Nutrient conservation plays an important role in plants adapted to infertile environments. Nutrients can be
conserved mainly by extending the life span of plant parts and/or by minimizing the nutrient content of those parts
that are abscissed. Together these two parameters (life span and resorption) define the mean residence time
(MRT) of a nutrient. In this review we summarize available information on nitrogen resorption and life span, and
evaluate their relationship to the MRT of nitrogen, both between and within species. Abundant information with
respect to nitrogen resorption efficiency and 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-specific
variation in leaf life span is much larger than intra-specific variation (factor of >200 compared with 2,
respectively), while resorption efficiency varies by about the same magnitude at both levels (factor of 3.8 compared
with 2.7, respectively). The importance of resorption efficiency in determining leaf-level MRT increases
exponentially towards and above the maximum resorption efficiency observed in nature. This effect is independent
of leaf life span, which may explain the lack of life-form related differences in resorption efficiency. When scaling
up from the leaf to the whole-plant level, fundamental differences in turnover rate among different plant organs
must be considered. Woody species invest c. 50% of their net productivity 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, nutrient turnover
of woody (evergreen and deciduous) plants is generally lower than that of herbaceous species (herbs and
graminoids) on a whole-plant basis. At the intra-specific level empirical data show that both biomass life span (i.e.
the inverse of biomass loss rate) and resorption efficiency are important sources of variation in MRT. However,
we argue that the relative importance of resorption efficiency in explaining variation in MRT is lower at the inter-specific level, whereas the reverse is true for life span. This is because variation in MRT and life span is much
larger at the inter-specific level compared with variation in resorption efficiency. Plant traits related to nutrient
conservation are discussed with respect to their implications for leaf structure, plant growth, competition,
succession and ecosystem nutrient cycling.