VAPOR · Unveiling the impacts of vapor pressure deficit on forest productivity

Atmospheric dryness, which is often quantified as a vapor pressure deficit (VPD), exerts significant influences on vegetation productivity and global carbon cycles. VPD is expected to rise in a warming world, but how this increased atmospheric dryness will impact the carbon uptake by vegetation remains largely unknown. One major challenge is the lack of long-term measurements, which limits studies to the recent decades when satellite and flux measurements are available. Moreover, disentangling the effects of VPD from other highly correlated factors such as soil moisture is difficult. This lack of understanding hampers our ability to make skillful predictions of how future VPD changes induced by climate change will affect vegetation productivity globally.The VAPOR project seeks to address the existing knowledge gaps through a multi-timescale approach. First, the sensitivity of tree growth to VPD variations will be quantified on the decade-to-century timescale by leveraging global long-term tree-ring measurements as a proxy for forest productivity. Second, the independent effects of VPD and soil moisture on forest productivity during heatwaves will be disentangled using sub-diurnal high-resolution satellite observations from the last decades. Finally, output from global coupled climate simulations will be evaluated based on the insights from the previous findings, and emergent constraints will be applied to mitigate uncertainties in future projects of forest productivity based on relationships emerging from the multi-model ensemble and current observations. The constrained model results will then be used to evaluate the implications of future VPD changes on forest productivity and functioning. By embarking on this endeavor, I aim to deepen the understanding of the influence of VPD on vegetation productivity, while concurrently developing and improving my own research capabilities.