Carbon balance under dual droughts: Disentangling the effects of soil and atmospheric moisture on tree sink and source dynamics in European forests.

Atmospheric drought (high vapor pressure deficit, VPD) and soil drought frequently co-occur, yet plants respond to these stressors through partially distinct physiological pathways, potentially altering source (photosynthesis) and sink (growth and respiration) activities. Despite significant advances in understanding drought impacts on photosynthesis and plant hydraulics, the independent and interactive effects of soil vs. atmospheric drought on whole-tree carbon balance, including non-structural carbohydrate (NSC) dynamics, allocation priorities, and growth, remain insufficiently understood.
This project aims to disentangle the effects of soil and atmospheric droughts on tree sink and source dynamics in European forests, by studying leaf-to-whole-tree ecophysiological processes that drive growth, carbon sequestration and tree mortality. Research will primarily be conducted in Pfynwald (Valais, Switzerland), as part of the VPDrought experiment, where mature Scots pines (Pinus sylvestris) are exposed to increased soil drought and reduced VPD.
Our study addresses the research question: how do soil and atmospheric droughts differentially affect tree carbon uptake and allocation to growth? We hypothesize that atmospheric drought primarily limits carbon assimilation via stomatal closure, leading to reduced growth and respiration (sink activity), whereas soil drought suppresses both carbon uptake (source activity) and sink strength (growth and respiration), leading to stronger shifts in carbon allocation and early reductions in sink demand. Soil drought restricts carbon transport to belowground sinks, while atmospheric drought promotes NSC accumulation in source tissues due to stomatal closure and reduced sink demand.
**Methods:**
Four campaigns (~4 days each) will be conducted at VPDrought during the summer of 2026-2028, covering four experimental treatments: 1) soil drought + ambient air, 2) soil drought + reduced VPD, 3) irrigation + ambient air, 4) irrigation + reduced VPD. Each treatment will include 6-9 replicates.
During the campaigns, gas exchange measurements will be performed, including net photosynthesis, stomatal conductance, temperature responses of photosynthesis and respiration, and A/Ci curves, using three LI-COR systems (LI-6800). Twig water potential (pre-dawn and midday) will be measured with a Scholander pressure chamber (PMS, model 1505D). Twig, needles, and root samples will be collected across treatments for non-structural carbohydrate (NSC) analyses. Stem respiration measurements will be taken using a custom-made chamber and analyzed with the LI-COR (LI-8100A). In addition, stem radial growth (measured with dendrometers) and leaf area development (using Lidar measurements) will be utilized as proxies for sink activity and carbon allocation to structural growth. Measurements will be repeated across all years, with additional assessments in 2027 and 2028, including resin flow and phloem transport efficiency using a 13C pulse labeling approach.