Leaf life span and nutrient resorption as determinants of plant nutrient conservation in temperate-arctic regions

Nutrient conservation plays an important role in plants adapted to infertil e environments. Nutrients can be conserved mainly by extending the life spa n of plant parts and/or by minimizing the nutrient content of those parts t hat are abscissed. Together these two parameters (life span and resorption) define the mean residence time (MRT) of a nutrient. In this review we summ arize available information on nitrogen resorption and life span, and evalu ate their relationship to the MRT of nitrogen, both between and within spec ies. Abundant information with respect to nitrogen resorption efficiency an d life span is available at the leaf level. By definition, woody evergreen plants have a much longer leaf life span than species of other life-forms. Conversely, differences in resorption efficiency among life-forms or among plants in habitats differing in soil fertility appear to be small. Inter-sp ecific variation in leaf life span is much larger than intra-specific varia tion (factor of >200 compared with 2, respectively), while resorption effic iency varies by about the same magnitude at both levels (factor of 3.8 comp ared with 2.7, respectively). The importance of resorption efficiency in de termining leaf-level MRT increases exponentially towards and above the maxi mum resorption efficiency observed in nature. This effect is independent of leaf life span, which may explain the lack of life-form related difference s in resorption efficiency. When scaling up from the leaf to the whole-plan t level, fundamental differences in turnover rate among different plant org ans must be considered. Woody species invest c. 50% of their net productivi ty into their low-turnover stems, while in herbaceous species the life span of stems is only slightly longer than that of leaves. As a result, nutrien t turnover of woody (evergreen and deciduous) plants is generally lower tha n that of herbaceous species (herbs and graminoids) on a whole-plant basis. At the intra-specific level empirical data show that both biomass life spa n (i.e. the inverse of biomass loss rate) and resorption efficiency are imp ortant sources of variation in MRT. However, we argue that the relative imp ortance of resorption efficiency in explaining variation in MRT is lower at the interspecific level, whereas the reverse is true for life span. This i s because variation in MRT and life span is much larger at the inter-specif ic level compared with variation in resorption efficiency. Plant traits rel ated to nutrient conservation are discussed with respect to their implicati ons for leaf structure, plant growth, competition, succession and ecosystem nutrient cycling.