Foliar demand and resource economy of nutrients in dry tropical forest species

Important phenological activities in seasonally dry tropical forest species occur within the hot-dry period when soil water is limiting, while the sub sequent wet period is utilized for carbon accumulation. Leaf emergence and leaf area expansion in most of these tree species precedes the rainy season when the weather is very dry and hot and the soil cannot support nutrient uptake by the plants. The nutrient requirement for leaf expansion during th e dry summer period, however, is substantial in these species. We tested th e hypothesis that the nutrients withdrawn from the senescing leaves support the emergence and expansion of leaves in dry tropical woody species to a s ignificant extent. We examined the leaf traits (with parameters such as lea f life span, leaf nutrient content and retranslocation of nutrients juring senescence) in eight selected tree species in northern India. The concentra tions of N, P and K declined in the senescing foliage while those of Na and Ca increased. Time series observations on foliar nutrients indicated a sub stantial amount of nutrient resorption before senescence and a 'tight nutri ent budgeting'. The resorbed N-mass could potentially support 50 to 100 % a nd 46 to 80 % of the leaf growth in terms of area and weight, respectively, across the eight species studied. Corresponding values for P were 29 to 10 0 % and 20 to 91 %, for K 29 to 100 % and 20 to 57 %, for Na 3 to 100 % and 1 to 54 %, and for Ca 0 to 32 % and 0 to 30 %. The species differed signif icantly with respect to their efficiency in nutrient resorption. Such inter specific differences in leaf nutrient economy enhance the conservative util ization of soil nutrients by the dry forest community. This reflects an ada ptational strategy of the species growing on seasonally dry, nutrient-poor soils as they tend to depend mon or less on efficient internal cycling and, thus, utilize the retranslocated nutrients for the production of new folia ge biomass in summer when the availability of soil moisture and nutrients i s severely limited.