Print PageLittle is known regarding leaf gas exchange during fall leaf color change and senescence in deciduous tree species. Hypotheses about the adaptive advantages of leaf color change during fall senescence have been proposed, yet no study to our knowledge has focused on photosynthetic gas exchange during a time of year that may last for weeks and often coincides with the driest time the year (post-summer). The purpose of our study was to measure changes in standard gas exchange parameters, net photosynthesis (A), leaf conductance (g), internal CO2 concentration (Ci), and water use efficiency (WUE, estimated from A/g values) during fall leaf color change and senescence in five deciduous tree species (Acer saccharum, Cornus florida, Ginkgo biloba, Prunus sp, and Prunus cerasifera). For this purpose, we recorded measurements on 2-4 individuals per species over the entire period of color change prior to leaf abscission (from late September to early November) using a LiCor LI-6400 portable photosynthesis system.
Results/Conclusions
Substantial declines in A of approximately 4.0 μmol m-2 s-1, in g of about 50 mmol m-2 s-1, and in A/g of over 100 µmol/mmol m2 s-1 occurred over the duration of the senescent period. The temporal dynamics of these declines and the relationships between A, g, and Ci were statistically identical (p < 0.01) in four of the five species sampled. While similar declines in A and g during autumn leaf senescence have been reported in a few studies, associated changes in WUE have not been evaluated (to our knowledge). We calculated WUE values from gas exchange data published previously and found similar declines in WUE that also occurred under dry conditions. Similar to other reports mentioning the possible adaptive benefits of low WUE in a variety of species and habitats, benefits during fall leaf senescence have not been proposed. Speculation about possible adaptive advantages of excessive transpiration during fall leaf senescence is presented here.
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