Gas exchange of juvenile and mature trees of Alnus jorullensis (Betulaceae) at sites with contrasting humidity in the Venezuelan Andes
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The reduction of photosynthetic rates with tree age has been proposed as a major driver of the productivity declination along ontogeny. It is not clear, however, how environmental humidity affects stomatal conductance and biochemical potential of photosynthesis in trees belonging to different age-classes. We assessed daily cycles of gas exchange on leaves of juvenile and mature individuals of the tropical high-elevation tree Alnus jorullensis Kunth (Betulaceae), at two sites with contrasting precipitation in the Venezuelan Andes. Photosynthesis and stomatal conductance were higher in juvenile trees during the morning and at noon in the mesic site, and were in general similar between age-classes in the wet site. Under light-saturating conditions, the net photosynthetic rate was similar between the age-classes at the wet site and higher for juvenile trees at the mesic site, whereas stomatal conductance did not differ between age-classes and was higher at the wet site. Daily cycles of gas exchange and a type II regression model between photosynthesis and intercellular CO2 concentration indicated that the better performance of juvenile trees at the mesic site was due to lower non-stomatal limitations. These results support the proposal that non-stomatal limitations—rather than stomatal ones—are involved in the decay of photosynthesis in mature trees, and suggest that such limitations may be evident only under drier conditions.
KeywordsAlnus High altitude Ontogeny Photosynthesis Venezuelan Andes
We thank Fermín Rada, Marino Cabrera, Gerardo Pérez, Elysaúl Rangel, and David Martínez for their assistance in the field. We appreciated the worthy contribution of the editor and three anonymous reviewers who helped us to significantly improve the manuscript from its early version. This research was supported by the Red Latinoamericana de Botánica (RLB), fellowship P5-1993. Frida I. Piper has a postdoctoral fellowship, Fondecyt 3080057. BBVA Prize for Conservation of Biodiversity in Latin America 2004 is also acknowledged. This paper is part of the research activities of the Institute of Ecology and Biodiversity (IEB) funded by P05-002 F ICM.
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