Impact of thinning on leaf economics, plant hydraulics, and growth dynamics

Integrating climate change concerns into forest management strategies remains challenging. Among management strategies, thinning has been proven to alter major components underlying the carbon and water cycles over the short term. However, the functional adaptability of managed forests to cope with long-term drought remains unclear. This study aims to quantify the influence of thinning on key plant functional traits for two pine species, and their impacts on the fundamental processes of tree growth and transpiration. We conducted an experimental silvicultural trial with varying thinning intensities in two Pinus sylvestris and Pinus nigra plantations with the following specific objectives: i) to assess the impact of thinning on major traits involved in the leaf economic spectrum (LES), and hydraulic relations, and ii) to understand which of these traits reflect fundamental aspects of plant growth and transpiration, and their relations with environment. We used a combination of dendrochronological (basal area increment, BAI), sapflow (F-d), and tree ring isotope (delta C-13) data, along with key functional and physiological traits including photosynthesis (A), leaf mass area (LMA), Huber value (H-v), sapwood density (W-d), and predawn to midday water potential (Psi(pd), Psi(md)). Our results revealed species-specific responses, with P. nigra exhibiting a conservative growth strategy, whereas P. sylvestris displayed increased growth after thinning. Despite thinning influenced both BAI and wood delta C-13 with notable species-dependent variations, we evidenced a general convergence in the relationships between LES traits and Hv across species, treatments and seasons (p < 0.01). BAI responses to thinning translated into predictable changes in key hydraulic traits and adaptive changes in physiological performance. For instance, BAI was inversely related to LMA and H-v. Consistent with LES, variations in LMA influenced A. In turn, seasonal variations in LMA scaled negatively with Psi(pd), and Psi(md) (p < 0.01). Such a functional adaptation suggests that thinning promotes growth while favouring forest resilience in the long term. This study provides practical implications for the implementation of a trait-based approach into silvicultural frameworks and anticipating the consequences of climate change for managed forest ecosystems.