Introduction to the Lecture：

Contrary to earlier expectations of increased forest growth under rising temperature and atmospheric CO2 in boreal forests, growth in many Nordic forests has declined in recent years. Due to the potentially vast negative consequences for both the economy, ecology, and carbon sequestration, a better understanding of the reasons for this decline is urgently needed. Several environmental factors limit tree growth, including temperature, CO2, soil moisture, humidity of the air, and soil nutrient availability. But although the separate effects of water limitation and soil nitrogen availability are well known, it remains a challenge to predict how trees manage their combined effects under novel conditions. To address this question, we introduce a new physiological model based on optimality principles which accounts for plasticity in stomatal conductance and leaf nitrogen concentration. This allows the model not only to capture gross primary productivity (GPP) and transpiration in response to weather conditions but also effects of soil moisture and nitrogen availability. We demonstrate the accuracy of the model compared to GPP estimates from eddy flux measurements and canopy transpiration in fertilized and unfertilized pine forest in northern Sweden. We also show that the response to increased soil nitrogen availability can be captured by a reduced carbon cost of N uptake and an increased leaf area per sapwood area. The results imply that the interactive effects of water and N limitation on GPP and water use in boreal pine forest can be explained by the trees striving for an optimal balance of maximizing growth while limiting hydraulic risk. This model provides a fundamental tool for analyzing the expected future growth of Nordic forests and potential management strategies.

Profile of the Speaker:

Philippe Ciais is a senior researcher working at Laboratoire des Sciences du Climat et de l’Environnement. He is an internationally recognized expert of the global carbon cycle and greenhouse gases emissions monitoring. He is the author of over a thousand of peer-reviewed publications including many in Nature, Science and high-impact journals (cited 230000 times). He is among the top 1% most-cited scientists in the fields of Geosciences and Ecology. He mentored 50 PhD students and 60 post-doctoral fellows and obtained the European Copernicus Medal and the Silver Medal of the National Center for Scientific Research in France. Past co-chair of the Global Carbon Project and lead of the carbon cycle chapter in the 5th IPCC Report. He co-leads the Carbon Monitor initiative that provides daily near real time CO2 emissions. Philippe Ciais is an elected member of the French National Academy of Sciences and an elected foreign member of the Chinese Academy of Science.