Estimating tree canopy water use via xylem sapflow in an old Norway spruce forest and a comparison with simulation-based canopy transpiration estimates

Abstract

Tree xylem sapflow rates of 140-year-old Norway spruce (Picea abies) were scaled to the stand level and compared to canopy transpiration predicted by the stand gas exchange model STANDFLUX. Variation in sapflux densities between individual sensors was high (coefficient of variance = 0.4) and included both variation within and between trees, but it was not different between two applied sapflow methodologies (radial flowmeter according to Granier, variable heating tissue heat balance method according to Cermak and Kucera). During the morning, a time-lag of typically 2 h elapsed between sapflow (E(f)) and predicted canopy transpiration rate (E(p)). During this time total water use was as high as 0.3 mm, which was less than the estimated capacity of easily available water in the tree canopy (0.45 mm, on average 14 L per tree). Canopy conductance derived from stand sapflow rates (g(f)) and from STANDFLUX (g(p)) was in the same range (g(tmax): 10 mm s-1), but a stronger decline with increasing vapor pressure deficit of the air (D) was observed for g(f) as compared to g(p) with current model parameterization. Tree water uptake measured by xylem sapflow was higher during spring and somewhat lower during summer compared with E(p). Seasonal sums of transpiration from April to October amounted to 108 and 103 mm season-1 for E(f) and E(p), respectively. Estimated tree water uptake during night increased with D up to 0.5 mm per dark period (on average 16 L per tree) which was 10-140 % of total daily flux. Because this flow rate did not increase with further increases in D during night, it is concluded that it reflects the refilling of easily exchangeable water in the trees rather than a rate of night transpiration.