黄土高原半干旱区柠条(Caragana korshinskii)树干液流动态及其影响因子

应用热脉冲技术在黄土高原神木县六道沟小流域于2006年6月13至25日测定了两种不同密度柠条(Caragana korshinskii)群落的树干液流动态.同时测量了土壤水分、太阳辐射、大气温度、相对湿度、风速、水汽压亏缺和作物参考蒸散等环境因子,并根据植物蒸腾的P-M公式,反推计算冠层导度.结果表明,除风速外,柠条树木液流与太阳辐射、大气温度、相对湿度、水汽压亏缺、作物参考蒸散均显著相关,且可用太阳辐射的线性表达式来估测.不同密度群落的日蒸腾量随叶面积指数增大而增加,叶面积指数为2.3的群落平均日蒸腾为3.83mm d-1m-2,而叶面积指数为1.1的林分平均日蒸腾1.64mm d-1m-2.冠层导度与气象因子关系复杂,当土壤水分不存在亏缺时,冠层导度与太阳辐射、大气温度、作物参考蒸散因子显著相关,与水汽亏缺和相对湿度因子无相关性;当土壤水分存在亏缺时,冠层导度与太阳辐射、大气温度、作物参考蒸散因子无相关关系,而与水汽亏缺和相对湿度因子显著相关.     

樱桃冠层导度特征及模拟

为了揭示樱桃冠层蒸腾、冠层导度对环境因子的响应规律,评价Jarvis模型在樱桃冠层尺度上应用的适用性,利用Granier热消散式探针连续监测了北京四季青果林所试验地3年生盆栽樱桃(Prunus avium L.)4-8月份蒸腾动态变化,同步监测了气象与土壤水分数据。以实测液流为基础,利用Penman-Monteith方程反推方法获取了长期连续冠层导度,在分析樱桃冠层蒸腾、冠层导度的动态变化规律的基础上,采用十字交叉法对多元回归模型与Jarvis模型进行参数率与误差分析,结果显示盆栽樱桃冠层蒸腾规律性强、时滞效应小,不同辐射条件下,冠层导度随水汽压亏缺增加呈负指数函数下降趋势,采用水汽压亏缺、光合有效辐射、气温的不同组合方式构建了多元回归和Jarvis冠层模型,模拟结果显示Jarvis模型精度高于多元回归模型,环境因子对模型精度的影响程度依次为:水汽压亏缺光合有效辐射气温,考虑了水汽压亏缺和太阳辐射的Jarvis模型精度最高,最低相对误差仅为12.12%,均方根误差为0.271。     

整树水力导度协同冠层气孔导度调节森林蒸腾

冠层气孔导度决定森林的蒸腾效率,它对驱动水汽移动的水汽应力的响应受树木水力结构的影响,并随水汽压亏缺上升和水力导度下降而降低,维持水势在最低阈值之上,避免出现水力灾变,调控冠层蒸腾。由于叶形和树冠结构的特点,部分脱耦联反映了湿润地区阔叶林冠层与大气的水汽交换特征,单纯以气孔导度的变化难以完整描述水分通量的调节规律,因而,需要考虑冠层气孔导度与水力导度协同控制冠层蒸腾的潜在机理。通过整合叶片气孔气体交换、树干液流、冠层微气象和其他环境因子的野外观测值,估测不同时间尺度的森林冠层气孔导度与大气的脱耦联系数和变异范围,以基于树干液流的冠层蒸腾,结合叶片/土壤水势梯度计算的水力导度,分析水力导度影响冠层气孔导度响应水汽压亏缺的敏感性,可以揭示和阐明水力导度和冠层气孔导度联合调节森林蒸腾的机理,对准确估测全球变化背景下森林对水资源利用的潜在生态效应有明显的理论意义。     

北京山区侧柏林冠层-大气蒸腾导度模拟及环境因子响应

蒸腾导度模型是衡量冠层-大气界面水汽输出的重要阻力模型,研究其特征及对环境因子的响应,为揭示森林冠层-大气界面水汽输出阻力机制提供理论依据。以首都圈森林生态系统定位观测研究站侧柏林为研究对象,采用TDP热探针法测定侧柏林树干液流密度,同步监测光合有效辐射、饱和水汽压差、气温、风速等主要环境因子,分析冠层导度和空气动力学导度的动态变化,构建冠层-大气蒸腾导度模型并模拟,明确冠层-大气蒸腾导度对各环境因子的响应关系。结果表明:蒸腾导度季节变化表现为非生长季与冠层导度趋势一致,生长季与空气动力学导度趋势一致,全年均为单峰趋势。冬季蒸腾导度与冠层导度保持较稳定差值(45 mol m^(-2 )s^-1左右),其他季节蒸腾导度与冠层导度、空气动力学导度的最大差值,均在各季节冠层导度、空气动力学导度的峰值水平。全年日均蒸腾导度冬季最大(86.92 mol m^(-2 )s^-1),其他季节较小且稳定(40—50 mol m^(-2 )s^-1之间)。在非生长季各环境因子对蒸腾导度的影响与对冠层导度的影响基本一致,温度为主要影响因子(r=-0.198),其他环境因子影响较小(r<0.1);在生长季中风速为主要影响因子(r=0.488),光合有效辐射(r=0.228)和饱和水汽压差(r=-0.299)的影响明显升高,温度的影响降低(r=0.114)。蒸腾导度模型较好的模拟了冠层-大气界面侧柏蒸腾不同季节的变化规律,阐明了各环境因子和冠层导度、空气动力学导度对蒸腾导度的影响机制,证实在生长季应重视空气动力学导度对蒸腾的影响。     

城市绿地冠层导度季节变化及其环境调控

冠层导度(canopy conductance,gc)是生态系统对环境响应的敏感性指标,探讨冠层导度对环境因素的响应模式对了解生态系统生产力的变化模式至关重要。城市绿地作为人为设计的复合生态系统,其冠层导度变化规律及其对环境因子的响应亟待明确。基于涡度相关方法,本研究调查了北京奥林匹克森林公园2012-2016年连续5年的冠层导度的季节动态变化及其对空气温度(Ta)、光合有效辐射(PAR)、饱和水汽压差(VPD)以及土壤含水量(VWC)等环境因子的响应,利用统计回归方法分析了环境因子对冠层导度的影响,最后分析了冠层导度对总生态系统生产力(GEP)的影响。结果表明:2012-2016年的冠层导度依次为3.97、3.28、2.13、3.95和5.07 mm·s^-1,5年平均值为3.69±1.99 mm·s^-1;季节变化表现为从4月开始逐渐升高,在7、8月达到最大值后逐渐降低;在季节尺度上,VWC和Ta是影响冠层导度的主要环境因子,冠层导度随它们的增加而增大;而PAR和VPD对冠层导度的影响存在年际间的差异;5年间,GEP随着冠层导度的升高显著增大;城市绿地中,季节尺度上土壤水分的增加和气温的升高显著增加冠层导度,从而促进GEP。     

辽西农林复合系统中杨树冠层导度特征

利用Granier热扩散式探针法对辽西杨树-玉米复合系统的杨树树干液流进行连续测定,并对环境因子(空气温度、空气湿度、净辐射、风速、土壤温度和土壤湿度)进行同步观测,结合Penman-Monteith方程计算冠层导度值.结果表明:研究区杨树冠层导度日变化呈“单峰型”曲线,季节变化表现为波动式下降趋势;冠层导度随着饱和水汽压差增加呈负对数下降,5-9月,冠层导度对水汽压差变化的敏感性逐渐下降;冠层导度与太阳辐射呈正相关关系;太阳辐射越大,冠层导度曲线下降幅度越大.不同月份,相同环境因子与冠层导度的相关程度不同.从整个生长季来看,与冠层导度相关性最显著的环境因子是饱和水汽压差.     

鼎湖山南亚热带天然针阔叶混交林臭氧吸收特征

针阔叶混交林是我国南亚热带针叶林向地带性常绿阔叶林演替的中间林分类型,为我国南亚地区主要森林类型,发挥着重要的生态系统服务功能。基于树干液流技术和对臭氧浓度的连续监测,评价该森林类型的臭氧吸收特征和能力有着重要的环境生态学意义。对鼎湖山天然针阔叶混交林优势种马尾松(Pinus manssoniana)、锥栗(Castanopsis chinensis)、木荷(Schima superba)和华润楠(Machilus chinensis)在自然环境条件下的臭氧吸收能力进行了分析研究。结果表明:在日尺度上,4个优势树种的冠层气孔对臭氧导度(GO_3)和臭氧吸收通量(FO_3)均呈单峰型曲线,其最大值的时间在干季(10月至竖年3月)比湿季(4月至9月)滞后;季节尺度上,臭氧浓度在湿季达到最大值48.94 n L/L,湿季GO_3、FO_3和年臭氧吸收累积量(accumulative stomatal O_3flux,AFst)均显著高于干季(P 0.01),华润楠的臭氧吸收能力最强,在干季和湿季可分别达1.11 nmol m~(-2)s~(-1)和1.71nmol m~(-2)s~(-1)。随着水汽压亏缺(VPD)增大,优势种GO_3降低。光合有效辐射(PAR)超过1500 umol m~(-2)s~(-1)时,优势树种GO_3和FO_3呈下降趋势。针阔叶混交林的年臭氧吸收累积量超过了保护森林树木所采用的临界阈值,可认为鼎湖山针阔叶混交林受臭氧危害的潜在风险较高。     

华南荷木林冠层气孔导度对水汽压亏缺的响应

冠层气孔导度(Gs)是量化气孔在冠层尺度水平上表现的参数,能够表征森林冠层表面水汽和能量交换的动态.本研究利用Granier树干液流测定系统,连续监测华南地区荷木林的树干液流,通过尺度转换和扩展获得冠层蒸腾速率,结合微气象观测值,以Pen-man-Monteith公式计算了Gs,并比较不同土壤水分条件下Gs对水汽压亏缺的响应.结果显示,Gs与气孔气体交换方法实测的叶片气孔导度(gs)日变化相似,单位转换数值大小与实测gs数量级一致.Gs对水汽压亏缺的响应在干季和湿季有明显差别:(1)在土壤水分充足的湿季(土壤含水量θ ＞33％),Gs对水汽压亏缺的响应更敏感(偏相关系数-0.316),而在干季(θ＜23％)则对光合有效辐射的响应更敏感(偏相关系数0.885).(2)荷木林冠层-大气脱耦联系数(Ω)在湿季接近l,干季则较湿季小,说明湿季叶片的界面层较厚,水汽压亏缺对Gs影响较小,而光合有效辐射是控制Gs的主要环境因子.     

北京市绿化树种紫玉兰的蒸腾特征及其影响因素

加快的城市绿化,尤其是干旱和半干旱的温带和热带地区,对水的大量需求给未来水资源的合理利用提出了严峻挑战。了解紫玉兰蒸腾特征及环境和生物因子对蒸腾的调控有助于城市绿化树种的选择和养护。2008.5－2009.5于北京市中心教学植物园内利用热消散技术研究不同时间尺度上15年生紫玉兰（Magnolia liliflora）的耗水特征,环境因子同步监测。本研究分析了紫玉兰小时、日、月和季节尺度上的蒸腾特征。日尺度上,春夏季节紫玉兰树干液流速率呈双峰型曲线,蒸腾午休现象明显;秋季树干液流速率呈单峰型曲线,液流启动时间延后而结束时间提前;冬季树干液流速率日变化格局不明显,夜间液流速率高于白天液流速率。年内尺度上,按月份,从3月开始上升,5月达到峰值后略有下降,9月开始显著降低;按季节,春季>夏季>秋季>冬季;按物候期,从发芽到开花展叶阶段,整树耗水迅速增加,叶速生期达到最大值,休眠期整树耗水显著降低。值得注意的是：紫玉兰蒸腾午休现象明显;春季耗水型树种。一天之中总辐射和水汽压亏缺是影响紫玉兰整树耗水的主要因子;加上大气温度是日尺度和月尺度上影响紫玉兰耗水的关键因子;叶面积指数在开花展叶期是影响紫玉兰耗水的关键因子。由于城市中的适时灌溉和研究期间降水充足,土壤水分对紫玉兰的耗水几乎没有影响。     

刺槐树冠光合作用的空间异质性

林木冠层是森林与外界环境相互作用最直接的部分,冠层光合作用是研究森林生产力的基础。为了深入了解冠层内部光合作用的差异性,以陕西省永寿县马莲滩流域阳坡和阴坡立地的刺槐林为研究对象,对比分析了光合速率(An)、蒸腾速率(E)、水分利用效率(WUE)、气孔导度(gs)、羧化效率(Vc)、水汽压亏缺(VPD)、气孔限制值(ls)、光合有效辐射(PAR)、空气温度(Ta)在树冠不同层次、不同方位,以及不同坡向之间的差异性。结果表明,刺槐树冠不同层次的光合作用差异性显著,大部分光合生理生态指标表现为:上中下。对于阳坡刺槐,VPD、Ta、gs、E是影响不同层次An的主要因子;对于阴坡刺槐,VPD、E、PAR是影响不同层次An的主要因子。光合作用在刺槐树冠的不同方位没有显著差异,大多数光合指标变化很小,E、ls、PAR、Ta是影响不同方位An的主要因子。对于刺槐冠层内部的任何层次或方位,阴坡刺槐具有更高的日均An、E、Vc、VPD、ls,而阳坡刺槐具有更高的日均WUE、gs、PAR、Ta。阳坡刺槐树冠中层西方和阴坡中层东方的日总光合速率值,可以分别代表阳坡和阴坡刺槐整个冠层的日总光合速率。研究认为,在冠层水平模拟和估计森林生产力时,必须考虑冠层光合作用的空间异质性,对于从单木到林分的尺度推演和模型拟合具有重要的意义。     

Tree water dynamics in a drying and warming world

Disentangling the relative impacts of precipitation reduction and vapour pressure deficit (VPD) on plant water dynamics and determining whether acclimation may influence these patterns in the future is an important challenge. Here, we report sap flux density (F_D), stomatal conductance (G_s), hydraulic conductivity (K_L) and xylem anatomy in piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees subjected to five years of precipitation reduction, atmospheric warming (elevated VPD) and their combined effects. No acclimation occurred under precipitation reduction: lower G_s and F_D were found for both species compared to ambient conditions. Warming reduced the sensibility of stomata to VPD for both species but resulted in the maintenance of G_s and F_D to ambient levels only for piñon. For juniper, reduced soil moisture under warming negated benefits of stomatal adjustments and resulted in reduced F_D, G_s and K_L. Although reduced stomatal sensitivity to VPD also occurred under combined stresses, reductions in G_s, F_D and K_L took place to similar levels as under single stresses for both species. Our results show that stomatal conductance adjustments to high VPD could minimize but not entirely prevent additive effects of warming and drying on water use and carbon acquisition of trees in semi‐arid regions.     

Sap flow of the southern conifer, <Emphasis Type="Italic">Agathis australis</Emphasis> during wet and dry summers

Key message

Analysis of sap flux density during drought suggests that the large sapwood and rooting volumes of larger trees provide a buffer against drying soil.

Abstract

The southern conifer Agathis australis is amongst the largest and longest-lived trees in the world. We measured sap flux densities (F _d) in kauri trees with a DBH range of 20–176 cm to explore differences in responses of trees of different sizes to seasonal conditions and summer drought. F _d was consistently higher in larger trees than smaller trees. Peak F _d was 20 and 8 g m^?2 s^?1 for trees of diameters of 176 and 20 cm, respectively, during the wet summer. Multiple regression analysis revealed photosynthetically active radiation (PAR) and vapour pressure deficit (D) were the main drivers of F _d. During drought, larger trees were more responsive to D whilst smaller trees were more responsive to soil drying. Our largest tree had a sapwood area of 3,600 cm². Preliminary analysis suggests stem water storage provides a buffer against drying soil in larger trees. Furthermore, F _d of smaller trees had higher R ² values for soil moisture at 30 and 60 cm depth than soil moisture at 10 cm depth (R ² = 0.68–0.97 and 0.55–0.67, respectively) suggesting that deeper soil moisture is more important for these trees. Larger trees did not show a relationship between F _d and soil moisture, suggesting they were accessing soil water deeper than 60 cm. These results suggest that larger trees may be better prepared for increasing frequency and intensity of summer droughts due to deeper roots and/or larger stem water storage capacity.

     

Effects of increased UV-B radiation and elevated levels of tropospheric ozone on physiological processes in European beech (Fagus sylvatica)

As a consequence of the ongoing reduction of the stratospheric ozone layer, the vegetation is exposed to increasing levels of UV-B radiation (280–320 nm). In addition ozone in the troposphere is a pollutant and also capable of affecting the photosynthetic machinery. In this study, 5-year-old European beech trees were exposed from 1 July to October 1993 to two levels of UV-B radiation and two levels of ozone, alone and in combination, in open-top chambers equipped with lamps. The simulated UV-B levels corresponded to either clear sky ambient level or a 14% decrease in the stratospheric ozone column over eastern Denmark, resulting in a 23% difference in biologically effective UV-B (UV-B_BE) irradiance. The maximum UV-Bbe given was 8.61 kJ m⁻² day⁻¹. The ozone levels were either the ambient (average 32 nl l⁻¹) or ambient with ozone addition (average resulting concentration 71 nl l⁻¹). Compared to the control treatment (ambient UV-B, ambient O₃) the elevated levels of UV-B and O₃ affected the trees negatively, expressed as declines in net photosynthesis (P_n), stomatal conductance (g_s), chlorophyll fluorescence (F_v/F_m) and acceleration of senescence, measured as yellowing of the leaves. The UV-B treatment induced stomatal closure before the other treatments did. The magnitude of the decreases in P_n and F_v/F_m occurred in the order: control 3 3. Compared to the control, the combination treatment with high levels accelerated the visual senescence processes by ca 27 days, while for high UV-B and O₃ alone, there was an acceleration by 14 and 21 days, respectively. UV-B and O₃ in combination enhanced the negative effects compared with UV-B and O₃ alone. The P_n and F_v/F_m results could be related to this acceleration process. The chamber effect was investigated by comparing the control plots with a plot without open-top chamber. The trees in the chambers showed a higher P_n and F_v/F_m and a 14-day delayed senescence compared to the trees outside, probably caused by higher temperatures, a more protected environment and altered conditions inside the chambers.     

Effects of nitrogen and phosphorus imbalance on photosynthetic traits of poplar Oxford clone under ozone pollution

Ozone (O₃) pollution and the availability of nitrogen (N) and phosphorus (P) in the soil both affect plant photosynthesis and chlorophyll (Chl) content, but the interaction of O₃ and nutrition is unclear. We postulated that the nutritional condition changes plant photosynthetic responses to O₃. An O₃-sensitive poplar clone (Oxford) was subject to two N levels (N0, 0 kg N ha^??1; N80, 80 kg N ha^??1), two P levels (P0, 0 kg P ha^??1; P80, 80 kg P ha^??1) and three levels of O₃ exposure (ambient concentration, AA; 1.5?×?AA; 2.0?×?AA) over a growing season in an O₃ free air controlled exposure (FACE) facility. The daily change of leaf gas exchange and dark respiration (R_d) were investigated at mid-summer (August). Chl a fluorescence was measured three times in July, August and September. At the end of the growing season, Chl content was measured. It was found that Chl content, the maximum quantum yield (F_v/F_m), Chl a fluorescence performance index (PI) and gas exchange were negatively affected by elevated O₃. Phosphorus may mitigate the O₃-induced reduction of the ratio of photosynthesis to stomatal conductance, while it exacerbated the O₃-induced loss of F_v/F_m. Nitrogen alleviated negative effects of O₃ on F_v/F_m and PI in July. Ozone-induced loss of net photosynthetic rate was mitigated by N in medium O₃ exposure (1.5?×?AA). However, such a mitigation effect was not observed in the higher O₃ level (2.0?×?AA). Nitrogen addition exacerbated O₃-induced increase of R_d suggesting an increased respiratory carbon loss in the presence of O₃ and N. This may result in a further reduction of the net carbon gain for poplars exposed to O₃.     

Branch cuvettes as means of ozone risk assessment in adult forest tree crowns: combining experimental and modelling capacities

The branch autonomy principle has been referred to extensively for using branch cuvettes as a technique of studying ozone (O₃) effects within the canopy of adult forest trees. However, this principle may not hold in general regarding biochemical interactions between O₃-impacted branches exposed inside cuvettes and neighbouring crown parts under the unchanged ambient O₃ regime. After reviewing relevant cuvette studies conducted to date, we will provide evidence that cuvette-exposed branches may serve, given awareness of outlined pre-requisites and restrictions, as surrogates for examining the crown-level response of trees to elevated O₃ regimes. Such a conclusion is based on the defence metabolism of branches, which seems to be autonomous to some extent from neighbouring crown sections. Cuvette studies may, therefore, be used to derive dose response functions as measures of O₃ sensitivity. On such grounds, also validation and improvement of stomatal O₃ uptake modelling becomes feasible. The branch-level approach, however, does not substitute whole-tree free-air O₃ fumigation and related flux assessments, as branches in view of representativeness and boundary layer characteristics represent one stage in scaling O₃ flux between leaf and tree level. Branch level-based flux scaling should be backed, therefore, by independent trunk sap-flow assessment techniques that offer derivation of FO₃ at the whole-tree level.     

Seasonal changes in the photosynthetic capacity of cones on a larch (Larix kaempferi) canopy

The seasonal changes of photosynthesis of cones of Japanese larch (Larix kaempferi Carr.) trees showed that gross photosynthetic rate of young cones (P _G) was 2–3 μmol m⁻² s⁻¹ at surface area unit and P _G/R _D (dark respiration of cones) peaked about 0.7 in the same period, indicating that 70 % of respiratory CO₂ was re-fixed. With maturation, P _G and P _G/R _D sharply decreased. Chlorophyll content in cones was 3–20 % of that in leaves, which made it a limiting factor for photosynthesis and its content was closely correlated with photosynthetic capacity. Although sunken and linearly arranged stomatal organs were found on the scale of young cones, differently from the significant regulation of leaf photosynthesis, these stomata tended to be non-functional since CO₂ is not limiting factor for cone photosynthesis. Thus photosynthesis of larch cones is an additional contribution to their development.     

Altitudinal Change in the Photosynthetic Capacity of Tropical Trees: A Case Study from Ecuador and a Pantropical Literature Analysis

In tropical mountains, trees are the dominant life form from sea level to above 4,000-m altitude under highly variable thermal conditions (range of mean annual temperatures: <8 to >28°C). How light-saturated net photosynthesis of tropical trees adapts to variation in temperature, atmospheric CO₂ concentration, and further environmental factors, that change along elevation gradients, is not precisely known. With gas exchange measurements in mature trees, we determined light-saturated net photosynthesis at ambient temperature (T) and [CO₂] (A _sat) of 40 tree species from 21 families in tropical mountain forests at 1000-, 2000-, and 3000-m elevation in southern Ecuador. We tested the hypothesis that stand-level averages of A _sat and leaf dark respiration (R _D) per leaf area remain constant with elevation. Stand-level means of A _sat were 8.8, 11.3, and 7.2?μmol?CO₂?m^?2?s^?1; those of R _D 0.8, 0.6, and 0.7?μmol?CO₂?m^?2?s^?1 at 1000-, 2000-, and 3000-m elevation, respectively, with no significant altitudinal trend. We obtained coefficients of among-species variation in A _sat and R _D of 20–53% (n?=?10–16 tree species per stand). Examining our data in the context of a pan-tropical A _sat data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based A _sat decreases in tropical mountains by, on average, 1.3?μmol?CO₂?m^?2?s^?1?per?km altitude increase (or by 0.2?μmol?CO₂?m^?2?s^?1 per K temperature decrease). The A _sat decrease occurred despite an increase in leaf mass per area with altitude. Local geological and soil fertility conditions and related foliar N and P concentrations considerably influenced the altitudinal A _sat patterns. We conclude that elevation is an important influencing factor of the photosynthetic activity of tropical trees. Lowered A _sat together with a reduced stand leaf area decrease canopy C gain with elevation in tropical mountains.     

Assessment of annual carbon exchange in a water-stressed Pinus radiata plantation: an analysis based on eddy covariance measurements and an integrated biophysical model

We used a combination of eddy flux, chamber and environmental measurements with an integrated suite of models to analyse the seasonality of net ecosystem carbon uptake (F_CO2) in an 8-year-old, closed canopy Pinus radiata D.Don plantation in New Zealand (42°52′ S, 172°45′ E). The analyses utilized a biochemically based, big-leaf model of tree canopy photosynthesis (A_c), coupled to multiplicative environmental-constraint functions of canopy stomatal conductance (G_c) via environmental measurements, a temperature-dependent model of ecosystem respiration (R_eco), and a soil water balance model. Available root zone water storage capacity at the measurement site is limited to about 50 mm for the very stony soil, and annual precipitation is only 660 mm, distributed evenly throughout the year. Accordingly the site is prone to soil moisture deficit throughout the summer. G _c and A_c obtained maximum rates early in the growing season when plentiful soil water supply was associated with sufficient quantum irradiance (Q_abs), and moderate air saturation deficit (D) and temperature (T). From late spring onwards, soil water deficit and D confined G_c and A_c congruously, which together with the solely temperature dependency of R_eco resulted in the pronounced seasonality in F_CO2. Reflecting a light-limitation of A_c in the closed canopy, modelled annual carbon (C) uptake was most sensitive to changes in Q_abs. However, Q_abs did not vary significantly between years, and changes in annual F_CO2 were mostly due to variability in summer rainfall and D. Annual C-uptake of the forest was 717 g C m^–2 in a near-average rainfall year, exceeding by one third the net uptake in a year with 20% less than average rainfall (515 g C m^–2).     

Interactions of elevated CO2, NH3 and O3 on mycorrhizal infection,gas exchange and N metabolism in saplings of Scots pine

Four-year-old saplings of Scots pine (Pinus sylvestris) (L.) were exposed for 11 weeks in controlled-environment chambers to charcoad-filtered air, or to charcoal-filtered air supplemented with NH₃ (40 g m^–3), O₃ (110 g m^–3 during day/ 40 g m^–3 during night) or NH₃+O₃. All treatments were carried out at ambient (259 L L^–1) and at elevated CO₂ concentration (700 L L^–1). Total tree biomass, mycorrhizal infection, net CO₂ assimilation (P_n), stomatal conductance (g_s), transpiration of the shoots and NH₃ metabolization of the needles were measured. In ambient CO₂ (1) gaseous NH₃ decreased mycorrhizal infection, without significantly affecting tree biomass or N concentration and it enhanced the activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in one-year-old needles; (2) ozone decreased mycorrhizal infection and the acitivity of GS in the needles, while it increased the activity og GDH; (3) exposure to NH₃+O₃ lessened the effects of single exposures to NH₃ and O₃ on reduction of mycorrhizal infection and on increase in GDH activity. Similar lessing effects on mycorrhizal infection as observed in trees exposed to NH₃+O₃ at ambient CO₂, were measured in trees exposed to NH₃+O₃ at elevated CO₂. Exposure to elevated CO₂ without pollutants did not significantly affect any of the parameters studied, except for a decrease in the concentration of soluble proteins in the needles. Elevated CO₂ _NH₃ strongly decreased root branching and mycorrhizal infection and temporarily stimulated P_n and g_s. The exposure to elevated CO₂+NH₃+O₃ also transiently stimulated P_n. The possible mechanisms underlying and integrating these effects are discussed. Elevated CO₂ clearly did not alleviate the negative effects of NH₃ and O₃ mycoorhiral infection. The significant reduction of mycorrhizal infection after exposure to NH₃ or O₃, observed before significant changes in gas exchange or growth occurred, suggest the use of mycorrhizal infection as an early indicator for NH₃ and O₃ induced stress.Abbreviations DW dry weight - FA filtered air - FA_a filtered air at ambient CO₂ - FA_e filtered air at elevated CO₂ - FW fresh weight - GDH glutamate dehydrogenase - GS glutamine synthetase - g_s stomatal conductance - P_n net CO₂ assimilation - RWR root weight ratio - SRL specific root length     

Regulation of water flux through trunks, branches, and leaves in trees of a lowland tropical forest

We studied regulation of whole-tree water use in individuals of five diverse canopy tree species growing in a Panamanian seasonal forest. A construction crane equipped with a gondola was used to access the upper crowns and points along the branches and trunks of the study trees for making concurrent measurements of sap flow at the whole-tree and branch levels, and vapor phase conductances and water status at the leaf level. These measurements were integrated to assess physiological regulation of water use from the whole-tree to the single-leaf scale. Whole-tree water use ranged from 379 kg day⁻¹ in a 35 m-tall Anacardium excelsum tree to 46 kg day⁻¹ in an 18 m-tall Cecropia longipes tree. The dependence of whole-tree and branch sap velocity and sap flow on sapwood area was essentially identical in the five trees studied. However, large differences in transpiration per unit leaf area (E) among individuals and among branches on the same individual were observed. These differences were substantially reduced when E was normalized by the corresponding branch leaf area:sapwood area ratio (LA/SA). Variation in stomatal conductance (g _s) and crown conductance (g _c), a total vapor phase conductance that includes stomatal and boundary layer components, was closely associated with variation in the leaf area-specific total hydraulic conductance of the soil/leaf pathway (G _t). Vapor phase conductance in all five trees responded similarly to variation in G _t. Large diurnal variations in G _t were associated with diurnal variation in exchange of water between the transpiration stream and internal stem storage compartments. Differences in stomatal regulation of transpiration on a leaf area basis appeared to be governed largely by tree size and hydraulic architectural features rather than physiological differences in the responsiveness of stomata. We suggest that reliance on measurements gathered at a single scale or inadequate range of scale may result in misleading conclusions concerning physiological differences in regulation of transpiration. Received: 1 October 1997 / Accepted: 6 March 1998