The climatic drivers of normalized difference vegetation index and tree-ring-based estimates of forest productivity are spatially coherent but temporally decoupled in Northern Hemispheric forests

Aim: Radial growth and foliage dynamics of trees both play a significant role in the terrestrial carbon cycle. Yet, crucial knowledge gaps exist in how these two growth components are linked. Our goal is to help bridge these gaps by providing a Northern Hemispheric survey of the connections between, and drivers of, inter-annual wood and canopy–landscape dynamics and phenology. Location: Northern (>30° N) forest ecosystems. Methods: We compared a multispecies network of ca. 700 annually resolved radial tree-growth records with the global inventory modelling and mapping studies-normalized difference vegetation index (GIMMS-NDVI) estimates of foliage greenness between 1982 and 2012. Tree-ring data were assimilated into the simple process-based Vaganov–Shashkin Lite model to derive xylem phenology on a monthly basis and were contrasted against NDVI estimates of canopy phenology. We additionally determined the response of all these vegetation measures to temperature and precipitation. Results: We found broad-scale agreement in the phenology and growing season climate response between radial tree growth and seasonally integrated canopy–landscape dynamics. On a monthly basis, however, a temporal asynchrony in the climate signals at mid- and high latitudes was observed, where the strongest climate response of the NDVI record occurred around leaf flush, whereas an early- to mid-growing season signal dominated the tree-ring growth. Main conclusions: Our comprehensive study helps to elucidate the unique contributions of foliar and radial growth to terrestrial carbon cycling and the time-scales at which they operate. Although we observed that both measures have similar overall climate constraints, these two growth components are sensitive to distinct seasonal windows. Our study suggests that joint assessment of both leaf and stem growth is required to address productivity of forests and demonstrates that these seasonal sensitivities must be considered before combining and interpreting these two metrics.