Thermophilization, climatic debt, and consequent declines in primary productivity

Species have optimal environmental conditions, and ongoing climate warming is reshaping community composition. In particular, many ecosystems exhibit thermophilization, a shift toward species adapted to warmer conditions. However, this process is often slower in forests, leading to a mismatch between community composition and ambient temperature, referred to as climatic debt. Despite increasing attention, its effects on forest productivity remain unclear. Quantifying tree community responses to warming is therefore essential for predicting future forest dynamics and informing biodiversity conservation. In this study, we analyzed natural forests across Japan using data from the 3rd and 4th National Forest Inventory periods (2009-2018). We first assessed compositional consistency between survey periods using the Bray-Curtis index and excluded plots with high dissimilarity ([≥] 0.8). Species-specific thermal optima were estimated using species distribution models and used to calculate the Community Temperature Index (CTI). Thermophilization was quantified as the temporal change in CTI, while climatic debt was defined as the difference between CTI and mean annual temperature. We then examined the relationship between climatic debt and changes in aboveground biomass, used as a proxy for productivity, using linear mixed-effects models. We found a mean thermophilization rate of 0.005 {degrees}C yr-{superscript 1}. Despite this shift, climatic debt increased at an average rate of -0.022 {degrees}C yr-{superscript 1}, indicating a growing mismatch between climate warming and community thermal composition. Although thermophilization showed no statistically significant association with stand structure, it tended to vary with the proportion of small-diameter trees, suggesting the influence of multiple interacting drivers. Importantly, increasing climatic debt was significantly associated with declines in forest primary productivity, even after accounting for stand structure and regional variation. These results demonstrate that delayed thermal adjustment of tree communities can constrain forest productivity under ongoing climate warming, highlighting the importance of evaluating community-level thermal responses for sustaining forest ecosystem functioning.