Experimental drought conditioning increases resilience to subsequent natural drought in Norway spruce

Key message

Exposure to water shortage causes acclimation to drought in 40-year-old Norway spruce.

Abstract

Growth and health of trees that are drought-naïve may be more affected by water shortage, which could increase the risk of mortality. This study evaluates whether drought conditioning can improve resilience to subsequent drought, where resilience is the ability to regain pre-drought levels of basal area growth. It also explores the xylem structure underlying the responses. Tree-ring samples were collected from Norway spruce trees subjected to throughfall exclusion in southern Sweden, which subsequently experienced a severe natural drought event. We compare growth and xylem structure in three treatments from a randomized complete block design: drought-conditioned trees with a 2-year artificial drought treatment and 2 years of recovery prior to the natural drought; a prolonged artificial drought that exaggerated the natural drought; and a control. Trees in the conditioned with recovery treatment were significantly more resilient than trees in the control and in the prolonged drought without recovery treatment. While not significant, drought-conditioned trees were trending toward xylem with thicker cell walls, and wider cell lumen. At the same time, the fortification of the xylem did not seem to impair long-term growth performance. These findings suggest that drought conditioning, after a period of recovery, can improve tree responses to subsequent droughts.