The supply of radiative energy to the Earth and its redistribution maintain a thermal equilibrium at the surface through a combination of the radiation balance and surface energy budget:
where K↓ and K↑ are the components of incoming and reflected shortwave radiation (W m−2); L↓ and L↑ are the components of incoming and emitted longwave radiation (W m−2); Q* is the net radiation (the excess energy remaining at the surface-atmosphere interface, W m−2); H, LE, and G are the sensible, latent, and ground heat fluxes (W m−2) that redistribute the bulk of the net radiation; and ΔS is the energy storage that represents a combination of energy sinks, including heat storage within the canopy biomass, heat and water vapor storage within the canopy airspace, and the energy consumed by photosynthesis [Oke, 1987]. Total energy storage typically accounts for less than 10% of the net radiation on an hourly basis [McCaughey and Saxon, 1998; Meesters and Vugts, 1996] and is generally negligible compared to the measurement accuracy of the primary redistribution processes.
 Since incoming shortwave radiation and column average sky temperature are relatively constant over short timescales near noon under clear-sky conditions, it follows from the radiation balance (equation (2)) that under these conditions, albedo and surface temperature have the greatest influence on net radiation [Betts and Ball, 1997]:
Here α is the surface albedo, σ is the Stefan-Boltzman constant (5.67 × 10−8 W m−2 K−4), T is absolute temperature (K), and ɛ is the emissivity.
 Net radiation is determined by stand properties, insolation, and moisture availability. Since it is the intention of this study to focus on the influence of stand characteristics on net radiation specifically, we have (1) normalized net radiation for each stand by its respective value of incoming shortwave radiation (Q*/K↓) to account for changes in incoming shortwave radiation between sites, (2) restricted our period of interest to 2 hours either side of local solar noon when fluxes are largest and there are no rapid changes in the components of the surface energy budget, and (3) selected periods of similar climatic conditions (clear-sky conditions and not immediately following rain) for the black spruce and tundra site pairs. The ratio Q*/K↓ is hereinafter referred to as the “radiation efficiency” (i.e., net radiation per unit of incoming shortwave radiation) [e.g., Baldocchi et al., 2000].
 The magnitude of the net longwave radiation (L* = L↓ − L↑) contribution to the radiation balance depends on how efficient a given canopy is at shedding energy to the atmosphere (the surface-atmosphere coupling), which, in turn, is related to canopy architecture [Jarvis and McNaughton, 1986]. Some key canopy attributes that influence the efficiency of energy transfer to the near-surface atmosphere are the relative surface area, density, and shape of the canopy elements. Consequently, if fire significantly affects the canopy architecture, it could alter the efficiency of energy transfer to the atmosphere. Two trademarks of spruce canopies are a low albedo and high surface roughness. The low albedo results from efficient radiation trapping [Stenberg et al., 1995] and dark canopy elements [Jarvis et al., 1976], and the high roughness is largely attributable to the irregular crown heights and tapered shape of the coniferous crowns [Baldocchi et al., 2000]. The crown shape plays at least two key roles in surface atmosphere exchange: (1) It enables turbulent mixing throughout the entire depth of the canopy, particularly in stands of low to moderate stem density, providing a highly efficient surface-atmosphere coupling for the exchange of heat and various scalars [Oke, 1987], and (2) it enables deep penetration of insolation throughout the canopy and the understory [Baldocchi et al., 1997], such that the energy is distributed over a large surface area.
 Tower-based microclimatic and eddy covariance measurements were used to characterize the radiation balance and energy budget above unburned and recently burned black spruce and tundra ecosystems in central and western Alaska, respectively (Table 1) [Chambers and Chapin, 2002; J. Beringer et al., Partitioning of surface energy exchanges along a tundra-forest vegetation transition: The importance of canopy structure, submitted to Agricultural and Forest Meteorology, 2005]. Both fires occurred early in the summer of 1999, and in both cases, measurements were made in July, less than a month after the fires. Although instrument heights differed according to the canopy heights, identical instrumentation was used for the measurements at each site. We used 15 m towers over the spruce forest stands and 10 m towers over the tundra sites. Primary radiation measurements were made as close as practical to the top of the towers (10–14 m) to minimize the potential of shading and to maximize the surface area within the effective sensor footprint [Schmid, 1997]. At the spruce sites, measurements of net radiation and incoming shortwave radiation were also made at ∼0.20 of the mean canopy height (∼0.50 m), which was above the understory vegetation of the unburned stand. Eddy covariance measurements were made at 4–5 m above the average canopy height (or ground) over the burned and unburned spruce forest stands and at 2–3 m above the surface in the tundra sites. Budget closure errors were slightly larger over the burned stands. Overall, discrepancies were less than 0.2Q* during times of positive net radiation (neglecting storage terms), indicating that advective effects were small and the measurement technique was satisfactory.
Table 1. Summary of Site Locations, Campaign Dates, Typical Midday Insolation (K↓), and Key Site Characteristics
|Name and location||Hajdukovich Creek (63°48′N, 145°06′W)||Donnelly Flats (63°54′N, 145°44′W)||Seward Peninsula (64°50.5′N, 163°41.6′W)||Seward Peninsula (65°12.15′N, 164°18.48′W)|
|Observation period (1999)||16 July to 1 August||3 July to 1 August||28 July to 3 August||28 July to 3 August|
|Incoming shortwave, W m−2||754||754||615||615|
|Radiation efficiency, Q*/K↓||0.76 ± 0.004||0.68 ± 0.003||0.68 ± 0.027||0.76 ± 0.002|
|Albedo, α||0.09 ± 0.0004||0.07 ± 0.0005||0.19 ± 0.001||0.05 ± 0.002|
|Roughness length, zo, m||0.70||0.09||0.04||0.01|
|Normalized heat flux, H/K↓||0.40||0.43||0.20||0.27|
 A combination of a three-dimensional ultrasonic anemometer (Gill Solent, horizontally symmetric) and a closed-path infrared gas analyser (LI-COR Inc., model 6262) were used to estimate the fluxes of sensible and latent heat via the eddy covariance method [Baldocchi et al., 1988]. A 3 mm internal diameter “Bev-A-Line” intake tube was used for the gas analyzer with an aspiration rate of ∼7 L min−1 that ensured turbulent flow in the sample line, and 1.5 m of externally insulated copper tubing was placed inline to minimize temperature-induced density fluctuations [Leuning and Judd, 1996]. The observations were logged at 10 Hz with a laptop computer.
 Climate sensors were scanned every 20 s, and 10 min averages were recorded on a data logger (Campbell Scientific Inc., CR10). Incoming and reflected shortwave as well as incoming and emitted longwave radiation were measured using pairs of pyranometers (Eppley Inc., model PSP) and pyrgeometers (Eppley Inc., model PIR), respectively. An independent estimate of net radiation above each surface was made using a Frischen-type net radiometer (Radiation and Energy Balance Systems (REBS), model Q7.1) with a wind speed–dependant dome-cooling correction applied to the results [Radiation and Energy Balance Systems, 1994]. Profiles of air temperature and water vapor content beneath the level of the sonic anemometer were measured using temperature/relative humidity probes (Vaisala, model HMP45C). A separate portable infrared sensor (Everest Interscience Inc., 4000ALCS) was also available for spot surface temperature estimates. Wind speed at the radiometer height, as well as near the canopy/surface, was measured using cup anemometers (R. M. Young, 03101). Ground heat flux was estimated via the combination method [Oke, 1987] using heat flux plates (REBS, HFT3) and integrated soil temperature measurements in the soil layer above the heat flux plate (REBS, PRT) at four representative locations. Ground heat flux for each tower site was estimated using the area-weighted average of ground heat fluxes measured in each of the representative microsite types (e.g., lichen-dominated versus moss-dominated microsites). The 10 min climate data were aggregated to 30 min blocks to match the eddy covariance data. Unless otherwise stated, values reported represent an average of the available 30 min data for 2 hours on either side of local solar noon. Depending on the measurement site, observation periods varied in duration from approximately 1 to 3 weeks (Table 1).
 A vegetation survey was conducted within the measurement footprint of each tower. These surveys indicated that the burned forest and tundra sites both had a canopy architecture (vegetation height and density) that was similar to their respective unburned control sites before fire.
 The mean height of trees greater than 1 m in the unburned black spruce (Picea mariana) stand was 2.7 m. As an indication of the unevenness of the canopy height, the maximum and 90th percentile heights were 7.9 and 4.9 m, respectively. The stem density of trees greater than 1 m was 13,800 ha−1 with a mean diameter at breast height (DBH) of ∼6 cm. The predominant ground cover was feathermoss and foliose lichens. The cover of herbaceous and woody understory species averaged 60%. At the burned spruce site, resprouting growth composed less than 1% of the total ground cover. The fire was severe, so all but the most substantial of the above ground biomass was consumed, leaving only the trunks of large trees and parts of thick branches. The mean height of the standing dead trees greater than 1 m remaining after fire was 3.9 m, with a stem density of only 4500 ha−1. Typically, 2–5 cm of ash and charred organic material remained above the mineral soil horizon. Patches of exposed mineral soil were evident, as well as patches of windblown loess that had accumulated within the moss/lichen layer and had been exposed by the fire. Overall, ∼80% of the surface within the measurement footprint was charred black, and the rest was a sandy color.
 The canopy of the unburned tundra site was typically 15–30 cm tall and consisted mainly of sedges (16%) and dwarf shrubs (13%). The dominant tundra ground cover species were fruticose lichens (31%) and Sphagnum moss (24%). The tundra burn was of moderate severity. Most of the lichen, green moss, and aboveground vascular vegetation were consumed, leaving a blackened organic surface. Hot, dry weather conditions at that site in June had desiccated the uppermost 10–15 cm of vegetation and organic matter, but the subsurface material was still frozen and saturated. As a result the burning was not very deep. At the time of measurements the only live vegetation present was resprouting leaves of the sedge Eriophorum vaginatum (3% cover) and small patches of unburned Sphagnum moss. Microscale variation in soil moisture status, chiefly a result of the distribution of Sphagnum moss and tussocks of Eriophorum vaginatum, resulted in a more heterogeneous burn than in the black spruce stand.
 Roughness length (zo) over each surface was estimated in the following manner:
where z is the measurement height (m), d is the displacement height (m), U is the mean wind speed at sonic height (m s−1), k is the Von Karman constant (0.4), and u* is the friction velocity (m s−1). Data were selected that most closely represented neutral stability (−0.2 < z/L < 0.2, where L is the Obukhov length), when the mean wind speed was greater than 1 m s−1. All times reported in this paper are local Alaskan time, and unless otherwise stated, the variability quoted about mean values is the standard error.