Biogeoclimatic influences on tree growth releases identified by the boundary line method in beech (Fagus sylvatica L.) populations of southern Europe

Abstract

Identifying tree growth releases is crucial to understand forest stand dynamics, their drivers (including climate change), and their interactions with other long-term ecosystem processes. We tested how bioclimate and geologic substrate affect the application of boundary line release criteria, which are based on scaling percent growth change to prior growth rate on a large number of samples, and then using a nonlinear function (e.g., negative exponential) to draw the maximum release potential (“boundary line”). Tree-ring series from 36 European beech (Fagus sylvatica L.) forests, comprising more than 130 thousand observations and spanning a range of elevations in the Italian Alps (200–1500m) and Apennines (400–1900m), were used to develop boundary lines for two geographical regions (Alps and Apennines) and three bioclimatic zones (high-mountain, mountain, low-elevation) within each region. In the Apennines, two distinct boundary lines were also developed for beech stands growing on sedimentary and volcanic soils. We uncovered different amounts of maximal growth releases for equal levels of prior growth when comparing the northern (central Europe) and southern (Italy) portion of the species’ geographic distribution. High elevation sites in the Apennines and the Alps had similar boundary lines, which in both cases remained well below the regional ones because of distinctly lower growth releases. A limiting effect of site fertility on growth releases was uncovered in the Alps for old-growth stands growing on poor soils (e.g. dolomite). To quantify the influence of bioclimate on growth releases we produced disturbance chronologies for the two oldest stands pertaining to the same bioclimatic zone according to regional and bioclimatic boundary lines. Moderate and major growth releases were severely underestimated when the regional boundary line was used rather than the one specifically computed for the bioclimatic zone. Since boundary lines were affected by biogeoclimate, reconstructing disturbance events from growth releases requires detailed analysis of tree growth records according to site conditions with special attention to extreme environments.