Omar Lopez, Krista Lopez, Rebecca Montgomery, and I are now completing our studies of how time of leafing and hydraulic conductance influence shade tolerance in trees of the southern Appalachians. Early leafers obtain a “spring subsidy” of carbon by photosynthesizing a few extra days or weeks under an open canopy, which may allow them to persist in microsites that are shadier in midsummer. To do so, however, they must have narrow xylem elements to weather late frosts.  Such elements resist cavitation but are hydraulically inefficient.  Late leafers with more efficient vessels may be able to sustain higher rates of transpiration (and hence photosynthesis) in sunnier microsites, even if they have higher whole-plant compensation points. We are testing these hypotheses through a series of field measurements and common-garden experiments on 17 tree species in Great Smoky Mountains National Park and Nantahala National Forest.

These hypotheses are related to my new model for adaptations in leaf phenology, which provides new explanations for several previously unexplained patterns in the distribution of evergreen vs. deciduous plants. This model shows that previous, widely cited, leaf-based models cannot account for many classic patterns, because those models fail to predict an advantage for multi-year evergreen leaves under any condition, given the relationship between photosynthetic rate and leaf longevity. New tests of the advantages of evergreen vs. deciduous leaves will require an integrated, whole-plant approach that focuses on the relationships among leaf phenology, photosynthetic rate, allocation belowground, hydraulic conductance, and stem support costs.

Rebecca Montgomery, Guillermo Goldstein, and I are studying the ecology and evolution of photosynthetic light adaptations in the Hawaiian lobeliads (see Adaptive Radiation).

I am currently working on new models for optimal stomatal conductance and allocation between leaves and roots, and for maximum tree height.