Inconsistent intraspecific pattern in leaf life span along nitrogen-supply gradient

Leaf life span (LLS) has long been hypothesized to plastically increase with decreasing nitrogen (N) supply from soil to maximize N retention, carbon assimilation, and fitness; however, accumulating evidence shows no consistent trend. The apparent inconsistencies are explained by a recent model that assumes LLS has a hump-shaped quadratic response to the N-supply gradient. The available evidence mostly originates from comparisons of LLS at only two levels of N availability, and the hypothesis remains unanswered.

We investigated LLS of two asteraceous forbs (Adenocaulon himalaicum and Xanthium canadense) experimentally grown at eight levels of N supply, which covered a range of N supply in their natural habitats. We additionally conducted a literature search to retrieve studies reporting LLS response along an N-supply gradient.

The LLS of neither species showed a hump-shaped response along the N-supply gradient. Past studies examining the LLS of an aquatic forb and terrestrial shrubs and trees along the N-supply gradient (more than four levels of N supply) also refuted the hypothesis.

The LLS of a single species exhibited neither an increase nor a hump-shaped response to decreased N supply in a variety of life forms. Comparisons at only a few N levels are misleading with regard to LLS response to N supply.