Climate lessons from the study of the boreal forest

We will discuss the physical processes that affect near-surface climate over the boreal forest, using analyses from the Boreal Ecosystem-Atmosphere Study (BOREAS). The boreal forest is marked by a very large seasonal cycle with below freezing temperatures for half the year. Consequently, water is unavailable for evaporation and photosynthesis until snow and ground melt, which occurs late in Spring. As a result, in April and early May, relative humidity (RH) is a minimum, the surface sensible heat flux is large and the daytime boundary layer (BL) is very deep, because of this unavailability of water. The situation reverses in the fall, when the ground is warmer than the cooling atmosphere, and mean RH is high and BL depths low. This asymmetry between spring and fall can be seen in both seasonal and diurnal cycles, and in the surface radiation balance, because of cloud-radiation feedback. The forest is heterogeneous, and there is a marked difference in summer in daytime evaporative fraction between the conifers and the deciduous forest and fens. However, above the forest, the daytime BL has a strong homogenizing effect, and it is the dominant coniferous forest that controls the mean BL depth. The impact of recent rainfall, stored on the canopy, in the surface moss layer, and in the top soil layer can be readily seen in summer, as BL depths rise on succeeding days without rain. A comparison of a wetland and a nearby jack-pine site shows the important role of the stomatal control by conifers on transpiration. Since evaporation goes down at high net radiation and low RH for conifer sites, it is clear that the low RH and high BL depth over the boreal forest are a direct consequence of stomatal control. At night, however, both temperature, relative humidity and CO2, are quite heterogeneous under the stable BL, but both the local and seasonal interaction between the vegetation and the CO2 balance of the atmosphere can be seen in the data . We show that the uncoupling of the stable BL at night inside the forest canopy occurs at low wind speeds and high outgoing net radiation.