Direct and indirect effects of spring phenology on forest transpiration

Abstract

Vegetation phenology serves as a crucial impact on terrestrial ecosystem, yet its long-term effects on hydrological process, especially seasonal transpiration changes, remain understudied compared to the extensive vegetation-carbon coupling. Here, leveraging long-term solar-induced chlorophyll fluorescence (SIF) and satellite-retrieved vegetation transpiration (TR) datasets, we assessed the response of vegetation transpiration to phenology change during the start of the growing season (SOS) and peak of season (POS) spanning 1982‒2018 in China. By decomposing the increase in cumulative TR during SOS and POS into two parts, i.e. the TR_pheno due to earlier SOS and TR_product driven by vegetation productivity, we found that a one-day SOS advancement directly increased TR_pheno by 1.80 ± 0.44 mm, while indirectly reduced TR_product by -1.08 ± 0.80 mm. This indirect effect was due to a higher spring SIF induced by earlier SOS, which accelerated TR_pheno but also exacerbated soil moisture depletion, limiting the subsequent TR_product. The direct and indirect effects of spring phenology on TR was found to strengthen over last 37 years using a 15-year moving window analysis. Additionally, differences among plant species underscored the role of plants hydraulic traits in regulating transpiration. Needleleaf forest exhibited a smaller increase in SIF and positive relationship between SIF and soil moisture, thereby mitigating the moisture constrain on TR_product increasing (coefficient = 0.11, p < 0.05). Our findings highlighted the phenological feedback on hydrological cycle through plant transpiration processes, emphasizing the importance of accounting for plant-environmental interactions in hydrological projections under climate change.