Litterfall N:P ratios increase throughout autumn, suggesting a temporal separation between N and P resorption

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In deciduous forests, autumn leaf fall constitutes the largest annual nutrient input to the forest floor, and is critical to the recycling of nitrogen and phosphorus. Concentrations of N and P in leaf litterfall are in large part determined by the amount of N and P resorbed by trees during leaf senescence.  Thus, plasticity in the relative resorption of N and P not only affects the flux of these nutrients to the forest floor, but their stoichiometric availability as well. Despite clear implications for the regulation of forest carbon and nutrient cycling, there are still many unknowns regarding the relationship between N and P resorption.  

Here we examine temporal shifts in N and P resorption proficiency (measured as litterfall concentration) in three tree species at the Hubbard Brook Experimental Forest. Leaf litter was collected weekly throughout the autumn for two years. Our results suggest a temporal separation between N and P resorption: most leaf N was resorbed early in the season, while P resorption occurred more gradually, leading to an increase in litter N:P ratios throughout the season. From the tree’s perspective, a temporal separation between N and P resorption could serve as a way to maintain stoichiometric balance between the two nutrients, ensuring that excess P is not resorbed relative to N supply.