Nutrient use strategies of plants of various life-forms in a subarctic environment : Nutrient conservation as an adaption to infertile habitats

Sammanfattning: Plant performance in relation to limiting nutrients was analysed using the concept of nutrient use efficiency (NUE). NUE can be decomposed into (1) the nutrient productivity (aNP) and (2) the mean residence time (MRT). In particular, this thesis addressed the hypothesis that plants adapt to infertile habitats by nutrient conservation (long MRT) rather than a high NUE.In a field study comparing species of contrasting life-forms a negative relationship between aNP and MRT was found. Consequently NUE i.e. the product of aNP and MRT, was relatively constant across species, habitats and life-forms. These results offered support to the hypothesis that adaptation to infertile habitats involves a long MRT rather than a high NUE per se. There appeared to be a trade-off between aNP and MRT, since both components of NUE could not be maximised together.Data from a garden experiment suggested that this trade-off is most obvious when evergreen species are compared with deciduous ones. Owing to adaptations to an unfavourable season, the structure and function of evergreen leaves differ from deciduous leaves. Consequently, evergreens show a low aNP a long MRT, while deciduous plants display the reverse pattern.Variation of MRT among species was closely related to leaf life span, while intra-specific variation was also related to resorption efficiency. The importance of resorption of leaf resources for shoot growth did not vary between evergreen and deciduous woody plants. As compared with vascular plant life-forms, a woodland bryophyte showed a nutrient use strategy similar to that of evergreens, i.e. a long MRT. A long shoot life span and efficient internal nutrient recycling achieved this.This thesis shows that the concept of NUE offers a powerful tool to study plant strategies with respect to nutrient limitation as well as the role of plant species in the ecosystem.