SOLWEIG 1.0 – Modelling spatial variations of 3D radiant fluxes and mean radiant temperature in complex urban settings

The mean radiant temperature, T_mrt, which sums up all shortwave and longwave radiation fluxes (both direct and reflected) to which the human body is exposed is one of the key meteorological parameters governing human energy balance and the thermal comfort of man. In this paper, a new radiation model (SOLWEIG 1.0), which simulates spatial variations of 3D radiation fluxes and T_mrt in complex urban settings, is presented. The T_mrt is derived by modelling shortwave and longwave radiation fluxes in six directions (upward, downward and from the four cardinal points) and angular factors. The model requires a limited number of inputs, such as direct, diffuse and global shortwave radiation, air temperature, relative humidity, urban geometry and geographical information (latitude, longitude and elevation). The model was evaluated using 7 days of integral radiation measurements at two sites with different building geometries – a large square and a small courtyard in Göteborg, Sweden (57°N) – across different seasons and in various weather conditions. The evaluation reveals good agreement between modelled and measured values of T_mrt, with an overall good correspondence of R ²?=?0.94, (p?相似文献   

Characteristics of the mean radiant temperature in high latitude cities—implications for sensitive climate planning applications

Knowledge of how the mean radiant temperature (T _mrt ) is affected by factors such as location, climate and urban setting contributes to the practice of climate sensitive planning. This paper examines how T _mrt varies within an urban setting and how it is influenced by cloudiness. In addition, variations of T _mrt in three high latitude cities are investigated in order to analyse the impact of geographical context and climate conditions. Results showed large spatial variations between sunlit and shaded areas during clear weather conditions, with the highest values of T _mrt close to sunlit walls and the lowest values in the areas shaded by buildings and vegetation. As cloudiness increases, the spatial pattern is altered and the differences are reduced. The highest T _mrt under cloudy conditions is instead found in open areas where the proportion of shortwave diffuse radiation from the sky vault is high. A regional comparison between three Swedish coastal cities showed that T _mrt during summer is similar regardless of latitudinal location. On the other hand, large differences in T _mrt during winter were found. Shadows, both from buildings and vegetation are the most effective measure to reduce extreme values of T _mrt . However, extensive areas of shadow are usually not desired within outdoor urban environments at high latitude cities. One solution is to create diverse outdoor urban spaces in terms of shadow and also ventilation. This would provide individuals with access to a choice of thermal environments which they can use to assist their thermal regulation, based on personal needs and desires.     

Comparison of different methods of estimating the mean radiant temperature in outdoor thermal comfort studies

Correlations between outdoor thermal indices and the calculated or measured mean radiant temperature T_mrt are in general of high importance because of the combined effect on human energy balance in outdoor spaces. The most accurate way to determine T_mrt is by means of integral radiation measurements, i.e. measuring the short- and long-wave radiation from six directions using pyranometers and pyrgeometers, an expensive and not always an easily available procedure. Some studies use globe thermometers combined with air temperature and wind speed sensors. An alternative way to determine T_mrt is based on output from the RayMan model from measured data of incoming global radiation and morphological features of the monitoring site in particular sky view factor (SVF) data. The purpose of this paper is to compare different methods to assess the mean radiant temperature T_mrt in terms of differences to a reference condition (T_mrt calculated from field measurements) and to resulting outdoor comfort levels expressed as PET and UTCI values. The T_mrt obtained from field measurements is a combination of air temperature, wind speed and globe temperature data according to the forced ventilation formula of ISO 7726 for data collected in Glasgow, UK. Four different methods were used in the RayMan model for T_mrt calculations: input data consisting exclusively of data measured at urban sites; urban data excluding solar radiation, estimated SVF data and solar radiation data measured at a rural site; urban data excluding solar radiation with SVF data for each site; urban data excluding solar radiation and including solar radiation at the rural site taking no account of SVF information. Results show that all methods overestimate T_mrt when compared to ISO calculations. Correlations were found to be significant for the first method and lower for the other three. Results in terms of comfort (PET, UTCI) suggest that reasonable estimates could be made based on global radiation data measured at the urban site or as a surrogate of missing SR data or globe temperature data recorded at the urban area on global radiation data measured at a rural location.     

Seasonal variations of CO2 and water vapour exchange rates over a temperate deciduous forest

Long-term and direct measurements of CO₂ and water vapour exchange are needed over forested ecosystems to determine their net annual fluxes of carbon dioxide and water. Such measurements are also needed to parameterize and test biogeochemical, ecological and hydrological assessment models. Responding to this need, eddy covariance measurements of CO₂ and water vapour were made ever a deciduous forest growing near Oak Ridge, TN, between April 1993 and April 1994. Periodic measurements were made of leaf area index, stomatal resistance, soil moisture and pre-dawn leaf water potential to characterize the gas exchange capacity of the canopy. Four factors had a disproportionate influence on the seasonal variation of CO₂ flux densities. These factors were photon flux densities (during the growing season), temperature (during the dormant season), leaf area index and the occurrence of drought The drought period occurred during the peak of the growing season and caused a significant decline in daily and hourly CO₂ flux densities, relative to observations over the stand when soil moisture was plentiful. The annual net uptake of carbon was calculated by integrating flux measurements and filling missing and spurious data with the relations obtained between measured CO₂ fluxes and environmental forcing variables. The net flux of carbon for the period between April 1993 and April 1994 was -525 g C m^?2 y^?1. This value represents a net flux of carbon from the atmosphere and into the forest. The net annual carbon exchange of this southern temperate broadleaved forest exceeded values measured over a northern temperate forest (which experiences a shorter growing season and has less leaf area) by 200 g C m^?2 y^?1 (cf. Wofsy et al 1993). The seasonal variation of canopy evaporation (latent heat flux) was controlled mostly by changes in leaf area and net radiation. A strong depression in evaporation rates was not observed during the drought Over a broadleaved forest large vapour pressure deficits promote evaporation and trees in a mixed stand are able to tap a variety of deep and shallow water sources.     

On the Reason for the Different Photosynthetic Rates of Seedlings of Pinus silvestris and Betula verrucosa

The growth and net photosynthetic properties of seedlings of Pinus silvestris L. and Betula verrucosa Ehrh., grown under identical conditions in a controlled environment chamber, were compared. The relative growth rate of birch was about twice that of pine. The rates of in situ net photosynthesis were 1.50 and 2.30 micromoles CO₂ meter⁻² second⁻¹ and the photosynthetic quantum yields under light-limiting conditions were 0.022 and 0.032 for pine and birch, respectively. The total leaf surface areas were used for calculating the CO₂ flux densities. The difference in the rates of in situ net photosynthesis depended equally on morphological and metabolic factors. It was assumed that a pronounced mutual shading and an unfavorable leaf inclination made the pine seedlings less efficient in absorbing the unidirectional light of the climate chamber than the broadleaved seedlings of birch. Both pine and birch were adapted to the growth conditions so the flux densities of absorbed quanta were rate-limiting for in situ net photosynthesis. It was concluded that the difference in the photosynthetic quantum yields (i.e. the linear slope of the photosynthetic light curve) of the two species defined the metabolically controlled part of the difference in the rate of in situ net photosynthesis. The quantum yield of pine was lower than that of birch and was partly explained by pine having a higher rate of photorespiration than birch. The remaining difference was most likely controlled by the properties of the chloroplast thylakoids, e.g. energy transfer efficiency between pigments, photosynthetic electron transport, or coupling between electron transport and photophosphorylation.     

Modelling radiation fluxes in simple and complex environments—application of the RayMan model

The most important meteorological parameter affecting the human energy balance during sunny weather conditions is the mean radiant temperature T_mrt. It considers the uniform temperature of a surrounding surface giving off blackbody radiation, which results in the same energy gain of a human body given the prevailing radiation fluxes. This energy gain usually varies considerably in open space conditions. In this paper, the model ‘RayMan’, used for the calculation of short- and long-wave radiation fluxes on the human body, is presented. The model, which takes complex urban structures into account, is suitable for several applications in urban areas such as urban planning and street design. The final output of the model is, however, the calculated T_mrt, which is required in the human energy balance model, and thus also for the assessment of the urban bioclimate, with the use of thermal indices such as predicted mean vote (PMV), physiologically equivalent temperature (PET) and standard effective temperature (SET*). The model has been developed based on the German VDI-Guidelines 3789, Part II (environmental meteorology, interactions between atmosphere and surfaces; calculation of short- and long-wave radiation) and VDI-3787 (environmental meteorology, methods for the human-biometeorological evaluation of climate and air quality for urban and regional planning. Part I: climate). The validation of the results of the RayMan model agrees with similar results obtained from experimental studies.     

The chemistry and transport of mercury in a small wetland in the Adirondack region of New York, USA

The biogeochemistry of Hg was evaluated in a small wetland in the Adirondack region of New York. Concentrations of total Hg (Hg_T) in streamwater draining the wetland showed little temporal variation. The annual areal watershed flux of Hg_T (2.2 µg/m²-yr) was considerably smaller than regional inputs of atmospheric deposition of Hg_T, indicating that the terrestrial environment is a net sink for atmospheric deposition of Hg_T. Drainage inputs of Hg_T were conservatively transported through the beaver impoundment. The annual flux of total methyl mercury (CH₃Hg⁺ _T was greater than literature values of atmospheric deposition suggesting that the watershed is a net source of CH₃Hg⁺ _T . Stream concentrations of CH₃Hg⁺ _T increased during low-flow summer conditions in a riparian wetland, and particularly at the outlet of the beaver impoundment. Net production of CH₃Hg⁺ _T occurred in the beaver impoundment (0.45 µg/m²-yr). Rates of net methylation for the beaver impoundment were comparable to values reported in the literature for wetlands.     

Influence of shoot structure on light interception and photosynthesis in conifers

The influence of shoot structure on net photosynthesis was evaluated under field conditions for the central Rocky Mountain (United States) conifers Picea engelmannii (Parry ex Engelm.), Abies lasiocarpa ([Hook] Nutt.), and Pinus contorta (Engelm.). In all species, the greater number of needles per unit stem length on sun shoots correlated with a smaller silhouette leaf area to total leaf area ratio (STAR). Decreased STAR was due primarily to greater needle inclination toward the vertical, plus some needle mutual shading. However, photosynthesis expressed on a total leaf area basis did not decrease in sun shoots (lower STAR) but remained nearly constant at approximately 3 micromoles per square meter per second over a wide range of STAR (0.1 to 0.3). Relatively low light saturation levels of 200 to 1400 microeinsteins per square meter per second and diffuse light to 350 microeinsteins per meter per second maintained photosynthetic flux densities in inclined and/or shaded needles at levels comparable to those in unshaded needles oriented perpendicular to the solar beam. As a result, net CO₂ uptake per unit stem length increased as much as 2-fold in sun shoots (low STAR) in direct proportion to increasing needle density.     

敦煌绿洲夏季典型晴天地表辐射和能量平衡及小气候特征

 利用“我国西北干旱区陆-气相互作用试验”加强期(IOP)在甘肃省敦煌绿洲观测的资料,系统地分析了夏季典型晴天敦煌地表辐射收支和地表能量平衡特征及小气候特征,结果发现：敦煌绿洲总辐射特别大,其峰值高达1 038.1 w·m-2;地表净辐射也高于其它地区,白天能超过600 w·m-2;在地表能量分配中,感热、潜热和地热流量的日积分值的量级相当,白天地热流量比潜热要大,几乎与感热相当;地表反照率除中午比较接近荒漠戈壁的值外,其它时候均明显比荒漠戈壁的值小;Bowen比在白天1～2之间,比一般灌溉地区要大。地表能量不平衡差额较大,这可能与水平热通量的影响有很大关系。另外,还首次发现了比较可观的下沉气流,部分证实了绿洲与荒漠之间存在的热力环流。     

Use of operative temperature and standard operative temperature models in thermal biology

Operative temperature (T_e) and standard operative temperature (T_es) models have been used to address ecological questions about the thermal biology of ectotherms and endotherms for over 25 years. This review focuses on the accuracy and use of T_e and T_es models in ecological and physiological studies. The utility of T_e and T_es models lie in the fact that they take a multivariate problem involving inputs of air temperature, ground temperature, solar radiation, and wind speed and map them into a single thermal metric on a spatial scale appropriate for the animal. The most reliable T_e models are copper casts that mimic the morphology and absorptivity of an animal. Simplified T_e models such as cylinders and spheres have been shown to produce errors in T_e as large as 12 °C under certain conditions and should only be used after careful calibration against a live animal. The accuracy of heated T_es models has been addressed in much less detail then that of T_e models. When calibrated and used under conditions of low solar radiation, heated taxidermic mounts and simplified T_es models produce errors in net heat production on the order of 5% or less. In order to provide reliable data, all types of models must be calibrated over an ecologically realistic range of environmental conditions experienced by the animal. This advice has been largely ignored in the literature, where 61% of the of studies examined failed to properly calibrate the models prior to use. Additionally, studies using these models tend to lack experimental rigor, using only one or two models to make measurements on 1 or 2 days of the active season. When used correctly, T_e and T_es models can be powerful tools for integrating the thermal environment experienced by an animal into a single metric that can address questions regarding the ecology, physiology, and behavior of endotherms and ectotherms. However, until investigators make the effort to use these models in a scientifically valid manner with proper calibration and experimental design their value to thermal biologists will be limited.     

Effect of atoms evaporated from the anode on the structure of the vacuum arc space charge sheath

The structure of the anode space charge sheath of a vacuum arc is studied with allowance for the dependence of the negative anode fall on the ratio of the directed electron velocity v ₀ to the electron thermal velocity v _T for different values of the flux density of atoms evaporated from the anode. Poisson’s equation for the sheath potential is solved taking into account the electron space charge, fast cathode ions, and slow ions produced due to the ionization of atoms evaporated from the anode. The kinetic equation for atoms and slow anode ions is solved with allowance for ionization in the collision integral. Analytic solutions for the velocity distribution functions of atoms and slow ions and the density of slow ions are obtained. It is shown that the flux of slow ions substantially affects the spatial distribution of the electric field E(z) in the sheath. As the flux density increases, the nonmonotonic dependence E(z) transforms into a monotonic one and the sheath narrows. For a given flux of evaporated atoms Πa, the increase in the ratio of the directed electron velocity to the electron thermal velocity leads again to a nonmonotonic dependence E(z). As z increases, the electric field first increases, passes through the maximum, decreases, passes through the minimum E _min, and then again increases toward the anode. There is a limiting value of the ratio (v ₀/v _T )* at which E _min(z) vanishes. At v ₀/v _T > (v ₀/V _T )*, the condition for the existence of a steady-state sheath is violated and the profiles of the field and potential in the sheath become oscillating. The dependence of (v ₀/v _T )* on the flux density of evaporated atoms Π _a is obtained. It is shown that the domain of existence of steady-state solutions in the sheath broadens with increasing Π _a .     

Spatial variability and major controlling factors of CO2 sink strength in Asian terrestrial ecosystems: evidence from eddy covariance data

Asian terrestrial ecosystems cover an extensive area characterized by a large variety in climates and ecosystem properties. The observations of ecosystem CO₂ flux in this area are increasing both in duration and spatial density, but no synthesis has yet been conducted. We surveyed CO₂ flux observation data obtained by eddy covariance methods at 49 sites in terrestrial Asia. The measurements at most sites (44 of 49) began after 2000. The net ecosystem uptake of CO₂ (NEE) varied greatly among sites and years and averaged −132.6±73.7, −250.1±206.1, and −180.1±361.7 g C m⁻² yr⁻¹, in boreal, temperate, and tropical Asia, respectively, and the coefficient of variation among sites increased from boreal to tropical Asia. The site-averaged annual NEE was correlated linearly with the mean annual temperature (T_air) and also correlated logarithmically with the precipitation. Multiple regression analysis and stepwise analysis indicated that photosynthetic active radiation (PAR) and T_air were the most significant predictors of the annual NEE. The study results suggest that Asian terrestrial ecosystems are currently significant net CO₂ sinks and that the sink strength is largely controlled by temperature, moisture, and light conditions.     

Lettuce plant growth and tipburn occurrence as affected by airflow using a multi-fan system in a plant factory with artificial light

A multi-fan system (MFS) for single culture beds was developed to improve the airflow in a plant factory with artificial light. The MFS had seven fans which were installed on both the front and back sides of culture beds to generate airflow from two opposite horizontal directions. The fans that push the air into the culture bed were air inlets while those that pull the air out of the culture bed were air outlets. In this study, three airflow patterns were evaluated: T₁, the front and back sides of the culture bed were air inlets; T₂, the front side was an air inlet and the backside was an air outlet; and T₃, both the front and back sides were air outlets. A culture bed with no MFS was used as a control (T₄). Lettuce growth and tipburn occurrence were evaluated and leaf boundary layer resistance (1/g_bv), sensible heat flux (S_h), and latent heat flux (L_h) of lettuce plants were estimated. The airflow pattern in T₁ improved the air velocity (V_a) by an average of 0.75 m s^-1 and a variation coefficient of 65%. The 1/g_bv decreased significantly with the increase in V_a, and the lowest value of 54.0 s m^-1 was observed in T₁. The low resistance to heat and moisture transfer enhanced the S_h and L_h of lettuce plants. The average S_h and L_h were 40% and 46% higher in T₁ compared with those in T₄. The fresh and dry weights of lettuce plants in T₁ were 1.13 and 1.06 higher than those in T₄, respectively. No tipburn occurrence was observed in lettuce plants grown under the MFS while five leaves per plant were injured with tipburn in T₄. The results indicated that improving the airflow can improve the growth of indoor cultured lettuce and alleviate the occurrence of tipburn due to the decrease in the 1/g_bv and the increase in the transpiration rate.     

Kok effect and the quantum yield of photosynthesis : light partially inhibits dark respiration

The linear response of photosynthesis to light at low photon flux densities is known to change abruptly in the vicinity of the light compensation point so that the quantum yield seems to decrease as radiation increases. We studied this `Kok effect' in attached sunflower (Helianthus annuus L. cv IS894) leaves using gas exchange techniques. The effect was present even though respiration was constant in the dark. It was observed at a similar photon flux density (7 to 11 micromole photons per square meter per second absorbed photosynthetically active radiation) despite a wide range of light compensation points as well as rates of photosynthesis. The effect was not apparent when photorespiration was inhibited at low pO₂ (1 kilopascal), but this result was complicated because dark respiration was quite O₂-sensitive and was partially suppressed under these conditions. The Kok effect was observed at saturating pCO₂ and, therefore, could not be explained by a change in photorespiration. Instead, the magnitude of the effect varied as dark respiration varied in a single leaf, and was minimized when dark respiration was minimized, indicating that a partial suppression of dark respiration by light is responsible. Quantum yields measured at photon flux densities between 0 and 7 to 11 micromole photons per square meter per second, therefore, represent the combined yields of photosynthesis and of the suppression of a component of dark respiration by light. This leads to an overestimate of the quantum yield of photosynthesis. In view of these results, quantum yields of photosynthesis must be measured (a) when respiration is constant in the dark, and (b) when dark respiration has been inhibited either at low pO₂ to eliminate most of the light-induced suppression of dark respiration or at photon flux densities above that required to saturate the light-induced suppression of dark respiration. Significant errors in quantum yields of photosynthesis can result in leaves exhibiting this respiratory behavior if these principles are not followed.     

Growth,allometry and shade tolerance of understory saplings of four subalpine conifers in central Japan

The conifers Abies veitchii, A. mariesii, Picea jezoensis var. hondoensis, Tsuga diversifolia dominate in subalpine forests in central Japan. We expected that species differences in shade tolerance and in aboveground and belowground architecture are important for their coexistence. We examined net production and carbon allocation of understory saplings. Although the four species allocated similar amounts of biomass to roots at a given trunk height, the root-zone area of T. diversifolia was greater than that of the three other species. T. diversifolia often dominates shallow soil sites, such as ridge and rocky slopes, and, therefore, a wide spread of lateral roots would be an adaptation to such edaphic conditions. Crown width and leaf and branch mass were greatest for T. diversifolia and A. mariesii, followed in order by A. veitchii and P. jezoensis var. hondoensis. Although leaf mass of P. jezoensis var. hondoensis was lowest among the four species, species differences were not found in the net production per sapling because net production per leaf mass was greatest for P. jezoensis var. hondoensis. The leaf lifespan was longer in the order A. mariesii, T. diversifolia, P. jezoensis var. hondoensis and A. veitchii. The minimum rate of net production per leaf mass required to maintain the current sapling leaf mass (MRNP_LM) was lowest in A. mariesii and T. diversifolia, and increased in the order of A. veitchii and P. jezoensis var. hondoensis. A. mariesii and T. diversifolia may survive in shade conditions by a lower MRNP_LM than the two other species. Therefore, species differences in aboveground and belowground architecture and MRNP_LM reflected their shade tolerance and regeneration strategies, which contribute to their coexistence.     

Spatiotemporal variations of T/ET (the ratio of transpiration to evapotranspiration) in three forests of Eastern China

Evapotranspiration (ET), which is comprised by evaporation from soil surface (E), transpiration (T) and evaporation from the intercepted water by canopy (EI), plays an important role in maintaining global energy balance and regulating climate. Quantifying the spatiotemporal variations of T/ET (the ratio of T to ET) can improve our understandings on the role of vegetation ecophysiological processes in climate regulation. Using eddy covariance measurements at three forest ecosystems (Changbaishan temperate broad-leaved Korean pine mixed forest (CBS), Qianyanzhou subtropical coniferous plantation (QYZ) and Dinghushan subtropical evergreen mixed forest (DHS)) in north–south transect of Eastern China (NSTEC), we run the revised Shuttleworth–Wallace model (S–W model), validated its performance with the water vapor fluxes measured at two layers, and quantified the spatiotemporal variations of T/ET. The S–W model performed well in simulating ET and T/ET. The mean value of annual T/ET at three forests during the observation period all exceeded 0.6. The diurnal variation of canopy stomal conductance (G_c) dominated that of T/ET. The seasonal dynamics of T/ET was mainly shaped by that of leaf area index (LAI), vapor pressure deficit (VPD) and air temperature (T_a) through altering G_c and the portion that the energy absorbed by canopy (P_EC) at temperate forest (CBS), while the seasonal dynamics of T/ET at subtropical forests (QYZ and DHS) were mainly affected by T_a, net radiation, VPD, and soil water content through altering G_c and soil surface conductance (G_s). The variation of mean annual G_c governed the interannual varaition and spatial variation of T/ET. Therefore, forests in Eastern China played an important role in regulating climate through T and G_c primarily affected the spatial and temproal variations of the role of forest T in regulating climate.     

The accuracy of the thermal dissipation technique for estimating sap flow is affected by the radial distribution of conduit diameter and density

There are conflicting reports on the accuracy of the thermal dissipation probe (TDP, the Granier method) measurement using the original formula, which is widely used to estimate the transpiration of individual trees and forest stands. In this article, six woody species of three wood types were used to study a possible association between TDP measurement accuracy and wood anatomical characteristics, including the vessel diameter and density, as well as sapwood depth. We found that TDP technique with Granier’s original equation underestimated the sap flux density in six species to various degrees, dependent on conduit size and sap flux. Our calibration using two conifers with small diameters and a high density of tracheids was relatively consistent with Granier’s calibration; however, because there were larger diameters and lower densities of vessels in the two diffuse-porous species, the original calibration significantly underestimated sap flow. Two ring-porous species had the largest diameters and lowest densities of vessels. In particular, Robinia pseudoacacia possessed the shallowest sap wood depth, less than a probe length. Our calibration for the ring-porous species, especially R. pseudoacacia, deviated far from the original calibration, which mostly underestimated the sap flow. The degree of underestimation was well associated with sap wood depth and the radial diameter and density distribution of conduits. Our results demonstrated that a new calibration must be operated for each species together with the sapwood depth determination and more probes may be applied for one stem in the field to obtain the more accurate sap flux. In addition, we investigated the effects of different environmental temperature and perfusing fluid composition on the TDP-based sap flux measurement. We found that an environmental temperature reduction from 25 to 0 °C did not alter the values of the maximum temperature difference (ΔT_m) between a heated probe and a reference probe when there was no sap flow, verifying that ΔT_m measured at night can be used as a reference in daytime.     

The Cooling Trend of Canopy Temperature During the Maturation,Succession, and Recovery of Ecosystems

The maximum exergy dissipation theory provides a theoretical basis for using surface temperature to measure the status and development of ecosystems, which could provide an early warning of rapid evaluation of ecosystem degradation. In the present study, we used the radiation balance of ecosystems to demonstrate this hypothesis theoretically. Further, we used empirical data to verify whether ecosystems gain more radiation, while lowering their surface temperatures, as they develop naturally. We analyzed 12 chronosequences from the FLUXNET database using meteorological data and heat fluxes. We included age, disturbance, and successional chronosequences across six climate zones. Net radiation (R _n) and the ratio of net radiation to global radiation (R _n/R _g) were used to measure the energy gain of the ecosystems. The maximum daily air temperature above the canopy (T _max) and thermal response number (TRN) were used to analyze the surface temperature trends with ecosystem natural development. The general trends of T _max, TRN, R _n, and R _n /R _g demonstrated that ecosystems become cooler and more stable, yet gain more energy, throughout their natural development. Among the four indicators, TRN showed the most consistent trends and highest sensitivity to ecosystem growth, succession, and recovery. Moreover, TRN was not significantly influenced by precipitation or wind. We propose that TRN can be used to rapidly evaluate or warn of ecosystem disturbance, senescence, and degradation without prior knowledge of species composition, nutrient status, and complex ecosystem processes.     

吉林不同生境防风夏季光合特性

采用LCpro+全自动便携式光合仪同时测定了长春、长岭和白城3个生境防风的光合作用及环境因子,研究不同生境防风净光合速率、蒸腾速率及水分利用效率等日变化特征及其与生态因子之间关系,并采用通径分析法分析各环境因子对防风净光合速率的直接和间接影响,探讨防风的光合生理规律,为进一步研究防风的光合生理提供依据。结果表明,各环境因子之间存在相互作用,在影响防风光合作用的环境因子中,光合有效辐射(PAR)是驱动因子,它的变化驱动了大气温度(Ta)、叶片温度(Tl)变化,Ta升高引起空气密度的变化是午间相对湿度(RH)和大气浓度CO2(Ca)降低的一个重要原因。环境因子中对Pn直接作用由大到小长春为TlTaPARCaRH,长岭为PARTlRHCaTa,白城为TaTlRHPARCa。长春、长岭PAR与Pn相关系数最大,而白城地区Ca与Pn相关系数最大,说明防风叶片对不同环境因子的反应有所不同。8月份3个地区防风净光合速率日变化均呈双峰型曲线,具有明显的光合"午休"现象。长春、长岭地区防风光合"午休"现象主要由气孔因素引起,白城地区防风光合"午休"现象主要由非气孔因素引起的。3个生境防风的Pn与Gs之间均呈显著的正相关关系,白城的Gs比长春、长岭地区的低44%左右,这是防风为适应高温干旱的环境而形成的生理反应。不同生境土壤含水量间差异显著,白城地区土壤水分含量很低,只有20.35%,仅是长春土壤含水量的56.50%,而空气温度较高,强辐射使气孔叶片蒸腾失水加剧,导致叶片含水量也很低,仅仅为56.60%,高温干旱使防风叶温升高,防风磷酸化、电子传递、酶活性等受到抑制,导致光合速率降低。因此,白城地区长期高温胁迫是限制防风Pn的主要原因。     

Crassulacean Acid Metabolism and Photochemical Efficiency of Photosystem II in the Adaxial and Abaxial Parts of the Succulent Leaves of Kalanchoë daigremontiana Grown at Four Photon Flux Densities

Kalanchoë daigremontiana, a species possessing crassulacean acid metabolism, was grown at four photon flux densities (1300, 400, 60, and 25 micromole photons per square meter per second). In leaves which had developed at 1300 and 400 micromole photons per square meter per second, CO₂ was mainly incorporated through the lower, shaded leaf surfaces, and the chlorenchyma adjacent to the lower surfaces showed a higher degree of nocturnal acid synthesis than the chlorenchyma adjacent to the upper surfaces. In leaves acclimated to 60 and 25 micromole photons per square meter per second, the gradient in CAM activity was reversed, i.e. more CO₂ was taken up through the upper than through the lower surfaces and nocturnal acidification was higher in the tissue next to the upper surfaces. Total net carbon gain and total nocturnal acid synthesis were highest in leaves which had developed at 400 micromole photons per square meter per second. Chlorophyll content was markedly reduced in leaves which had developed at 1300 micromole photons per square meter per second, especially in the exposed adaxial parts. There was also a sustained reduction in photosystem II photochemical efficiency as indicated by measurements of the ratio of variable over maximum chlorophyll a fluorescence. These findings suggest that, at high growth photon flux densities, the reduced activity of the exposed portions of these succulent leaves is caused by (a) the adverse effects of excess light, (b) together with a genotypic component which favors CO₂ uptake and acid synthesis in the abaxial (lower) leaf parts even when light is not or only marginally excessive. This latter component is predominant at medium photon flux densities, e.g. at 400 micromole photons per square meter per second. It becomes overridden, however, under conditions of deep shade when strongly reduced light levels in the abaxial parts of the leaf chlorenchyma severely limit photosynthesis.