COMBINED EFFECTS OF LIGHT AND WATER AVAILABILITY ON PHOTOSYNTHESIS AND GROWTH OF ARISAEMA-HETEROPHYLLUM IN THE FOREST UNDERSTORY AND AN OPEN SITE

Photosynthetic characteristics, leaf longevity and biomass accumulatio n of a threatened herb species, Arisaema heterophyllum, were studied i n the understory of a riparian forest and at a neighboring deforested open site for 3 years in order to understand the combined effects of l ight and water availability. Light availability was 2- to 4-fold highe r at the deforested than at the forest site during the growing season of the species, and precipitation varied considerably over the 3 years . Despite the difference in water availability among the years (dry in 1994 and 1996, and wet in 1995), the species showed a strong acclimat ion to the different light environments. Light-saturated assimilation rate on a leaf area basis, leaf mass area ratio (LMA), and relative gr owth rate (RGR) were all higher at deforested site. While a positive c orrelation between individual RGR and microsite light availability was found in the wet year, no correlation was found in the dry years, and mean RGR was significantly lower in the dry year at both sites. Leaf longevity, photosynthetic capacity on a leaf mass basis, dark respirat ion rate, and leaf conductance, varied considerably from year to year, especially in the plants at the open site, probably depending on wate r availability. In the dry years plants at the deforested sites showed a lower photosynthetic rate and leaf conductance under unwatered than under watered conditions. These results suggest that the water availa bility in a given year may strongly affect light acclimation and annua l RGR of the herb species in natural habitats, even under mesic climat e conditions.