Microclimate and Energy Balances of Three Douglas Fir Forests in Southern Washington

Micrometeorology stations were installed on a scaffolding tower in 20 and 40 year-old-Douglas-fir forests, respectively, in 1998 and 1999 to continuously measure net radiation (Rn), incoming PAR, air temperature (Ta) at 6 vertical heights, and relative humidity (h) above the canopies, and at 2 m above the ground under the canopies. Additional measurements of soil temperature (Ts) and moisture (M), litter temperature (Tl) and soil heat fluxes (G) were taken at 30-min intervals. Between June and October, a 3-D sonic anemometer and a LICOR6262 IRGA (i.e., eddy-covariance system, 10 Hz) were used to measure and calculate sensible heat (H) and latent heat (L). Combined with microclimate and energy data measured on an 83 m canopy crane in a 500 year-old forest, this study focused on comparisons of individual microclimatic variables and major energy terms and enclosures. Young stands appeared to have higher M, higher temporal variation in Ta and Ts, higher h, and better energy enclosure (>82%), while the old growth had higher spatial variation, and high advective flows of major energy terms. Both diurnal and seasonal changes in microclimate and energy terms were examined. Latent heat, for example, seemed to be related to many environmental variables. An exponential relationship with (Ta-Ts)/Ts (i.e., rate of temperature change) appeared appropriate when vapor pressure deficit was less than 4 g.m-3, but this relationship was weak when VPD exceeded 4 g.m-3.