Transpiration response to soil moisture in pine and spruce trees in Sweden

Variation in transpiration and conductance between individual trees of Scots pine and Norway spruce was investigated in a mixed 50-year-old stand in central Sweden. Daily transpiration rates were measured by the tissue heat balance method on five trees of each species during a dry, warm growing season. Daytime averages of sapflow, climatic variables and soil water content were used to fit an empirical model of tree conductance for each tree. Conductance per unit needle area was about twice as high in pine as in spruce, while equal-sized trees transpired similarly in both species. Conductance generally decreased more steeply with increasing vapour pressure deficit and increased faster with increasing light in pine than in spruce, although one individual spruce behaved more like the pines. Inclusion of a linear or exponential function for air temperature improved the model for pine, but of the spruces, only one tree showed a clear temperature dependency. The response to decreasing soil water content varied widely; the spruces tended to be more sensitive to drought than the pines. When the drought was at its worst, no sapflow could be detected in some of the trees. On average, the reduction in transpiration began when ca. 80% of the extractable water in the rooting zone had been depleted.