Optimaliseren van klimaatslim bosbeheer: het in balans brengen van droogteresiliëntie van bomen en microklimaatbuffering in gematigde bossen

Climate change poses major challenges to temperate forests, with rising prevalence and severity of disturbances threatening tree vitality, microclimate integrity, and biodiversity. A proposed strategy to enhance tree resilience to drought is increasing the thinning intensity, but evidence remains inconsistent, particularly in deciduous forests. Additionally, thinning alters forest structures, an important driver of the forest microclimate. These changes can have cascading effects on the rest of the forest ecosystem, such as altering forest-related species communities. Research integrating thinning effects on multiple forest aspects is, however, lacking. Therefore, this dissertation aimed to investigate the potential of thinning to balance tree growth, drought resilience, forest microclimate stability, and macroarthropod biodiversity in European beech (Fagus sylvatica) and pedunculate oak (Quercus robur) dominated forests in central Belgium. Chapter 2 investigated the effects of thinning on annual tree growth and stability of growth under drought using tree-ring analyses and historical standing stock reconstructions. Results demonstrated that growth stability remained unaffected by stand density for both European beech and pedunculate oak. Stand density reductions might dampen the interannual growth variability related to drought severity, although this effect was inconsistent. Standing stocks of ~500 m³/ha (&asymp;30 m²/ha basal area) produced the highest individual growth rates, outperforming denser and sparser stands. Thinning, thus, enhances tree growth but provides limited relief from drought stress. Chapter 3 employed daily dendrometer measurements to analyze intra-annual dynamics of tree water deficit and growth across two contrasting years. European beech showed reduced drought stress at intermediate competition levels (~30 to 35 m²/ha basal area), whereas oak remained insensitive, possibly reflecting its lower drought-susceptibility and already open stand context. Growth, however, improved consistently with reduced competition for beech, while oak growth remained stable. These results confirm the limited potential of thinning to reduce drought stress and highlight its role in supporting individual tree productivity. The inconsistent drought stress effects may be because thinning benefits can be offset by higher individual tree water requirements linked to canopy expansion and evaporative demand increases caused by rising local temperatures after thinning. Chapter 4 examined forest microclimate and macroarthropod communities in relation to stand structure. Canopy cover emerged as the primary driver of the forest's thermal buffering capacity, with closed stands strongly decoupling forest conditions from macroclimate fluctuations. Reductions in canopy cover not only weakened temperature buffering but could amplify heat extremes (canopy cover < 60%), underscoring the importance of dense canopies for climate regulation. Wet soils, associated with low basal areas, cooled the forest, albeit to a lesser extent than canopy cover. Biodiversity responses were complex and species-specific: woodlice thrived under warm and moist microclimates, while the ground beetles we caught in the forest favored structural openness but were negatively affected by higher temperatures overall. These contrasting responses highlight the role of habitat heterogeneity in supporting diverse species communities. Chapter 5 integrated the dendrometer and microclimate data in a single structural equation model. Both tree growth and soil moisture benefited from basal area reductions, whereas microclimate stability most strongly relied on canopy cover. Tree growth and soil moisture cooled forest-floor temperatures, although this cooling was limited compared to the warming associated with canopy reductions. Management trade-offs, thus, became explicit, with no configuration that could optimize all ecosystem services simultaneously. Collectively, these results reveal fundamental trade-offs: thinning enhances growth but does not consistently alleviate drought stress, while closed canopies remain crucial for microclimate buffering. Biodiversity, meanwhile, benefits from habitat heterogeneity. These trade-offs necessitate a mosaic-like approach with diversified management strategies across the landscape at various spatial scales to reconcile timber production, biodiversity, and climate regulation, creating multifunctional and resilient forest landscapes.