The seasonal water and energy exchange above and within a boreal aspen forest

The seasonal water and energy exchange of a boreal aspen forest underlain b y a hazelnut understory is described. Measurements of above-aspen latent an d sensible heat, short-wave and net radiation, and photosynthetically activ e radiation are compared to those measured above the hazelnut understory. U nderstory radiation measurements were cn made using a tram system. Energy s torage at each measurement height was determined, and measurements of the s oil moisture, temperature, and heat flux were made using an array of probes . The mean annual air temperature and total precipitation during 1994 were 1. 2 degreesC and 488.4 mm, respectively, above the 1951-1980 average -0.2 deg reesC and total 462.6 mm. There was a pronounced seasonal development of le aves. with the maximum Leaf area index of the hazelnut (3.3 m(2) m(-2)) exc eeding that of the aspen (2.3 m(2) m(-2)). Beneath-aspen radiation decrease d exponentially as the aspen leaf area increased, and the calculated effect ive extinction coefficients decreased as the plant area index increased. At full aspen leaf 27, 23, and 20% of the above-aspen short-wave, net, and ph otosynthetically active radiation, respectively, reached the hazelnut. The diurnal energy balance at both heights showed pronounced seasonal trends. S ensible heat from the forest door dominated during the leaf-free period, wh ereas latent heat from the overstory dominated during the leafed period. Th e fraction of the annual precipitation evaporated was 82-91%, with 67-68%, 26-28%, and 4-7% originating from the aspen, hazelnut, and soil, respective ly. Over the leafed period, soil water was depleted from the root zone (0-6 0 cm depth) and accumulated between the 61-123 cm depth, overall resulting in a deficit of 34.7 mm between 0-123 cm depths. This soil water balance co mpared well with the daily integrated difference between precipitation and eddy-covariance determined measurements of evaporation. (C) 2001 Elsevier S cience B.V. All rights reserved.