AUTUMNAL ALLOCATION OF PHOSPHORUS IN BLACK ALDER, EASTERN COTTONWOOD, AND WHITE BASSWOOD

Changes in leaf and current year bark phosphorus (P) concentrations of actinorhizal black alder (Alnus glutinosa (L.) Gaertn.), eastern cottonwood (Populus deltoides Bartr. ex. Marsh.), and white basswood (Tilia heterophylla Vent.) were determined during autumn on a mine-spoil site and a contrasting prairie site. During the last 2 weeks of sampling (4 weeks for alder at the prairie site), leaf P concentrations decreased by 43% for black alder and 23% for eastern cottonwood at the mine-spoil site and by 30% for black alder and 26% for white basswood at the prairie site. Concurrent increases in bark P concentration were observed only for eastern cottonwood and white basswood. Changes in leaf, bark, and root P of potted black alder seedlings were also followed in a parallel experiment. Bud set was associated with the onset of increasing leaf P concentration and content and stabilizing bark P content, while roots continued to accumulate P. The ratio of bark P content to root P content as well as the ratio of bark dry weight to root dry weight increased during the last stages of leaf senescence. Our results suggest that black alder can resorb a large proportion of its leaf P during leaf senescence, particularly when P availability is low. Both twigs and roots served as storage tissues for P, with roots acting as a primary sink in early autumn and bark acting as a sink for subsequently resorbed leaf P.