The intriguing paradox of leaf lifespan responses to nitrogen availability

Abstract

1. Here we address two main questions which have received little attention: the variability of leaf lifespan (LLS) response to nitrogen (N) availability and the frequent apparent divergence between evolutionary and plastic responses of LLS to N availability. 2. By analysing numerous ecophysiological studies conducted in laboratories and in the field, we show that: (i) there is no constancy among studies and species in the LLS response to N availability; and (ii) in contrast to what is expected in an evolutionary perspective (i.e. species with shorter LLS growing on high N conditions), species that delay leaf senescence and thus increase LLS in response to high N availability are more numerous than species that behave oppositely. 3. We propose that in a given species, N sink-source interactions in the plant and exogenous N availability regulate LLS and induce a curvilinear plastic response of LLS to N availability, the maximum LLS occurring when the strength of sink activity on endogenous N reserves is minimal. We suggest that LLS responses to N supply could depend on where the plants were located on the gradient of the strength of sink activity, which depends on soil N availability. We provide several possible explanations for why long but not short LLS are selected in N-depleted habitats, even though short LLS seems to increase certain aspects of plant fitness. Particularly, we attempt to resolve the apparent conflict between plastic and evolutionary LLS responses in the light of the relationship between LLS and nutrient source-sink interactions and also between LLS and mean residence time of N in the plant. 4. Our new concept helps to explain many of the discrepancies of plastic and evolutionary LLS responses to N availability seen in the literature. © 2011 The Authors. Functional Ecology © 2011 British Ecological Society.