Energy balance and canopy conductance of a boreal aspen forest: Partitioning overstory and understory components

Abstract

Both species showed a decrease in canopy conductance as the saturation deficit increased and both showed an increase in canopy conductance as the photosynthetic active radiation increased. There was a linear relationship between forest leaf area index and forest canopy conductance. The timing, duration, and maximum leaf area of this deciduous boreal forest was found to be an important control on transpiration at both levels of the canopy. The full-leaf hazelnut daytime mean Priestley and Taylor [1972] a coefficient of 1.22 indicated transpiration was largely energy controlled and the quantity of energy received at the hazelnut surface was a function of aspen leaf area. The full-leaf aspen daytime mean c• of 0.91 indicated some stomatal control on transpiration, with a directly proportional relationship b6tWeen forest leaf area and forest canopy conductance, varying c• during much of the season through a range very sensitive to regional scale transpiration and surface-convective boundary layer feedbacks. IntroductionThe boreal forest represents one of the world's largest yet least understood ecosystems. Of the estimated 48.5 million km 2 total land area of the world's forests, 12.0 (25%) is covered by boreal forest, second only to the 17.0 (35%) covered by tropical rain forest. Boreal forest net primary productivity (ex---1 pressed as dry matter) accounts for an estimated 9.6 Gt yr (13%) of the world's forests 73.9, exceeding both temperate deciduous 8.4 (11%) and temperate coniferous 6.5 (9%) forests [Salisbury and Ross, 1978].•University of British Columbia, Vancouver, Canada. 2Atmospheric Environment Service, Downsview, Ontario, Canada.•Yale University, New Haven, Connecticut. Continuing with the work presented by Black et al. [1996], the objectives of this paper are (1) to describe the diurnal and seasonal patterns of the aspen overstory and hazelnut understory energy balance, (2) to describe the diurnal and seasonal patterns of canopy water vapor conductances for both the aspen and the hazelnut, and (3) to relate the canopy conductances to the ambient meteorological conditions at both the canopy and the regional levels. Site DescriptionThe study site (

Carbon dioxide and energy flux partitioning between the understorey and the overstorey of a maritime pine forest during a year with reduced soil water availability

“…The evaporation from the whole forest as well as from the soil, whether it is covered or not by a photosynthetically active understorey, is generally higher in and dry air conditions (Loustau and Cochard, 1991). At our site evaporation from the understorey represents more than 70% of net radiation in dry air conditions (Table 2), as was already observed over a transpiring vegetative understorey (Blanken et al, 1997). In 'summer' and wet air conditions, these rates can even be closer to 100% whatever the soil water status is, as observed under a Jack pine stand by Baldocchi et al (2000).…”

“…The feasibility of eddy-flux measurements just above the forest floor has also been considered in various types of forests like temperate broad-leaved, boreal coniferous, coniferous and deciduous forests (e.g. Baldocchi and Vogel, 1996;Blanken et al, 1997;Lamaud et al, 2001;Launiainen et al, 2005;Wilson et al, 2000). Such measurements have two well-known limitations: (i) flux estimates are usually representative of a much smaller area than those performed above the canopy (Baldocchi, 1997;Wilson and Meyers, 2001); (ii) the underlying assumptions of the EC method are not expected to be generally valid in the conditions of highly non-stationary and turbulent flow regime prevailing there (Blanken et al, 1998;Baldocchi et al, 2000).…”

How plant functional-type, weather, seasonal drought, and soil physical properties alter water and energy fluxes of an oak–grass savanna and an annual grassland

“…With regards to comparative literature on λE/λE eq over forests, our well-watered data agree well with measurements from over a temperate deciduous forest , where λE/λE eq , on average, equals 0.70. In contrast, slightly greater values of λE/λE eq (0.91) have been reported for a boreal aspen forest (Blanken, 1997). Many plant physiologists prefer to relate soil water deficits in terms of soil water potential (ψ s ), a thermodynamic measure of water availability (Hsiao, 1973).…”

Available energy and energy balance closure at four coniferous forest sites across Europe

“…Note that the accuracy of J C relies on the quality of GPP data. Some of the energy fixed by photosynthesis is steadily released by respiration and is already accounted for in sensible heat storage change J H and heat storage change of biomass J veg (Blanken et al 1997;Oncley et al 2007). J C adds energy to the storage and should be therefore positive during day and is zero during periods without photosynthesis, i.e.…”