Urban Tree Species Show the Same Hydraulic Response to Vapor Pressure Deficit across Varying Tree Size and Environmental Conditions

Background

The functional convergence of tree transpiration has rarely been tested for tree species growing under urban conditions even though it is of significance to elucidate the relationship between functional convergence and species differences of urban trees for establishing sustainable urban forests in the context of forest water relations.

Methodology/Principal Findings

We measured sap flux of four urban tree species including Cedrus deodara, Zelkova schneideriana, Euonymus bungeanus and Metasequoia glyptostroboides in an urban park by using thermal dissipation probes (TDP). The concurrent microclimate conditions and soil moisture content were also measured. Our objectives were to examine 1) the influence of tree species and size on transpiration, and 2) the hydraulic control of urban trees under different environmental conditions over the transpiration in response to VPD as represented by canopy conductance. The results showed that the functional convergence between tree diameter at breast height (DBH) and tree canopy transpiration amount (E _c) was not reliable to predict stand transpiration and there were species differences within same DBH class. Species differed in transpiration patterns to seasonal weather progression and soil water stress as a result of varied sensitivity to water availability. Species differences were also found in their potential maximum transpiration rate and reaction to light. However, a same theoretical hydraulic relationship between G _c at VPD = 1 kPa (G _cref) and the G _c sensitivity to VPD (−dG _c/dlnVPD) across studied species as well as under contrasting soil water and R _s conditions in the urban area.

Conclusions/Significance

We concluded that urban trees show the same hydraulic regulation over response to VPD across varying tree size and environmental conditions and thus tree transpiration could be predicted with appropriate assessment of G _cref.     

Transpiration efficiency and apparent cuticular transpiration in some c3 and c4 plants

Amphistomatous C₃ (Nicotiana tabacum L., Datura stramonium L.) and C₄ (Sorghum saccharatum Pers. and Zea mays L.) species were examined to find how (if at all) their inherent differences in water-use economy are reflected in apparent cuticular transpiration or vice versa. Transpiration efficiency (TE) was calculated from steady state photosynthesis (A) and transpiration (E) rates estimated for the upper side of the leaf after light induction of stomata opening. Apparent cuticular transpiration (’E_c) was measured as the part of transpiration which was not eliminated by convective counteraction of the air stream passing across the amphistomatous leaf: total pressure difference (AP) across the leaf was increased and the minimal value of E_ΔPτ0 was taken as the apparent cuticular transpiration rate (’E_c). ’E_c was treated relative to E at AP equal to zero (E_GDP=0), E’_cr. Measurements were carried out under two leaf-air vapour pressure differences (VPD). E_r (i.e. E_GDPτ0/E_GDP=0) versus GDP patterns differed qualitatively between the investigated C₃ and C₄ plants. TE increased and ’E_cr decreased from tobacco, stramony, maize to sorghum for both VPD of air. ’E_cr and TE were approximately linearly related, the slope being dependent on VPD. The increase in VPD resulted in larger E and slightly smaller epidermal conductance (g) at GDP equal to zero. Both E’_cr and E’_cr decreased markedly at the same time especially, for species with high TE. The results were considered as an indirect confirmation that E’_c values estimated by the technique used reflect species-specific differences in external peristomatal and cuticular vapour loss, at least in a relative sense.     

Sap flow characteristics and responses to summer rainfall for Pinus tabulaeformis and Hippophae rhamnoides in the Loess hilly region of China

As a major driving element of the structure and function of arid and semiarid ecosystems, rainfall is the essential factor limiting plant biological processes. To clarify the characteristics of transpiration and responses to summer rainfall, sap flow density (F_d) of Pinus tabulaeformis and Hippophae rhamnoides was monitored using thermal dissipation probes. In addition, midday leaf water potential (ψ_m) and leaf stomatal conductance (G_s) were also analyzed to determine water use strategies. The results indicated that the diurnal variation in the normalized F_d values exhibited a single‐peak curve for P. tabulaeformis, while H. rhamnoides showed multiple peaks. The normalized F_d for P. tabulaeformis remained relatively stable regardless of rainfall events. However, there was also a significant increase in the normalized F_d for H. rhamnoides in response to rainfall in June and August (p < .05), although no significant differences were observed in July. The normalized F_d values for P. tabulaeformis and H. rhamnoides fitted well with the derived variable of transpiration, an integrated index calculated from the vapor pressure deficit and solar radiation (R_s), using an exponential saturation function. The differences in fitting coefficients suggested that H. rhamnoides showed more sensitivity to summer rainfall (p < .01) than P. tabulaeformis. Furthermore, during the study period, P. tabulaeformis reduced G_s as soil water decreased, maintaining a relatively constant ψ_m; while H. rhamnoides allowed large fluctuations in ψ_m to maintain G_s. Therefore, P. tabulaeformis and H. rhamnoides should be considered isohydric and anisohydric species, respectively. And more consideration should be taken for H. rhamnoides in the afforestation activities and the local plantation management under the context of the frequently seasonal drought in the loess hilly region.     

半干旱区城市环境下油松林分蒸腾特征及其影响因子

在城市环境下,由于不透水地面面积的增加,土壤-植物-大气之间水汽循环减弱,水汽调节能力差,因而研究城市树木蒸腾对环境因子的响应对于城市进行合理的水汽调节具有重要意义。于2017年生长季,在内蒙古呼和浩特市区树木园内选择58年生油松(Pinus tabulaeformis Carr.)作为研究树种,采用热扩散法测定其树干液流,并同步监测气象因子和土壤含水量变化,利用彭曼公式计算冠层气孔导度。结果表明:(1)生长季内,油松林分蒸腾存在明显日、月变化,晴天天气下林分蒸腾日变化呈单峰曲线,月林分蒸腾量5月最大,其次是7月、8月、6月和9月,分别为20.96、19.89、18.09、17.25 mm和7.49 mm。(2)油松林分蒸腾与饱和水汽压差、太阳总辐射、土壤含水量和风速均存在极显著相关关系(P0.01),太阳总辐射、饱和水汽压差和土壤含水量是影响林分蒸腾的主要环境因子(R~2=0.47、R~2=0.31和R~2=0.16),风速对林分蒸腾的影响程度最小(R~2=0.12);不同降雨量对林分蒸腾的影响作用不同,10 mm以上的日降雨量对油松林分蒸腾作用明显。(3)除环境因子外,油松叶片气孔通过响应环境变化控制蒸腾作用,当饱和水汽压差1.5 kPa时,叶片气孔对饱和水汽压差的响应更敏感;当太阳总辐射250 W/m~2时,叶片气孔对蒸腾起促进作用,超过该阈值,叶片气孔关闭从而抑制树木蒸腾。     

Variations in bulk canopy conductance of an irrigated olive (Olea europaea L.) orchard

Measurements of transpiration (E_p) and bulk canopy conductance (G_c) of an irrigated olive orchard during three years are presented here. Transpiration was obtained subtracting the hourly soil evaporation (E_s, calculated with a specific model) from the total evapotranspiration of the orchard measured with eddy covariance. G_c was obtained inverting the Penman–Monteith model for the transpiration flux. The orchard G_c showed a seasonal pattern of variation, very similar in all the three years of experiment. G_c is very low in the beginning of the growing cycle (spring), then it rises slowly until a maximum until the beginning of July. From that point on, it shows a sharp fall in August, leading it to approximately half the maximum value of July. During Autumn, high conductances can be reached again, when favourable conditions are met. To help discerning the direct effect of the meteorological environment on the stomata, an analysis with a stomatal conductance model was performed. The Jarvis model was optimised on small periods of G_c data in different times of the year, and used to obtain a simulated G_c for a standard environment. This modelled-normalized G_c showed essentially the same seasonal pattern than the measured G_c, indicating that the source of seasonal variation is not due to the direct environmental effect on the stomatal system.     

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.     

Phreatophytes under stress: transpiration and stomatal conductance of saltcedar (Tamarix spp.) in a high-salinity environment

Background and aims

We sought to understand the environmental constraints on an arid-zone riparian phreatophtye, saltcedar (Tamarix ramosissima and related species and hybrids), growing over a brackish aquifer along the Colorado River in the western U.S. Depth to groundwater, meteorological factors, salinity and soil hydraulic properties were compared at stress and non-stressed sites that differed in salinity of the aquifer, soil properties and water use characteristics, to identify the factors depressing water use at the stress site.

Methods

Saltcedar leaf-level transpiration (E_L), LAI, and stomatal conductance (G_S) were measured over a growing season (June–September) with Granier and stem heat balance sensors and were compared to those for saltcedar at the non-stress site determined in a previous study. Transpiration on a ground-area basis (E_G) was calculated as E_L?×?LAI. Environmental factors were regressed against hourly and daily E_L and G_S at each site to determine the main factors controlling water use at each site.

Results

At the stress site, mean E_G over the summer was only 30 % of potential evapotranspiration (ET_o). G_S and E_G peaked between 8 and 9 am then decreased over the daylight hours. Daytime G_S was negatively correlated with vapor pressure deficit (VPD) (P?<?0.05). By contrast, E_G at the non-stress site tracked the daily radiation curve, was positively correlated with VPD and was nearly equal to ET_o on a daily basis. Depth to groundwater increased over the growing season at both sites and resulted in decreasing E_G but could not explain the difference between sites. Both sites had high soil moisture levels throughout the vadose zone with high calculated unsaturated conductivity. However, salinity in the aquifer and vadose zone was three times higher at the stress site than at the non-stress site and could explain differences in plant E_G and G_S.

Conclusions

Salts accumulated in the vadose zone at both sites so usable water was confined to the saturated capillary fringe above the aquifer. Existence of a saline aquifer imposes several types of constraints on phreatophyte E_G, which need to be considered in models of plant water uptake. The heterogeneous nature of saltcedar E_G over river terraces introduces potential errors into estimates of ET by wide-area methods.     

Transpiration of <Emphasis Type="Italic">Cyclobalanopsis glauca</Emphasis> (syn. <Emphasis Type="Italic">Quercus glauca</Emphasis>) stand measured by sap-flow method in a karst rocky terrain during dry season

Seasonal drought may have a high impact on the karst ecosystem. The transpiration from Cyclobalanopsis glauca (syn. Quercus glauca) stand on a rocky hilly slope in South China was measured during the dry period of 2006 by using the Granier’s sap-flow method. During the experimental period, maximum sap flux density (J _s) ranged from 20 to 40 g H₂O m⁻² s⁻¹ according to diameter of breast height (DBH) of individual trees. On sunny days, daily transpiration varied between 3.4 and 1.8 mm day⁻¹. Transpiration of C. glauca was closely correlated to the radiation, air temperature, and vapor pressure deficit (VPD). Soil moisture was a very important factor influencing transpiration. The very low soil water content might result in low stand transpiration even when VPD is high, but high soil water content might also result in low transpiration if it was low VPD. However, VPD rather than soil moisture, affected largely the stand transpiration under high soil water content. The amount of transpiration was much more than that of the total soil moisture loss during the continuous sunny days, indicating that the dry shallow soils were probably not the only source for root-uptake water. C. glauca grows deep roots through the rock fissures of epikarst, indicating that epikarst might be another main source for sustaining transpiration in response to dry demand in autumn. Therefore, a large amount of deep roots of karst species would be a very important hydraulic connecting from the epikarst to above ground by transpiration, which would promote the biogeochemical process in a karst system.     

亚热带岩溶区典型常绿和落叶树种的蒸腾特征及其对环境因子的响应

植物蒸腾是水循环的重要组成部分,为了解亚热带岩溶区树木的蒸腾耗水情况,探究气候和水文地质条件对植物蒸腾的影响,运用Granier热耗散探针技术,对亚热带岩溶区次生林内的常绿树种女贞(L.lucidum)和落叶树种刺槐(R.pseudoacacia)的树干液流进行了连续监测,并同步监测了气象因子及土壤含水率(SMC),探讨在不同时间尺度下两种生活型树种的蒸腾特征及其对环境因子的响应。结果表明：(1)在季节尺度下,影响两树种整树蒸腾量(ET)的主要因子为太阳辐射强度(Rs)、气温(T)和水汽压亏缺(VPD);女贞蒸腾量(ET_L)表现为夏季(1.29 kg/h)>春季(0.57 kg/h)>冬季(0.15 kg/h)>秋季(0.13 kg/h),刺槐蒸腾量(ET_R)表现为夏季(0.90 kg/h)>春季(0.31 kg/h)>秋季(0.16 kg/h)>冬季(0.04 kg/h)。(2)在日尺度下,晴天两树种ET呈现出明显的单峰日变化,且主要影响因子均为T、VPD和Rs;但由于常绿和落叶树种的生理特征差异,降雨时...     

黄土高原水土保持林对土壤水分的影响

黄土高原植被恢复的限制因素主要是土壤水分,植被与土壤水分关系的研究对黄土高原植被恢复具有重要意义.2008年7月1日至2009年10月31日间采用EnviroSMART土壤水分定位监测系统以每30min监测1次的频度,对晋西黄土区刺槐人工林地、油松人工林地、次生林地的土壤水分变化进行了研究.研究得出:次生林地0-150 cm土层中平均蓄水量为331.95mm,刺槐人工林地为233.85 mm,有整地措施的油松人工林地为314.85mm,刺槐人工林比次生林多消耗的98.10mm土壤水分主要来源于80 cm以下土层.次生林主要消耗0-80 cm土层的水分,而人工林不但对0-80 cm土层水分的消耗量大于次生林,对深层土壤的消耗也较次生林大,这将有可能导致人工林地深层土壤的“干化”.在土壤水分减少期(11-1月)刺槐人工林土壤水分的日均损耗量为0.86mm、油松人工林为0.82 mm、次生林为0.84 mm.土壤水分缓慢恢复期(2-5月)刺槐人工林地土壤水分的恢复速度0.90mm/d,油松人工林地为0.53 mm/d、次生林地为0.79 mm/d.土壤水分剧烈变化期(5-10月)刺槐人工林地土壤水分含量的极差为95.71mm,油松人工林地为179.1mm,次生林地为72.03mm.在干旱少雨的黄土高原进行植被恢复时,应多采取封山育林等方式,依靠自然力量形成能够与当地土壤水资源相协调的次生林,是防止人工植被过度耗水形成“干化层”、保障水土保持植被持续发挥生态服务功能的关键.     

Up-scaling to stand transpiration of an Asian temperate mixed-deciduous forest from single tree sapflow measurements

Species diversity in mixed forest stands is one of the factors that complicate up-scaling of transpiration from individual trees to stand level, since tree species are architecturally and functionally different. In this study, thermal dissipation probes were used to measure sap flow in five different tree species in a mixed-deciduous mountain forest in South Korea. Easily measurable tree characteristics that could serve to define individual tree water use among the different species were employed to scale up transpiration from single trees to stand level. Tree water use (TWU) was derived from sap flux density (SFD) and sapwood area (SA). Canopy transpiration E was scaled from TWU while canopy conductance (g _c) was computed from E and VPD. SFD, TWU and g _c were correlated with tree diameter at breast height (DBH) for all the five measured species (SFD: R ² = 0.21, P = 0.036; TWU: R ² = 0.83, P < 0.001; g _c: R ² = 0.63, P < 0.001). Maximum stand transpiration (E) during June, before the onset of the Asian monsoon rains, was estimated at 0.97 ± 0.12 mm per day. There was a good (R ² = 0.94, P < 0.0001) agreement between measured and estimated E using the relationship between TWU and DBH. Our study shows that using functional models that employ converging traits among species could help in estimating water use in mixed forest stands. Compared to SA, DBH is a better scalar for water use of mixed forest stands since it is non-destructive and easily obtainable.     

黄土丘陵区人工林刺槐和油松凋落叶在不同降雨时期的分解特征

凋落物分解是维持生态系统养分循环和能量流动的关键过程,但在雨热同期的黄土丘陵区,不同降雨时期凋落物基质质量动态对该区不同树种凋落物分解速率的影响还不清楚。采用凋落物分解袋法,基于野外原位分解实验分析黄土丘陵区主要人工林刺槐（Robinia pseudoacacia Linn.）和油松（Pinus tabulaeformis Carr.）凋落叶在不同降雨时期的分解特征和分解过程中凋落叶基质质量的变化与分解速率之间的关系。研究结果发现：（1）经过391 d的分解,刺槐凋落叶的平均质量损失速率为（51.0±8.44）mg/d,显著地高于油松凋落叶（36.7±4.83）mg/d;雨季期间两树种凋落叶的质量损失速率均显著地高于旱季,其中夏季多雨期间凋落叶的质量损失速率最高,冬季微量降雨期间质量损失速率最低。（2）在整个分解过程中两树种凋落叶C和N含量都表现为净释放且主要发生在雨季,P含量表现为释放与富集交替进行;刺槐凋落叶C/N比、C/P比和N/P比呈波动的趋势,油松凋落叶C/N比则显著地增加且在夏季多雨期出现峰值,C/P比呈波动的状态,N/P比变化较小。（3）不同降雨时期刺槐凋落叶的质量损失速率与凋落叶P含量动态显著正相关,与C含量、C/P比和N/P比动态显著负相关。油松凋落叶质量损失速率与C/N比动态显著正相关,与C、N含量动态显著负相关,与N/P比动态呈负二次函数的关系。这些结果说明黄土丘陵区刺槐和油松凋落叶在不同降雨时期分解速率之间的差异显著且两树种凋落叶的分解都集中在雨季期间;此外凋落叶分解主要受到凋落叶N含量和N/P比动态变化的制约,与刺槐凋落叶相比,N含量与N/P比对油松凋落叶的限制作用更强。     

Transpiration and canopy conductance in an inner alpine Scots pine (Pinus sylvestris L.) forest

Canopy transpiration (E_c) of a 150-year-old Pinus sylvestris L. stand in an inner Alpine dry valley, Tyrol, Austria was estimated throughout two growing seasons 2011 and 2012 by means of xylem sap flow measurements. Although there were prolonged periods of limited soil water availability, E_c did not show a clear trend with respect to soil water availability and averaged 0.4 ± 0.19 mm day⁻¹ under conditions of non-limiting soil water availability and 0.37 ± 0.17 mm day⁻¹ when soil water availability was limited. This is because canopy conductance declined significantly with increasing evaporative demand and thus significantly reduced tree water loss. The growing season total of E_c was 74 mm and 88 mm in 2011 and 2012, respectively, which is significantly below the values estimated for other P. sylvestris forest ecosystems in Central Europe, and thus reflecting a strong adaptation to soil drought during periods of high evaporative demands.     

Environmental and physiological regulation of transpiration in tropical forest gap species: the influence of boundary layer and hydraulic properties

Environmental and physiological regulation of transpiration were examined in several gap-colonizing shrub and tree species during two consecutive dry seasons in a moist, lowland tropical forest on Barro Colorado Island, Panama. Whole plant transpiration, stomatal and total vapor phase (stomatal + boundary layer) conductance, plant water potential and environmental variables were measured concurrently. This allowed control of transpiration (E) to be partitioned quantitatively between stomatal (g _s) and boundary layer (g _b) conductance and permitted the impact of invividual environmental and physiological variables on stomatal behavior and E to be assessed. Wind speed in treefall gap sites was often below the 0.25 m s^–1 stalling speed of the anemometer used and was rarely above 0.5 m s^–1, resulting in uniformly low g _b (c. 200–300 mmol m^–2 s^–1) among all species studied regardless of leaf size. Stomatal conductance was typically equal to or somewhat greater than g _b. This strongly decoupled E from control by stomata, so that in Miconia argentea a 10% change in g _s when g _s was near its mean value was predicted to yield only a 2.5% change in E. Porometric estimates of E, obtained as the product of g _s and the leaf-bulk air vapor pressure difference (VPD) without taking g _b into account, were up to 300% higher than actual E determined from sap flow measurements. Porometry was thus inadequate as a means of assessing the physiological consequences of stomatal behavior in different gap colonizing species. Stomatal responses to humidity strongly limited the increase in E with increasing evaporative demand. Stomata of all species studied appeared to respond to increasing evaporative demand in the same manner when the leaf surface was selected as the reference point for determination of external vapor pressure and when simultaneous variation of light and leaf-air VPD was taken into account. This result suggests that contrasting stomatal responses to similar leaf-bulk air VPD may be governed as much by the external boundary layer as by intrinsic physiological differences among species. Both E and g _s initially increased sharply with increasing leaf area-specific total hydraulic conductance of the soil/root/leaf pathway (G _t), becoming asymptotic at higher values of G _t. For both E and g _s a unique relationship appeared to describe the response of all species to variations in G _t. The relatively weak correlation observed between g _s and midday leaf water potential suggested that stomatal adjustment to variations in water availability coordinated E with water transport efficiency rather than bulk leaf water status.     

Transpiration efficiency of the grapevine cv. Semillon is tied to VPD in warm climates

Water‐use efficiency in grapevines is dependent on the aerial and below‐ground environment of the plant. Specifically, transpiration efficiency, the ratio of net carbon fixation to water loss, may be influenced by soil moisture and the leaf‐to‐air vapour pressure deficit (VPD) in the soil–plant–atmosphere continuum. The interactive effect of these abiotic parameters, however, has not been suitably investigated in field‐grown grapevines. Accordingly, gas exchange of an anisohydric variety, Semillon, was assessed across a number of vineyards in two warm grape‐growing regions of New South Wales (NSW) to ascertain how soil moisture and VPD interact to affect transpiration efficiency at the leaf level. Leaf gas exchange measurements demonstrated that the rate of transpiration (E) was driven by VPD, particularly under high soil moisture. Both high VPD and low soil moisture decreased photosynthesis (A) and instantaneous leaf transpiration efficiency (A/E). Increased intrinsic leaf transpiration efficiency (A/g) in response to drying soil was limited to vines growing in a non‐irrigated vineyard. In this site, A/g was negatively related to vine water status. VPD did not have a substantial influence on A/g in any vineyard. While VPD is the main driver for A/E, soil moisture is an important determinant of A/g. Under high VPD, stomatal closure in Semillon leaves was not substantial enough to suitably curtail transpiration, and as a consequence A/E declined. These data indicate that in warm climates, irrigation scheduling of anisohydric varieties must take into account both VPD and soil moisture so that vine water status can be maintained.     

塔里木河下游河岸柽柳林冠层导度变化特征及模拟

冠层导度(G_c)对植被的蒸腾和光合作用具有重要影响。利用涡度相关仪器实测了塔里木河下游河岸柽柳林地的蒸散发,以及气象因子(温度、湿度、总辐射、光和有效辐射),并利用Penman-Monteith公式计算了柽柳林在2013年生长季的冠层导度。结果显示:柽柳林冠层导度日变化过程在8:00左右迅速增大,于10:30左右达到最大值,之后缓慢下降,18:00左右快速降低;柽柳林冠层导度季节变化过程总体显示,展叶期缓慢上升,落叶期迅速下降,生长盛期缓慢波动下降;研究区,叶面积指数(LAI)是影响柽柳冠层导度季节变化的主要因素,其次为温度(T)、光合有效辐射(PAR)、总辐射(S)、空气饱和差(VPD);四元线性回归方程可以较好地拟合冠层导度与各因子的关系,利用2013年奇数天数据建立回归方程,对偶数天冠层导度值进行模拟和验证,RMSE值为0.169 mm/s,NSE值为0.814,达到了较高的模拟精度。     

黄土高原油松冠层气孔导度和蒸腾变化特征与模拟

油松是黄土高原重要的造林树种,模拟其冠层气孔导度和蒸腾对区域水量平衡计算和人工林可持续经营具有重要意义。基于2015—2018年TDP(Thermal dissipation probes)方法所测得液流数据,分析了黄土高原地区油松冠层平均气孔导度(g_c)与冠层蒸腾(Tr)的变化特征与影响因素,并采用Penman-Monteith公式和Jarvis型气孔导度模型模拟了其g_c和Tr的变化过程,结果表明：(1)该地区油松g_c和Tr日内变化均呈现单峰型,日均蒸腾耗水量为(1.25±0.57) mm/d,生长季(4—10月)总蒸腾耗水量均值为195.47 mm。(2)g_c的日内变化受太阳辐射(Rad)驱动(偏相关系数为0.65),当Rad高于300 W/m²时,驱动作用减弱;g_c的日内变化受水汽压亏缺(VPD)控制(偏相关系数为-0.41),随VPD的增加而降低;g_c的日际变化受土壤水分限制(偏相关系数为0.46),当根区相对有效含水率(RE...     

Increased leaf area dominates carbon flux response to elevated CO2 in stands of Populus deltoides (Bartr.)

We examined the effects of atmospheric vapor pressure deficit (VPD) and soil moisture stress (SMS) on leaf‐ and stand‐level CO₂ exchange in model 3‐year‐old coppiced cottonwood (Populus deltoides Bartr.) plantations using the large‐scale, controlled environments of the Biosphere 2 Laboratory. A short‐term experiment was imposed on top of continuing, long‐term CO₂ treatments (43 and 120 Pa), at the end of the growing season. For the experiment, the plantations were exposed for 6–14 days to low and high VPD (0.6 and 2.5 kPa) at low and high volumetric soil moisture contents (25–39%). When system gross CO₂ assimilation was corrected for leaf area, system net CO₂ exchange (SNCE), integrated daily SNCE, and system respiration increased in response to elevated CO₂. The increases were mainly as a result of the larger leaf area developed during growth at high CO₂, before the short‐term experiment; the observed decline in responses to SMS and high VPD treatments was partly because of leaf area reduction. Elevated CO₂ ameliorated the gas exchange consequences of water stress at the stand level, in all treatments. The initial slope of light response curves of stand photosynthesis (efficiency of light use by the stand) increased in response to elevated CO₂ under all treatments. Leaf‐level net CO₂ assimilation rate and apparent quantum efficiency were consistently higher, and stomatal conductance and transpiration were significantly lower, under high CO₂ in all soil moisture and VPD combinations (except for conductance and transpiration in high soil moisture, low VPD). Comparisons of leaf‐ and stand‐level gross CO₂ exchange indicated that the limitation of assimilation because of canopy light environment (in well‐irrigated stands; ratio of leaf : stand=3.2–3.5) switched to a predominantly individual leaf limitation (because of stomatal closure) in response to water stress (leaf : stand=0.8–1.3). These observations enabled a good prediction of whole stand assimilation from leaf‐level data under water‐stressed conditions; the predictive ability was less under well‐watered conditions. The data also demonstrated the need for a better understanding of the relationship between leaf water potential, leaf abscission, and stand LAI.     

六盘山典型森林伴随降水的总有机碳(TOC)通量变化特征

在六盘山香水河小流域,选择6种典型森林样地,测定了2011年生长季的大气降水、穿透水、干流、枯落物渗漏水和主根系层(0—30 cm深)土壤渗漏水的总有机碳(TOC)浓度及其相应的通量变化。结果表明,在降水转化为由穿透雨和干流组成的林下降水中,所有样地的TOC浓度都不同程度地增大;虽然林冠截持使林下降水减小,但因雨水淋洗和与林冠发生碳交换,各样地林下降水携带的生长季TOC通量(kg/hm2)(华北落叶松人工林132.28、华山松次生林106.56、油松人工林94.10、灌木林79.49、桦木林66.52、辽东栎次生林63.01)都比林外降水(53.17)不同程度地明显增大,整体看来,林冠的TOC淋出作用在针叶林很大,在阔叶林较弱。在6种森林样地的枯落物层渗漏水中,其TOC浓度彼此相差不大,平均为24.51 mg/L,高于林冠穿透水的TOC浓度;受枯落物截持部分降水及与枯落物TOC交换的影响,4个样地枯落物渗漏水的TOC通量(kg/hm2)(桦木次生林84.35、野李子灌丛129.35、辽东栎次生林79.21、油松人工林114.93)都比其林下降水TOC通量增加了,但华北落叶松人工林和华山松次生林的TOC通量分别降至90.76和104.90 kg/hm2。在测定的华北落叶松人工林和华山松次生林的主根系层(0—30 cm)土壤渗漏水中,TOC浓度均低于枯落物渗漏水;由于水量减小和与土壤发生碳交换,土壤渗漏水的TOC通量均显著低于枯落物渗漏水,两个林分样地分别降至43.04和66.33 kg/hm2。整体来看,林外降水携带的TOC输入通量在林地TOC输入中占有重要地位,林冠的TOC淋洗使其程度不同地增加TOC通量,枯落物层具有增加或减少TOC通量的作用,但主根系层土壤会显著减少TOC输出通量,所以是固定TOC的重要场所。