Canopy CO2 Research Articles

Circadian rhythms regulate the environmental responses of net CO2 exchange in bean and cotton canopies

Studies on the dependence of the rates of ecosystem gas exchange on environmental parameters often rely on the up-scaling of leaf-level response curves (‘bottom-up’ approach), and/or the down-scaling of ecosystem fluxes (‘top-down’ approach), where one takes advantage of the natural diurnal covariation between the parameter of interest and photosynthesis rates. Partly independent from environmental variation, molecular circadian clocks drive ∼24h oscillations in leaf-level photosynthesis, stomatal conductance and other physiological processes in plants under controlled laboratory conditions. If present and of sufficient magnitude at ecosystem scales, circadian regulation could lead to different results when using the bottom-up approach (where circadian regulation exerts a negligible influence over fluxes because the environment is modified rapidly) relative to the top-down approach (where circadian regulation could affect fluxes as it requires the passage of a few hours). Here we dissected the drivers of diurnal net CO2 exchange in canopies of an annual herb (bean) and of a perennial shrub (cotton) through a set of experimental manipulations to test for the importance of circadian regulation of net canopy CO2 exchange, relative to that of temperature and vapor pressure deficit, and to understand whether circadian regulation could affect the derivation of environmental flux dependencies. Contrary to conventional wisdom, we observed how circadian regulation exerted controls over net CO2 exchange that were of similar magnitude to the controls exerted by direct physiological responses to temperature and vapor pressure deficit. Diurnal patterns of net CO2 exchange could only be explained by considering effects of environmental responses combined with circadian effects. Consequently, we observed significantly different results when inferring the dependence of photosynthesis over temperature and vapor pressure deficit when using the top-down and the bottom up approaches.

Loss of photosynthetic efficiency in the shade. An Achilles heel for the dense modern stands of our most productive C4crops?

The wild progenitors of major C4 crops grew as individuals subjected to little shading. Today they are grown in dense stands where most leaves are shaded. Do they maintain photosynthetic efficiency in these low light conditions produced by modern cultivation? The apparent maximum quantum yield of CO2 assimilation (ΦCO2max,app), a key determinant of light-limited photosynthesis, has not been systematically studied in field stands of C4 crops. ΦCO2max,app was derived from the initial slope of the response of leaf CO2 uptake (A) to photon flux (Q). Leaf fractional light absorptance (α) was measured to determine the absolute maximum quantum yield of CO2 assimilation on an absorbed light basis (ΦCO2max,abs). Light response curves were determined on sun and shade leaves of 49 field plants of Miscanthus × giganteus and Zea mays following canopy closure. ΦCO2max,app and ΦCO2max,abs declined significantly by 15-27% (P<0.05) with canopy depth. Experimentally, leaf age was shown unlikely to cause this loss. Modeling canopy CO2 assimilation over diurnal courses suggested that the observed decline in ΦCO2max,app with canopy depth costs 10% of potential carbon gain. Overcoming this limitation could substantially increase the productivity of major C4 crops.

Reduced crown root number improves water acquisition under water deficit stress in maize (Zea mays L.).

In this study we test the hypothesis that maize genotypes with reduced crown root number (CN) will have greater root depth and improved water acquisition from drying soil. Maize recombinant inbred lines with contrasting CN were evaluated under water stress in greenhouse mesocosms and field rainout shelters. CN varied from 25 to 62 among genotypes. Under water stress in the mesocosms, genotypes with low CN had 31% fewer crown roots, 30% deeper rooting, 56% greater stomatal conductance, 45% greater leaf CO2 assimilation, 61% net canopy CO2 assimilation, and 55% greater shoot biomass than genotypes with high CN at 35 days after planting. Under water stress in the field, genotypes with low CN had 21% fewer crown roots, 41% deeper rooting, 48% lighter stem water oxygen isotope enrichment (δ(18)O) signature signifying deeper water capture, 13% greater leaf relative water content, 33% greater shoot biomass at anthesis, and 57% greater yield than genotypes with high CN. These results support the hypothesis that low CN improves drought tolerance by increasing rooting depth and water acquisition from the subsoil.

Using satellite‐derived optical thickness to assess the influence of clouds on terrestrial carbon uptake

AbstractClouds scatter direct solar radiation, generating diffuse radiation and altering the ratio of direct to diffuse light. If diffuse light increases plant canopy CO2 uptake, clouds may indirectly influence climate by altering the terrestrial carbon cycle. However, past research primarily uses proxies or qualitative categories of clouds to connect the effect of diffuse light on CO2 uptake to sky conditions. We mechanistically link and quantify effects of cloud optical thickness (τc) to surface light and plant canopy CO2 uptake by comparing satellite retrievals of τc to ground‐based measurements of diffuse and total photosynthetically active radiation (PAR; 400–700 nm) and gross primary production (GPP) in forests and croplands. Overall, total PAR decreased with τc, while diffuse PAR increased until an average τc of 6.8 and decreased with larger τc. When diffuse PAR increased with τc, 7–24% of variation in diffuse PAR was explained by τc. Light‐use efficiency (LUE) in this range increased 0.001–0.002 per unit increase in τc. Although τc explained 10–20% of the variation in LUE, there was no significant relationship between τc and GPP (p &gt; 0.05) when diffuse PAR increased. We conclude that diffuse PAR increases under a narrow range of optically thin clouds and the dominant effect of clouds is to reduce total plant‐available PAR. This decrease in total PAR offsets the increase in LUE under increasing diffuse PAR, providing evidence that changes within this range of low cloud optical thickness are unlikely to alter the magnitude of terrestrial CO2 fluxes.

Effects of elevated soil CO2 concentration on growth and competition in a grass-clover mix

To investigate potential environmental affects in the context of carbon dioxide (CO2) leakage from Carbon Capture and Storage (CCS) schemes. The ASGARD (Artificial Soil Gassing and Response Detection) facility was established, where CO2 can be injected into the soil in replicated open-air field plots. Eight plots were sown with a grass-clover mix, with four selected for CO2 treatment while four were left as controls. Observations of sward productivity throughout the study allowed three effects to be distinguished: a direct stress response to soil gassing, limiting productivity in both species but with a greater effect on the clover; competition between the grass and clover affected by their differential stress responses; and an overall temporal trend from dominance by clover to dominance by grass in CO2 treatments. The direct effect of soil CO2 (or associated oxygen (O2) deprivation due to the high levels of CO2 in the soil) gave estimated reductions in productivity of 42% and 41% in grass, compared to 66% and 32% for clover in the high and low CO2 gassed zones respectively. Canopy CO2 increased by 70 parts per million (ppm) for every 1% increase in soil CO2 and a significant positive response of stomatal conductance in clover was observed; although carbon acquisition by the plants should not therefore be impeded, the reduction in productivity of the gassed plants is indicative of carbon-based metabolic costs probably related to soil CO2 affecting root physiology. Biomass measurements made after gassing has ceased indicated that recovery of vegetation was close to complete after 12 months.

Carbon assimilation and water use efficiency of a perennial bioenergy crop (Cynara cardunculus L.) in Mediterranean environment

Here we investigate how cardoon (Cynara cardunculus L.), energetic crop cultivated under Mediterranean climate in rainfed conditions, is adapted to the environment. Two main resources used for producing biomass are analysed in detail: water (H2O) and carbon dioxide (CO2). Following micrometeorological approach, the eddy covariance technique has been used for monitoring H2O and CO2 exchanges between canopy and atmosphere in order to investigate the dynamics of the cardoon growth at field level and to compute the Gross Primary Production (GPP). The dynamics of canopy CO2 assimilation in terms of GPP, evapotranspiration (ET) and water use efficiency (WUEGPP, as ratio between seasonal GPP and seasonal ET and WUEagro as ratio between yield and seasonal ET) were analysed during three successive growth seasons in a south Italy site. The environmental drivers of CO2 assimilation and ET were analysed at instantaneous scale. The crop showed increasing resource use efficiency along the three seasons of experiment for all considered resources: in particular, for the last two seasons cumulated GPP increased and cumulated ET decreased. It seemed to require a season for its establishment to the environment, improving the use of water and CO2 assimilation in the second and third season.

Daytime and Phenological Characteristics of O3 and CO2 Fluxes of Winter Wheat Canopy Under Short-Term O3 Exposure

To analyze the daytime and phenological variations of canopy O3 and CO2 uptake of winter wheat, the canopy fluxes of wheat plants were measured using a chamber system with four different O3 levels (0, 40, 80, and 120 nmol mol−1) being applied. During the daytime (7:30–18:00 hours), canopy fluxes usually peaked around noon in early growing stages, while a generally decreasing trend from morning to afternoon was observed in the later stages. O3 and CO2 fluxes were positively and negatively correlated with O3 concentration, respectively. Significant differences were observed in O3 fluxes but CO2 fluxes among O3 treatments. Photosynthetically active radiation (PAR) and vapor pressure deficit (VPD) could affect canopy gas uptake in opposite ways. On the phenological timescale, both O3 and CO2 fluxes followed the variation of leaf area index (LAI) with the maximum occurring simultaneously at the booting stage. The daytime mean fluxes varied from −10.6 to −17.2 nmol m−2 s−1 for O3 and from −5.9 to −19.6 μmol m−2 s−1 for CO2. Quantitatively important O3 deposition (−3.1∼−11.6 nmol m−2 s−1) was also observed at night with the ratios being about 40∼70 % relative to the daytime O3 fluxes for most measuring days, which indicates a significant contribution from non-stomatal components to canopy O3 removal. This study confirms that environmental variables and plant phenology are important factors in regulating canopy O3 and CO2 uptake. O3 exposure (≤120 nmol mol−1) could not significantly affect the CO2 uptake of wheat canopy in a short time (ca. 10 min).

Effects of Vertical Gradient of Leaf Nitrogen Content on Canopy Photosynthesis in Tall and Dwarf Cultivars of Sorghum

The vertical profile of leaf nitrogen (N) content per unit leaf area (NLA) is important for increasing crop productivity via optimizing N use for canopy photosynthesis. To investigate the effects of plant height on the optimality, we analyzed the NLA profiles with respect to light gradient twice during vegetative growth in canopies of tall and dwarf cultivars of sorghum. The gradients of the NLA profiles relative to the light gradients were similar in the two cultivars although the vertical light gradient was steeper in the dwarf cultivar with doubled leaf area density (LAD). This suggests that light attenuation is more influential on the NLA profile than is plant height or LAD. The advantage of the observed NLA profile for the canopy CO2 uptake as compared to the uniform NLA profile was similar in the cultivars except when the N allocation rate to the canopy decreased relative to the leaf area expansion in the dwarf cultivar. These results suggested that the optimality of the advantage of the NLA gradient may not be directly influenced by the plant height, but by the balance between N allocation and leaf area expansion. The balance may be altered by the difference in the biomass allocation in the shoot between the tall and dwarf sorghum. These factors are to be taken into consideration in breeding programs that target stature in order to potentially increase production in sorghum.Key words:

Temporal dynamics of oxygen isotope compositions of soil and canopy CO2 fluxes in a temperate deciduous forest

AbstractPartitioning of CO2 exchange into canopy (FA) and soil (FR) flux components is essential to improve our understanding of ecosystem processes. The stable isotope C18OO can be used for flux partitioning, but this approach depends on the magnitude and consistency of the isotope disequilibrium (Deq), i.e., the difference between the isotope compositions of FR (δA) and FA (δR). In this study, high temporal resolution isotopic data were used (1) to test the suitability of existing steady state and nonsteady models to estimate H218O enrichment in a mixed forest canopy, (2) to investigate the temporal dynamics of δA using a big‐leaf parameterization, and (3) to quantify the magnitude of the C18OO disequilibrium (Deq) in a temperate deciduous forest throughout the growing season and to determine the sensitivity of this variable to the CO2 hydration efficiency (θeq). A departure from steady state conditions was observed even at midday in this study, so the nonsteady state formulation provided better estimates of leaf water isotope composition. The dynamics of δR was mainly driven by changes in soil water isotope composition, caused by precipitation events. Large Deq values (up to 11‰) were predicted; however, the magnitude of the disequilibrium was variable throughout the season. The magnitude of Deq was also very sensitive to the hydration efficiencies in the canopy. For this temperate forest during most of the growing season, the magnitude of Deq was inversely proportional to θeq, due to the very negative δR signal, which is contrary to observations for other ecosystems investigated in previous studies.

CO&amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt; uptake of a mature &amp;lt;i&amp;gt;Acacia mangium&amp;lt;/i&amp;gt; plantation estimated from sap flow measurements and stable carbon isotope discrimination

Abstract. A simple, nondestructive method for the estimation of canopy CO2 uptake is important for understanding the CO2 exchange between forest and atmosphere. Canopy CO2 uptake (FCO2) of a subtropical mature \\textit{A. mangium} plantation was estimated by combining sap flow measurements and stable carbon isotope discrimination (Δ) in Southern China from 2004 to 2007. The mechanistic relationship linking FCO2, Δ in leaf sap, and sap flow-based canopy stomatal conductance (Gs) was applied in our study. No significant seasonal variations were observed in Δ or in the ratio of the intercellular and ambient CO2 concentrations (Ci/Ca), although diurnal Ci/Ca varied between sunlit and shaded leaves. A sensitivity analysis showed that estimates of FCO2 were more sensitive to dynamics in Gs than in Ca and Δ. By using seasonally and canopy averaged Ci/Ca values, we obtained an acceptable estimate of FCO2 compared to other estimates. FCO2 exhibited similar diurnal variation to that of Gs. Large seasonal variation in FCO2 was attributed to the responsiveness of Gs to vapor pressure deficit, photosynthetically active radiation, and soil moisture deficit. Our estimate of FCO2 for a mature A. mangium plantation (2.13 ± 0.40 gC m−2 d−1) approached the lower range of values for subtropical mixed forests, probably due to lower mean canopy stomatal conductance, higher Ci/Ca, and greater tree height than other measured forests. Our estimate was also lower than values determined by satellite-based modeling or carbon allocation studies, suggesting the necessity of stand level flux data for verification. Qualitatively, the sap flux/stable isotope results compared well with gas exchange results. Differences in results between the two approaches likely reflected variability due to leaf position and age, which should be reduced for the combined sap flux and isotope technique, as it uses canopy average values of Gs and Ci/Ca.

Field Evaluation of Open System Chambers for Measuring Whole Canopy Gas Exchanges

The ability to monitor whole canopy CO2 and H2O fluxes of crop plants in the field is needed for many research efforts ranging from plant breeding to the study of climate change effects on crops. Four portable, transparent, open system chambers for measuring canopy gas exchanges were field tested on well‐watered and fertilized cotton (Gossypium hirsutum L.) over 6 d at Lubbock, TX, in 2010. Our objective was to (i) characterize changes in canopy microclimate variables due to chamber enclosure and (ii) evaluate chamber‐to‐chamber variability in canopy gas‐exchange parameters on ground as well as leaf area basis. Chamber wall materials reduced photosynthetically active radiation (PAR) by about 13%. Programmable data loggers controlled variable speed fans and, with one minor exception, limited heat buildup in the chambers to a maximum of &lt;1.0°C above ambient air temperature. Differentials between incoming and outgoing atmospheric CO2 and H2O concentrations were used to calculate canopy net assimilation (A) and transpiration (E) at 10‐s intervals using solenoid valve actuated sample lines connected to an infrared gas analyzer. Water use efficiency (WUE = A/E) was then calculated and these data were averaged over both 5‐min and hourly intervals. Coefficient of variation (CV) for midday A, E, and WUE averaged 12.3, 9.7, and 5.8%, respectively. Expressing these parameters on a canopy leaf area basis produced similar CV results. These results will help guide experimental design for future research using these or similar chambers.

Effects of fire on carbon fluxes of a calcareous grassland

Frequent fires on drought-prone grasslands, such as pastures in the sub-Mediterranean region, can induce large post-fire variations of CO2 fluxes between the ecosystem and the atmosphere. Consequently, substantial changes in the C-cycle can be expected. In our research, we studied post-fire carbon fluxes (net canopy CO2 exchange (NE) and canopy respiration) on calcareous karst grassland after two spring fire events, a natural one in 2010 and a man-induced (anthropogenic) one in 2011. Canopy chamber measurements performed at short regular time intervals throughout the season revealed the rapid recovery of NE after the initial loss of C-sink strength. The long-term effects of the natural fire were largely masked by an early-season drought. In contrast, the burned areas did not reach the productivity of non-disturbed sites until the end of the season after the anthropogenic fire in 2011, when the post-burning period was characterised by favourable growing conditions. The similar NE values could be explained by a significant reduction of respiration at burned areas. Our research showed that C-sink strength of grassland after a fire disturbance can be re-established quite rapidly, after a month, although a full recovery (regeneration of plant cover, C fluxes) can take more time. The re-establishment is largely dependent on environmental conditions (soil water availability).

Simulation of canopy CO2/H2O fluxes for a rubber (Hevea brasiliensis) plantation in central Cambodia: The effect of the regular spacing of planted trees

We developed a soil-vegetation-atmosphere transfer (SVAT) model applicable to simulating CO2 and H2O fluxes from the canopies of rubber plantations, which are characterized by distinct canopy clumping produced by regular spacing of plantation trees. Rubber (Hevea brasiliensis Müll. Arg.) plantations, which are rapidly expanding into both climatically optimal and sub-optimal environments throughout mainland Southeast Asia, potentially change the partitioning of water, energy, and carbon at multiple scales, compared with traditional land covers that are being replaced. Describing the biosphere–atmosphere exchange in rubber plantations via SVAT modeling is, therefore, important to understanding the impacts on environmental processes. The regular spacing of plantation trees creates a peculiar canopy structure that is not well represented in most SVAT models, which generally assume a non-uniform spacing of vegetation. Herein we develop a SVAT model applicable to a rubber plantation and an evaluation method for its canopy structure, and examine how the peculiar canopy structure of rubber plantations affects canopy CO2 and H2O exchanges. Model results are compared with measurements collected at a field site in central Cambodia. Our findings suggest that it is crucial to account for intensive canopy clumping in order to reproduce observed rubber plantation fluxes. These results suggest a potentially optimal spacing of rubber trees to produce high primary productivity and water use efficiency.

Controls on seasonal patterns of maximum ecosystem carbon uptake and canopy-scale photosynthetic light response: contributions from both temperature and photoperiod

Most models of photosynthetic activity assume that temperature is the dominant control over physiological processes. Recent studies have found, however, that photoperiod is a better descriptor than temperature of the seasonal variability of photosynthetic physiology at the leaf scale. Incorporating photoperiodic control into global models consequently improves their representation of the seasonality and magnitude of atmospheric CO2 concentration. The role of photoperiod versus that of temperature in controlling the seasonal variability of photosynthetic function at the canopy scale remains unexplored. We quantified the seasonal variability of ecosystem-level light response curves using nearly 400 site years of eddy covariance data from over eighty Free Fair-Use sites in the FLUXNET database. Model parameters describing maximum canopy CO2 uptake and the initial slope of the light response curve peaked after peak temperature in about 2/3 of site years examined, emphasizing the important role of temperature in controlling seasonal photosynthetic function. Akaike's Information Criterion analyses indicated that photoperiod should be included in models of seasonal parameter variability in over 90% of the site years investigated here, demonstrating that photoperiod also plays an important role in controlling seasonal photosynthetic function. We also performed a Granger causality analysis on both gross ecosystem productivity (GEP) and GEP normalized by photosynthetic photon flux density (GEP n ). While photoperiod Granger-caused GEP and GEP n in 99 and 92% of all site years, respectively, air temperature Granger-caused GEP in a mere 32% of site years but Granger-caused GEP n in 81% of all site years. Results demonstrate that incorporating photoperiod may be a logical step toward improving models of ecosystem carbon uptake, but not at the expense of including enzyme kinetic-based temperature constraints on canopy-scale photosynthesis.

Modeling Canopy CO2 Exchange in the European Russian Arctic

In this study, we use the coupled photosynthesis-stomatal conductance model of Collatz et al. (1991) to simulate the current canopy carbon dioxide exchange of a heterogeneous tundra ecosystem in European Russia. For the parameterization, we used data obtained from in situ leaf level measurements in combination with meteorological data from 2008. The modeled CO2 fluxes were compared with net ecosystem exchange (NEE), measured by the eddy covariance technique during the snow-free period in 2008.The findings from this study indicated that the main state parameters of the exchange processes were leaf area index (LAI) and Rubisco capacity (V cmax). Furthermore, this ecosystem was found to be functioning close to its optimum temperature regarding carbon accumulation rates. During the modeling period from May to October, the net assimilation was greater than the respiration, leading to a net accumulation of 58 g C m-2. The model results suggest that the tundra ecosystem could change from a carbon sink to a carbon source with a temperature rise of only 2–3 °C. This is due to the fact that, in the continental Arctic, a global warming of a few degrees might restrict the net assimilation, due to high temperatures, whereas the respiration is predicted to be enhanced. However, future changes in vegetation composition and growth, along with acclimation to the new thermal regime, might facilitate the assimilation to counterbalance the carbon losses.

Tracking carbon within the trees

Tracking carbon within the trees

Optimal crop canopy architecture to maximise canopy photosynthetic CO2 uptake under elevated CO2 - a theoretical study using a mechanistic model of canopy photosynthesis.

Canopy architecture has been a major target in crop breeding for improved yields. Whether crop architectures in current elite crop cultivars can be modified for increased canopy CO2 uptake rate (Ac) under elevated atmospheric CO2 concentrations (Ca) is currently unknown. To study this question, we developed a new model of canopy photosynthesis, which includes three components: (i) a canopy architectural model; (ii) a forward ray tracing algorithm; and (iii) a steady-state biochemical model of C3 photosynthesis. With this model, we demonstrated that the Ac estimated from 'average' canopy light conditions is ~25% higher than that from light conditions at individual points in the canopy. We also evaluated theoretically the influence of canopy architectural on Ac under current and future Ca in rice. Simulation results suggest that to gain an optimal Ac for the examined rice cultivar, the stem height, leaf width and leaf angles can be manipulated to enhance canopy photosynthesis. This model provides a framework for designing ideal crop architectures to gain optimal Ac under future changing climate conditions. A close linkage between canopy photosynthesis modelling and canopy photosynthesis measurements is required to fully realise the potential of such modelling approaches in guiding crop improvements.

Water- and Plant-Mediated Responses of Ecosystem Carbon Fluxes to Warming and Nitrogen Addition on the Songnen Grassland in Northeast China

BackgroundUnderstanding how grasslands are affected by a long-term increase in temperature is crucial to predict the future impact of global climate change on terrestrial ecosystems. Additionally, it is not clear how the effects of global warming on grassland productivity are going to be altered by increased N deposition and N addition.Methodology/Principal FindingsIn-situ canopy CO2 exchange rates were measured in a meadow steppe subjected to 4-year warming and nitrogen addition treatments. Warming treatment reduced net ecosystem CO2 exchange (NEE) and increased ecosystem respiration (ER); but had no significant impacts on gross ecosystem productivity (GEP). N addition increased NEE, ER and GEP. However, there were no significant interactions between N addition and warming. The variation of NEE during the four experimental years was correlated with soil water content, particularly during early spring, suggesting that water availability is a primary driver of carbon fluxes in the studied semi-arid grassland.Conclusion/SignificanceEcosystem carbon fluxes in grassland ecosystems are sensitive to warming and N addition. In the studied water-limited grassland, both warming and N addition influence ecosystem carbon fluxes by affecting water availability, which is the primary driver in many arid and semiarid ecosystems. It remains unknown to what extent the long-term N addition would affect the turn-over of soil organic matter and the C sink size of this grassland.

Effect of salinity on canopy water vapor conductance of young and 3-year old Jatropha curcas L.

With increasing demand for biofuels, jatropha is considered as one of the biofuel crops, which is suitable for growth under adverse conditions including drought and salinity which mainly occur in arid and semi-arid environment. We report here to what extent salt stress affected water loss, canopy water vapor conductance, leaf growth and Na and K concentrations of leaves of 3-year old and young plants. Adult (on peat substrate) and young (in hydroponic system) plants were exposed to seven (0–300 mmol NaCl L−1) and five salt levels (0–200 mmol NaCl L−1), during 20 and 6 days, respectively.In both experiments, plants responded rapidly to salt stress by reducing water loss. The threshold value of responses was between 0 and 5 dS m−1. Leaf area increment of young jatropha had a threshold value of 5 dS m−1 implying that jatropha responds sensitive to external salt application in term of canopy development, conductance and CO2 assimilation rate. The rapid decrease of canopy conductance indicates that jatropha is effectively protecting leaves from massive Na import into leaves over short time periods. Our data indicate that use of wastewater in (semi-)arid areas should be carefully considered if high productivity is the key of plantation management.

Effect of inter-annual climate variability on evapotranspiration and canopy CO2 exchange of a tropical rainforest in Peninsular Malaysia

We investigated inter-annual variation of canopy CO2 exchange (NEE) and evapotranspiration during a 7-year period over a lowland Dipterocarp forest in Pasoh, Peninsular Malaysia, using the eddy covariance method. Annual rainfall fluctuated between 1,451 and 2,235 mm during this period. Annual evapotranspiration estimated by energy budget correction and gap filling using the relationship between latent heat and available energy was 1,287 ± 52 mm. Despite inter-annual variation in rainfall, annual evapotranspiration was stable, except for a slight decrease in the driest year (2009). Evapotranspiration was roughly related to the amount of available energy, but was regulated by stomatal closure to prevent excessive water loss at high vapour pressure deficit. Even during dry periods, no significant decrease in evapotranspiration occurred, as water was supplied from soil layers deeper than 0.5 m. Ecosystem respiration (RE) increased with soil water content. Daytime NEE was also stable during the 7 years, despite climate variability. Afternoon inhibition of canopy photosynthesis was seen every month. Daytime NEE did not become more negative with increasing solar radiation, or with increasing soil water content. During dry periods, gross primary production (GPP) and thus canopy gross photosynthesis decreased slightly, coupled with decreased daytime RE. In this forest, variability in rainfall pattern resulted in seasonal and inter-annual variability in micrometeorology; evapotranspiration, photosynthesis, and RE responded to these changes, and compensated for each other and/or other components of micrometeorology, resulting in rather stable annual evapotranspiration and NEE, even during a very dry year associated with an El Nino Southern Oscillation (ENSO) event.