The significance of resorption of leaf resources for shoot growth in evergreen and deciduous woody plants from a subarctic environment

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Five evergreen and four deciduous species of woody plants were experimentally defoliated to address the question of whether the relative importance of resource resorption from leaves to support new growth varies between these life-forms or whether the species show individualistic response patterns. The deciduous and evergreen species were defoliated before leaf senescence in autumn and before resource mobilisation in early summer, respectively. Responses were measured in terms of growth emerging the season following defoliation. Averaged across all species, defoliation had general effects on growth and nutrient status of new shoots. Shoot, leaf and stem weights, leaf area, leaf dry weight per unit area, shoot dry weight per unit phosphorus (P) and shoot nitrogen (N) and P pool sizes were reduced in defoliated shoots. Shoot P concentrations in defoliated plants were increased in comparison with controls; leaf area per unit shoot dry weight increased, particularly in evergreens. Leaf number, leaf N and P per unit leaf area, shoot N concentrations and shoot dry weight per unit N were not significantly affected by the treatment but some of these parameters showed large variation among species. However, there were no significant life-form specific responses of the parameters measured. In contrast, marked differences were found among species in their response to defoliation. The resorption of dry matter, N and P from old leaves to support new growth appears to be equally important in deciduous and evergreen woody species despite morphological, physiological and phenological differences between 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.

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