Variability in radial increment can predict an abrupt decrease in tree growth during forest decline: Tree-ring patterns of Abies alba Mill. in near-natural forests

Abstract

Tree-ring patterns are indicators of a tree's responses to various stressors and may be employed to predict forest decline. The primary aim of this study was to analyse and model tree-ring growth in Abies alba Mill. trees from older (OG; A. alba trees aged 136–300 years) and younger (YG; A. alba trees aged 45–135 years) generations in Central Europe in Poland, with particular attention paid to growth trends before, during, and after growth decline. The hypothesis was as follows: the radial increment variation can predict an abrupt growth decrease during growth decline. A novel approach was introduced to analyse growth trends based on a generalised additive model (GAM). The first derivative of the GAM spline and the Bayesian confidence intervals show inflection points and periods of significant growth changes. Three clusters for A. alba trees in OG (OG-A, OG-B, and OG-C) and four clusters for A. alba trees in YG (YG-A, YG-B, YG-C, and YG-D) were distinguished. Tree-ring widths of A. alba showed minimum values between 1980 and 1985. During this period, the mean tree-ring widths of A. alba in OG and YG were less than 1.0 and 1.5 mm, respectively. Generally, in the investigated area, A. alba decline began after 1960 (1965), and A. alba recovery began after 1980 (1985). The results of this study indicate that an abrupt growth reduction is typically a consequence of tree-ring width dynamics prior to decline. The radial increment variation indicates the degree of A. alba growth decline, and when the number and size of changes in tree-ring growth dynamics increases, the probability of a low radial increment occurring during A. alba decline increases. Early detection of A. alba trees at risk of decline may facilitate efforts to enhance the resistance of stands to disturbances and climate changes.