Implications of Leaf Longevity, Leaf Nutrient Re-Absorption and Translocation for the Resource Economy of Five Evergreen Plant Species

Abstract

The distribution of evergreen plants in seasonal environments coincides with low soil nutrient availability. Evergreenness has, therefore, often been considered as an adaptation to nutrient deficiency through several proposed mechanisms, among them: 1) Long internal retention time of nutrients accomplished by extended leaf longevity, combined with large fractional re-absorption of leaf nutrients from senescing leaves, leading to high assimilation of carbon per unit invested nutrient. 2) Transport of nutrients from stores in old leaves as they senesce to young leaves expanding at the same time, reducing the need for new nutrient uptake, and for construction of internal stores elsewhere. This study showed that neither of the proposed mechanisms was likely to explain the evergreenness of five species (Dryas octopetala, Rhododendron lapponicum, Empetrum hermaphoditum, Ledum palustre, and Cistus monspeliensis) of contrasting distribution (Arctic to Mediterranean) and leaf habit (synchronous and sequential leaf development). Nutrient re-absorption, except in R. lapponicum and L. palustre, was in the lower part of the range reported previously for deciduous and evergreen species. Since most species also had short mean leaf longevity (about 1.5 yr) and slow development of the new leaves during the first year, it follows that the residence time of nutrients was generally low. It is unlikely therefore that the evergreen species gained more carbon per unit nutrients than many deciduous species. Nutrients were not translocated from old to new leaves as these developed, with one possible exception (L. palustre). Removal of old leaves (i.e., leaf nutrient stores) in spring before the break of the new leaf bud led in general to slower growth but unchanged transport of nutrients to the new leaves. Thus their growth was limited by carbon rather than by nutrient shortage. High internal efficiency of nutrient use was therefore rejected as a proximate cause for the adaptation of evergreens to nutrient deficient habitats. The adaptation of at least evergreens with relatively short leaf longevity to nutrient deficiency may instead be more related to a slow pace of nutrient uptake balanced with a slow release rate of nutrients from nutrient deficiency soils.