Hanshi's talk at the Ecological Society of America 2025

Hanshi’s abstract: Land surface phenology (LSP) describes the intra-annual variations of vegetation growth and is crucial for assessing the impacts of global change on terrestrial ecosystem services and functions. While temporal variations in LSP and its response to environmental changes have been widely studied, its spatial variation—particularly in fall phenology, remain understudied at continental scales. Understanding the timing of leaf senescence is crucial for monitoring growing season length and many ecosystem functions dependent on it. Canopy nutrients, such as nitrogen (N), phosphorus (P), and potassium (K), influence biological processes and could potentially affect the timing of leaf senescence. However, few studies have explicitly linked fall phenology with nutrient concentrations. This study aims to fill this knowledge gap by examining: 1) to what extent could canopy nutrients capture fall phenology across broad biogeographic gradients? and 2) whether this relationship operates through their influence on growing-season productivity (i.e., canopy nutrients-productivity-autumn phenology pathway)? We synthesized multi-source remote sensing data within a Bayesian hierarchical framework to quantify the response of fall phenology to key canopy nutrients indices, including the nitrogen: phosphorus (N:P) ratio and nitrogen: potassium (N:K) ratio by controlling other factors such as climate and canopy height across 25 forest National Ecological Observatory Network (NEON) sites. We also conducted structural equation models to quantify our interested pathway.

We find that in 80% of sites, a higher N:P ratio was associated with earlier leaf senescence at the continental scale, supporting a live fast, die young strategy: ample N promotes rapid growth and high photosynthetic rates, while P limitation shortens leaf lifespan. Conversely, a higher N:K ratio was associated with delayed leaf senescence in 60% of sites, likely because K is not the primary limiting factor, in contrast to N P ratios, where P limitation accelerates leaf aging. Additionally, we also found that canopy nutrients indirectly influence leaf aging by regulating productivity (i.e., GPP), as evidenced by six scenarios involving in N:P ratio-GPP-autumn phenology relationship and seven scenarios including in N K ratio-GPP-autumn phenology. Our findings suggest that canopy nutrients should be incorporated into spatial fall phenological models to better capture their role in fall senescence modeling. Integrating these effects will enhance our understanding of phenological changes and their feedback to the land-atmosphere interactions and biogeochemical cycles.