Quantifying Drivers of Root Depth and Distribution in European Forests: Species, Soil, and Climate Effects

Deep rooting is a critical trait for drought tolerance, yet quantitative knowledge of root distributions across tree species and soil properties remains limited [1, 2]. This study characterises fine-root distribution patterns and maximum rooting depths in mono and mixed species forests based on ~2,000 soil profiles from Switzerland and it is intended to extend the study to the continental scale with root, tree and soil data from ICP Forests’ Level I and II plots.
Root presence was recorded semi-quantitatively along soil profiles together with maximum rooting depths and associated soil and stand properties. Species specific traits and soil properties were analysed, and root distribution curves (beta curves: Y=1-β^d) were modelled to derive species- and soil-specific rooting patterns [3]. Trait-specific beta curves were then compared and analysed for site, stand, and soil properties, such as for topographic, and climate data, focusing on profiles only deeper than 1m soil depth, to avoid skewed beta calculations.
In monospecific stands (dominant species >50% canopy cover), linear models (LMs) explained 29.7% of beta variance across profiles. Tree species identity and soil density were the strongest contributors, while mean annual precipitation exhibited pronounced non-linear effects. Model parsimony improved strongly when tree species identity was aggregated into angiosperms and gymnosperms, although explanatory power decreased slightly to 27.4% of explained beta variance. On average, angiosperms showed a more homogenous fine-root distribution pattern (median β = 0.933) than gymnosperms (median β = 0.888).
In contrast, in mixed species stands, LMs explained 22.1% of beta variance. Tree species identity and soil type emerged as the primary drivers. In comparison, mixed species stands were more difficult to analyse and interpret than monospecific stands due to their higher structural and ecological complexity. Notably, strong collinearity was observed among soil type, hydromorphic condition, and soil density in both monospecific and mixed species stands.
Subsequently, we plan to integrate data from ICP Forests sites to test whether these relationships hold across broader climatic and edaphic gradients. With these results we aim to improve mechanistic modelling of soil water availability, root water uptake, and forest development under current and future climate conditions.

References

[1] Meusburger, K., Trotsiuk, V., Schmidt-Walter, P., Baltensweiler, A., Brun, P., Bernhard, F., Gharun, M., Habel, R., Hagedorn, F., Köchli, R., Psomas, A., Puhlmann, H., Thimonier, A., Waldner, P., Zimmermann, S., & Walthert, L. (2022). Soil–plant interactions modulated water availability of Swiss forests during the 2015 and 2018 droughts. Global Change Biology, 28, 5928–5944. DOI: 10.1111/gcb.16332.

[2] Pietig, K., Kotowska, M., Coners, H., Mundry, R., & Leuschner, C. (2026). Deep rooting revisited: Comparing the rooting patterns of European beech, Sessile oak, Scots pine, and Douglas fir in sandy soil to 3.8 m depth. Forest Ecology and Management, 600, 123288. DOI: 10.1016/j.foreco.2025.123288

[3] Gale, M. R. & Grigal, D. F. (1987). Vertical root distributions of northern tree species in relation to successional status. Can. J. For. Res. 17: 829-834.