WHY DO LEAVES HAVE TEETH? EXPERIMENTAL INSIGHTS AND PALEOCLIMATIC IMPLICATIONS

Analysis of leaf shape in fossil floras has been, for almost 90 years, the most reliable method for reconstructing terrestrial paleotemperatures from before the Pleistocene. In particular, the strong positive correlation observed in living forests between mean annual temperature and the proportion of plant species that have untoothed leaf margins is widely applied to fossil leaf assemblages. In this context, it is striking that the selective and mechanistic bases of the correlation are poorly known: why do leaves have teeth, and why are they less likely to have teeth in warmer climates? The leading hypothesis states that leaf teeth serve as “hot spots” for carbon fixation and water loss, thereby maximizing carbon gain in shorter, colder growing seasons. To quantitatively address this issue, we tracked the seasonal patterns of photosynthesis and transpiration in the leaf margins and whole laminae of 24 woody dicots native to south-central Pennsylvania. Photosynthesis inside teeth was highest in the young leaves of the first cohort, ~30 % higher than the bulk laminae, and then declined as the leaves matured to rates lower than the bulk laminae. This initial pulse in leaf-tooth photosynthesis was not observed in subsequent cohorts. Transpiration in the teeth of young leaves was similar to that in the bulk laminae, but then also subsequently declined relative to the bulk laminae as the leaves matured. No clear patterns in photosynthesis or transpiration were observed for the margins of untoothed species, making the interpretation of data from toothed species difficult. Nevertheless, enhanced early-season photosynthesis in toothed species may be adaptive for short growing seasons in cold environments. If so, better correlations between leaf characters and temperature variables may be possible by incorporating parameters such as number and total area of teeth.