Canopy nutrient balance predicts autumn senescence across U.S. forests.

Abstract

Terrestrial ecosystems are changing rapidly under global nutrient enrichment, yet how canopy nutrients balance shapes the seasonal end of photosynthesis, a critical control on ecosystem carbon cycling, remains unclear. Rising anthropogenic nitrogen inputs are shifting many ecosystems towards relative phosphorus limitation, with uncertain consequences for autumn phenology. Here, we combined airborne imaging spectroscopy with remotely sensed phenology to test whether canopy nutrient ratios predict spatial variations in autumn senescence across National Ecological Observatory Network sites in the United States. Higher canopy N:P is consistently associated with earlier senescence in 20 out of 25 sites, whereas higher N:K has opposite effects in 19 sites after controlling for climate, elevation, dominant species, and canopy structure. Growing-season productivity is generally related to later senescence, but only modestly mediates the direct nutrient-phenology associations. These findings add a phenological dimension to the functional impacts of nutrient imbalance on terrestrial ecosystems and demonstrate that nutrients provide predictive power beyond climate alone. Continued nitrogen deposition is expected to increase relative phosphorus limitation, a shift that could be associated with earlier senescence and shorter growing season. Incorporating canopy nutrients into phenological models may therefore improve forecasts of canopy duration and carbon fluxes under ongoing nutrient enrichment.