Impacts of meteorological drought on peak vegetation productivity of grasslands from perspectives of canopy structure and leaf physiology

Abstract

Frequent drought events greatly decrease the gross primary productivity (GPP) and disturb the carbon dynamics of ecologically fragile grassland ecosystems. While GPP is controlled both by canopy structure-related fraction of absorbed photosynthetically active radiation and leaf physiology-related light use efficiency, the underlying mechanisms of drought-induced decline in photosynthetic capacity remain unclear in grassland ecosystems. Here, we used ground-observed maximum GPP based on flux tower (GPP_peak) and satellite-retrieved maximum solar-induced chlorophyll fluorescence (SIF_peak) within each year as the proxy of photosynthetic capacity of alpine grasslands (AG) and temperate grasslands (TG) in China. We first decomposed GPP_peak and SIF_peak into radiation, structure, and physiological components, and compared their relative changes in drought years versus normal years at both local and regional scales. Results showed that declines in GPP_peak and SIF_peak under drought were more pronounced in TG due to its more arid climates. In TG, canopy structure was the main component driving photosynthesis loss because of the smaller proportion of carbon allocation to leaves under drought. However, in AG, the decline in leaf physiology was the primary component. Structural equation modeling revealed that changes in structure and physiological components dominated the spatial variance of SIF_peak response to drought in TG and AG, respectively. The alteration in environmental factors under drought could explain most of the spatial variance of changes in canopy structure and leaf physiology. These findings enhance understanding of the mechanisms behind grassland photosynthetic response to drought, providing insights critical for predicting ecosystem carbon balance under climate change.