Changes in the radial growth of trees in relation to biogeomorphic processes in an old‐growth forest on flysch, Czechia

Abstract

Tree radial growth is influenced by individual tree abilities, climate, competition, disturbance regimes, as well as biogeomorphic processes – including biomechanical interactions between trees and soil. Trees are actively involved in hillslope dynamics, both responding to and affecting many (bio)geomorphic processes. Using dendrochronology, we studied feedbacks associated with tree–soil–landscape formation, specifically relationships between hillslope processes, biomechanical effects of trees in soils, tree microhabitat conditions and their morphological adaptations, in the flysch zone of the Carpathians. We visually evaluated stem shape, microhabitat conditions and the biomechanical effects of 1663 trees. Cores were taken in four growing directions from 224 individuals of European beech (Fagus sylvatica L.). In a set of 193 cross‐dated beeches, average tree‐ring widths and tree eccentricities in all directions were calculated and analysed in relation to the biogeomorphic impacts of trees. Some significant drivers of tree radial growth and sources of stem eccentricity were detected. The radial growth of trees on which deadwood was leaning was markedly limited. In contrast, trees with exposed roots expressed the highest growth rates. This clearly suggests that root exposure may not be an effect of ‘exogenous’ soil creep, but may rather result from individually intensifying tree growth due to fine‐scale disturbance dynamics. The response of biomechanical tree–soil interactions in tree radial growth weakened with increasing stem diameter, reflecting the stabilizing role of larger trees. The significance of calendar year on radial growth suggests seasonality in the dynamic component of soil creep. Tree eccentricity was observed mainly in the downslope direction, which suggests a relatively complex effect of biomechanics on stem tilting. © 2020 John Wiley & Sons, Ltd.

Hillslope Processes Affect Vessel Lumen Area and Tree Dimensions

“…The TRW variation of trees in our study gradually decreased with the increasing stem diameter, assuming the stabilization effect of trees in soil ( Šilhán, 2019 ; Kašpar et al, 2020 ). It is commonly accepted that enhanced tree stability is connected with an increasing root biomass penetrating the deeper soil horizons (e.g., Di Iorio et al, 2005 ; Šilhán, 2017 ), which can be increased by the tree weight.…”

“…Therefore, a constant decrease in hydraulic parameters across the stem appears to be a more robust proxy than a comparison of TRWs ( Supplementary Figure 5 ). This is especially true when considering the complexity of biomechanical interactions of trees in soil (mainly of smaller trees), which can force the eccentric growth in other directions than would be generally expected ( Wistuba et al, 2015 ; Malik et al, 2016 ; Kašpar et al, 2020 ).…”

“…All the samples were taken using an increment Pressler’s borer. From each tree, a total of four cores were taken, one downslope, one upslope, and two in contour directions ( Figure 1B ; as shown in ref Kašpar et al, 2020 ), at approximately 50 cm above the surface (measured from the contour). Each sample was visually checked and only samples without rot and within an estimated distance from the pith lower than 3 cm ( Applequist, 1958 ) were accepted for further analysis.…”

“…To quantify the eccentricity of radial stem growth, we calculated a variation coefficient from the widths of corresponding tree-rings (TRW variation). We chose this approach due to its dimensionless and because slow hillslope processes (such as creep) may be manifested in different parts of the stem with the changing tree size ( Kašpar et al, 2020 ); as shown in Supplementary Figure 1 .…”

“…Tree growth is mainly controlled by the climate ( Rossi et al, 2016 ) and local disturbances (e.g., Frelich, 2002 ). However, on unstable slopes, the growth of trees may be significantly disturbed by the hillslope processes (e.g., Šilhán, 2017 , 2019 ; Tumajer and Treml, 2019 ; Kašpar et al, 2020 ). Frequent stem tilting leads to a continuous restoration of the stem position toward vertical ( Harker, 1996 ; Šilhán, 2015 ).…”