Abstract ID: 3.10720 | Accepted as Talk | Talk/Oral | TBA | TBA

Gross primary productivity (GPP) is a key driver of the current net land carbon sink. However, climate change—along with the increasing frequency of extreme events—may significantly alter GPP, particularly in mountain regions, which are very vulnerable to extreme events like drought. Understanding how the GPP responds to droughts in mountain regions is therefore crucial. Since GPP cannot be measured directly, it must be inferred via proxies or modelling, introducing substantial uncertainties, which limit the ability to predict GPP responses to climate change driven stressors such as drought. A promising proxy of GPP are measurements of carbonyl sulfide (COS) fluxes, as COS is taken up by plants in parallel to CO2 but in contrast to the latter generally not emitted. In this study, we measured COS and CO₂ fluxes of two woody mountain species, Pinus sylvestris and Juniperus communis, under controlled and varying drought conditions (soil water content ranging from 40 % to 2 %). As water stress intensified, the uptake of both COS and CO₂ declined due to reductions in stomatal conductance. These flux responses to drought were of greater magnitude in P. sylvestris than in J. communis. Interestingly, diurnal variations in GPP and autotrophic respiration were more pronounced in semi-stressed plants, which regulated stomatal conductance and closure following an optimality principle, compared to heavily stressed or control groups. The CO₂ uptake velocity decreased more rapidly than COS uptake under increasing stress in both species due to the increasing biochemical limitations of photosynthesis. The main driver of the diurnal cycle of the COS and CO2 exchange was PAR in both species and all stress levels but with increasing light saturation points as drought intensified. Daily means were mostly driven by the water availability. This study provides valuable insights into how mountain species respond to drought stress, improving our understanding of GPP dynamics under extreme conditions, essential for refining carbon cycle models and improving predictions on the impacts of climate change on terrestrial carbon sinks.