Mitigation of drought by thinning: Short-term and long-term effects on growth and physiological performance of Norway spruce (Picea abies)

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.