鼎湖山针阔叶混交林小气候调节效应

森林小气候监测对于评价森林对全球气候变化的响应与反馈有重要意义。选取2010年鼎湖山针阔叶混交林通量塔林内气象资料及其附近气象站空旷地同时段内气象资料,对两站点的气温、湿度、地温等小气候因子的日变化及月变化进行分析和对比。结果表明:(1)混交林年均气温、大气相对湿度和地表温度分别为19.8℃、86.5%、20℃,空旷地分别为22.1℃、79.1%、24℃。混交林内年均气温与地温分别比空旷地低2.3℃、4℃,年均相对湿度比空旷地高7.4%,土壤各层湿度明显高于空旷地,表现出良好的降温、增湿、涵养水源的小气候效应。(2)二者气温、大气湿度、土壤温度的差值均为白天大于晚上,在10:00—17:00差异明显,表明针阔叶混交林的降温和增湿作用白天大于夜晚。气温和土壤温度差值在湿季大于干季,而相对湿度差值在干季大于湿季,表明针阔叶混交林的降温作用湿季大于干季,增湿作用干季大于湿季。这种小气候调节作用是由南亚热带特殊的气候条件及植被类型决定的。     

鼎湖山针阔叶混交林4种优势树种树干液流特征

运用Granier热消散式探针法,对鼎湖山自然保护区针阔叶混交林4种优势树种(马尾松、木荷、锥栗和广东润楠)的树干液流密度进行了长期连续观测,并同步监测林分的环境因子(光合有效辐射(PAR)、空气温度(T)、空气湿度(RH)、土壤含水量)。结果表明:(1)4种优势树种的边材面积(As)与胸径(DBH)均存在显著相关关系(P0.05);(2)各优势树种树干液流均呈现"昼高夜低"单峰曲线,且液流速率存在明显的季节性差异;(3)无论湿季还是干季,光合有效辐射(PAR)和水汽压亏缺(VPD)均为控制蒸腾的主要驱动因子;(4)4种优势树种湿季平均日蒸腾量高于干季,马尾松、木荷、锥栗、广东润楠湿季平均日蒸腾量分别为29.52、39.29、30.40、9.41 kg H2O/d,干季分别为20.91、24.84、24.26、8.43 kg H2O/d,干季和湿季的平均日蒸腾量(kg H2O/d)大小均为木荷锥栗马尾松广东润楠,这种种间差异是由边材面积大小和树种本身的生物学特性共同决定的。     

南亚热带针阔叶混交林生态系统水分利用效率

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,研究其生态系统水分利用效率有利于预测环境变化对生态系统碳水过程的影响。基于我国南亚热带鼎湖山站2005—2010年涡度相关法通量数据及相应气象观测数据,分析了演替中期针阔叶混交林的生态系统水分利用效率的变化特征和主要环境因子对其影响作用。结果表明:(1)年尺度上,鼎湖山针阔叶混交林生态系统年平均水分利用效率为(2.85±0.22)g C·kg~(-1)H_2O,季节尺度上呈单峰变化,夏季低,春冬高,秋季次之,最低月均水分利用效率出现于2009年7月,为1.45 g C·kg~(-1)H_2O,最高月2006年1月平均水分利用效率为4.75 g C·kg~(-1)H_2O,研究期间,系统水分利用效率呈现出降低的波动趋势。(2)与环境因子的相关分析表明,年尺度上,生态系统水分利用效率变化的主要驱动因子为光合有效辐射(PAR)、水汽压亏缺(VPD)及气温。气温、VPD对干季的典型月份(11月—翌年1月)WUE影响显著(P0.001),PAR对湿季的典型月份(6—9月)影响显著(P0.001)。研究结果有助于明晰南亚热带地区森林生态系统碳水耦合程度,为气候变化下生态系统的管理提供服务。     

鼎湖山不同演替阶段森林土壤水分时空变异研究

土壤水分作为森林生态系统水分蓄库的主体,森林土壤水分储量及其时空动态与变异对揭示区域植被恢复与气候变化背景下的森林生态系统水文过程响应与服务功能变化机制具有重要意义。本研究以南亚热带地区典型森林植被演替序列马尾松人工林（Pinus massoniana coniferous forest,PF）-马尾松针阔叶混交林（mixed Pinus massoniana/broad-leaved forest,MF）-季风常绿阔叶林（monsoon evergreen broad-leaved forest,MEBF）为研究对象,依托中国生态系统研究网络森林样地建设与监测统一规范对鼎湖山森林生态系统定位站站区内分布的上述森林类型土壤水分的长期定位观测（2005-2015年）,通过分析各演替阶段森林土壤不同土层（0-15、15-30、30-45、45-60、60-75和75-90 cm）土壤体积含水量观测数据,探究该区域森林植被恢复过程中的土壤水分变化及其时空变异。结果表明：在雨热同期且干湿季明显的南亚热带地区,鼎湖山森林土壤储水量及其时间动态受降雨量的影响显著,森林土壤层对降雨具有强烈的调蓄和稳定作用,伴随PF→MF→MEBF自然演替进程,调蓄水分能力逐步增强。林型间,由初期阶段PF到顶级群落MEBF,森林土壤水分储量逐渐提高,且演替后期林型相对于早期林型,土壤储水量均呈现为较小的年际与年内变幅。干、湿季而言,干季时林型间的土壤储水量差异大于湿季,干季时MEBF和MF土壤含水量分别是PF的1.33倍和1.11倍。从土壤含水量的干、湿季期间变异来看,不同林型各土层土壤含水量的变异系数大小均表现为干季大于湿季;垂直剖面方向上,突出表现为无论干湿季MEBF各层土壤含水量变异均比其他两种林型较为缓和,充分体现了MEBF优越的土壤水分时空调配能力。整体上,伴随PF→MF→MEBF自然演替进程,土壤水分储量及其稳定性逐步提升。     

南亚热带森林植被恢复演替序列的土壤有机碳氮矿化

采用室内培养的方法,分析了南亚热带鼎湖山森林植被恢复演替序列不同阶段代表性森林—马尾松林、针阔叶混交林和季风常绿阔叶林土壤(0~10cm)CO2、CH4排放/吸收和有机氮矿化的差异.结果表明:3种森林土壤培养52周的CO2-C累积排放量分别为(30.66±3.36)、(58.17±7.25)和(59.31±13.58)mg·kg-1,而其中的65.12%、64.41%和64.12%均在前9周被排放;马尾松林土壤的CO2-C累积排放量一直显著小于针阔叶混交林和季风常绿阔叶林;用相符的二库动力学模型模拟的活性库和惰性库的碳矿化速率均呈递减趋势;土壤培养52周吸收CH4的累积量、培养20周有机氮净矿化量和净硝化量均为马尾松林<针阔叶混交林<季风常绿阔叶林(P<0.05),净矿化的有效氮以硝态氮为主.说明森林植被类型的变化改变了土壤有机碳的分解速率,这是其影响土壤有机碳含量的一种内在方式.     

鼎湖山主要森林生态系统地表CH4通量

利用静态箱-气相色谱法对鼎湖山3种处于演替不同阶段的森林类型-季风常绿阔叶林、针阔叶混交林和马尾松林-的地表CH₄通量进行了为期一年的原位观测和研究,结果表明:3种林型地表吸收CH₄通量按从大到小的顺序为:季风林>混交林>松林,不同林型间的CH₄通量差异与森林土壤的性质有密切关系,即土壤容重越小、有机质含量与土壤温度和湿度都没有明显的相关性,但在旱季土壤温度成为控制地表CH₄通量的主要因子.     

鼎湖山针阔叶混交林凋落物CO2释放对模拟酸雨的响应

该研究2011年1月开始在鼎湖山针阔叶混交林(混交林)进行模拟酸雨实验,设置4个不同处理水平,即对照(CK)(pH为4.5左右的天然湖水)、T_1(pH=4.0)、T_2(pH=3.25)和T_3(pH=2.5)。2013年1—12月对不同酸雨强度处理下的森林凋落物CO_2释放速率进行为期1 a的连续观测,探讨酸雨对混交林凋落物C排放的影响。结果表明:凋落物CO2释放通量在对照样方为(1 507.41±155.19) g CO_2·m~(-2)·a~(-1),其中湿季和旱季分别占年通量的68.7%和31.3%。模拟酸雨抑制了森林凋落物CO_2释放,与CK相比,T_2和T_3处理下的CO_2释放通量分别显著降低15.4%和42.7%(P0.05);且这种抑制作用具有季节差异性,处理间的显著差异只出现在湿季。凋落物CO_2释放速率与土壤温度和土壤湿度分别呈显著指数相关和显著直线相关,同时,酸雨处理降低了凋落物CO_2释放的温度敏感性。混交林凋落物CO_2释放在模拟酸雨下的抑制效应与土壤累积酸化而导致的土壤微生物活性变化有关,表现为模拟酸雨作用下土壤pH值和微生物量碳显著下降。上述结果说明酸雨是影响混交林土壤碳循环的重要因子之一。     

浙江西天目山主要森林类型的苔藓多样性比较

苔藓是森林的重要组分, 是森林保护区的重要保护对象, 在物种资源和生态系统功能维护中有重要作用。该研究以浙江西天目山国家自然保护区内7种主要森林类型(落叶矮林、落叶阔叶林、常绿-落叶阔叶混交林、常绿阔叶林、针阔混交林、针叶林和竹林)内的苔藓植物为对象, 调查了32个10 m × 10 m的样地, 记录地面生苔藓植物盖度和树附生苔藓植物多度, 采用重要值、相似性系数、多样性指数分析了森林类型间的苔藓植物多样性差异。共采集969份标本, 隶属41科84属142种, 其中苔类植物13科18属33种, 藓类植物28科66属109种, 优势科为灰藓科、青藓科和羽藓科。2种混交林(常绿-落叶阔叶混交林和针阔混交林)的物种丰富度和多样性指数均高于其余5种森林, 其中物种丰富度以针阔混交林最高, 苔藓植物多样性则以常绿-落叶阔叶混交林最高, 竹林两者均为最低。海拔等环境因子较为接近的植被类型的苔藓植物多样性相似性较高, 常绿阔叶林与针叶林相似性最高, 而落叶矮林和竹林相似性最小。     

南亚热带森林演替过程中土壤线虫群落结构变化

森林演替会通过改变植物群落组成和土壤环境影响土壤生物群落, 反过来, 土壤生物群落的变化也会对生态系统的演替产生反馈作用, 但迄今南亚热带森林演替过程中土壤生物群落的变化特征尚不清晰。本研究以广东省鼎湖山的南亚热带森林演替序列(马尾松(Pinus massoniana)林-针阔叶混交林-季风常绿阔叶林)为对象, 研究了森林演替过程中土壤线虫多样性和群落结构的动态变化及其影响因素。通过采集不同演替阶段的土壤样品, 分析和比对了不同演替阶段土壤线虫的多度、多样性、群落组成、土壤线虫生态指数以及土壤理化性质的差异。结果表明: (1)在南亚热带森林演替过程中, 针阔叶混交林和季风常绿阔叶林土壤线虫的α多样性显著高于马尾松林, 但土壤线虫总数和各营养类群多度及其相对丰度并无显著变化; (2)针阔叶混交林中土壤线虫富集指数显著高于马尾松林, 表明其土壤养分状况要好于马尾松林, 而季风常绿阔叶林土壤线虫结构指数较高, 表明其受干扰程度较低; (3)针阔叶混交林的土壤含水量和土壤理化性质(除土壤总磷含量)已达到季风常绿阔叶林的水平, 但两者的土壤pH值均显著低于马尾松林, 而土壤pH值和土壤含水量是影响土壤线虫群落动态变化的主要因素。综上所述, 南亚热带森林中土壤线虫多度、多样性和群落结构对森林演替的响应略有不同, 演替过程中土壤环境因素的趋同是导致针阔叶混交林和季风常绿阔叶林中土壤线虫多样性和群落特征相似的主要原因。     

中国东部森林样带典型森林水源涵养功能

通过对我国东部森林样带四个森林生态系统定位研究站（长白山站、北京站、会同站和鼎湖山站）的九种森林类型水源涵养监测数据的分析,研究了水热梯度下不同森林生态系统水源涵养功能。结果表明：在生长季的5-10月份,各森林类型的水源涵养特性表现出较大差异。林冠截留率的大小依次为：阔叶红松林＞杉木林＞常绿阔叶林＞针阔混交林＞季风常绿阔叶林＞落叶阔叶混交林＞马尾松林＞落叶松林＞油松林,最高的长白山站阔叶红松林的截留率是最低的北京站油松林的2.2倍。森林降雨截留量与林外降雨量呈显著的正相关,林冠截留率与降雨量呈显著负相关。枯落物最大持水深（5-10月份）以北京站落叶阔叶林最大,为6.0mm;鼎湖山站的季风常绿阔叶林最小,为1.0mm。0-60cm土层蓄水量最大的是会同站的人工杉木林,为247mm;最小的是北京站的落叶松林,仅为45.5mm;林分总持水量依次为：杉木林＞阔叶红松林＞常绿阔叶林＞针阔混交林＞季风常绿阔叶林＞落叶阔叶混交林＞马尾松林＞落叶松林＞油松林。各林分总持水量主要集中在土壤层,占总比例的90%以上。     

Carbon and water balance in Polylepis sericea,a tropical treeline species

Polylepis sericea trees grow well above the continuous forest line in the Venezuelan Andes. In these environments, extreme daily temperature ranges can occur at any time of the year and trees experience a 4 month dry period. The purpose of this work was to study carbon and water relations of this species in the field during wet and dry seasons in order to understand this species' success at such high altitudes. Leaf gas exchange (portable system in open mode) and leaf water potential (pressure chamber) were measured at 1–2 h intervals during several daily courses at 4000 m elevation in the Páramo de Piedras Blancas. CO₂ assimilation versus leaf temperature curves were also obtained for this species in the laboratory. Clear differences in the measured parameters were observed between seasons. For a wet season day, maximum CO₂ assimilation rate was 7.4 mol m^-2 s^-1 and leaf conductance was relatively constant (approximately 100 mmol m^-2 s^-1)In the dry season day, maximum CO₂ assimilation rate was 5.8 molm^-2 s^-1 and leaf conductance was close to 60 mmolm^-2 s^-1. Minimum leaf water potentials measured were -1.3 MPa for the wet and -2.2 MPa for the dry season. The CO₂ assimilation-leaf temperature relationship showed a 13.4°C leaf temperature optimum for photosynthesis with maximum and minimum compensation points of 29.5 and -2.8°C, respectively. Maximum night-time respiration was relatively high (2.7 (imol) m^-2 s^-1)Our results show thatP. sericea maintains a highly positive carbon balance through all daily courses, even though there is a slight water stress effect during the dry season; this suggests that its carbon assimilation machinery is well adapted to the low temperatures and seasonal water stress found in the high tropical mountains.     

Seasonal variation in energy fluxes and carbon dioxide exchange for a broad-leaved semi-arid savanna (Mopane woodland) in Southern Africa

We studied the seasonal variation in carbon dioxide, water vapour and energy fluxes in a broad‐leafed semi‐arid savanna in Southern Africa using the eddy covariance technique. The open woodland studied consisted of an overstorey dominated by Colophospermum mopane with a sparse understorey of grasses and herbs. Measurements presented here cover a 19‐month period from the end of the rainy season in March 1999 to the end of the dry season September 2000. During the wet season, sensible and latent heat fluxes showed a linear dependence on incoming solar radiation (I) with a Bowen ratio (β) typically just below unity. Although β was typically around 1 at low incoming solar radiation (150 W m^?2) during the dry season, it increased dramatically with I, typically being as high as 4 or 5 around solar noon. Thus, under these water‐limited conditions, almost all available energy was dissipated as sensible, rather than latent heat. Marked spikes of CO₂ release occurred at the onset of the rainfall season after isolated rainfall events and respiration dominated the balance well into the rainfall season. During this time, the ecosystem was a constant source of CO₂ with an average flux of 3–5 μmol m^?2 s^?1 to the atmosphere during both day and night. But later in the wet season, for example, in March 2000 under optimal soil moisture conditions, with maximum leaf canopy development (leaf area index 0.9–1.3), the peak ecosystem CO₂ influx was as much as 10 μmol m^?2 s^?1. The net ecosystem maximum photosynthesis at this time was estimated at 14 μmol m^?2 s^?1, with the woodland ecosystem a significant sink for CO₂. During the dry season, just before leaf fall in August, maximum day‐ and night‐time net ecosystem fluxes were typically ?3 μmol m^?2 s^?1 and 1–2 μmol m^?2 s^?1, respectively, with the ecosystem still being a marginal sink. Over the course of 12 months (March 1999–March 2000), the woodland was more or less carbon neutral, with a net uptake estimated at only about 1 mol C m^?2 yr^?1. The annual net photosynthesis (gross primary production) was estimated at 32.2 mol m^?2 yr^?1.     

华南地区5种人工幼林的土壤呼吸及其季节性变化

为了解华南人工林的碳固存机制,对广东鹤山的尾叶桉(Eucalyptus urophylla)纯林、30种树种混交林、10种树种混交林、红椎(Castanopsis hystrix)纯林、厚荚相思(Acacia crassicarpa)纯林5种人工林(林龄2–5 a)的土壤总呼吸(Rs)和自养呼吸(Ra)的季节变化进行了研究。结果表明,从2007年到2012年,5种人工林的Rs为81.3~103.9 mg C m–2h–1,Ra为11.2~22.3 mg C m–2h–1,自养呼吸贡献率(RC)为12.4%~26.9%,且5种人工林间的Rs、Ra及RC差异不显著。5种人工林湿季的Rs均显著大于干季的,平均高出311.4%;Ra、RC的季节性差异不显著。湿季土壤温度与Rs具有显著相关性,土壤温度解释了90.2%的变异,而两者关系在干季不显著。人工林间的微环境和土壤条件差异不明显,可能是由于造林时间短,土壤还处于干扰的恢复过程中,导致人工林间土壤呼吸差异不显著。     

鼎湖山针阔叶混交林优势种树干液流特征及其与环境因子的关系

运用Granier热扩散式探针法,于2010年干湿季对鼎湖山自然保护区针阔混交林4种优势树种马尾松、锥栗、木荷和广东润楠的树干液流密度进行连续监测,并同步观测气温、相对湿度和光合有效辐射等环境因子的变化,研究其树干液流特征及其对环境因子的响应.结果表明:在干湿季,4种优势树种的树干液流速率日变化均呈“昼高夜低”的典型单峰曲线,阔叶树锥栗、木荷和广东润楠的平均液流速率和峰值以及日液流量均显著大于针叶树马尾松;马尾松、锥栗、木荷和广东润楠的最大树干液流密度分别为29.48、38.54、51.67、58.32g H2O·m-2·s-1.优势树种树干液流速率的变化与环境因子的昼夜变化存在时滞;液流速率变化与光合有效辐射、水汽压亏缺和气温等环境因子的变化呈显著正相关,其中湿季以光合有效辐射为主导因子,干季以气温为主导因子.     

Fluxes of carbon, water and energy over Brazilian cerrado: an analysis using eddy covariance and stable isotopes

We present the energy and mass balance of cerrado sensu stricto (a Brazilian form of savanna), in which a mixture of shrubs, trees and grasses forms a vegetation with a leaf area index of 1·0 in the wet season and 0·4 in the dry season. In the wet season the available energy was equally dissipated between sensible heat and evaporation, but in the dry season at high irradiance the sensible heat greatly exceeded evaporation. Ecosystem surface conductance g_s in the wet season rose abruptly to 0·3 mol m^?2 s^?1 and fell gradually as the day progressed. Much of the total variation in g_s was associated with variation in the leaf-to-air vapour pressure deficit of water and the solar irradiance. In the dry season the maximal g_s values were only 0·1 mol m^?2 s^?1. Maximal net ecosystem fluxes of CO₂ in the wet and dry season were –10 and –15 μmol CO₂ m^?2 s^?1, respectively (sign convention: negative denotes fluxes from atmosphere to vegetation). The canopy was well coupled to the atmosphere, and there was rarely a significant build-up of respiratory CO₂ during the night. For observations in the wet season, the vegetation was a carbon dioxide sink, of maximal strength 0·15 mol m^?2 d^?1. However, it was a source of carbon dioxide for a brief period at the height of the dry season. Leaf carbon isotopic composition showed all the grasses except for one species to be C₄, and all the palms and woody plants to be C₃. The CO₂ coming from the soil had an isotopic composition that suggested 40% of it was of C₄ origin.     

Land use and season affect fluxes of CO2, CH4, CO,N2O,H2 and isotopic source signatures in Panama: evidence from nocturnal boundary layer profiles

Conversion of tropical rainforests to pastures and plantations is associated with changes in soil properties and biogeochemical cycling, with implications for carbon cycling and trace gas fluxes. The stable isotopic composition of ecosystem respiration (δ¹³C_R and δ¹⁸O_R) is used in inversion models to quantify regional patterns of CO₂ sources and sinks, but models are limited by sparse measurements in tropical regions. We measured soil respiration rates, concentrations of CO₂, CH₄, CO, N₂O and H₂ and the isotopic composition of CO₂, CH₄ and H₂ at four heights in the nocturnal boundary layer (NBL) above three common land‐use types in central Panama, during dry and rainy seasons. Soil respiration rates were lowest in Plantation (average 3.4 μmol m^?2 s^?1), highest in Pasture (8.3 μmol m^?2 s^?1) and intermediate in Rainforest (5.2 μmol m^?2 s^?1). δ¹³C_R closely reflected land use and increased during the dry season where C₃ vegetation was present. δ¹⁸O_R did not differ by land use but was lower during the rainy than the dry season. CO₂ was correlated with other species in approximately half of the NBL profiles, allowing us to estimate trace gas fluxes that were generally within the range of literature values. The Rainforest soil was a sink for CH₄ but emissions were observed in Pasture and Plantation, especially during the wet season. N₂O emissions were higher in Pasture and Plantation than Rainforest, contrary to expectations. Soil H₂ uptake was highest in Rainforest and was not observable in Pasture and Plantation during the wet season. We observed soil CO uptake during the dry season and emissions during the wet season across land‐use types. This study demonstrated that strong impacts of land‐use change on soil–atmosphere trace gas exchange can be detected in the NBL, and provides useful observational constraints for top‐down and bottom‐up biogeochemistry models.     

High rates of net ecosystem carbon assimilation by Brachiara pasture in the Brazilian Cerrado

To investigate the consequences of land use on carbon and energy exchanges between the ecosystem and atmosphere, we measured CO₂ and water vapour fluxes over an introduced Brachiara brizantha pasture located in the Cerrado region of Central Brazil. Measurements using eddy covariance technique were carried out in field campaigns during the wet and dry seasons. Midday CO₂ net ecosystem exchange rates during the wet season were ?40 μmol m^?2 s^?1, which is more than twice the rate found in the dry season (?15 μmol m^?2 s^?1). This was observed despite similar magnitudes of irradiance, air and soil temperatures. During the wet season, inferred rates of canopy photosynthesis did not show any tendency to saturate at high solar radiation levels, with rates of around 50 μmol m^?2 s^?1 being observed at the maximum incoming photon flux densities of 2200 μmol m^?2 s^?1. This contrasted strongly to the dry period when light saturation occurred with 1500 μmol m^?2 s^?1 and with maximum canopy photosynthetic rates of only 20 μmol m^?2 s^?1. Both canopy photosynthetic rates and night‐time ecosystem CO₂ efflux rates were much greater than has been observed for cerrado native vegetation in both the wet and dry seasons. Indeed, observed CO₂ exchange rates were also much greater than has previously been reported for C₄ pastures in the tropics. The high rates in the wet season may have been attributable, at least in part, to the pasture not being grazed. Higher than expected net rates of carbon acquisition during the dry season may also have been attributable to some early rain events. Nevertheless, the present study demonstrates that well‐managed, productive tropical pastures can attain ecosystem gas exchange rates equivalent to fertilized C₄ crops growing in the temperate zone.     

Oxygen-dependent electron transport and protection from photoinhibition in leaves of tropical tree species

The roles of photorespiration and the Mehlerperoxidase pathway in sustaining electron transport and protection from photoinhibition were studied in outer canopy leaves of two species of tropical trees: the drought-deciduous Pseudobombax septenatum (Jacq.) Dug. and the evergreen Ficus insipida Willd. Ficus had a higher photosynthetic capacity than Pseudobombax and also a greater capacity for light-dependent electron transport under photorespiratory conditions (in the absence of CO₂). As a consequence, in the absence of CO₂, Ficus was able to maintain a largely oxidized electron-transport chain at higher photon flux densities than Pseudobombax. Under the same light conditions, photoinhibition (reduction in F_v/F_m) was always greater in Pseudobombax than Ficus, was increased when leaves were exposed to 2% O₂ in nitrogen compared to 21% O₂ in CO₂-free air, but was not increased by the absence of CO₂. Rates of electron transport due to the Mehler-peroxidase pathway (assessed in 2% O₂ in nitrogen) ranged between 16–40 mol · m^–2·s^–1 in both species. As the dry season approached and Pseudobombax neared leaf senescence there was a decline in the capacity for photorespiratory flux to maintain electron transport in Pseudobombax, but not in Ficus. Ratios of light-dependent electron transport to net CO₂ fixation for Pseudobombax, Ficus and two other species in the field, Luehea seemannii Tr. & Planch, and Didymopanax morototoni (Aubl.) Dec. & Planch., ranged from 6.2 (Ficus) to 16.7 (Pseudobombax). High in-situ rates of photorespiration combined with the decreased capacity of Pseudobombax for photorespiratory flux as the dry season approached indicates a decreased capacity to protect against photooxidative damage. This may contribute to the promotion of leaf senescence in Pseudobombax during the transition from wet to dry season.Abbreviations F_v/F_m ratio of variable to maximum chlorophyll a fluorescence - NPQ nonphotochemical fluorescence quenching - PFD photon flux density - Q_A primary electron acceptor of PSII This research was supported by a grant from the Mellon Foundation. We thank Monica Mejia and Juan Posada for assisting with the fluorescence measurements and Aurelio Virgo for assisting with the field CO₂-exchange measurements.     

Land-use change effects on local energy, water, and carbon balances in an Amazonian agricultural field

To study how changing agricultural practices in the eastern Amazon affect carbon, heat and water exchanges, a 20 m tower was installed in a field in August 2000. Measurements include turbulent fluxes (momentum, heat, water vapor, and CO₂) using the eddy covariance (EC) approach, soil heat flux, wind, and scalar profiles (T, q, and CO₂), soil moisture content, terrestrial, total solar radiation, and photosynthetically active radiation (PAR, 400–700 nm). At the beginning of the measurements, in September 2000, the field was a pasture. On November 2001, the pasture was burned, plowed, and planted in upland (nonirrigated) rice. Calm nights were the norm in this site. Anomalously low values of net ecosystem exchange (NEE) were found using the EC method, even when the common criterion u^*<0.2 m s^?1 was used to identify and exclude poor performance nights. We observed more plausible values of NEE using criterion u^*<0.08 m s^?1, indicating that the criterion must be revised downward for flow over surfaces smoother than forests. However, even using the lower threshold, u^* was lower than this limit for 82% of nights, and this led to nocturnal respiration underestimates. We compensate for this difficulty by estimating the respiration rate using the nocturnal boundary layer budget method. Land‐use change from pasture to rice cultivation strongly affected both diurnal rates of turbulent exchange but also the pattern of seasonal variation. Seasonal wet and dry season differences in vegetation state were clearly detected in the albedo and PAR‐albedo. These reflectivity changes were accompanied by modified net radiative flux, turbulent heat flux and evaporation rates. The highest evaporation rate was observed during the rice crop, when the field had total evaporation approximately half the precipitation input, less than that of the surrounding forest. Effects of the land‐cover changes were also detected in the carbon budget. For the pasture, the maximum CO₂ uptake occurred in May, appreciably delayed from the start of the rainy season. After the field was plowed and the soil was exposed and there was efflux of CO₂ to the atmosphere day and night for an extended period. Highest values of carbon uptake occurred during the rice plantation. Although the upland rice took up carbon at double the rate of the pasture that it replaced, the field was left fallow for much of the year, during the dry season.     

Apparent carboxylation efficiency and relative stomatal and mesophyll limitations of photosynthesis in an evergreen cerrado species during water stress

Miconia albicans, a common evergreen cerrado species, was studied under field conditions. Leaf gas exchange and pre-dawn leaf water potential (Ψ_pd) were determined during wet and dry seasons. The potential photosynthetic capacity (P _Npmax) and the apparent carboxylation efficiency (ε) dropped in the dry season to 28.0 and 0.7 %, respectively, of the maximum values in the wet season. The relative mesophyll (L_m) and stomatal (L_s) limitations of photosynthesis increased, respectively, from 24 and 44 % in the wet season to 79 and 57 % at the peak of the dry season when mean Ψ_pd reached −5.2 MPa. After first rains, the P _Npmax, ε, and L_m recovered reaching the wet season values, but L_s was maintained high (63 %). The shallow root system growing on stonemason limited by lateral concrete wall to a depth of 0.33 m explained why extreme Ψ_pd was brought about. Thus M. albicans is able to overcome quickly the strains imposed by severe water stress.