Relations between Potential Evapotranspiration, Sap Flow and Maximum Daily Stem Shrinkage of Spruce Trees in Growth Stages of a Pole- and Trunk-mature Forest During Irrigation Experiment

On both plots (25 and 90 years old forests), subjected in 2009 to an irrigation experiment, significant differences between the irrigated and control group of trees were observed. Several drought periods affected transpiration, stem circumference changes and their mutual correlations. During the periods with sufficient availability of soil water, tight correlation between measured sap flow and estimated potential evapotranspiration was observed. In the irrigation period, the control groups trees showed decreased sap flow whilst the slope of their regression axis related to potential evapotranspiration was milder. Maximum daily stem shrinkage positively correlated with sap flow. With lower availability of soil water their values per a sap flow unit were increasing indicating a decrease of water potential in conducting and surrounding tissues not backward saturated by sufficient water intake. The correlations were similarly manifested in the irrigation period, more distinctly in the younger stand.