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 by a hazelnut understory is described. Measurements of above-aspen latent and sensible heat, short-wave and net radiation, and photosynthetically active radiation are compared to those measured above the hazelnut understory. Understory radiation measurements were made using a tram system. Energy storage at each measurement height was determined, and measurements of the soil 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 degrees C and 488.4 mm, respectively, above the 1951-1980 average -0.2 degrees C and total 462.6 mm. There was a pronounced seasonal development of leaves, with the maximum leaf area index of the hazelnut (3.3 m2 m-2) exceeding that of the aspen (2.3 m2 m-2). Beneath-aspen radiation decreased exponentially as the aspen leaf area increased, and the calculated effective 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 photosynthetically active radiation, respectively, reached the hazelnut. The diurnal energy balance at both heights showed pronounced seasonal trends. Sensible heat from the forest floor dominated during the leaf-free period, whereas latent heat from the overstory dominated during the leafed period. The fraction of the annual precipitation evaporated was 82-91%, with 67-68%, 26-28%,and 4-7% originating from the aspen, hazelnut, and soil, respectively. Over the leafed period, soil water was depleted from the root zone (0-60 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 compared well with the daily integrated difference between precipitation and eddy-covariance determined measurements of evaporation.