variation via integrated ecological modelling and genetic analysis to assist tree species management

Background/Question/Methods

Macro-scale intraspecific variation of abundance, mortality and regeneration of four eastern US tree species (Tsuga canadensis, Betula lenta, Liriodendron tulipifera and Quercus prinus) are assessed to evaluate the importance of environmental heterogeneity and demographic responses among three climatic zones - cold-adapted leading region; well-adapted middle region and warm-adapted trailing region. Ecologically meaningful species regional groups are mapped based on decision tree rules and univariate and multivariate patterns of species demography are related to environmental variables (climate, soil and topography). Forest inventory analysis data are used for deriving demographic variables and USDA plant hardiness zones for delineating climatic zones.

 In addition, we compare environmental heterogeneity to genetic variation (defined by microsatellite markers) in eastern hemlock (Tsuga canadensis) and test if we can derive climatically relevant potential adaptive zones based on evolutionary lineages. We investigate if patterns of abundance/mortality/regeneration of hemlock are consistent with expected trends based on genetic analysis. We use genetic clusters and disjunct populations to look for associations between environmental heterogeneity and genetic similarity, and in regions affected by the Hemlock Wooly Adelgid. Genetic indices are incorporated into decision-tree based ensemble methods to test for their significance compared to environmental variables.

Results/Conclusions

The analysis reveals a pattern where moisture based variables become more important in the warm-adapted trailing region compared to the cold-adapted leading zone. Higher mortality and lower regeneration patterns in the warm trailing zone point to its vulnerability to growing season temperature and precipitation changes that are bound to exacerbate under future climates.  

Multivariate analysis of abundance, mortality and regeneration allows identification of regions where these factors can simultaneously result in species vulnerability, and also the ability to depict environmental rules that lead to them.

The study highlights the importance of taking into account intra-specific variation of fitness measures in order to understand differential adaptation based on environmental and genetic heterogeneity. Combining environmental and genetic data using climatic zones via a common statistical framework allows for understanding the evolutionary basis for ecological change and in defining better management units with adaptive potential.