Global Patterns of Nitrogen and Phosphorus in Reproductive and Leaf Litterfall

Forest nutrient availability, foliar nutrient concentrations, and primary productivity across the globe have been previously investigated to understand large-scale biogeochemical and ecosystem processes. While previous studies have focused on nitrogen (N) and phosphorus (P) concentrations and fluxes of foliage, the concentrations and fluxes of these elements in reproductive organs have not yet been synthesized at the global scale despite the importance of fecundity and fitness in biogeochemical cycles. Here, we tested three hypotheses related to trends in reproductive and leaf litterfall at the global scale, including two hypotheses related to concentrations of N and P, and a third hypothesis related to fluxes of N and P. The temperature/precipitation hypothesis posits that N and P availability is greatest in warm and wet climates and the substrate-age hypothesis suggests young high-latitude soils are more N limited compared to older, highly weathered soils at low latitudes that are more P limited. The productivity and fecundity hypothesis posits that both rates of primary productivity and allocation to reproductive tissues decrease from tropical to temperate and boreal latitudes, resulting in greater fluxes of N and P in tropical than temperate and boreal ecosystems. To test these hypotheses and elucidate patterns of reproductive allocation of nutrients in forest ecosystems, we present a global-scale data synthesis of both reproductive and leaf litterfall concentrations and fluxes of N and P. We found that reproductive litterfall N and P concentrations both increased with mean annual temperature and most closely aligned with the temperature/precipitation hypothesis. In support of the productivity and fecundity hypothesis, we found that reproductive litterfall fluxes of N and P also increased toward lower latitudes. Our global median estimates of the percent contribution of reproductive litterfall to leaf plus reproductive litterfall for N were 13.0% (interquartile range: 4.6–20.0%) and for P were 16.1% (6.8–25.9%). Our results demonstrate that reproductive litterfall fluxes of N and P are important components of litterfall and contribute to our understanding of the mechanisms that limit both N and P across the globe.