AIR AND SOIL MICROCLIMATES OF DECIDUOUS WOODLAND COMPARED TO AN OPEN SITE

Forest microclimate has been monitored continuously for more than 3 yr at two sites in deciduous woodland at Wytham Woods, Oxford, UK. Autom atic weather stations provided hourly data for wind speed, photosynthe tically active radiation (PAR), vapor pressure deficit (VPD) and, in p articular, temperature at different heights above the ground and at di fferent depths in the soil. These data were compared with values from an open sire at the same location. Air temperatures could be up to 3 d egrees C lower under the canopy than at the open site under sunny cond itions in summer, but the differences averaged 0.6 and 0.9 degrees C d uring the summer months and barely differed over winter. Effects of fo rest cover on soil temperatures were much greater and under the woodla nd they neither fell below 0 degrees C nor rose above 20 degrees C eve n at 5 mm depth, in contrast to temperatures under bare earth and gras s. The temperature of a grass tussock at the soil surface under the ca nopy was intermediate between soil and air temperature; in comparison to grassland plants at an open site it experienced a much smaller rang e of temperatures. The differences between both soil and air temperatu re under the canopy and in the open were significantly correlated with solar radiation, presence or absence of canopy leaves and wind speed. Soil water content had no significant effect on soil temperature, par tly because thermal diffusivity was relatively insensitive to it and p artly because soil heat flux was small. Seasonal changes in canopy lea f cover caused a large peak of PAR under the canopy in spring prior to bud burst in trees and a much smaller one in the autumn after leaf fa ll. Wind speed under the canopy was also affected by the presence of l eaves and was proportionally lower in summer and autumn than winter an d spring. (C) 1998 Elsevier Science B.V.