Phenological responses under compound atmospheric and soil droughts

Vegetation phenology, the timing of recurrent plant developmental events, is widely recognized as a sensitive indicator of climate change and plays a critical role in regulating ecosystem functioning and carbon uptake. A mechanistic understanding of phenological drivers is therefore essential for accurately predicting the onset and cessation of tree growing seasons and their implications for forest carbon dynamics.

While the effects of external environmental cues, such as temperature and precipitation, on vegetation phenology have been extensively studied, the role of internal physiological processes remains poorly understood. In particular, carbon source–sink activities, including photosynthetic carbon assimilation, allocation, and utilization, and the associated dynamics of non-structural carbohydrates (NSCs) across plant organs, may play a critical role in regulating phenological development. Under future climate change, the increasing frequency and intensity of compound climate extremes, particularly concurrent atmospheric and soil droughts, are expected to alter carbon assimilation, allocation, and storage. However, how these drought-induced changes in carbon source–sink dynamics influence tree phenology remains largely unknown.

To address this knowledge gap, we will leverage the unique VPDrought experimental platform in Pfynwald, Switzerland, to investigate spring phenological responses of Scots pine (Pinus sylvestris) under contrasting atmospheric and soil drought conditions. Spring phenology will be monitored through repeated manual observations and PhenoCam imagery. Complementary field campaigns will quantify needle photosynthetic performance and seasonal NSC dynamics across different plant organs. By integrating phenological observations with physiological measurements and organ-specific NSC dynamics, we aim to disentangle the individual and interactive effects of atmospheric and soil drought on spring phenology and identify the underlying carbon source–sink mechanisms.