Poplar nutrient resorption in fall or drought: influence of nutrient status and clone

Nutrient mobilization in hybrid poplars was studied in two greenhouse experiments. Half the clones in each experiment were drought resistant and half were drought sensitive. In experiment 1, four clones were grown at three levels of N, two levels of P, and with four drought treatments. In experiment 2, six clones received two levels of N and three levels of K. Well-watered trees, trees subjected to drought conditions once, and hardened drought-stressed trees were harvested in late summer, and other wellwatered trees were harvested in fall. Mobilization of nutrients differed with speed and intensity of drought. During gradual drought in the first experiment, N and P were resorbed but not K or Cu. In experiment 2, drought was too rapid for resorption. Instead, nutrients became more concentrated: increased N facilitated hardening and osmotic adjustment, while K moved out of the leaves on rewatering. In fall, N, P, Cu, and K were resorbed. Increased K supply increased fall resorption efficiency of Cu, N, and P but not K. Increased N supply improved fall resorption proficiency of K at low levels of supply. However, clonal productivity on dry sites was more related to leaf retention in drought and in fall than to resorption efficiency.