THE SIGNIFICANCE OF RESORPTION OF LEAF RESOURCES FOR SHOOT GROWTH IN EVERGREEN AND DECIDUOUS WOODY-PLANTS FROM A SUB-ARCTIC ENVIRONMENT

Five evergreen and four deciduous species of woody plants were experim entally defoliated to address the question of whether the relative imp ortance of resource resorption from leaves to support new growth varie s between these life-forms or whether the species show individualistic response patterns. The deciduous and evergreen species were defoliate d before leaf senescence in autumn and before resource mobilisation in early summer, respectively. Responses were measured in terms of growt h emerging the season following defoliation. Averaged across all speci es, defoliation had general effects on growth and nutrient status of n ew shoots. Shoot, leaf and stem weights, leaf area, leaf dry weight pe r unit area, shoot dry weight per unit phosphorus (P) and shoot nitrog en (N) and P pool sizes were reduced in defoliated shoots. Shoot P con centrations in defoliated plants were increased in comparison with con trols, leaf area per unit shoot dry weight increased, particularly in evergreens. Leaf number, leaf N and P per unit leaf area, shoot N conc entrations and shoot dry weight per unit N were not significantly affe cted by the treatment but some of these parameters showed large variat ion among species. However, there were no significant life-form specif ic responses of the parameters measured. In contrast, marked differenc es were found among species in their response to defoliation. The reso rption of dry matter, N and P from old leaves to support new growth ap pears to be equally important in deciduous and evergreen woody species despite morphological, physiological and phenological differences bet ween these life-forms. The magnitude of growth depression in response to defoliation across all species was not correlated to the amount of dry matter or nutrients removed, to the species' resorption efficiency or to the potential benefit of resorption in terms of future growth. Rather the species responded to defoliation in an individualistic way.