NUTRITIONAL CONTROLS OVER NITROGEN AND PHOSPHORUS RESORPTION FROM ALASKAN BIRCH LEAVES

Little is known about factors that control nutrients loss from plants. We measured N (nitrogen) and P (phosphorus) resorption from leaves of Alaskan birch (Betula papyrifera) trees that differed strongly in their tissue nutrient status. We then experimentally manipulated those factors that we thought were most likely to control nutrient resorption. Trees of high nutrient status had larger maximum leaf N and P pools, translocated more N and P out of leaves, and left larger N and P pools in abscised leaves than did trees on infertile soils. However, plant nutrient status had no effect on the efficiency of N or P resorption (i.e., the proportion of the maximum nutrient pool resorbed from leaves during autumn senescence) except for low efficiency of P resorption in the highly fertile lawn. Prolonging the time that a leaf remained attached to the tree had no effect on N and P resorption efficiency. Leaching by autumn rains accounted for 25% of the N disapperance and 55% of the P disappearance from senescing leaves in a high-fertility field. Weakening of source-sink interactions by shading senescing leaves or by removing catkins (a sink for carbohydrates and nutrients) significantly reduced resorption efficiency. On the basis of our results and previous work, we conclude that nutrient resorption efficiency is influenced more strongly by carbohydrate flux from leaves (i.e., source-sink interactions) than by factors governing hydrolysis of nutrient-containing fractions in leaves.