Stand transpiration and sapflow density in relation to weather, soil moisture and stand characteristics

Sapflow density was measured in six stands in a boreal forest in central Sweden, to assess its dependence on soil moisture and stand characteristics. The stands were mixed and pure Scots pine and Norway spruce stands, which were between 34 and 105 years old. Sapflow was measured in 12 trees per stand using the Granier method during two contrasting growing seasons; one warm and dry and one wet and cool. The canopy conductance of the stands was estimated by the inverse of the Penman-Monteith equation, using time-lag-adjusted sapflow as input. Maximum canopy conductance varied between 8 mm s(-1) and 88 mm s(-1) for the stand with the lowest and highest conductance, respectively. Transpiration was higher in the dry, warm season, mean values for the different stands ranging between 1.30 to 4.64 mm day(-1) during July to September. The corresponding range in the wet, cool season was 0.95 to 2.65 mm day(-1). Besides climatic factors, stand age, stem density and diameter explained most of the variation in sapflow density. By use of multiple regression analysis for 5-day periods it was possible to estimate sapflow density and transpiration for a larger area of the forest. This upscaled area) transpiration was compared with evaporation measured by an eddy-correlation system located centrally in the area. It was shown that areal transpiration constituted 78% of total evaporation in the warm, dry season and 52% in the wet, cool season. It was not possible to establish with confidence a critical limit for soil water at which transpiration began to be reduced, mainly because of wide scatter in the relationship between potential and actual transpiration.