Assessing seasonal drought stress response in Norway spruce (Picea abies (L.) Karst.) by monitoring stem circumference and sap flow

Summer drought frequency is expected to increase with climate change in forested regions of Europe. To examine the physiological impacts of low soil moisture on Norway spruce [Picea abies (L.) Karst.], we conducted an irrigation experiment in a Norway spruce-dominated forest. We monitored sap flow (Q(s)), stem circumference and soil water potential ((w)), measured needle water potential ((l)), and estimated potential evapotranspiration (PET) in control and irrigated plots. Soil water availability influenced the response of Q(s) to PET and the impact of Q(s) on maximum daily stem shrinkage (MDS). The positive relationship between Q(s) and PET was constrained below a threshold (w) near -0.3MPa. MDS was higher beyond this threshold, for a given value of Q(s). Higher MDS and lower tree water status (W) were observed at low (w) in control plants, suggesting the lower water potential of stems' conducting tissues. Stem circumference increase (SCI) was 62% lower in control trees following the irrigation treatment. Slight SCI recovery was observed in these trees in response to early autumn rainfall, which caused W to return to its predrought state. The results demonstrate that low water availability not only reduced Q(s), W, SCI, (l) and increased MDS but also altered their mutual relations. Copyright (c) 2014 John Wiley & Sons, Ltd.