Canopy Exchange Model Research Articles

The Combined Impact of Canopy Stability and Soil NOx Exchange on Ozone Removal in a Temperate Deciduous Forest

AbstractDry deposition is an important ozone sink that impacts ecosystem carbon and water cycling. Ozone dry deposition in forests is regulated by vertical transport, stomatal uptake, and non‐stomatal processes including chemical removal. However, accurate descriptions of these processes in deposition parameterizations are hindered by sparse observational constraints on individual sink terms. Here we quantify the contribution of canopy‐atmosphere turbulent exchange and chemical ozone removal by soil‐emitted nitric oxide (NO) to ozone deposition in a North‐Italian broadleaf deciduous forest. We apply a multi‐layer canopy exchange model to interpret campaign observations of nitrogen oxides (NOx = NO + NO2) and ozone exchange above and inside the forest canopy. Two state‐of‐science parameterizations of in‐canopy vertical diffusivity, based on above‐canopy wind speed or stability, do not reproduce the observed exchange suppressed by canopy‐top radiative heating, resulting in overestimated dry deposition velocities of 10%–19% during daytime. Applying observation‐derived vertical diffusivities in our simulations largely resolves this overestimation. Soil emissions are an important NOx source despite the observed high background NOx levels. Soil NOx emissions decrease the gradient between canopy and surface layer NOx mixing ratios, which suppresses simulated NOx deposition by 80% compared to a sensitivity simulation without soil emissions. However, a sensitivity analysis shows that the enhanced chemical ozone sink by reaction with soil‐emitted NO is offset by increased vertical ozone transport from aloft and suppressed dry deposition. Our results highlight the need for targeted observations of non‐stomatal ozone removal and turbulence‐resolving deposition simulations to improve quantification and model representation of forest ozone deposition.

Atmospheric iodine, selenium and caesium depositions in France: II. Influence of forest canopies

Estimation of the canopy influence on atmospheric inputs of iodine (I), selenium (Se) and caesium (Cs) in terrestrial ecosystems is an essential condition for appropriate biogeochemical models. However, the processes involved in rain composition modifications after its passage through forest canopy have been barely studied for these elements. We monitored I, Se and Cs concentrations in both rainfall and throughfall of fourteen French forested sites throughout one year, and estimated dry deposition and canopy exchange fluxes for these elements, as well as speciation of I and Se. Comparison of rainfall and throughfall elemental composition highlighted an important impact of forest canopy on both (i) concentrations and fluxes of I, Se and Cs, and (ii) I and Se species. For the three elements, most of their throughfall concentrations were higher than corresponding rainfall. The increase of throughfall elemental fluxes was mostly due to dry deposition for I and Se although the canopy exchange model revealed some sorption within the canopy in most cases; for Cs, foliage leaching was most influencing. Regarding speciation, iodine species in rainfall were highly modified by forest canopy with an important increase of unidentified I proportion in throughfall (on average 49 and 82% in rainfall and throughfall, respectively), possibly due to washoff of dry deposition and/or to transformation into organic forms. Similarly, while rainfall was composed of 26–54% of inorganic Se, inorganic species were undetectable in throughfall. This dataset represents key information to improve modelling of I, Se and Cs cycling within forest ecosystems.

Measurement report: Leaf-scale gas exchange of atmospheric reactive trace species (NO<sub>2</sub>, NO, O<sub>3</sub>) at a northern hardwood forest in Michigan

Abstract. During the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) campaign from 21 July to 3 August 2016, field experiments on leaf-level trace gas exchange of nitric oxide (NO), nitrogen dioxide (NO2), and ozone (O3) were conducted for the first time on the native American tree species Pinus strobus (eastern white pine), Acer rubrum (red maple), Populus grandidentata (bigtooth aspen), and Quercus rubra (red oak) in a temperate hardwood forest in Michigan, USA. We measured the leaf-level trace gas exchange rates and investigated the existence of an NO2 compensation point, hypothesized based on a comparison of a previously observed average diurnal cycle of NOx (NO2+NO) concentrations with that simulated using a multi-layer canopy exchange model. Known amounts of trace gases were introduced into a tree branch enclosure and a paired blank reference enclosure. The trace gas concentrations before and after the enclosures were measured, as well as the enclosed leaf area (single-sided) and gas flow rate to obtain the trace gas fluxes with respect to leaf surface. There was no detectable NO uptake for all tree types. The foliar NO2 and O3 uptake largely followed a diurnal cycle, correlating with that of the leaf stomatal conductance. NO2 and O3 fluxes were driven by their concentration gradient from ambient to leaf internal space. The NO2 loss rate at the leaf surface, equivalently the foliar NO2 deposition velocity toward the leaf surface, ranged from 0 to 3.6 mm s−1 for bigtooth aspen and from 0 to 0.76 mm s−1 for red oak, both of which are ∼90 % of the expected values based on the stomatal conductance of water. The deposition velocities for red maple and white pine ranged from 0.3 to 1.6 and from 0.01 to 1.1 mm s−1, respectively, and were lower than predicted from the stomatal conductance, implying a mesophyll resistance to the uptake. Additionally, for white pine, the extrapolated velocity at zero stomatal conductance was 0.4±0.08 mm s−1, indicating a non-stomatal uptake pathway. The NO2 compensation point was ≤60 ppt for all four tree species and indistinguishable from zero at the 95 % confidence level. This agrees with recent reports for several European and California tree species but contradicts some earlier experimental results where the compensation points were found to be on the order of 1 ppb or higher. Given that the sampled tree types represent 80 %–90 % of the total leaf area at this site, these results negate the previously hypothesized important role of a leaf-scale NO2 compensation point. Consequently, to reconcile these findings, further detailed comparisons between the observed and simulated in- and above-canopy NOx concentrations and the leaf- and canopy-scale NOx fluxes, using the multi-layer canopy exchange model with consideration of the leaf-scale NOx deposition velocities as well as stomatal conductances reported here, are recommended.

Modelling the effect of the 2018 summer heatwave and drought on isoprene emissions in a UK woodland.

Projected future climatic extremes such as heatwaves and droughts are expected to have major impacts on emissions and concentrations of biogenic volatile organic compounds (bVOCs) with potential implications for air quality, climate and human health. While the effects of changing temperature and photosynthetically active radiation (PAR) on the synthesis and emission of isoprene, the most abundant of these bVOCs, are well known, the role of other environmental factors such as soil moisture stress are not fully understood and are therefore poorly represented in land surface models. As part of the Wytham Isoprene iDirac Oak Tree Measurements campaign, continuous measurements of isoprene mixing ratio were made throughout the summer of 2018 in Wytham Woods, a mixed deciduous woodland in southern England. During this time, the United Kingdom experienced a prolonged heatwave and drought, and isoprene mixing ratios were observed to increase by more than 400% at Wytham Woods under these conditions. We applied the state-of-the-art FORest Canopy-Atmosphere Transfer canopy exchange model to investigate the processes leading to these elevated concentrations. We found that although current isoprene emissions algorithms reproduced observed mixing ratios in the canopy before and after the heatwave, the model underestimated observations by ~40% during the heatwave-drought period implying that models may substantially underestimate the release of isoprene to the atmosphere in future cases of mild or moderate drought. Stress-induced emissions of isoprene based on leaf temperature and soil water content (SWC) were incorporated into current emissions algorithms leading to significant improvements in model output. A combination of SWC, leaf temperature and rewetting emission bursts provided the best model-measurement fit with a 50% improvement compared to the baseline model. Our results highlight the need for more long-term ecosystem-scale observations to enable improved model representation of atmosphere-biosphere interactions in a changing global climate.

Modelling bidirectional fluxes of methanol and acetaldehyde with the FORCAsT canopy exchange model

Abstract. The FORCAsT canopy exchange model was used to investigate the underlying mechanisms governing foliage emissions of methanol and acetaldehyde, two short chain oxygenated volatile organic compounds ubiquitous in the troposphere and known to have strong biogenic sources, at a northern mid-latitude forest site. The explicit representation of the vegetation canopy within the model allowed us to test the hypothesis that stomatal conductance regulates emissions of these compounds to an extent that its influence is observable at the ecosystem scale, a process not currently considered in regional- or global-scale atmospheric chemistry models.We found that FORCAsT could only reproduce the magnitude and diurnal profiles of methanol and acetaldehyde fluxes measured at the top of the forest canopy at Harvard Forest if light-dependent emissions were introduced to the model. With the inclusion of such emissions, FORCAsT was able to successfully simulate the observed bidirectional exchange of methanol and acetaldehyde. Although we found evidence that stomatal conductance influences methanol fluxes and concentrations at scales beyond the leaf level, particularly at dawn and dusk, we were able to adequately capture ecosystem exchange without the addition of stomatal control to the standard parameterisations of foliage emissions, suggesting that ecosystem fluxes can be well enough represented by the emissions models currently used.

Scalar Concentration Profiles in the Canopy and Roughness Sublayer

The mean flow and scalar concentration profiles within and above a tall canopy are well known to violate the standard boundary-layer flux-gradient relationships. We present a theory for the scalar concentration profile that is comprised of a canopy exchange model coupled to a modified surface-layer model. The coupling between the two components and the modifications to the surface-layer profiles are formulated through the mixing-layer analogy for the flow at canopy top. This analogy provides an additional length scale—the vorticity thickness—upon which the profiles depend and a set of criteria that allows a reduction in the empiricism associated with earlier forms in the literature. Predictions of the mean scalar concentration profiles are shown to match observations over a wide range of diabatic stabilities for both potential temperature and water vapour.

Dry deposition of particles and canopy exchange: Comparison of wet, bulk and throughfall deposition at five forest sites in Italy

The contribution of dry deposition to the total atmospheric input of acidifying compounds and base cations is of overwhelming importance. Throughfall measurements provide an estimate of the total deposition to forest soils, including dry deposition, but some uncertainties, related to the canopy interaction processes, affect this approach. We compared the concentrations and the fluxes of the main ions determined in wet-only, bulk and throughfall samples collected at five forest sites in Italy. The contribution of coarse particles deposited onto the bulk samplers was of prime importance for base cations, representing on average from 16% to 46% of the bulk deposition. The extent of this dry deposition depended on some geographical features of the sites, such as the distance from the sea and the annual rainfall. The possibility of applying specific bulk/wet ratios to estimate the wet deposition proved to be limited by the temporal variability of these ratios, which must be considered together with the spatial variability. A direct comparison of the dry contribution deriving from the bulk–wet and the throughfall–wet demonstrated that an extensive natural surface (forest canopy) performs better than a small synthetic surface (funnel of the bulk sampler) in collecting dry deposition of SO 4 2−, NO 3 − and Na +. The canopy exchange model was applied to both bulk and wet data to estimate the contribution of dry deposition to the total input of base cations, and the uncertainty associated to the model discussed. The exclusive use of bulk data led to a considerable underestimation of base cation dry deposition, which varies among the study sites.

Uncertainties in measurement and modelling of net ecosystem exchange of a forest

At Hyytiälä, SMEAR II measurement station in Southern Finland, the eddy covariance (EC) measurements of ecosystem exchange were performed above a Scots pine forest with two closely positioned flux measurement systems. The separation distance of about 30 m between the two systems surrounded by a homogeneous stand of the radius 200 m implies practically the same micrometeorological flux measurement source area, but partial decorrelation and different observations of turbulence and fluxes due to spatial separation. Thus, these fluxes provide two not fully independent estimates of net ecosystem exchange (NEE) of the same forest. NEE, obtained from the EC measurements, was intercompared with modelled values for periods consisting of several months of observations. The measurements as well as model values were studied for random errors as well as systematic differences. The process-based canopy exchange model was as successful as direct EC measurements in estimation of NEE in terms of random errors and systematic biases in NEE and serves as a useful tool for studying the canopy processes and gap-filling of missing values in measurements. As the EC technique is not free of possible error sources, modelling is also useful for evaluation of flux measurements. The important conclusion of current study is that the main uncertainty of long-term ecosystem NEE does not possibly result from underestimation of night-time observations as commonly believed, but could be instead related to day-time observations. For the given site, the yearly uncertainty of annual NEE was estimated to be 80 gC m −2 year −1, originating mainly from the uncertainty of day-time observations.

Atmospheric N- and S-fluxes to a spruce forest—Comparison of inferential modelling and the throughfall method

The vertical transport of reactive nitrogen gases such as NH 3 and HNO 3, and of NH 4 and NO 3 particle fluxes, and the dry deposition of SO 2 and sulphate particles was studied within the BMBF-project VERTIKO (Vertical Transport under Complex Natural Conditions). An inferential model, based on measurements of meteorological and air chemistry parameters and the throughfall method, using wet precipitation and throughfall measurements as input data for the canopy budget model, were used for dry deposition estimates. The deposition calculations were performed with the newly compiled SVAT model SPRUCEDEP. This model uses 0.5 h meteorological data, and ambient air concentration figures with a time resolution of 0.5 h (SO 2, NO, NO 2) and 2 weeks (HNO 2, HNO 3, NH 3, particulate NO 3 - , SO 4 2 - , and NH 4 + ). Compensation points in the inferential model were assumed to be 0 ppb v for NH 3 and 1.7 ppb v for NO x . Annual dry deposition rates were estimated for oxidised N-species (NO, NO 2, HNO 2, HNO 3, pNO 3) to amount to about 6.1 kg ha −1, for reduced N (NH 3, pNH 4) to 6.4 kg ha −1, and for S (SO 2, pSO 4) to 10.1 kg ha −1 for the years 2002–2004. The relative contribution of dry to total (dry+wet) deposition was 63% for sulphur, 61% for oxidised nitrogen, and 57% for reduced nitrogen. Total S-deposition and oxidised N-species showed good agreement with data from throughfall measurements and a canopy exchange model. For reduced N-species, the modelling approach yielded a considerably higher deposition in the vegetation period which could be explained by the inability of canopy throughfall measurements to account for stomatal uptake of NH 3 and probably microbial utilisation. The good agreement of the two methods suggests that a combination of the two modelling approaches may be an alternative to cost-intensive micrometeorological measurements for the determination of dry deposition fluxes on weekly or biweekly basis and long-term monitoring of total deposition.

Comparing independent estimates of carbon dioxide exchange over 5 years at a deciduous forest in the southeastern United States

At a deciduous forest in the southeast United States (Walker Branch Watershed, Oak Ridge, Tennessee), as at other sites with tall vegetation and/or complex terrain, it is difficult to temporally integrate eddy covariance data to obtain long‐term estimates of net ecosystem exchange of carbon dioxide (NEE), primarily because of suspected systematic nocturnal errors. Therefore, although eddy covariance data can be invaluable, additional tools such as empirical gap‐filling methods, independent measurements of CO2 flux using chambers, and simulations using canopy process models are often necessary to obtain reliable annual carbon uptake estimates. Two independent approaches for estimating annual NEE using these tools at the Walker Branch site are discussed. One approach is to cumulatively sum the full set of eddy covariance measurements over time. The second approach is to sum the output of NEE from a biophysical canopy exchange model (CANOAK). CANOAK incorporates independent chamber measurements on leaves, soil, and stems and is driven using the observed canopy architecture, meteorology, soil water content, and soil temperatures to predict NEE. Both methods estimate similar trends and magnitudes of daytime (daylight hours) soil respiration and NEE over 5 years. Both methods also suggest similar differences among years (interannual variability). These two estimates of NEE are used to address possible measurement bias errors at this site and to provide plausible estimates of annual NEE. The estimated mean annual NEE at this site is −574 g C m−2 yr−1 between 1995 and 1999, ranging from −470 g C m−2 y−1 (1995) to −629 g C m−2 yr−1 (1999) (negative NEE indicates uptake by forest).

Atmospheric deposition and canopy exchange processes in alpine forest ecosystems (northern Italy)

Throughfall and bulk precipitation chemistry were studied for five years (June 1994–May 1999) at two high elevation forest sites (Val Gerola and Val Masino) which were known to differ in terms of tree health, as assessed by live crown condition. The ion concentration of bulk precipitation samples did not differ significantly between sites, except for Mg 2+, while the throughfall concentrations differed in the measured values of H +, N-NO 3 −, Cl −, Na +, K +, DOC and weak organic acids. The results of the application of the canopy exchange model indicated a higher contribution from the dry deposition of N-NO 3 −, N-NH 4 + and H + at Val Gerola, where the damage symptoms were more evident. In addition, the canopy leaching of Ca 2+, K + and weak organic acids were 47%, 21% and 27% higher at Val Gerola than at Val Masino. Annual SO 4 2− deposition fluxes (21.3 kg ha −1 yr −1 at Val Masino and 23.6 kg ha −1 yr −1 at Val Gerola) were similar to those reported for moderately polluted European and U.S. sites. Annual N loads were 13.6 and 13.1 kg ha −1 yr −1 in the bulk input, and 15.0 and 18.0 kg ha −1 yr −1 in throughfall inputs, at Val Masino and Val Gerola, respectively. The contribution of the organic fraction to the total N atmospheric deposition load is significant, constituting 17% of the bulk flux and 40% of the throughfall flux. Measured nitrogen loads exceed the critical nutrient loads by several kg N ha −1 at both stations. In particular the nitrogen throughfall load at Val Gerola was about 3 times higher than the critical values.

Seasonal and annual deposition rates of sulphur, nitrogen and chloride species to an oak forest in north-eastern austria (wolkersdorf, 240 m a.s.l.)

Dry deposition estimates of sulphur, nitrogen and chloride components to an oak forest in north-eastern Austria were performed using an “Inferential model.” The deposition calculations were performed with a modified Hicks et al. (1985) model based on chemical measurement data with 0.5 h (SO 2, NO, NO 2) and 24 h (HCl, HNO 3, NH 3, particulate chloride, nitrate, sulphate and ammonium) time resolution. Annual dry deposition rates of sulphur (SO 2+pSO 4) were estimated to amount around 13 kg ha -1, of oxidised nitrogen (NO+NO 2+HNO 3+pNO 3) 8.4 kg ha -1, of reduced nitrogen (NH 3+pNH 4) 5.6 kg ha -1 and of chloride (HCl+pCl) 2.1 kg ha -1. The relative contribution of dry to total (dry+wet) deposition is 68% for sulphur, 77% for oxidised nitrogen, 55% for reduced nitrogen and 40% for chloride species. Total deposition of sulphur and reduced nitrogen species showed good agreement with data from throughfall measurements and a canopy exchange model. For oxidised nitrogen species the inferential modeling approach yielded a considerably higher deposition which could be explained by the inability of the canopy exchange model to account for stomatal uptake of NO 2. A comparison of dry and wet fluxes of sulphur and nitrogen species at European and U.S. sites indicates that in both environments large regional differences occur. Thus the hypothesis, that European forests receive substantially higher inputs of “major ions” than U.S. forests ( Lindberg et al., 1990) cannot be generalised. “Dry+wet” deposition of nitrogen species at two Austrian sites is well within the range of deposition at 11 forested sites in the U.S., while at one Austrian site it is slightly higher than the highest N-deposition reported for U.S. sites. Compared sites from the U.S. are from the Mountain Cloud Chemistry Project and the Integrated Forest Study ( Lovett and Lindberg, 1993).

A canopy budget model to assess atmospheric deposition from throughfall measurements

A canopy exchange model is presented which allows atmospheric deposition to be estimated from longterm throughfall and precipitation measurements. For a forest in the Netherlands, the combination of throughfall measurements and this model resulted in deposition estimates which were similar to deposition estimates derived from micrometeorological measurements and inferential modeling, deposition of NOy being the only exception. Unfortunately, several basic assumptions in the canopy exchange model are not properly evaluated, which up to now limits its application. Suggestions are made on how the model can be improved.