Ecological genetics of adaptive phenotypic traits for three white pine species from the Lake Tahoe Basin, USA: Implications for tree responses and evolutionary change

Background/Question/Methods

In terrestrial ecosystems identifying adaptive phenotypic variation of ecologically relevant plant traits may permit us to detect the sensitivity (e.g., narrowly versus broadly adapted), resiliency, or vulnerability of populations, as well as improve our general understanding of species’ responses to environmental change.  Genetic variation within and among populations has evolved as a result of abiotic and biotic influences imposing selective pressures on individuals inhabiting heterogeneous environments.  In montane landscapes, environmental heterogeneity is a key feature and can occur over short physical distances (<1 km), influencing important environmental (temperature, precipitation, geology, soil properties, disturbance regimes) and biological (species interactions, forest conditions, diversity) factors and subsequent selective pressures. In a common garden experiment, we evaluated phenotypic variation within (narrow-sense heritability, h²) and among (Q_ST) populations in height growth, phenology (bud flush), resource partitioning (root:shoot ratio), water-use efficiency (δ¹³C), and foliar nitrogen (δ¹⁵N) for three important forest tree species (sugar pine, western white pine, and whitebark pine) in the Sierra Nevada. In addition relationships between environmental factors (climate and soil) and quantitative traits were determined.

Results/Conclusions

Across the Lake Tahoe Basin we observed significant differences in phenotypic variation among populations and families (within populations) for most traits measured for all three species.  Narrow-sense heritability within populations varied considerably between species and traits, with moderate to high h² for bud flush, resource allocation (r:s), and δ¹³C. Population differentiation and Q_ST values for traits among species ranged from low to moderate.  These values (both h² and Q_ST) indicate a moderate degree of local adaptation at the scale of the Lake Tahoe Basin, a heterogeneous montane landscape spanning 1,300 km².  Significant correlations were found between traits and environmental (climate and soil) principal components.  Temperature and precipitation are key determinants of adaptation in forest trees, yet edaphic factors such as geology and associated soil properties may be just as important in influencing phenotypic variation and local adaptation. Considerable genetic variation was found among and within populations for a number of ecologically relevant plant traits for all three species– potentially allowing populations of white pines in the Lake Tahoe Basin to respond to the selective pressures of environmental change.