Spatial variability of micro-climatic conditions within a mid-latitude deciduous forest

Micro-climatic differences within forests exert important controls, notably on understory composition, wildlife habitat, and many biogeochemical proce sses. This study documents the spatial variability and temporal evolution o f micro-climatic conditions (air temperature, relative humidity, solar radi ation and wind speed) within a mid-latitude deciduous forest, over 3 growin g seasons (1995 to 1997). For all sites, in all years, conditions change ma rkedly at the start of the growing season (Days 120 to 145), simultaneous w ith onset of leaf-out and the development of full canopy cover. Below the f ully developed canopy, radiation and wind speed are significantly reduced, both in magnitude and duration, while relative humidity increases. Within t he forested ravines, spatial differences in daily maximum air temperature r ange from 0.5 to 4.1 degrees C (average of 2 degrees C) and in minimum air temperatures from 0 to 4.5 degrees C (average of 1 degrees C). Within the f orest, solar radiation varies by 100 W m(-2) (when maximum values are <200 W m(-2)), minimum relative humidity varies by 10 to 15%, and wind speed by 0.5 m s(-1). Local weather conditions have a strong influence on the spatia l and temporal variability of all the micro-climatic variables considered. In general, differences within the forest are greater on clear, sunny days. Once the canopy closes, the effects of topography and associated aspect/ge ometry dominate over micro-scale differences due to canopy structure. Less radiation, lower air temperatures, higher relative humidity, and higher win d speeds all are documented at the bottom of the ravines. The differences i n micro-climatic conditions measured within the forest are of the same orde r as those measured in previous studies contrasting open and forest sites. Such spatial variability should be considered in studies of ecological and biogeochemical processes in secondary growth deciduous forests.