H. H. Neumann

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.

Leaf area measurements based on hemispheric photographs and leaf-litter collection in a deciduous forest during autumn leaf-fall

Abstract Predicted relationships between leaf area index ( LAI ) and gap frequency using the Poisson, binomial and Markov theoretical models of canopy geometry (Nilson, 1971) were used to estimate LAI from hemispherical photographs taken during the autumn leaf-fall period in a maple-aspen forest in southern Ontario, Canada. Both the binomial and Markov models require specification of a parameter to describe the clumpiness of the leaf distributions. For the binomial model, the parameter value given by Baldocchi et al. (1985) for an oak-hickory forest in Tennessee was used, while for the Markov model, a means for estimating the required parameter was developed based on conditional gap probabilities deduced from the hemispheric photographs. LAI estimates derived from the theoretical models were compared with values obtained from leaf-litter collection as the canopy went from fully leafed to leafless ( LAI range 0–∼5.1). Comparisons were obtained for 6 days over this period. Estimates from the models were based on photographs taken at the bottom of the canopy at nine locations within a 15 × 15-m plot. Analysis of photographs was performed using a video camera and a commercial image analyser. Poisson model-derived results were appreciably less than the LAI from leaf-litter collection for values of the latter greater than ∼2, while results from the binomial and the Markov models compared favourably with leaf-litter collection for LAI >2. The apparent failure of the Poisson model at the relatively larger LAI s was attributed to the expected clumpiness of leaf distributions in deciduous forests. For LAI s less than ∼1, the estimates from all models exceeded the leaf-litter collection values, reflecting the influence of tree branches on the gap frequencies recorded in the hemispheric photographs. An estimate of 0.5 was obtained for the woody element area index based on the mean collection LAI estimates. For the Markov model, this difference showed little trend with decreasing leaf area, but for the binomial model, the corresponding difference tended to increase as LAI decreased, probably reflecting a change in the true clumpiness parameter from the full canopy value as leaf area decreased. Estimates of leaf area density profiles were also obtained. These were derived from hemispheric field-of-view photographs taken at seven levels from a tower within the forest. Only the equivalent of one photograph at each level for any measurement day could be obtained, which increased the inherent uncertainty of the estimates relative to the entire canopy estimates, which were based on nine photographs. With a little subjective smoothing of the LAI profiles produced by application of the Markov model, physically consistent leaf area density profiles were produced which appeared reasonable based on the few measurements of such profiles that have been reported.

Influence of foliar density and thermal stability on profiles of Reynolds stress and turbulence intensity in a deciduous forest

Observations were made of turbulence in an extensive deciduous forest on level terrain using a vertical array of seven three-dimensional sonic anemometer/thermometers within and above the canopy. Data were collected through the period of leaf fall and over a range of thermal stabilities. A bulk canopy drag coefficient was nearly independent of the density of the forest but decreased greatly with the onset of nocturnal stability. The depth of penetration of momentum into the forest increased with leaf fall but, again, was greatly curtailed by stable conditions. Turbulent velocities decreased with increasing depth in the forest but relative turbulence intensities increased to mid-canopy levels. Leaf density influenced turbulence levels but not as strongly as did thermal stability. Thermal effects were adequately described by the single parameter h/L, where h is the canopy height and L is the Monin-Obukhov length. The longitudinal and vertical velocity correlation coefficient was larger in magnitude than expected in the upper layers of the forest but decreased to a small value in the lowest layers where the Reynolds stress was small. The ratio Σw/u*, where u* is the local friction velocity, reflected changes in the uw correlation, becoming smaller than usual in the upper canopy layers. It is believed that these effects result from the intermittent, spatially coherent structures that are responsible for a large fraction of the momentum flux to the forest.

An investigation of the ADOM dry deposition module using summertime O3 measurements above a deciduous forest

Abstract The dry deposition module in the Acid Deposition and Oxidant Model (ADOM) has been isolated from the complete model to accept local meteorological measurements in order to provide local estimates of dry deposition velocities. These are compared with observed O 3 data obtained during the summer of 1988 over a deciduous forest. The comparison shows that the model systematically overpredicts by about 70% the dry deposition velocity for two selected days and for the average diurnal variations, obtained from the entire observed time series for July and August of 1988. The canopy resistance, to transfer of O 3 , is the major contributor to the models total resistance for O 3 . In addition to the possible errors due to the “Big Leaf” approximation, sensitivity studies show that key parameters for reducing the systematic bias in the model are the Leaf Area Index, as well as mesophyll, stomatal, ground and cuticle resistances. For dry deposition velocities of SO 2 , the model also shows sensitivity to these parameters as well as the canopy wetness. A brief discussion on some mathematical approximations in the model is provided.

The Validity of Similarity Theory in the Roughness Sublayer Above Forests

Flux-gradient relationships based upon similarity theory have been reported to severely underestimate scalar fluxes in the roughness sublayer above forests, as compared to independent flux estimates (for example, eddy covariance or energy balance measurements). This paper presents the results of a unique three-month investigation into the validity of similarity theory in the roughness sublayer above forests. Eddy covariance and flux-gradient measurements of carbon dioxide (CO2) exchange were compared above a mixed deciduous forest at Camp Borden, Ontario, both before and after leaf senescence. The eddy covariance measurements used a Li-Cor infrared gas analyzer, and the flux-gradient (similarity theory) measurements featured a tunable diode laser Trace Gas Analysis System (TGAS). The TGAS resolved the CO2 concentration difference to 300 parts per trillion by volume (ppt) based upon a half-hour sampling period. The measured enhancement factor γ (the ratio of independent flux estimates, in this case eddy covariance, to similarity theory fluxes) was smaller and occurred closer to the canopy than in most previous investigations of similarity theory. Very good agreement between the eddy covariance and similarity theory fluxes was found between 1.9 and 2.2 canopy heights (hc), and the mean enhancement factors measured before and after leaf senescence were 1.10 plusmn; 0.06 and 1.24 ± 0.07, respectively. Larger discrepancies were measured closer to the canopy (1.2 to 1.4 hc), and mean enhancement factors of 1.60 ± 0.10 and 1.82 ± 0.11 were measured before and after leaf senescence, respectively. Overall, the Borden results suggest that similarity theory can be used within the roughness sublayer with a greater confidence than previously has been believed.

Eddy correlation measurements of methane fluxes using a tunable diode laser at the Kinosheo Lake tower site during the Northern Wetlands Study (NOWES)

As part of the Canadian Northern Wetlands Study (NOWES) measurements of methane flux were made at the Kinosheo Lake tower site for a 1-month period during the 1990 summer intensive. The measurements were made with a diode-laser-based methane sensor using the eddy correlation technique. Measurements of the methane fluxes were made at two levels, 5 or 18 m. Approximately 900 half-hour average methane flux measurements were obtained. Weak temporal and diurnal trends were observed in the data. Fluxes averaged over the study period showed an overall methane emission of 16 mg CH{sub 4} m{sup {minus}2} d{sup {minus}1} with a daytime average of 20 mg CH{sub 4} m{sup {minus}2} d{sup {minus}1} and a nighttime average of 9 mg CH{sub 4} m{sup {minus}2} d{sup {minus}1}. The effect of emission footprint was evident in the data. A strong relationship between the daily average methane flux and wet bog temperature at 20-cm depth was observed. 41 refs., 6 figs.

Ozone deposition onto a deciduous forest during dry and wet conditions

Abstract Results of an experiment conducted to quantify the ozone deposition onto a deciduous forest stand in an acid-precipitation-impacted area of Canada are presented and discussed. The ozone deposition data were obtained above and within the forest canopy. The deposition process was affected by solar radiation, wind speed and ambient ozone concentration. Solar radiation was likely acting through its influence on stomatal opening and wind speed through its effects on bulk boundary layer resistance. Ozone deposition deep in the canopy was negligibly small compared with that in the upper canopy. The difference is ascribed to a larger biological sink for ozone in the upper canopy and to a lack of efficient transport in the lower canopy. Substantial ozone deposition was measured while the forest canopy remained wet with either dew or rain water, during night-time and daytime conditions. This is contrary to assumptions made in some deposition models that ozone uptake is reduced when foliage is wet.

Ambient biogenic hydrocarbons and isoprene emissions from a mixed deciduous forest

Experiments were conducted during the growing season of 1993 at a mixed deciduous forest in southern Ontario, Canada to investigate the atmospheric abundance of hydrocarbons from phytogenic origins, and to measure emission rates from foliage of deciduous trees. The most abundant phytogenic chemical species found in the ambient air were isoprene and the monoterpenes α-pinene and β-pinene. Prior to leaf-bud break during spring, ambient hydrocarbon mixing ratios above the forest remained barely above instrument detection limit (∼20 parts per trillion), but they became abundant during the latter part of the growing season. Peak isoprene mixing ratios reached nearly 10 parts per billion (ppbv) during mid-growing season while maximum monoterpene mixing ratios were close to 2 ppbv. Both isoprene and monoterpene mixing ratios exhibited marked diurnal variations. Typical isoprene mixing ratios were highest during mid-afternoon and were lowest during nighttime. Peak isoprene mixing ratios coincided with maximum canopy temperature. The diurnal pattern of ambient isoprene mixing ratio was closely linked to the local emissions from foliage. Isoprene emission rates from foliage were measured by enclosing branches of trees inside environment-controlled cuvette systems and measuring the gas mixing ratio difference between cuvette inlet and outlet airstream. Isoprene emissions depended on tree species, foliage ontogeny, and environmental factors such as foliage temperature and intercepted photosynthetically active radiation (PAR). For instance, young (<1 month old) aspen leaves released approximately 80 times less isoprene than mature (>3 months old) leaves. During the latter part of the growing season the amount of carbon released back to the atmosphere as isoprene by big-tooth and trembling aspen leaves accounted for approximately 2% of the photosynthetically fixed carbon. Significant isoprene mixing ratio gradients existed between the forest crown and at twice canopy height above the ground. The gradient diffusion approach coupled with similarity theory was used to estimate canopy isoprene flux densities. These canopy fluxes compared favorably with values obtained from a multilayered canopy model that utilized locally measured plant microclimate, biomass distribution and leaf isoprene emission rate data. Modeled isoprene fluxes were approximately 30% higher compared to measured fluxes. Further comparisons between measured and modeled canopy biogenic hydrocarbon flux densities are required to assess uncertainties in modeling systems that provide inventories of biogenic hydrocarbons.

Nocturnal mixing in a forest subcanopy

The vertical structure of the flow in the old aspen canopy in BOREAS is examined in terms of thermocouple profiles and sonic anemometers above, within, and below the aspen canopy. The data are composited for different periods in order to isolate seasonal changes of the canopy and sun angle. On clear nights, a strong surface inversion develops in the lower part of the subcanopy in contrast to more closed canopies where strong stratification does not develop in the subcanopy. On clear nights with weak winds, a second weaker inversion develops at the top of the aspen canopy. On average, the subcanopy is very stable in the early evening and becomes less stable later in the evening. This appears to be due to a general increase in wind speed above the canopy during the night. On some of the nights, the stability of the flow in and above the canopy suddenly decreases in association with cold air advection. The characteristics of these events are examined. The vertical structure of the heat and momentum flux below and above the canopy are examined. The drag coefficient for the subcanopy stress exhibits a maximum at neutral stability and systematically decreases with increasing subcanopy stability and also decreases slowly with increasing instability. Possible explanations for this unexpected decrease with instability are examined. ©2000 Elsevier Science B.V. All rights reserved.

An evaluation of the regional acid deposition model surface module for ozone uptake at three sites in the San Joaquin Valley of California

Plants and soils act as major sinks for the destruction of tropospheric ozone, especially during daylight hours when plant stomata open and are thought to provide the dominant pathway for the uptake of ozone. The present study, part of the California Ozone Deposition Experiment, compares predictions of the regional acid deposition model ozone surface conductance module with surface conductance data derived from eddy covariance measurements of ozone flux taken at a grape, a cotton, and a grassland site in the San Joaquin Valley of California during the summer of 1991. Results indicate that the model (which was developed to provide long-term large-area estimates for the eastern United States) significantly overpredicts the surface conductance at all times of the day for at least two important types of plant cover of the San Joaquin Valley and that it incorrectly partitions the ozone flux between transpiring and nontranspiring components of the surface at the third site. Consequently, the model either overpredicts or inaccurately represents the observed deposition velocities. Other results indicate that the presence of dew does not reduce the rate of ozone deposition, contradicting to model assumptions, and that model assumptions involving the dependency of stomata upon environmental temperature are unnecessary. The effects of measurement errors and biases, arising from the presence of the roughness sublayer and possible photochemical reactions, are also discussed. A simpler model for ozone surface deposition (at least for the San Joaquin Valley) is proposed and evaluated.