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. Automatic weather stations provided hourly data for wind speed, photosynthetically active radiation (PAR), vapor pressure deficit (VPD) and, in particular, temperature at different heights above the ground and at different 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 degrees C lower under the canopy than at the open site under sunny conditions in summer, but the differences averaged 0.6 and 0.9 degrees C during the summer months and barely differed over winter. Effects of forest cover on soil temperatures were much greater and under the woodland they neither fell below 0 degrees C nor rose above 20 degrees C even at 5 mm depth, in contrast to temperatures under bare earth and grass. The temperature of a grass tussock at the soil surface under the canopy was intermediate between soil and air temperature; in comparison to grassland plants at an open site it experienced a much smaller range of temperatures. The differences between both soil and air temperature 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, partly because thermal diffusivity was relatively insensitive to it and partly because soil heat flux was small. Seasonal changes in canopy leaf 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 fall. Wind speed under the canopy was also affected by the presence of leaves and was proportionally lower in summer and autumn than winter and spring. (C) 1998 Elsevier Science B.V.