Observed strong atmospheric water constraints on forest photosynthesis using eddy covariance and satellite-based data across the Northern Hemisphere

Abstract

Soil water deficit and high atmospheric dryness (vapor pressure deficit, VPD) are major environmental limitations on carbon uptake of terrestrial ecosystems. However, it is still unclear how climate seasonality influences seasonal soil water supply and atmospheric water demand, and consequently limits plant photosynthesis. Here, we analyzed the impacts of the seasonal radiation-rainfall coupling on soil moisture limitations versus atmospheric dryness limitations on plant photosynthesis across the Northern Hemisphere north of 15°N, using the eddy covariance data of 83 forest sites and multiple satellite-based data. Our results show that forest photosynthesis is strongly reduced by low soil water availability that is accompanied by a high atmospheric dryness during warm seasons for sites and regions where there is a strong negative covariation between radiation and rainfall availability, which we denote as asynchronous climate. However, under climates with positive covariation between radiation and rainfall availability, i.e. synchronous climate, forest photosynthesis experiences only a small soil water stress, but tends to be limited by high atmospheric dryness during warm seasons. Both the site and regional analyses imply that atmospheric dryness exhibits stronger constraints on forest photosynthesis in synchronous climate over a larger area than in asynchronous climate across the Northern Hemisphere.