CR 15:137-149 (2000)  -  doi:10.3354/cr015137

ABSTRACT: Micro-climatic differences within forests exert important controls, notably on understory composition, wildlife habitat, and many biogeochemical processes. This study documents the spatial variability and temporal evolution of micro-climatic conditions (air temperature, relative humidity, solar radiation and wind speed) within a mid-latitude deciduous forest, over 3 growing seasons (1995 to 1997). For all sites, in all years, conditions change markedly at the start of the growing season (Days 120 to 145), simultaneous with onset of leaf-out and the development of full canopy cover. Below the fully developed canopy, radiation and wind speed are significantly reduced, both in magnitude and duration, while relative humidity increases. Within the forested ravines, spatial differences in daily maximum air temperature range from 0.5 to 4.1°C (average of 2°C) and in minimum air temperatures from 0 to 4.5°C (average of 1°C). Within the forest, 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 spatial 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/geometry dominate over micro-scale differences due to canopy structure. Less radiation, lower air temperatures, higher relative humidity, and higher wind speeds all are documented at the bottom of the ravines. The differences in micro-climatic conditions measured within the forest are of the same order 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.

KEY WORDS: Micro-climate · Below-canopy · Mid-latitude deciduous forest