Michael D. Novak

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

The energy balance components were measured throughout most of 1994 in and above a southern boreal aspen (Populus tremuloides Michx.) forest (53.629°N 106.200°W) with a hazelnut (Corylus cornuta Marsh.) understory as part of the Boreal Ecosystem-Atmosphere Study. The turbulent fluxes were measured at both levels using the eddy-covariance technique. After rejection of suspect data due to instationarity or inhomogeneity, occasional erratic behavior in turbulent fluxes and lack of energy balance closure led to a recalculation of the fluxes of sensible and latent heat using their ratio and the available energy. The seasonal development in leaf area was reflected in a strong seasonal pattern of the energy balance. Leaf growth began during the third week of May with a maximum forest leaf area index of 5.6 m 2 m -2 reached by mid-July. During the full-leaf period, aspen and hazelnut accounted for approximately 40 and 60% of the forest leaf area, respectively. Sensible heat was the dominant consumer of forest net radiation during the preleaf period, while latent heat accounted for the majority of forest net radiation during the leafed period. Hazelnut transpiration accounted for 25% of the forest transpiration during the summer months. During the full-leaf period (June 1 to September 7) daytime dry-canopy mean aspen and hazelnut canopy conductances were 330 mmol m -2 s -1 (8.4 mm s -1 ) (70% of the total forest conductance) and 113 mmol m -2 s -1 (2.9 mm s -1 ) (24% of the total forest conductance), respectively. Maximum aspen and hazelnut canopy conductances were 1200 mmol m -2 s -1 (30 mm s -1 ) and 910 mmol m -2 s -1 (23 mm s -1 ), respectively, and maximum stomatal conductances were 490 mmol m -2 s -1 (12.5 mm s -1 ) and 280 mmol m -2 s -1 (7 mm s -1 ), aspen and hazelnut, respectively. 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] α 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 α of 0.91 indicated some stomatal control on transpiration, with a directly proportional relationship between forest leaf area and forest canopy conductance, varying α during much of the season through a range very sensitive to regional scale transpiration and surface-convective boundary laver feedbacks.

E - ε modelling of turbulent air flow downwind of a model forest edge

A two-dimensional E-ε model, which included the effects of plant-atmosphere interaction, was used to simulate air flow downwind of forest edges for the purpose of predicting the microclimate in forest openings. A suitable set of wall functions was selected to consider the aerodynamic effects of the ground in the opening. The model with discretization and parameter schemes was validated using a set of data from a wind-tunnel experiment. The simulated wind speed and turbulence kinetic energy closely agreed with the measured values. After validation, the model was used to predict eddy diffusivity in the lee of the forest edge. The modelled spatial distribution of the eddy diffusivity agreed in general with that calculated using wind-tunnel measurements. The usefulness and limitations of the E-ε model are discussed.

Wind Tunnel And Field Measurements Of Turbulent Flow In Forests. Part I: Uniformly Thinned Stands

Many forest management methods alterstand density uniformly. The effectsof such a change on the wind andturbulence regimes in the forest arecritical to a number of processes governingthe stability of the stand and itsmicroclimate. We measured wind speed andturbulence statistics with a Dantec tri-axialhot-film probe in model forests of variousdensities (31–333 trees m-2), created byremoving whole trees in a regular pattern in awind tunnel, and compared them with similarmeasurements made with propeller anemometers insimilarly thinned plots (156–625 trees ha-1)within a Sitka spruce stand in Scotland. The results agree well, in general, with measurements made inother such studies with diverse canopy types.The systematic variations with density and verticalleaf-area distribution (which differed betweenwind-tunnel and field trees) in our work can explainmuch of the variability shown in scaled profiles ofbasic turbulence statistics reported in theliterature. The wind tunnel and field results are shown to be in good agreement overalldespite the difference in vertical leaf-areadistribution. Within-canopy and isolated-treedrag coefficients in the wind tunnel showthat tree-scale shelter effects increase astree density increases. The measurements indicatethat turbulence in the canopy is dominated bylarge-scale structures with dimensions of the sameorder as the height of the canopy as found inother studies but suggest that inter-tree spacing also modulates the size of these structures. These structures are associated with the sweeps that dominatemomentum exchange in the canopy and it is thisfact that allows the tri-axial probe to operate sowell despite the relatively narrow range of anglesin which the wind vector is correctly measured. Theratio of streamwise periodicity of these structuresto vorticity thickness varies systematically withtree density in the range 2.7–5.1, which spans theexpected range of 3.5–5 found in a laboratorymixing-layer, suggesting that tree spacing imposes another relevant length scale. This test andothers show that the results are in agreement withthe idea that canopy turbulence resembles that of a mixing layer even though they disagree with, and challenge the linear relationship between, streamwise periodicity andshear length scale presented recently in theliterature. The measurements are also in goodoverall agreement with simple drag models presented recently by other researchers.

Coherent eddies and temperature structure functions for three contrasting surfaces. Part I: Ramp model with finite microfront time

Air temperature time series within and above canopies reveal ramp patternsassociated with coherent eddies that are responsible for most of thevertical transport of sensible heat. Van Atta used a simple step-changeramp model to analyse the coherent part of air temperature structurefunctions. However, his ocean data, and our own measurements for aDouglas-fir forest, straw mulch, and bare soil, reveal that even withoutlinearization his model cannot account for the observed decrease of thecubic structure function for small time lag. We found that a ramp model inwhich the rapid change at the end of the ramp occurs in a finite microfronttime can describe this decrease very well, and predict at least relativemagnitudes of microfront times between different surfaces. Averagerecurrence time for ramps, determined by analysis of the cubic structurefunction with the new ramp model, agreed well with values determined usingthe Mexican Hat wavelet transform, except at lower levels within theforest. Ramp frequency above the forest and mulch scaled very well withwind speed at the canopy top divided by canopy height. Within the forest,ramp frequency did not vary systematically with height. This is inaccordance with the idea that large-scale canopy turbulence is mostlygenerated by instability of the mean canopy wind profile, similar to aplane mixing layer. The straw mulch and bare soil experiments uniquelyextend measurements of temperature structure functions and ramp frequencyto the smallest scales possible in the field.

Spatial and temporal variability of CO2 concentration and flux in a boreal aspen forest

In conjunction with eddy covariance measurements of CO2 fluxes at the 39.5-m height over a 21.5-m-tall boreal aspen stand in northern Saskatchewan, CO2 concentration was measured at eight heights in order to calculate net ecosystem exchange. During both leafless and full-leaf periods, daytime vertical CO2 concentration gradients above 9 m were weak (<0.2 μmol mol−1 m−1), but were strong below this height. Little change in CO2 storage in the air column below 39.5 m occurred during much of the daytime, while around sunrise and sunset CO2 storage changed mainly below 9 m. For the rest of the night, over 85% of the increase in CO2 storage occurred above 9 m. On some calm nights during the growing season, CO2 also accumulated below 9 m resulting in a sudden upward CO2 flux at 39.5 m following the resumption of mixing 2–3 hours after sunrise. A 10-day experiment was conducted to determine the spatial variability of CO2 flux in the trunk space. Two eddy covariance systems were mounted just above the understory about two tree heights apart. The correlation between CO2 fluxes were poor even under unstable (daytime) conditions, suggesting a relatively heterogeneous understory and soil. In contrast, the correlation between water vapor fluxes was high (r2 = 0.70) in unstable conditions. However, average daytime and nighttime CO2 fluxes over the 10 days agreed to within 5%. This suggests that partitioning net ecosystem exchange between overstory and understory on an hourly basis using a single-understory eddy covariance system is inadvisable; however, partitioning probably can be done quite reliably using 5-day average fluxes.

Coherent eddies and temperature structure functions for three contrasting surfaces. Part II: Renewal model for sensible heat flux

Sensible heat, latent heat, and other scalar fluxes cannot be measuredwithin short dense canopies, e.g., straw mulches, with standard approachessuch as eddy correlation, Bowen ratio-energy balance, aerodynamic, andvariance methods. However, recently developed surface renewal models, thatare based on the fact that most of the turbulent transfer within and abovecanopies is associated with large-scale coherent eddies, which are evidentas ramp patterns in scalar time series, offer a feasible solution. Wepresent a new air renewal model that calculates sensible heat flux atdifferent heights within and above a canopy from the average cubictemperature structure function, sampled at a moderate rate, and measuredaverage friction velocity. The model is calibrated and tested with datameasured above and within a Douglas-fir forest and above a straw mulch andbare soil. We show that the model describes half-hour variations ofsensible heat flux very well, both within the canopy and roughnesssublayers and in the inertial sublayer, for stable and unstable atmosphericconditions. The combined empirical coefficient that appears in the modelhas an apparently universal value of about 0.4 for all surfaces andheights, which makes application of the model particularly simple. Themodel is used to predict daytime and nighttime sensible heat flux profileswithin the straw mulch and within a small bare opening in the mulch.

Theoretical determination of the surface energy balance and thermal regimes of bare soils

An analytical theory that determines the thermal regimes in the soil and the thermal and moisture regimes in the atmosphere for bare surfaces is derived. Both soil and atmospheric thermal properties are assumed to be power functions of depth and height, respectively. Evaporation is determined using a surface resistance to vapour flow. Fourier superposition is used to represent nonsinusoidal variations in time due to effects such as variable cloud cover. The theory is in acceptable agreement with micrometeorological measurements made at two bare soil sites of contrasting surface bulk density. It is concluded that the surface resistance model for evaporation is applicable to bare soils which remain wet at depth, particularly if their surface is loosened. The theory is used to predict the diurnal thermal regimes of saturated and dry sand, loam, and peat soils.

Turbulent exchange processes within and above a straw mulch. Part II : Thermal and moisture regimes

This paper, the second of a two-part series, reports on measurements of thermal and moisture regimes, including sensible and latent heat flux densities, and the complete energy balance made above and within a 10 t ha 1 (6.6 cm high) barley-straw mulch in both normal and artificially wetted states. Soil, mulch, and air temperatures were measured with fine-wire thermocouples, the sensible heat flux was determined with an air renewal model from the cubic structure function of measured air temperature fluctuations, water-vapour pressures were measured within and above the mulch with capacitance sensors, the (energy-limited) latent heat flux below the mulch was measured with a custom-made tension-plate system and latent heat fluxes at heights above the soil surface were determined from additional measurements of weight loss or gain of the mulch elements, net radiation flux was measured above the mulch and downcoming radiation measured below the mulch with thermopile radiometers, soil heat flux was determined with a custom-made thermopile plate installed at the 1 cm depth and corrected for heat storage in the soil above the plate, and heat storage within the mulch was determined from measured mulch element temperatures. During daytime on fine summer days, the source of the most of the sensible heat flux is in the upper third of the mulch, which corresponds to where the upper-surface mulch element temperature exceeds air temperature. Sensible heat is transferred counter to the local vertical gradient near the middle of the mulch, which puts a lower limit of about 2 cm on the size of the eddies responsible for most of the transfer. Near the bottom of the mulch, the sensible heat flux is small and directed downward, in accordance with the strong air temperature inversion within the mulch. The largest source of latent heat during daytime is from the underlying soil surface except early in the morning when evaporation of dew near the top of the mulch dominates. Turbulence within the mulch enhances latent heat transfer above that due to molecular diffusion by 2‐6 times, with mild dependence on wind speed. During nighttime, sensible heat flux within the mulch was small but latent heat flux was a large component in the energy balance, which was attributed in part to the unstable conditions that then existed within the mulch. The sum of sensible, latent, soil, and storage heat fluxes (all measured independently) at all heights within and above the mulch are in good agreement with net radiation flux determined using a radiation transfer model developed for the mulch under normal (non-wetted) conditions. The largest effect of artificially wetting the mulch was on latent heat flux below the mulch which became negative, indicating condensation, during daytime. For the data after the irrigation, net radiation flux from the radiation transfer model is no longer in good agreement with the sum of sensible, latent, soil, and storage heat fluxes near the bottom of the mulch. Agreement

Turbulent exchange processes within and above a straw mulch. Part I: Mean wind speed and turbulent statistics

Mulching is a technique widely used to conserve soil and moderate its microclimate. Modelling transfer processes in mulches is limited by our lack of understanding of turbulent exchange within the mulch. This paper, the first of a two-part series, reports on measurements of wind and turbulence made above and within a 10 t ha 1 barley-straw mulch using custom-made hot-wires and a tri-axial hot-film probe. Wind regimes within the mulch during daytime (relatively high wind) and nighttime (low wind) differ greatly. During daytime, 10 min average horizontal wind speeds at all levels in the mulch (where they vary nearly exponentially with height) correlate well with (near-logarithmic profile) wind speeds above the mulch and are not affected by the strong temperature inversion existing in the mulch. During nighttime, 10 min average horizontal wind speeds within the mulch are decoupled from (poorly correlated with) wind speeds in the generally stable air above the mulch. Unstable conditions in the mulch at night lead to free convection, which explains the good correlation of 10 min average wind speeds at all heights within the mulch and the high evaporation rates we measured below the mulch. Under high wind conditions most of the drag occurs very near the top of the mulch which behaves as an aerodynamically smooth surface similar to a bare soil. Turbulence within the mulch is of high intensity and is dominated by intermittant gusts, with the extreme values described by a Gumbel distribution. The frequency of the gusts agrees reasonably well with that found for laboratory mixing layers. The wind and turbulence regimes in the mulch resemble in many ways those in plant canopies much larger in height and lower in leaf area density.

A comparison of parametrizations of canopy conductance of aspen and Douglas‐fir forests for class

Abstract Canopy conductance (gc) of an old boreal aspen forest and a west coast Douglas‐fir forest was calculated from the inversion of the Penman‐Monteith (PM) equation with above‐canopy water vapour flux measurements. Values of aspen gc agreed reasonably well with those obtained by scaling up from leafstomatal conductance measurements. Comparison of values of gc obtained from the CLASS (Canadian LAnd Surface Scheme) parametrization with values of Douglas‐fir gc in 1983 and 1984 calculated from the PM equation showed that the CLASS parametrization (based on the Jarvis‐Stewart (JS) model) worked well at high soil water potential (ψ), but underestimated gc at low ψ. In the case of the aspen forest during a wet growing season in 1994, the CLASS parametrization underestimated gc for high values of incident photosynthetic photon flux density. The effectiveness of three parametrizations of gc, developed using linear or non‐linear least squares analysis, was evaluated for the two forests. The first (based on the JS model), related gc to the product of several independent limiting functions, the second (based on the Ball‐Woo‐drow‐Berry (B WB) model related gc to the product of canopy net assimilation rate and canopy surface relative humidity divided by canopy surface CO2 concentration, and the third (based on a modified form of the BWB (MBWB) model) was the same as the second except that the relative humidity was replaced by the reciprocal of air vapour pressure deficit. For both forests, the JS parametrization gave the highest r2 and lowest root mean square (RMS) error. The RMS error of the MBWB parametrization was less than that of the BWB parametrization because the latter underestimated gc during the morning. With the incorporation of the new JS and MBWB parametrizations into CLASS, better estimates of the latent heat flux (QE) from the aspen and Douglas‐fir forests were obtained on half‐hourly and daily bases than with the original CLASS parametrization. The JS parametrization gave better estimates than the MBWB parametrization. Both models parametrized using 1994 data from the aspen forest were successfully applied to the same stand in 1996, which also had a relatively wet growing season. Both models parametrized using data from the Douglas‐fir forest were also applied to four other similar‐aged Douglas‐fir forests but with different values of the leaf area index. Under conditions of minimal water stress, better estimates of QE were obtained for three of the four forests using both parametrizations. In the case of the fourth forest, none of the parametrizations gave satisfactory estimates. This was likely because the initial conditions of soil water content and ψ used in CLASS for the gravelly soil was significantly overestimated as a result of not taking the stone content into account. For conditions of high water stress, which occurred in two of the forests, none of the parametrizations gave satisfactory estimates. However, when the ψ limiting function in the JS parametrization was replaced by that developed from measurements made in the other two forests, the JS parametrization gave reasonable estimates of QE. In the case of the MBWB parametrization, we were unable to adjust the ψ limiting function due to the lack of measurements of canopy net assimilation rate at these two sites.