Not always in sync: Environmental drivers of the relationship between tree radial growth and vegetation indices in...

Radial growth and canopy physiological status are key factors influencing forest biomass accumulation and carbon sink capacity. Tree-ring width (TRW) and vegetation indices (VIs) are widely used proxies to monitor these processes and serve as complementary datasets for assessing forest productivity and health. However, the relationship between TRW and VIs remains poorly understood, particularly regarding the environmental drivers underlying its variability. This study examined site-level relationships between TRW and four VIs; Normalized Difference Vegetation Index (NDVI), Chlorophyll Index Green (CIGR), Normalized Difference Water Index (NDWI), and Plant Phenology Index (PPI) each sensitive to different aspects of canopy physiological status. To identify key environmental drivers affecting these relationships including stand characteristics, climate, soil, and topography, we used Generalized Additive Models (GAMs). Results showed considerable variability in the direction and strength of TRW-VIs correlations across sites, driven mainly by spatial heterogeneity and differences among indices. GAMs revealed that stand characteristics (site index and stand age), climatic drivers (aridity index and temperature/precipitation seasonality), and topographic factors had the strongest effects on TRW-VIs relationships. These drivers influence the degree to which radial growth is synchronized with canopy status, often resulting in spatial decoupling. Additionally, our findings underscore the complexity of linking in situ growth measurements with remotely sensed VIs and caution against assuming a consistent TRW-VIs relationship across diverse environments. By advancing the understanding of environmental drivers that shape radial growth-canopy status relationships, this study contributes to more robust frameworks for assessing forest productivity and health across spatial scales.