Search

Predicted intense and prolonged drought events challenge forest management. Thinning is debated as a silvicultural measure for reducing drought risk in densely established forest stands. We report on a thinning experiment in a 26-year-old Norway spruce stand (Picea abies), comprising of two thinning intensities and one unthinnned control. The removal of 43% (moderate thinning, MT) and 67% (heavy thinning, HT) of the initial basal area led to increased water availability during the entire three year observation period. Stand-level transpiration (Es) was decreased by about 25% upon moderate, and by about 50% upon heavy thinning during the first year after the interventions had been carried out. However, differences in Es across the treatments decreased within three years after thinning mainly due to increased single-tree transpiration and additional understory evapotranspiration at HT. Nevertheless, due to lower interception and transpiration on the thinned plots three years after treatment MT and HT still showed a substantial surplus in extractable soil water. The results showed that the main determinants concerning the extent of the mitigation effect with increasing thinning intensity were the available soil water storage capacity and the emerging understory vegetation. We conclude that repeated moderate thinning, through enhancing the water availability to the remaining trees, can mitigate drought risk in young spruce stands and thus, represent a viable silvicultural measure in anticipating possible water limitations due to climate change.

We hypothesize that reductions in stand density through thinning improve the recovery of radial stem growth in Norway spruce trees ( Picea abies ) from severe drought. However, thinning may not lead to higher relative radial growth during drought. Annual stem growth and stable carbon and oxygen isotopes in early- and latewood were assessed in trees from heavily thinned (HT), moderately thinned (MT) and un-thinned control stands at two sites in southern Germany. Physiological performance of trees as inferred from stable isotope analysis was used to interpret annual stem growth in response to the drought events in 1976 and 2003. Only in recently thinned stands, trees maintained growth probably through higher soil water availability during the drought year when compared to controls. In contrast, thinning improved the growth recovery in the years following the drought irrespective of the time span between thinning and drought. We conclude that thinning improves drought recovery response in the short and long term and should be considered as an effective management strategy to increase drought tolerance of Norway spruce stands.