The dependence of water potential in shoots of Picea abies on air and soil water status

Where there is sucient water storage in the soil the water potential …Wx† in shoots of Norway spruce [Picea abies (L.) Karst.] is strongly governed by the vapour pressure de®cit of the atmosphere, while the mean minimum values of Wx usually do not drop below ÿ1:5 MPa under meteorological conditions in Estonia. If the base water potential …Wb† is above ÿ0:62 MPa, the principal factor causing water de®ciency in shoots of P. abies may be either limited soil water reserves or atmospheric evaporative demand depending on the current level of the vapour pressure de®cit. As the soil dries the stomatal control becomes more ecient in preventing water losses from the foliage, and the leaf water status, in turn, less sensitive to atmospheric demand. Under drought conditions, if Wb falls below ÿ0:62 MPa, the trees' water stress is mainly caused by low soil water availability. Further declines in the shoot water potential (below ÿ1:5MPa) can be attributed primarily to further decreases in the soil water, i.e. to the static water stress.