Changes in radial growth of earlywood in Quercus crispula between 1970 and 2004 reflect climate change

Earlywood width in Quercus crispula increased from 1970 to 2004 without changes of vessel anatomy and ring growth. The increase in diameter of a tree stem is an important indicator of forest productivity. Xylem traits, such as the number and cross-sectional area of earlywood vessels, are also critical parameters of forest growth because of the physiological and structural contribution of xylem to the growth of the tree stem. Forest productivity appears to be affected by climate change and, indeed, trees might be expected to acclimate to gradual long-term climate change. The aim of this study was to identify long-term changes in increases in stem diameter and in earlywood vessels by examining tree rings of Quercus crispula. Focusing on 20 mature specimens of Q. crispula, we examined annual ring growth from 1970 to 2004 and measured earlywood traits, namely, the width, cross-sectional area (henceforth referred to as area) and number of earlywood vessels, by digital image analysis. We developed a hierarchical Bayesian model for detection of long-term trends in these traits. We found that earlywood width, as well as the total number and area of earlywood vessels, increased during the 35 years under analysis. One possible cause of these changes might be the long-term elevation of temperatures in early spring, which determine the timing of the onset of cambial reactivation from winter dormancy. In contrast to the long-term changes, short-term, yearly changes in earlywood traits fluctuated to a smaller extent than yearly changes in tree ring width. Therefore, the observed long-term changes in earlywood appear to represent acclimation to long-term climate change.