Physiological evidence for climate limitations of oak distributions at local and regional scales

Abstract

Understanding the extent to which physiological tolerances of climate may limit plant distributions is critical to predicting the effects of a changing climate. This dissertation research focuses on how responses to drought and cooling influence species ranges among oaks (Quercus L.), a globally-distributed woody genus that is highly diverse within the Americas. I used functional and physiological traits to investigate correlations between cold and drought resistances and climate at two scales: 1) local species elevation limits in a semi-arid montane system and 2) regional range limits among North and Central American oaks. In Chapter 1, I found that a trade-off between the leaf-level drought resistance traits of avoidance (leaf abscission) and desiccation recovery (leaf capacitance), and not stem freezing tolerance, influenced species sorting by elevation in a semi-arid mountain system in the southwestern US. In Chapter 2, I found that stem drought tolerance (xylem vulnerability) is correlated with aridity of climate of origin among oaks from across the Americas, but that the seasonality of precipitation best predicted leaf level drought avoidance (leaf habit and stomatal closure). Finally, in Chapter 3, I found that oak species in the Americas have different leaf level cooling responses (chlorophyll fluorescence measurements of photosynthetic stress and yield) that were correlated with minimum temperatures in their climate of origin, but that ability to acclimate to cold temperatures was best predicted by leaf phenology, not climate of origin. I also found that we could predict chlorophyll fluorescence measurements with models created from hyperspectral reflectance data measured on the same leaves. These models included regions of important biological significance, including wavelengths corresponding with reflectance of photosynthetic and protective pigments. Overall, there I found significant evidence that species distributions are strongly influenced by climate. Oak species have suites of traits as mechanisms of drought resistance, and these strategies are correlated with not only overall aridity in their current habitats, but with the seasonality of precipitation. Oak species may also be able to acclimate to cooler temperatures outside of those commonly experienced in their current range. Oak species within the Americas and the mountainous, arid southwest US, may be vulnerable to range shifts as global temperatures continue to rise and precipitation regimes change.