Why do toothed leaves correlate with cold climates? Gas exchange at leaf margins provides new insights into a classic paleotemperature proxy

Leaf teeth are conspicuous and often diagnostic features of many plant species. In mesic environments with sufficient nutrient resources, the percentage of toothed species in extant floras generally correlates negatively with temperature; consequently, fossil leaf teeth are widely used to estimate continental paleotemperatures. However, the function of leaf teeth with respect to climate is poorly understood. Here, we test the hypothesis that teeth enhance rates of carbon uptake at the beginning of the growing season when temperatures are limiting. We measure the seasonal patterns of leaf-margin photosynthesis and transpiration for 60 woody species from two temperate regions with differing climates (Pennsylvania and North Carolina). Three significant results are, first, physiological activity at leaf margins is greatest early in the growing season (first 30 d); second, toothed margins are more active with respect to photosynthesis and transpiration than untoothed margins; finally, leaf margins are more active in species native to colder Pennsylvania. The toothed species increase transpiration and photosynthate production early in the growing season relative to untoothed species and do so more in the Pennsylvania sample, maximizing carbon gain when temperature is limiting but moisture and nutrient availability are not. This mechanism may provide a proportionally increasing selective advantage to toothed species with decreasing temperature that is reflected in empirical correlations used for paleotemperature estimation.