Forest canopy transformation of atmospheric deposition

Solute fluxes to the ground in open plots and under the forest canopy of different species were investigated in a number of long-term ecosystem studies in West Germany. From the canopy flux balance, rates of interception deposition and canopy/deposition interactions were assessed. Chemically, both open precipitation and throughfall are dilute solutions of H₂SO₄ and HNO₃ and their salts. For the sites investigated, mean pH in bulk precipitation ranged from 4.1 to 4.6, and in throughfall from 3.4 to 4.7. The increase in acidity after canopy passage at most sites indicates considerable interception deposition of strong acids to the forest stands, exceeding the rate of H+ buffering in the canopy. Evidence for buffering processes can be directly deduced from the fact that on sites with high soil alkalinity and high foliage base status, throughfall pH is usually higher than precipitation pH. Furthermore, the same idea can be concluded from changes in solution composition after canopy passage: the H⁺/SO _inf4 ^sup2? ratio is decreasing at most sites, while alkali earth cations from exchange processes occur in throughfall (Ca²⁺/SO _inf4 ^sup2? ratio increases). Solution composition and element flux data are presented for each of the sites, and the regional, orographical and site specific (species composition, ecosystem state) differentiations are discussed. A method for the assessment of total deposition and of canopy interactions such as H⁺-buffering and cation leaching is described, and results of calculations are shown. From these calculations it is concluded that forest ecosystems in Germany receive mean H⁺ loads of ca. 1 to 4 keq H⁺ · ha^?1 · a^?1 from atmospheric deposition. Acidity deposition rates seem to be related to a few key factors such as regional characteristics and ecosystem characteristics.     

Relative contributions of sedimentation and impaction to deposition of particles in a crop canopy

Lycopodium spores were released steadily into the air during 20–30 min from a line source positioned within a wheat crop. The spores were trapped on sticky strips held at angles, π, of 0, 30, 60 and 90° with respect to the horizontal and oriented to face the mean wind direction and on sticky, vertical glass rods. The aerial spore concentration, C, was measured by small suction traps. Deposits of the spores on wheat leaves were obtained from sections of leaves whose posture in the canopy was nearly horizontal, nearly vertical, or at angles between 30 and 60°. Number of spores per m² for all trapping surfaces were obtained by counting under a microscope. Experiments were conducted on seven different days, encompassing friction velocities, u^*, of 0.27–0.50 m s⁻¹. The rate of deposition on angled surfaces, D(π), was given approximately by D(θ) = D(0) cos (θ) + D(90) sin (θ), where D(0) and D(90) were the observed rate of deposit on horizontal and on vertical surfaces, respectively. Below mid-canopy height, inertial impaction of spores was negligible, so that D(90) = 0 for all the trap surfaces. There, D(0) was mainly due to sedimentation and was very nearly equal to v_s·C, where v_s is the settling speed of the spore in still air. Near the top of the canopy, deposition on sticky surfaces was enhanced by inertial impaction and turbulent deposition, so that D(0) was about twice that expected from sedimentation and D(90) was about five times larger than expected from inertial impaction at the mean wind speed. Nevertheless, considering the vertical distribution of leaf area and the angles of leaves in a wheat canopy, the rate of deposition of spores for the entire depth of a wheat canopy can be calculated with a probable underestimation of only 20% by simply assuming sedimentation on horizontally projected area and impaction on vertically projected area.     

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.     

Mercury in forest canopy throughfall water and its relation to atmospheric deposition

A seasonal variation of both particle and gaseous Hg concentrations in the atmosphere is present in south-western Sweden. An average gaseous Hg level of 3.7 ng m⁻³ is found in winter, compared to 2.8 ng m⁻³ in summer. A weak decreasing south-north gradient for gaseous Hg in air over the Nordic countries is also present, with yearly average values from 3.2 to 2.8 ng m⁻³. A gradient for particulate Hg is less clear. An air parcel trajectory sector classification of gaseous Hg levels in air, and to some extent the particulate associated Hg, clearly demonstrates the increased concentrations in the southern sectors, especially in south-western Sweden where the gaseous Hg increase is about I ng m⁻³. These observations are consistent with an influence from the European continent. The average concentrations of Hg in precipitation at the various stations show a pronounced decreasing south-north gradient. A major portion of the total Hg present in precipitation is associated with particles. For the southern stations, a strong correlation between Hg and sulfate, or pH, is present suggesting a connection between Hg in precipitation and anthropogenic activities.     

Measurements of atmospheric deposition and internal circulation of base cations to a forested catchment area

The dry deposition of base cations to a Norway spruce stand was estimated by multiplying the ratio of the ion deposition to the sodium deposition on a surrogate surface with the dry deposition of sodium on the forest stand. The method can in principle only be applied to species that are present only in particles, but the method gives reasonable results when tested on ions that are also dry deposited in other forms (SO ₄ ^2– . NO ₃ ^– and NH ₄ ⁺ ). The atmospheric input and especially the dry deposition of base cations is an important replacement for the loss of base cations from the soil by run-off. The calculated internal circulation of K⁺ and Ca²⁺ showed maxima synchronously with rainfall maxima and constitute 71% and 53%, respectively, of the net throughfall deposition. The internal circulation of Ca²⁺ was almost equal to the SO₂ uptake.     

Measurements of wet and dry deposition in a Northern Hardwood forest

Inputs of wet and dry deposition were monitored at the Huntington Forest in the Adirondack Mountains of New York for two years in the open and beneath the canopy of a northern hardwood forest. In the open, ion flux estimates were similar using wet-only weekly (NADP protocol) and event collections, but bulk collections were higher for all ions except H⁺, which was much lower. These differences were due to the contribution of dry deposition and possible biotic alterations in bulk collectors. Dry deposition was estimated using air concentrations and ion-specific depositional velocities modeled with meteorological data, and contributed substantially to the input of all ions [H⁺ (45%), Na⁺ (24%), K⁺ (22%), NH₄ ⁺ (12%), Ca²⁺ (58%), Mg²⁺(43%), NO₃ ^? (55%), Cl^? (27%) and SO₄ ^?2 (26%)]. Dry input of base cations was dominated by coarse particles, whereas gaseous inputs were more important for S and NO₃ ^?. Atmospheric concentrations of SO₂ and inputs of SO₄ ^2? and H⁺ were lower at this site than sites closer to point sources of S gas emission. The importance of estimating atmospheric inputs was examined using examples of elemental budgets. For example, different estimates of the contribution of dry deposition of SO₄ ^2? (9–21 meq m^?2 y^?1) resulted in conclusions ranging from no net retention to a net loss of this element. Such differences have important implications in assessing the current and future role of atmospheric inputs in affecting elemental cycling.     

Modelling ozone deposition fluxes: The relative roles of deposition and detoxification processes

A new Model of Ozone Deposition and Detoxification (MODD) is presented. This model describes stomatal ozone uptake and deposition on external plant surfaces and soil; it accounts for diurnal variability of detoxification processes and reactive ozone uptake on cuticular waxes and soil surface. The mechanistic modelling of plant defense reactions is based on the Plöchl et al. (2000) detoxification model in which the dynamics of apoplast chemistry are considered. To estimate ozone deposition fluxes on cuticular waxes and soil surface, we use a revised version of the Morrison and Nazaroff (2002) model developed to account for ozone uptake on material surfaces. This model which has been fully integrated with a soil-plant-atmosphere continuum model ensures a complete coupling between stomatal conductance and O₃ exchanges between leaves and the atmosphere. The observed diurnal variations in stomatal conductance which largely control the influx of O₃ into the leaf are well reproduced. Model simulations point out that the pool of ascorbate located in the mesophyll cell wall plays a significant role in the detoxification of O₃. Besides stomatal conductance, it is the key process involved in the control of ozone flux to the cell wall. A decrease in the pool of ascorbate lengthens the chemical lifetime of O₃ in the cell wall then the virtual apoplastic resistance is found to increase with decreasing ascorbate. Although the atmospheric ozone concentration increases as the weather becomes hot and dry, the virtual apoplastic resistance follows the same trend, indicating a decrease of the ascorbate pool in the mesophyll cell wall. Results also indicate that for the pre-senescence period 57% of the ozone is deposited onto the cuticular surfaces, 4% on soil and only 37% is absorbed by stomata. The comparison of modelled and measured data reported in this study indicates that the model is capable of predicting the major features of the patterns of total ozone flux.     

基于无人机高光谱影像的马铃薯叶绿素含量估测

[目的]叶绿素含量高低反映植被的健康状况与光合能力.研究准确、有效地将冠层影像反演为叶绿素含量的技术参数,以便经济快速、实时地监测作物生长状况.[方法]田间试验于2018-2020年在内蒙古阴山北麓马铃薯主产区进行,设置氮肥梯度处理,在马铃薯块茎膨大期和淀粉积累期,测定试验地马铃薯植株SPAD值,通过线性关系将其转化成...     

Atmospheric deposition in forest edges measured by monitoring canopy throughfall

Atmospheric dry deposition in two forest edges was studied by means of monitoring canopy throughfall in Douglas Fir stands. Throughfall fluxes in the first 50 to 100 m of forest edges were found to be substantially higher than fluxes in the interior of forest stands. Sodium and chloride showed the steepest throughfall flux gradients. Ions important for soil acidification and eutrophication showed relatively less steep but still significant gradients. The mean increase of the throughfall flux at 10 m, with respect to the flux at 200 m from the forest edge amounted to 150% for Na⁺, 119% for Cl^?, 54% for S0₄ ^2?, 38% for NO₃ ^? , and 39% for NH₄ ⁺ The enhancement of dry deposition in forest edges strongly depends on wind velocity and wind direction during dry deposition. Particularly trees in forest edges exposed to prevailing wind directions receive relatively large amounts of dry deposition.     

Characteristics of an ozone deposition module II: Sensitivity analysis

An ozone deposition module is currently being developed which will allow the estimation of stomatal fluxes of ozone into a number of vegetation types. This model is designed to be linked into a regional chemical-transport model for use within the European Monitoring and Evaluation Programme (EMEP), to provide information on possible risks to vegetation across Europe. This paper investigates the sensitivity of this model to some of the important input parameters, for two land-cover classes (temperate coniferous forests, temperate cereals). Of the factors contributing to the stomatal conductance considered within this study, those controlling soil moisture seem to be the biggest source of uncertainty. As ozone damage is believed to be driven by flux into the leaf rather than by ambient concentrations, this study suggests that flux modelling may be a practical alternative to the use of the AOT40 concept which is currently used to guide assessments of vegetation risk.     

Inferring ozone deposition on agricultural surfaces: An application to herbaceous and tree canopies

A combined gradient transfer-inferential technique was tested to evaluate ozone deposition on two agricultural systems, an herbaceous crop (soybean) and a fruit tree orchard (peach). The measurements were performed at a rural site in northern Italy's Po valley, where summer ozone concentration values often register levels exceeding the national health standard because of the presence of industrial facilities. Canopy resistance, which is mainly the result of stomatal regulation, proved in both cases the most important component of total resistance to dry deposition. The fruit tree canopy evinced a more marked propensity for ozone uptake. The average deposition velocity on soybean was 0.43 cm s^?1, and a mean value about three-fold higher was calculated on peach.     

Spatial variability in nutrient deposition under an oak/beech canopy

Nutrient deposition in throughfall and stemflow were measured on a two-weekly basis in an oak/beech forest in the eastern part of the Netherlands. The large number of sampling points (11 throughfall and 8 stemflow collectors) enabled an analysis of the spatial variability in ion deposition. Spatial variability of ion depositions in throughfall revealed wide variation both between dissolved constituents as between seasons for single dissolved constituents. On a two weekly basis coefficients of variation ranged from 20–72% with highest values generally occurring for ortho-P and H and during spring. In contrast coefficients of variation based upon annual deposition totals ranged from 9–31%. Ion depositions in stemflow showed a much higher spatial variability with coefficients of variation for annual totals ranging from 46–115% with highest values again reported for ortho-P and H. The results on the ion-depositions in throughfall are summarized in a scheme in which for a given dissolved constituent and a desired accuracy interval, the needed number of sampling points is indicated.     

Dry deposition velocity of O3 over a vineyard obtained from models and observations: The 1991 California ozone deposition experiment

Measurements were made of concentrations and flux densities (using the eddy correlation technique) for O₃ over a vineyard for 20 days during July and August 1991. These were compared with modelled dry deposition velocities (V _d), using (a) a module in the air quality model known as ADOM (Acid Deposition and Oxidant Model), which was modified to apply over a specific site, (b) a version of the ADOM module that employs a modified canopy resistance and (c) versions of ADOM which use two new canopy resistance formulations that are referred to as Wesely and Massman, respectively. The Massman parameterization is valid only locally since it was tuned to the site's ozone data. Here it is used as a benchmark for model comparisons. The observed V _dhad an average value of about 0.5 cm s^?1 during the day and about 0.2 cm s^?1 at night. Compared to the modified ADOM, the new parameterizations yielded results that were in better agreement with the observations at night. During the daytime, the original ADOM and the Wesely estimates were much larger than the observations, the Massman values were slightly smaller, and the modified ADOM showed a better agreement. We speculate that the underestimation of the Massman V _dvalues during the day may have been caused by the ADOM aerodynamic resistance rather than the Massman canopy resistance. It is hypothesized that the original ADOM module and its modified version may need a revised aerodynamic or an additional canopy resistance at night in order to bring the estimates closer to the observations.     

The role of organic horizons and canopy to modify the chemistry of acidic deposition in some forest ecosystems

To clarify the mechanisms of pH buffering in forest ecosystems, field observations of pH and ionic concentrations in precipitation (R), throughfall (Tf), stemflow (Sf), and leachates from organ c horizons (Lo) were conducted for three years at three stands in Tomakomai (TK) and Teshio (TS) in Hokkaido, northern Japan. Weighted mean rates of H⁺ input as wet deposition at TK and TS were estimated in the range from 0.3 to 1.0 and 0.4–0.6 kmol_c ha^?1 y^?, respectively. While the net H⁺ flux was reduced significantly by the forest canopy, net fluxes of other ions by throughfall, especially for Na⁺, Cl^?, and SO₄ ^2?, were apparently greater than those by precipitation. The canopy modification of the H⁺ flux was more remarkable under deciduous stands than under coniferous stands, suggesting that the efficiency of conifers as the collectors of dry deposition is greater than that of deciduous ones. More than 50% of H⁺ flux due to throughfall was absorbed by the organic horizons and the weighted mean pH of Lo at TK and TS was in the range from 4.9 to 5.5 and 5.0–5.5, respectively. Results from field observation and field leaching experiments, showed that the major H⁺ sinks of the organic horizons are exchange reaction of Ca²⁺, Mg²⁺ and K⁺. Organic acids or organo-metallic complexes of lower pK(=5.0–5.5) played a significant role as counter anions in O horizons leachate in coniferous forests. Our results indicate the importance of biogeochemical modifications in the canopy and organic horizon in acid buffering mechanisms of forest ecosystems.     

多尺度特征融合的柑橘冠层施药沉积量分类模型

针对传统农作物冠层施药沉积量分类模型分类准确率低、网络模型参数量大且运算速度慢的问题,该研究提出一种改进的SPP-Net-Inception-v4模型。该模型通过构建稀疏网络结构平衡各个模型子网间的计算量,利用3个Inception模块生成施药沉积量在柑橘冠层热红外图像的稠密有效特征数据;在模型的卷积层与全连接层间创新性接入空间金字塔池化网络（Spatial Pyramid Pooling Network, SPP-Net）,进行一次历遍提取热红外图像特征信息,再通过空间池化操作融合3种池化方式提取的多尺度特征,实现柑橘冠层热红外图像施药沉积量表现特征的提取与融合。搭建多环境因素自主控制试验环境,模拟无人机低空采集柑橘冠层热红外图像,应用3个分类模型进行对比试验,试验结果表明,SPP-Net-Inception-v4模型与Inception-v4和ResNet-152两种模型相比,准确率分别提高1.58%和3.26%,模型训练完成冻结后占用计算机存储空间大小分别降低13%和24%,表明SPP-Net-Inception-v4模型在降低模型规模的基础上,提高了柑橘树冠层施药沉积量分类的准确率,可为精准农业航空中无人机植保技术的进一步发展提供参考。     

Seasonal and diurnal variation in the deposition velocity of ozone over a spruce forest in Denmark

The flux of O₃ was measured by the eddy-correlation method over Norway spruce in periods when the trees had a very low activity, periods with optimum growth, and periods with water stress. The aerodynamic resistance (r _a ), viscous sub-layer resistance (r _b ) and surface resistance (r _c ) to O₃ were calculated from meteorological parameters and the deposition velocity. The canopy stomatal resistance to O₃ was calculated from measurements of the water vapour flux. The deposition velocities showed a diurnal pattern with night-time values of 3.5 mm s^–1 and day-time values of 7 mm s^–1, when the trees had optimal growth conditions. The surface resistance was highly dominating in day-time and the influence of meteorology low. In night-time the surface resistance to 03 was lower than the canopy stomatal resistance. A low surface resistance was also found in winter-time, when the activity of the trees was low. The surface resistance increased when the trees were subject to water stress. It is concluded that stomatal uptake is an important parameter for the deposition of O₃. However, other processes such as destruction of O₃ at surfaces, reaction with NO emitted from the soil, and reactions with radicals produced from VOC's emitted from the forest, should also be taken into consideration.     

Estimates of atmospheric deposition and canopy exchange for three common tree species in the Netherlands

During one year, dry and wet deposition onto thirty forest stands is studied by sampling throughfall and bulk precipitation. Nine measurement sites are situated in Douglas fir (Pseudotsuga menziesii Mirb. Franco) stands, ten in Scotch pine (Pinus sylvestris L.) and eleven in Oak (Quercus robur L.) stands. Because the stands are situated in each other's proximity (i.e. within a radius of approximately 1.4 km) it is assumed that they experience an approximately equal air pollution load. For the acidifying compounds SO₄ ^2?, NO₃ ^? and NH₄ ⁺ spatial variability in wet deposition was small within the area studied. Dry deposition, as estimated by net throughfall, displayed a much higher spatial variability. Significant differences existed between tree species and growing seasons. Douglas fir mostly displayed the highest, Oak the lowest and Scotch pine intermediate values for net throughfall fluxes of acidifying compounds. The annual net throughfall fluxes for nitrogen compounds were significantly higher for the coniferous tree species than the broadleaved tree species. For SO₄ ^2?, however, Oak showed a relatively high throughfall flux during the summer. By comparing the temporal pattern of net throughfall fluxes between the three tree species it was concluded that considerable canopy leaching occurred for SO₄ ^2?, Mg⁺, PO₄ ^3?, HCO₃ ^? and K⁺ in Oak stands during the sprouting of leaves in spring. From surface wash experiments in the laboratory it is concluded that canopy leaching of these ions may also be enhanced when Oak leaves are infected by Oak mildew, a fungal disease caused by the fungus Microshaera aliphilitoides.     

A graphical extrapolation method to determine canopy resistance from measured temperature and humidity profiles above a crop canopy

The profile of vapour pressure in Monteith's extrapolation method is replaced by the profile of dew-point temperature. The canopy resistance can then be obtained directly by a graphical extrapolation method from measured temperature and humidity profiles above a crop canopy. The effect of choosing different excess resistances on the canopy resistance thus obtained is discussed.     

小型无人直升机喷雾参数对杂交水稻冠层雾滴沉积分布的影响

为了初探小型无人直升机航空喷施雾滴在水稻冠层沉积分布规律,主要通过不同的飞行参数研究了不同喷雾作业参数对水稻冠层的雾滴沉积分布的影响。该试验以HY-B-10L型单旋翼电动无人机搭载北斗定位系统UB351绘制作业轨迹,以质量分数为5‰的丽春红2R水溶液模拟生长调节剂喷施沉积情况,以图像处理软件DepositScan来分析靶区和非靶区的雾滴沉积参数得出雾滴的沉积分布结果。结果表明：3次试验中的雾滴沉积分布趋势均相似,且飞行高度和飞行速度对靶区内采集点上雾滴平均沉积量影响均显著,对雾滴沉积均匀性影响并不显著。3次试验中靶区的雾滴沉积量随着高度的增加而减少,总雾滴沉积量分别为2.380、1.905、1.156μL/cm2,采集点的平均沉积量分别为0.198、0.159、0.064μL/cm2;在作业高度为1.92 m时雾滴沉积平均均匀性最佳,且非靶区的雾滴漂移总量最少,为0.174μL/cm2。另外,第1、2条采集带上靶区内的雾滴沉积量均明显多于作业速度较大的第三条采集带上的雾滴沉积量,3次试验中,第一、2条采集带上雾滴沉积总量的平均值分别高于第3条采集带上雾滴沉积总量的184.27%、53.51%、72.31%;且由于外界风场的影响,作业航线下风向的雾滴沉积量和漂移距离均大于作业航线上风向的雾滴沉积量和漂移距离;以及由于飞行速度的影响,非靶区航线下风向第3条雾滴采集上的雾滴漂移量均大于第1、2条雾滴采集带上的雾滴漂移量。该结果较好地全面揭示了作业参数对航空喷施雾滴沉积分布结果的影响,并从风场因素方面推测了对雾滴沉积的影响,对药液的合理喷施、提高喷施效率具有十分重要的指导意义。