Energy and XXXX flux densities above and below a temperate broad-leaved forest and a boreal pine forest.

@entity505 @entity551 @entity544 @entity515
Fluxes of @entity544 , water vapor and energy were measured above and below a temperate broad-leaved forest and a boreal jack pine (Pinus banksiania Lamb.) forest by the eddy covariance method. The aim of the work was to examine differences between the biological and physical processes that control the fluxes of mass and energy over these disparate forest stand types. @entity505 and latent heat flux (LE) densities over the temperate broad-leaved forest were about three times larger than those observed over the boreal forest. Available energy was the key variable modulating LE over the temperate broad-leaved forest, whereas LE over the boreal jack pine stand was sensitive to variations in water @entity551 ( @entity551 ) and available energy. It was also noted that @entity551 had different impacts on transpiration rates of the two forest stands. Increasing @entity551 forced a negative feedback on jack pine transpiration, whereas transpiration rates of the well-watered broad-leaved forest responded favorably to increasing @entity551 . @entity544 flux densities over the broad-leaved forest stand were more sensitive to changes in absorbed photosynthetic photon flux density than those over the boreal forest. The efficiency of @entity515 uptake over the jack pine stand was reduced, in part, because the low leaf area of the stand caused a sizable fraction of available quanta to be absorbed by nonphotosynthetic organs, such as limbs and trunks. Over both forest stands, variations in photosynthetic photon flux density of photosynthetically active radiation (Q(P)) explained only 50 to 60% of the variance of @entity515 exchange rates. Consequently, caution should be exercised when scaling @entity505 fluxes to regional scales based on unmodified, satellite-derived indices. The more open nature of the boreal jack pine forest caused water vapor, @entity515 and heat fluxes at the forest floor to be a significant component of whole canopy mass and energy exchange rates. About 20 to 30% of net canopy mass and energy exchange occurred at the forest floor. Much smaller rates of mass and energy exchange occurred under the temperate broad-leaved forest.
