SEASONAL VARIATION IN LEAF LITTER NUTRIENT CONCENTRATIONS OF VALDIVIAN RAINFOREST TREES

Resorption of nutrients from senescing tissues is an im-portant aspect of the nutrient economy of perennial plants, and a major influence on ecosystem nutrient dynamics. Attempts to interpret the adaptive significance of variation in nutrient resorption efficiency have met with little suc-cess, leading some scientists to emphasize the need for more work on the biochemical and physiological basis of resorp-tion. One hypothesis suggests than variation in resorption is controlled by sink-source effects within the plant. Ac-cording to this hypothesis, we might expect resorption to be positively correlated with plant growth rates. This could be expected not only for comparisons among species or populations, but also when comparing conspecific individu-als growing at different rates, or the same plant during dif-ferent phases of the annual growth cycle. We measured sea-sonal variation in leaf litter nitrogen and phosphorous con-centrations of four evergreen tree species of the Valdivian rainforest, and tested predictions of the sink hypothesis. Relatively low mean N:P ratios of litter (ranging from 5.5 to 9.9 among the four species) suggest nitrogen limitation of tree growth. N and P concentrations showed nearly two-fold seasonal variation in all four species. All species showed low resorption proficiency (high litter [N]) in win-ter, consistent with the sink-source hypothesis. However, most species showed their lowest litter concentrations of both nutrients (highest resorption proficiency) in autumn. As growth sink strength should be low in autumn, this find-ing appears to be inconsistent with the sink hypothesis