Microclimatological profiles in grass cover in relation to biological problems



Selected climatic profiles, as measured by adapted meteorological techniques in the Angus area, were found in general to conform with work done elsewhere either in grass or corn fields. Seasonal and diurnal differences in air-temperature stratification were related to height, density and mode of growth of crop, and solar altitude. The presence of a matted basal layer of dead grass in wild grassland, or the co-existence of a clover crop in hay and silage fields, was noted to be important in maintaining a cool, humid atmosphere next to the ground. Such a zone was favourable to insects when arid conditions obtained at higher levels. In the upper regions where more edible grass leaves extend, wind was shown to be a most significant factor. This was due to the sudden reduction of wind speed within the crop. A steep gradient of wind speed, from zero to 90 or 100 cm/sec, might thus commonly be expected in the region some 40 cm to 15 cm below the upper surface of the stand under a variety of wind and grass conditions. Such a gradient could make available to an insect so situated a considerable range of body temperatures and states of water loss during periods of insolation. Field experiments, however, suggested that convective cooling often extended beyond the depth at which radiation intensity fell off greatly, since, in grass, insect body-temperatures were frequently close to that of the surrounding air. The latter, however, was usually markedly different to that noted in a Stevenson screen. Wind conditions were also noted to be significant biologically wherever experimental procedure required the enclosure of grassland or the proximity of wind-shielding hedges or walls. The ensuing alteration of microclimate particularly as regards air temperature might seriously upset the original purpose of such experiments, a fact noted by workers investigating other crops. Whilst figures quoted might provide a guide to the order of magnitude of some of the microclimatic phenomena encountered in the Angus area due regard should be paid to the lack of uniformity in most grass stands and the sometimes rapid variability of weather conditions.