Reconstructing Ecosystem-Scale Vegetation Activity Across the Terrestrial Mediterranean using Tree-Ring Width Data

Connecting radial tree-growth variables with remotely-sensed vegetation indices provides a foundation for using tree-rings as proxies for ecosystem primary productivity over large space and long time scales. Here we explore the association between tree-ring width and Normalized Difference Vegetation Index (NDVI) records across the Mediterranean. In contrast with most previous tree-ring/remote sensing studies, which have focused on temperature-limited boreal and taiga environments, we assess a large network of drought-sensitive tree-ring width chronologies as proxies for ecosystem-scale `greening', which in this region is largely controlled by moisture availability across vegetation cover types. We find that precipitation, elevation, and land-cover type interact to generate a statistical relationship between radial tree growth and NDVI. Specifically, tree-ring chronologies at low-elevation dry sites are strongly correlated with NDVI during the winter (maximum) precipitation season. In these settings land cover is dominated by grass- and shrublands, suggesting tree-ring width operates as a proxy for broader ecosystem-scale vegetation activity as captured by NDVI. Interactions between climate, geography, and land cover modify the extent to which tree-ring data and NDVI are linked across the Mediterranean, and may be capitalized upon to fine-tune spatial reconstructions of vegetation activity here and in other water-limited environments.