When do dendrometric rules fail? Insights from 20 years of experimental thinnings on sessile oak in the GIS Coop network

Abstract

ContextLowering stand density has been suggested to adapt forests to warmer and drier conditions. Whether common dendrometric rules used to guide growth models and support silviculture are still valid at these densities lower than usual needs be tested. This includes (1) estimating the stand growth-density relationship over wider density gradients (‘Langsaeter’s rule’), (2) testing the stability of site index, and (3) ‘Eichhorn’s rule’ (relating site index to stand growth) at lower densities. Additionally, environmental changes call for a more direct inclusion of environmental drivers in growth models. Material and methodsWe investigated the effects of relative stand density (RDI) on stand growth of even-aged stands of sessile oak. We also analyzed effects of climatic conditions to assess whether sessile oak is already at a critical stage that requires adapting forest management. We used a network of silvicultural trials designed to test the effects of maximum density gradients on stand dynamics along climatic gradients. ResultsAs predicted by Langsaeter’s rule, gross stand growth increased with RDI then saturated for RDI > 0.4–0.5, mortality began after crown closure, and diameter increment decreased non-linearly with RDI. Dominant height growth first increased with RDI then saturated for RDI > 0.8. Positive effects of RDI on both stand and height growth caused Eichhorn’s rule to be stable over a large range of RDI (0.25–1). Effects of climate predictors were weak and only visible for radial growth indices. They however suggested antagonistic effects of water and temperature constraints in summer and autumn. DiscussionLangsaeter’s and Eichhorn’s rules were found valid over a large range of RDI values. We suggest using them to guide growth models and management. However, assumptions behind the use of site index as a proxy for stand productivity were invalidated. Weak and opposed effect of temperature in summer and autumn makes the outcome of climate warming on sessile oak growth uncertain.