Declining soil fertility does not increase leaf lifespan within species: evidence from the Franz Josef chronosequence, New Zealand

Leaf lifespan varies widely among plant species, from a few weeks to >40 years. This variation is associated with differences in plant form and function, and the distribution of species along resource gradients. Longer leaf lifespans increase the residence time of nutrients and are one mechanism by which plants conserve nutrients; consequently, leaf lifespan should increase within species with declining soil nutrient availability. The Franz Josef chronosequence is a series of post-glacial surfaces along which soil fertility declines strongly with increasing soil age. We used this fertility gradient to test whether leaf lifespans of six common indigenous woody species increased as soil nutrient availability declined. Leaf lifespan varied from 12.4 months in Coprosma foetidissima (Rubiaceae) to 47.1 months in Pseudopanax crassifolius (Araliaceae). These leaf lifespans sample 12% of the full range of leaf lifespans reported globally and occupy a relatively conservative portion of global leaf trait space. Contrary to our expectations, leaf lifespan of two species (Pseudopanax crassifolius and Prumnopitys ferruginea) decreased by 44-61% with increasing soil age and there were no other relationships between soil age and leaf lifespan. Across all species, leaf nutrient residence times increased by 85% for N and 90% for P with declining soil fertility, but this was caused by increased nutrient resorption efficiency rather than by increased leaf longevity. These data demonstrate that plants increase leaf nutrient resorption efficiency rather than leaf lifespan as a within- species response to long-term declines in soil fertility.

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