COS 38-5 - Climate sensitivity of tree growth shifts across the distributional range of a tropical tree species

Abstract: Tropical forest productivity and carbon sequestration are strongly linked to climatic variation. Tropical tree-ring studies have shown that annual tree growth is often negatively related to temperature and positively to precipitation. However, tropical dendrochronological studies are too sparse to infer climate sensitivity across large climatic gradients and cannot elucidate underlying physiological processes, while both aspects are important to predict tree growth responses to climate change. Here we apply a mechanistic modeling approach to assess climate responses at a continental scale and study the underlying physiological processes that precede these effects.

We used a source-limited, process-based tree growth model (IBTREE) to simulate climate responses of annual woody productivity (i.e., tree rings) for Toona ciliata across its distributional area. The simulated interannual variation in stem growth was validated using the mean time series of six climatically distinct tree-ring sites. Model simulations were then scaled up to include the entire climate envelope of T. ciliata based on GBIF records, yielding 174 climatically distinct sites. For each site, we assessed the sensitivity of annual stem growth variation to each climate variable and studied the underlying physiology.

We found strong correspondence between simulated and observed interannual variation in stem growth for all six tree ring sites. Across the entire distribution, the effect temperature was positive at the cooler and negative at the warmer sites. Temperature effects were modulated by rainfall, with warm and wet sites showing weaker negative effects. Rainfall effects were consistently positive but more pronounced at drier and warmer sites. Across climate space, simulated growth variation was mainly driven by rainfall at the hotter and drier edge and temperature in the cooler and wetter sites. Climate effects on stem growth were best explained by the net balance between photosynthesis and maintenance respiration, as well as the usage of reserves during months with a negative carbon balance.

In conclusion, climate sensitivity shifts across the distributional area, with strong negative temperature effects driving annual growth variation in the cooler and wetter sites and strong positive precipitation effects driving variability in the warmer and drier sites. These patterns are not only driven by net photosynthesis but also by the depletion of reserves. If tree physiology does not acclimate to the changing climate, we expect that rising temperature will further exacerbate negative temperature effects and increase water limitation in intermediate and warm sites, and may induce water limitation in sites that are currently not water limited.