Long-term measurements of stand water uptake in Swedish boreal forest

Long-term measurements of sap flow were performed to estimate actual transpiration of a mixed pine/spruce sub-boreal stand. There was a variation in water use for the growth periods of 1994-1996 that reflected the hydro-climatic conditions in these years. The variability of water fluxes at tree level was generally high. This forced an application of a ratio 'stand to sample trees' instead of a scaling curve when extrapolating fluxes to a stand level. For about 60-year-old-stand with the leaf area index (LAI) of 4-6 and the basal area of 29 m(2), transpiration reached the maximum of 3.6 mm and the mean of 1 mm day(-1) for about 180-day long growth period. There were differences in water use for pine and spruce. Quantitatively most important was the difference in spring periods when transpiration of pines only gradually increased to reach proportions correlated tightly with evaporative demand. On the contrary, spruce transpired in accord with the evaporative conditions already at the beginning of the growth period, provided soil moisture was not limiting the uptake. Pine seemed to be more drought-tolerant compared to spruce. Though there were some obvious differences in water use between the two species, the quantitative differences were small with respect to the seasonal water budget The experience with the long-term application of the tissue heat balance (THB) method is discussed, (C) 1999 Elsevier Science B.V. All rights reserved.