Reduction In Gross Primary Productivity Research Articles

The sensitivity of wet and dry tropical forests to climate change in Bolivia

AbstractBolivia's forests contribute to the global carbon and water cycle, as well as to global biodiversity. The survival of these forests may be at risk due to climate change. To explore the associated mechanisms and uncertainties, a regionally adapted dynamic vegetation model was implemented for the Bolivian case, and forced with two contrasting climate change projections. Changes in carbon stocks and fluxes were evaluated, factoring out the individual contributions of atmospheric carbon dioxide ([CO2]), temperature, and precipitation. Impacts ranged from a strong increase to a severe loss of vegetation carbon (cv), depending on differences in climate projections, as well as the physiological response to rising [CO2]. The loss of cv simulated for an extremely dry projection was primarily driven by a reduction in gross primary productivity, and secondarily by enhanced emissions from fires and autotrophic respiration. In the wet forest, less precipitation and higher temperatures equally reduced cv, while in the dry forest, the impact of precipitation was dominating. The temperature‐related reduction of cv was mainly due to a decrease in photosynthesis and only to lesser extent because of more autotrophic respiration and less stomatal conductance as a response to an increasing atmospheric evaporative demand. Under an extremely dry projection, tropical dry forests were simulated to virtually disappear, regardless of the potential fertilizing effect of rising [CO2]. This suggests a higher risk for forest loss along the drier southern fringe of the Amazon if annual precipitation will decrease substantially.

The Influence of Chronic Ozone Exposure on Global Carbon and Water Cycles

Abstract Ozone (O3) is a phytotoxic greenhouse gas that has increased more than threefold at Earth’s surface from preindustrial values. In addition to directly increasing radiative forcing as a greenhouse gas, O3 indirectly impacts climate through altering the plant processes of photosynthesis and transpiration. While global estimates of gross primary productivity (GPP) have incorporated the effects of O3, few studies have explicitly determined the independent effects of O3 on transpiration. In this study, the authors include effects of O3 on photosynthesis and stomatal conductance from a recent literature review to determine the impact on GPP and transpiration and highlight uncertainty in modeling plant responses to O3. Using the Community Land Model, the authors estimate that present-day O3 exposure reduces GPP and transpiration globally by 8%–12% and 2%–2.4%, respectively. The largest reductions were in midlatitudes, with GPP decreasing up to 20% in the eastern United States, Europe, and Southeast Asia and transpiration reductions of up to 15% in the same regions. Larger reductions in GPP compared to transpiration decreased water-use efficiency 5%–10% in the eastern United States, Southeast Asia, Europe, and central Africa; increased surface runoff more than 15% in eastern North America; and altered patterns of energy fluxes in the tropics, high latitudes, and eastern North America. Future climate predictions will be improved if plant responses to O3 are incorporated into models such that stomatal conductance is modified independently of photosynthesis and the effects on transpiration are explicitly considered in surface energy budgets. Improvements will help inform regional decisions for managing changes in hydrology and surface temperatures in response to O3 pollution.

Estimation and analysis of gross primary production of soybean under various management practices and drought conditions

Gross primary production (GPP) of croplands may be used to quantify crop productivity and evaluate a range of management practices. Eddy flux data from three soybean (Glycine max L.) fields under different management practices (no-till vs. till; rainfed vs. irrigated) and Moderate Resolution Imaging Spectroradiometer (MODIS) derived vegetation indices (VIs) were used to test the capabilities of remotely sensed VIs and soybean phenology to estimate the seasonal dynamics of carbon fluxes. The modeled GPP (GPPVPM) using vegetation photosynthesis model (VPM) was compared with the GPP (GPPEC) estimated from eddy covariance measurements. The VIs tracked soybean phenology well and delineated the growing season length (GSL), which was closely related to carbon uptake period (CUP, R2=0.84), seasonal sums of net ecosystem CO2 exchange (NEE, R2=0.78), and GPPEC (R2=0.54). Land surface water index (LSWI) tracked drought-impacted vegetation well, as the LSWI values were positive during non-drought periods and negative during severe droughts within the soybean growing season. On a seasonal scale, NEE of the soybean sites ranged from −37 to −264gCm−2. The result suggests that rainfed soybean fields needed about 450–500mm of well-distributed seasonal rainfall to maximize the net carbon sink. During non-drought conditions, VPM accurately estimated seasonal dynamics and interannual variation of GPP of soybean under different management practices. However, some large discrepancies between GPPVPM and GPPEC were observed under drought conditions as the VI did not reflect the corresponding decrease in GPPEC. Diurnal GPPEC dynamics showed a bimodal distribution with a pronounced midday depression at the period of higher water vapor pressure deficit (>1.2kPa). A modified Wscalar based on LSWI to account for the water stress in VPM helped quantify the reduction in GPP during severe drought and the model’s performance improved substantially. In conclusion, this study demonstrates the potential of integrating vegetation activity through satellite remote sensing with ground-based flux and climate data for a better understanding and upscaling of carbon fluxes of soybean croplands.

Evaluation of MODIS GPP over a complex ecosystem in East Asia: A case study at Gwangneung flux tower in Korea

Moderate Resolution Imaging Radiometer (MODIS) gross primary productivity (GPP) has been used widely to study the global carbon cycle associated with terrestrial ecosystems. The retrieval of the current MODIS productivity with a 1×1km2 resolution has limitations when presenting subgrid scale processes in terrestrial ecosystems, specifically when forests are located in mountainous areas, and shows heterogeneity in vegetation type due to intensive land use. Here, we evaluate MODIS GPP (MOD17) at Gwangneung deciduous forest KoFlux tower (deciduous forest; GDK) for 2006–2010 in Korea, where the forests comprise heterogeneous vegetation cover over complex terrain. The monthly MODIS GPP data overestimated the GDK measurements in a range of +15% to +34% and was fairly well correlated (R=0.88) with the monthly variability at GDK during the growing season. In addition, the MODIS data partly represented the sharp GPP reduction during the Asian summer monsoon (June–September) when intensive precipitation considerably reduces solar radiation and disturbs the forest ecosystem. To examine the influence of subgrid scale heterogeneity on GPP estimates over the MODIS scale, the individual vegetation type and its area within a corresponding MODIS pixel were identified using a national forest type map (∼71-m spatial resolution), and the annual GPP in the same area as the MODIS pixel was estimated. This resulted in a slight reduction in the positive MODIS bias by ∼10%, with a high degree of uncertainty in the estimation. The MODIS discrepancy for GDK suggests further investigation is necessary to determine the MODIS errors associated with the site-specific aerodynamic and hydrological characteristics that are closely related to the mountainous topography. The accuracy of meteorological variables and the impact of the very cloudy conditions in East Asia also need to be assessed.

Sensitivity of vegetation indices and gross primary production of tallgrass prairie to severe drought

Drought affects vegetation photosynthesis and growth. Many studies have used the normalized difference vegetation index (NDVI), which is calculated as the normalized ratio between near infrared and red spectral bands in satellite images, to evaluate the response of vegetation to drought. In this study, we examined the impacts of drought on three vegetation indices (NDVI, enhanced vegetation index, EVI, and land surface water index, LSWI) and CO2 flux from three tallgrass prairie eddy flux tower sites in the U.S. Gross primary production (GPP) was also modeled using a satellite-based Vegetation Photosynthesis Model (VPM), and the modeled GPP (GPPVPM) was compared with the GPP (GPPEC) derived from eddy covariance measurements. Precipitation at two sites in Oklahoma was 30% below the historical mean in both years of the study period (2005–2006), while the site in Illinois did not experience drought in the 2005–2007 study period. The EVI explained the seasonal dynamics of GPP better than did NDVI. The LSWI dropped below zero during severe droughts in the growing season, showing its potential to track drought. The result shows that GPP was more sensitive to drought than were vegetation indices, and EVI and LSWI were more sensitive than NDVI. We developed a modified function (Wscalar), calculated as a function of LSWI, to account for the effect of severe droughts on GPP in VPM. The GPPVPM from the modified VPM accounted for the rapid reduction in GPP during severe droughts and the seasonal dynamics of GPPVPM agreed reasonably well with GPPEC. Our analysis shows that 8-day averaged values (temperature, vapor-pressure deficit) do not reflect the short-term extreme climate events well, suggesting that satellite-based models may need to be run at daily or hourly scales, especially under unfavorable climatic conditions.

Drought impact on vegetation productivity in the Lower Mekong Basin

The Lower Mekong Basin (LMB) has a typical monsoon climate, with high temperatures and an uneven distribution of precipitation throughout the year. This climate, combined with the geographic position of the LMB, has led to an increase in the frequency of extreme weather events over last decade. However, few previous studies have used remote-sensing data to investigate the impact of such weather events, particularly severe droughts, on biological productivity in the LMB. To address this, we assessed the impact of drought on vegetation productivity in the LMB during 2000–2011 using MOD17 products. Several drought events were identified during this period. Of these, the most severe occurred during 2005 and 2010, although the 2005 drought was both more extensive and more intense. Net primary productivity (NPP) exhibited considerable variation during 2000–2011: the droughts in 2005 and 2010 reduced NPP by 14.7% and 8.4%, respectively. The impact of drought on NPP in 2005 was much greater than that in 2010, likely owing to the longer duration and larger deficit of precipitation in 2005 (which lasted from winter 2004 to spring 2005). Our results demonstrate that severe drought had a greater impact on NPP than mild drought, especially for forests, woodlands, and shrublands. Comparatively, little variation in NPP was found for croplands, even under drought conditions, which were attributed to the wide use of irrigation and the exploitation of water sources during drought periods. Moreover, multi-season croplands in Vietnam experienced only a small reduction in gross primary productivity (GPP) in 2005 compared to one-season croplands in Cambodia, which can be related to the shorter growing periods of the former impacted by droughts.

Monitoring the effects of rapid onset of drought on non-irrigated maize with agronomic data and climate-based drought indices

The 2003 growing season at Mead, NE began with moist and relatively cool conditions that persisted through most of June. During this moist phase of the season, soil water and parameters such as evapotranspiration (ET) and gross primary productivity (GPP) were nearly identical between a rainfed maize site (RMS) and an irrigated maize site (IMS). A drying phase began in late June, causing decline in soil water at RMS and the necessity of irrigation treatments at IMS. The drying phase turned into a “stressed” phase by early August, as only 10mm of precipitation fell in a 40-day period between mid-July and late August. Conditions at RMS began to deteriorate even more rapidly after maize entered the critical reproductive stage, as the depletion of soil water led to (implied) reductions in stomatal conductance, which led to significant reductions in ET and GPP, compared to the well-watered IMS. Two drought indices, the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI), were utilized to show the effectiveness of short-term indices at detecting flash drought versus field measurements. Results showed that both the 1-month SPI and the 1-month SPEI were quite sensitive to the onset of the flash drought and closely followed the decline in soil water and other biophysical parameters at RMS relative to IMS. Significant precipitation returned and led to some recharge prior to harvest but was far too late to be of any help to the maize at RMS, as the yield difference of 6.3Mg/ha between RMS and IMS revealed the detrimental effects of a rapid onset of drought during the critical reproductive stage of maize.

Climate Change and Water Use Partitioning by Different Plant Functional Groups in a Grassland on the Tibetan Plateau

The Tibetan Plateau (TP) is predicted to experience increases in air temperature, increases in snowfall, and decreases in monsoon rains; however, there is currently a paucity of data that examine the ecological responses to such climate changes. In this study, we examined the effects of increased air temperature and snowfall on: 1) water use partitioning by different plant functional groups, and 2) ecosystem CO2 fluxes throughout the growing season. At the individual plant scale, we used stable hydrogen isotopes (δD) to partition water use between shallow- and deep-rooted species. Prior to the arrival of summer precipitation (typically mid-July), snowmelt was the main water source in the soils. During this time, shallow and deep-rooted species partitioned water use by accessing water from shallow and deep soils, respectively. However, once the monsoon rains arrived, all plants used rainwater from the upper soils as the main water source. Snow addition did not result in increased snowmelt use throughout the growing season; instead, snowmelt water was pushed down into deeper soils when the rains arrived. At the larger plot scale, CO2 flux measurements demonstrated that rain was the main driver for net ecosystem productivity (NEP). NEP rates were low during June and July and reached a maximum during the monsoon season in August. Warming decreased NEP through a reduction in gross primary productivity (GPP), and snow additions did not mitigate the negative effects of warming by increasing NEP or GPP. Both the isotope and CO2 flux results suggest that rain drives productivity in the Nam Tso region on the TP. This also suggests that the effects of warming-induced drought on the TP may not be mitigated by increased snowfall. Further decreases in summer monsoon rains may affect ecosystem productivity, with large implications for livestock-based livelihoods.

Tropospheric ozone reduces carbon assimilation in trees: estimates from analysis of continuous flux measurements

High ground-level ozone concentrations are typical of Mediterranean climates. Plant exposure to this oxidant is known to reduce carbon assimilation. Ozone damage has been traditionally measured through manipulative experiments that do not consider long-term exposure and propagate large uncertainty by up-scaling leaf-level observations to ecosystem-level interpretations. We analyzed long-term continuous measurements (>9 site-years at 30min resolution) of environmental and eco-physiological parameters at three Mediterranean ecosystems: (i) forest site dominated by Pinus ponderosa in the Sierra Mountains in California, USA; (ii) forest site composed of a mixture of Quercus spp. and P. pinea in the Tyrrhenian sea coast near Rome, Italy; and (iii) orchard site of Citrus sinensis cultivated in the California Central Valley, USA. We hypothesized that higher levels of ozone concentration in the atmosphere result in a decrease in carbon assimilation by trees under field conditions. This hypothesis was tested using time series analysis such as wavelet coherence and spectral Granger causality, and complemented with multivariate linear and nonlinear statistical analyses. We found that reduction in carbon assimilation was more related to stomatal ozone deposition than to ozone concentration. The negative effects of ozone occurred within a day of exposure/uptake. Decoupling between carbon assimilation and stomatal aperture increased with the amount of ozone pollution. Up to 12-19% of the carbon assimilation reduction in P. ponderosa and in the Citrus plantation was explained by higher stomatal ozone deposition. In contrast, the Italian site did not show reductions in gross primary productivity either by ozone concentration or stomatal ozone deposition, mainly due to the lower ozone concentrations in the periurban site over the shorter period of investigation. These results highlight the importance of plant adaptation/sensitivity under field conditions, and the importance of continuous long-term measurements to explain ozone damage to real-world forests and calculate metrics for ozone-risk assessment.

Extreme late-summer drought causes neutral annual carbon balance in southwestern ponderosa pine forests and grasslands

We assessed the impacts of extreme late-summer drought on carbon balance in a semi-arid forest region in Arizona. To understand drought impacts over extremes of forest cover, we measured net ecosystem production (NEP), gross primary production (GPP), and total ecosystem respiration (TER) with eddy covariance over five years (2006–10) at an undisturbed ponderosa pine (Pinus ponderosa) forest and at a former forest converted to grassland by intense burning. Drought shifted annual NEP from a weak source of carbon to the atmosphere to a neutral carbon balance at the burned site and from a carbon sink to neutral at the undisturbed site. Carbon fluxes were particularly sensitive to drought in August. Drought shifted August NEP at the undisturbed site from sink to source because the reduction of GPP (70%) exceeded the reduction of TER (35%). At the burned site drought shifted August NEP from weak source to neutral because the reduction of TER (40%) exceeded the reduction of GPP (20%). These results show that the lack of forest recovery after burning and the exposure of undisturbed forests to late-summer drought reduce carbon sink strength and illustrate the high vulnerability of forest carbon sink strength in the southwest US to predicted increases in intense burning and precipitation variability.

Reconciling leaf physiological traits and canopy flux data: Use of the TRY and FLUXNET databases in the Community Land Model version 4

The Community Land Model version 4 overestimates gross primary production (GPP) compared with estimates from FLUXNET eddy covariance towers. The revised model of Bonan et al. (2011) is consistent with FLUXNET, but values for the leaf‐level photosynthetic parameterVcmaxthat yield realistic GPP at the canopy‐scale are lower than observed in the global synthesis of Kattge et al. (2009), except for tropical broadleaf evergreen trees. We investigate this discrepancy betweenVcmaxand canopy fluxes. A multilayer model with explicit calculation of light absorption and photosynthesis for sunlit and shaded leaves at depths in the canopy gives insight to the scale mismatch between leaf and canopy. We evaluate the model with light‐response curves at individual FLUXNET towers and with empirically upscaled annual GPP. Biases in the multilayer canopy with observedVcmaxare similar, or improved, compared with the standard two‐leaf canopy and its lowVcmax, though the Amazon is an exception. The difference relates to light absorption by shaded leaves in the two‐leaf canopy, and resulting higher photosynthesis when the canopy scaling parameterKn is low, but observationally constrained. Larger Kndecreases shaded leaf photosynthesis and reduces the difference between the two‐leaf and multilayer canopies. The low modelVcmaxis diagnosed from nitrogen reduction of GPP in simulations with carbon‐nitrogen biogeochemistry. Our results show that the imposed nitrogen reduction compensates for deficiency in the two‐leaf canopy that produces high GPP. Leaf trait databases (Vcmax), within‐canopy profiles of photosynthetic capacity (Kn), tower fluxes, and empirically upscaled fields provide important complementary information for model evaluation.

Carbon dioxide emissions from spring ploughing of grassland in Ireland

Grassland re-seeding or land-use change requires ploughing, which may enhance carbon dioxide (CO 2) emissions from soil. This study observed the short to intermediate-term (37 days) effects of ploughing on CO 2 emissions from poorly drained grassland using automated soil respiration chambers. Immediately after ploughing, a brief peak in CO 2 emissions from soil occurred with a maximum observed flux of 6.91 g CO 2 m −2 h −1. Contradictory to other reported results, ecosystem respiration after ploughing was lower on the ploughed than on the grass site. After including estimates of photosynthesis in the analysis, ploughing led to significantly higher net CO 2 emissions than from grassland. The main mechanism of C loss during ploughing was most likely due to a reduction in gross primary production rather than enhanced soil respiration.

Dynamics of carbon exchange in a Eucalyptus forest in response to interacting disturbance factors

Carbon (C) sinks created by forests depend on the balance between C uptake through photosynthesis and loss through respiration. This balance varies depending on the relative effect of environmental drivers on these processes. Components and dynamics of the C cycle were measured in a native Eucalyptus delegatensis forest at Tumbarumba in south-eastern Australia during conditions of average rainfall (1998–2001) and droughts (2002–2003 and 2006–2007). In 2002–03 there were interacting disturbance factors of dry conditions and insect damage that reduced the photosynthetically active leaf area in the canopy. Conditions during the droughts included reduced soil moisture content, higher temperatures and increased vapour pressure deficit. Similarly low soil moisture contents occurred during both drought periods, but lasted for longer in 2002–03. The combined impact of drought stress and insect damage resulted in markedly reduced growth (45–80%) and higher mortality of trees (5–60%). Impacts were variable across the 50,000ha of forest, with mortality greatest in stands with normally highest growth rates, and in locations with greatest reductions in soil moisture. Growth rates were reduced during 2002–03, most severely in 2003–04, and recovering in 2004–05. Mortality remained high in 2004–05 indicating the prolonged effect of the stress conditions. The total C pool in the forest is 483tCha−1 with net C uptake of −6.7tCha−1yr−1 in the growing season prior to the insect attack (2001–02). Under conditions of drought and insect disturbance in 2003 the forest released a total of 1.7tCha−1 over 8 months, while under conditions of drought alone carbon uptake was −6.5tCha−1yr−1 in the growing season year of 2006–07 and −5.6tCha−1yr−1 in the calendar year of 2007. Interacting stress factors of drought and insect damage resulted in a large imbalance in the C budget with a 26% reduction in gross primary productivity and a 9% reduction in ecosystem respiration, whereas drought alone had a much lesser effect. Drought conditions result in (1) weather conditions that break the synchronisation of insects with parasites and predators resulting in insect outbreaks, (2) moisture stress that predisposes trees to attack by insects, and (3) moisture stress that restricts leaf regeneration after damage. Climate change and climatically induced changes in disturbance regimes may affect the processes of photosynthesis and respiration differently and hence change the balance of net ecosystem C exchange. Reduced sink strength of forests will lead to positive C cycle–climate feedbacks, which enhance the greenhouse effect and climate change.

Summer drought influence on CO 2 and water fluxes of extensively managed grassland in Germany

Net ecosystem CO 2 exchange (NEE) and water fluxes of extensively managed grassland in Germany were measured in a dry year, 2003 and a normal year, 2004 using eddy covariance. Empirical descriptions of canopy capacities (light utilization efficiency and canopy capacity for maximum carboxylation) and inherent water use efficiency (IWUE*) were computed to indicate the grassland's response to environmental and management conditions. Compared with 2004, 2003 was characterized by a 2.5 °C rise in summer mean temperature and an annual precipitation deficit of 286 mm, which was almost one-third below the average precipitation of the normal year 2004 and the long-term annual average (1959–2005). Annual net water balance (precipitation minus evapo-transpiration) in 2003 and 2004 were 85 mm and 361 mm, respectively. Slightly higher IWUE* in 2003 indicates the grassland's sensitivity to drought. The grassland provided an annual gross primary production (GPP) and ecosystem respiration ( R eco) of 1022 and 728 g C m −2 respectively in 2003, as compared to their corresponding values of 1233 and 973 g C m −2 in 2004. Drought-induced biomass senescence caused GPP reduction in 2003, which in turn affected the ecosystem respiration. Light explained more than 80% of NEE variability during canopy development and as a result, annual NEE in 2003 and 2004 were −295 and −260 g C m −2, respectively. Taking into account the exported carbon by biomass harvest, we calculated net biome productivity (NBP) of −113 g C m −2 yr −1. We conclude that this managed grassland acted as a moderate carbon sink, despite the difference in environmental and management conditions.

Influence of the Asian monsoon on net ecosystem carbon exchange in two major ecosystems in Korea

Abstract. Considering the feedback in radiation, temperature, and soil moisture with alterations in rainfall patterns, the influence of the changing monsoon on Net Ecosystem CO2 Exchange (NEE) can be critical to the estimation of carbon balance in Asia. In this paper, we examined CO2 fluxes measured by the eddy covariance method from 2004 to 2008 in two major ecosystems in the KoFlux sites in Korea, i.e., the Gwangneung Deciduous forest (GDK) and the Haenam Farmland (HFK). Our objectives were to identify the repeatability of the mid-season depression of NEE encountered at the two sites based on the single-year observation, and to further scrutinize its cause, effect, and interannual variability by using multi-year observations. In both GDK and HFK sites, the mid-season depression of NEE was reproduced each year but with different timing, magnitude, and mechanism. At the GDK site, a predominant factor causing the mid-season depression was a decreased solar radiation and the consequent reduction in Gross Primary Productivity (GPP) during the summer monsoon period. At the HFK site, however, the monsoonal effect was less pronounced and the apparent mid-season depression was mainly a result of the management practices such as cultivation of spring barley and rice transplantation. Other flux observation sites in East Asia also showed a decline in radiation but with a lesser degree during the monsoon season, resulting in less pronounced depression in NEE. In our study, the observed depressions in NEE caused both GDK and HFK sites to become a weaker carbon sink or even a source in the middle of the growing season. On average, the GDK site (with maximum leaf area index of ~5) was a weak carbon sink with NEE of −84 gC m−2 y−1. Despite about 20% larger GPP (of 1321 gC m−2y−1) in comparison with the GDK site, the HFK site (with maximum leaf area index of 3–4) was a weaker carbon sink with NEE of −58 gC m−2 y−1 because of greater ecosystem respiration (of 1263 gC m−2 y−1). These NEE values were near the low end of the ranges reported in the literature for similar ecosystems in mid-latitudes. With the projected trends of the extended length of monsoon with more intensive rainfalls in East Asia, the observed delicate coupling between carbon and hydrological cycles may turn these key ecosystems into carbon neutral.

Estimating the reduction in gross primary production due to mountain pine beetle infestation using satellite observations

The impact of disturbance on national and global forest carbon budgets is considered large enough to shift forests into acting as carbon sinks or sources. While the role of fire on the landscape and its impact is relatively well understood, the role of insect infestation is less well known. The ongoing outbreak of mountain pine beetle (Dendroctonus ponderosae) in western Canada is impacting over 9.2 million ha of forest as of 2006. Using satellite-derived (Moderate Resolution Imaging Spectroradiometer (MODIS)) annual estimates of gross primary production (P g), in combination with survey data indicating infestation extent and severity, the reduction in P g attributable to this major disturbance is estimated. Results indicate that the infestation between 2002 and 2005 resulted in a reduction in the rate of carbon accumulation of between 60–100 g C m−2 year−1, which corresponds well with previously modelled values, reducing the pre-outbreak P g of the stands by approximately 15–20%, with productivity losses generally proportional to the locally accumulated severity of insect activity. Continued monitoring using MODIS based approaches may offer ongoing opportunities to estimate the landscape-level rates of recovery from the outbreak.

Separation of net ecosystem exchange into assimilation and respiration using a light response curve approach: critical issues and global evaluation

AbstractThe measured net ecosystem exchange (NEE) of CO2 between the ecosystem and the atmosphere reflects the balance between gross CO2 assimilation [gross primary production (GPP)] and ecosystem respiration (Reco). For understanding the mechanistic responses of ecosystem processes to environmental change it is important to separate these two flux components. Two approaches are conventionally used: (1) respiration measurements made at night are extrapolated to the daytime or (2) light–response curves are fit to daytime NEE measurements and respiration is estimated from the intercept of the ordinate, which avoids the use of potentially problematic nighttime data. We demonstrate that this approach is subject to biases if the effect of vapor pressure deficit (VPD) modifying the light response is not included. We introduce an algorithm for NEE partitioning that uses a hyperbolic light response curve fit to daytime NEE, modified to account for the temperature sensitivity of respiration and the VPD limitation of photosynthesis. Including the VPD dependency strongly improved the model's ability to reproduce the asymmetric diurnal cycle during periods with high VPD, and enhances the reliability of Reco estimates given that the reduction of GPP by VPD may be otherwise incorrectly attributed to higher Reco. Results from this improved algorithm are compared against estimates based on the conventional nighttime approach. The comparison demonstrates that the uncertainty arising from systematic errors dominates the overall uncertainty of annual sums (median absolute deviation of GPP: 47 g C m−2 yr−1), while errors arising from the random error (median absolute deviation: ∼2 g C m−2 yr−1) are negligible. Despite site‐specific differences between the methods, overall patterns remain robust, adding confidence to statistical studies based on the FLUXNET database. In particular, we show that the strong correlation between GPP and Reco is not spurious but holds true when quasi‐independent, i.e. daytime and nighttime based estimates are compared.

Seasonal and interannual patterns of carbon and water fluxes of a poplar plantation under peculiar eco-climatic conditions

This paper reports 3 years of eddy covariance measurements (2002–2004) on a poplar plantation ( Populus × canadensis Moench, Clone I-214) in Northern Italy. We analyzed seasonal and interannual variability of the net ecosystem exchange (NEE), gross primary production (GPP), ecosystem respiration ( R ECO) and evapotranspiration (ET) in relation to different meteorological and environmental conditions experienced by the plantation and to natural disturbances. In particular, during 2003 climatic conditions were exceptionally severe, with the highest mean air temperatures and the lowest precipitations observed in the study area in the last 50 years. In addition, during the late summer of 2004, the plantation was attacked by fall webworm larvae ( Hyphantria cunea). The cumulated NEE during the three growing season (April–September) was −752.8, −626.7 and −702.7 g C m −2 for 2002, 2003 and 2004, respectively. In the period June–August 2003, NEE was 35% and 29% higher than the NEE measured in the same period in 2002 and 2004, respectively, resulting in a lower net carbon uptake. As R ECO did not show large differences over the three growing seasons, the reduction in net carbon sequestration of June–August 2003 is mainly explained by a reduction of GPP (−18% and −17% in 2002 and 2004, respectively). The cumulated ET over the three growing seasons was 388, 471, 484 kg H 2O m −2 for 2002, 2003 and 2004, respectively. This behaviour was due to the fact that the position of the water table remained close to the roots, therefore supplying enough water to the plantation, even during the driest period. The analysis of light curve parameters, their residuals and canopy conductance suggests that the reduction of CO 2 uptake during summer 2003 was mainly controlled by stomatal and non-stomatal limitations due to high temperatures, directly and in particular indirectly (i.e. through D) rather than by soil water stress. In conclusion, our study shows that in the presence of a heat-wave, a significant reduction of net productivity during summer may occur even in the absence of marked soil water stress. In addition to this sensitivity to high temperatures, which will likely increase in the future, the effects of natural disturbances may add further uncertainties, thus suggesting caution in the evaluation of the potential carbon sequestration of these ecosystems.

Environmental controls on photosynthetic production and ecosystem respiration in semi-arid grasslands of Mongolia

The Mongolian steppe zone comprises a major part of East Asian grasslands. The objective of this study was to separately evaluate the quantitative dependencies of gross primary production (GPP) and ecosystem respiration ( R eco) on the environmental variables of temperature, moisture, radiation, and plant biomass in a semi-arid grassland ecosystem. We determined GPP and R eco using transparent and opaque closed chambers in a grassland dominated by Poaceae species in central Mongolia during five periods: July 2004, May 2005, July 2005, September 2005, and June 2006. Values of GPP were linearly related to live aboveground biomass (AGB) enclosed by the chamber. The amount of GPP per unit ground area differed among the study periods, whereas GPP normalized by the amount of AGB did not differ significantly among the periods, suggesting that plant production per unit green biomass did not depend on the phenological stage. GPP/AGB fit well a rectangular hyperbolic light–response curve for all the study periods. When the air and soil were dry, considerable reduction in GPP was observed. The GPP/AGB ratio was also expressed as individual functions of air temperature, vapor pressure deficit, and volumetric soil water content. R eco was exponentially related to the soil temperature and the relationship was modified by soil moisture. The amount of R eco and its temperature sensitivity ( Q 10) declined with decreasing soil moisture. Sharp increases of R eco after rainfall events were observed. The values of R eco, even including the rain-induced pulses, were expressed well as a bivariate function of soil temperature and soil moisture near the soil surface.

Ten years of fluxes and stand growth in a young beech forest at Hesse, North-eastern France

• Water and carbon fluxes, as measured by eddy covariance, climate, soil water content, leaf area index, tree biomass, biomass increment (BI), litter fall and mortality were monitored for 10 successive years in a young beech stand in Hesse forest (north-eastern France) under contrasting climatic and management conditions. • Large year-to-year variability of net carbon fluxes (NEE) and to a lesser extent, of tree growth was observed. The variability in NEE (coefficient of variation, CV = 44%) was related to both gross primary production (GPP) and to variations in total ecosystem respiration (TER), each term showing similar and lower interannual variability (CV = 14%) than NEE. Variation in the annual GPP was related to: (i) the water deficit duration and intensity cumulated over the growing season, and (ii) the growing season length, i.e. the period of carbon uptake by the stand. Two thinnings occurring during the observation period did not provoke a reduction in either GPP, water fluxes, or in tree growth. Interannual variation of TER could not be explained by any annual climatic variables, or LAI, and only water deficit duration showed a poor correlation. Annual biomass increment was well correlated to water shortage duration and was significantly influenced by drought in the previous year. • The relationship between annual NEE and biomass increment (BI) was poor: in some years, the annual carbon uptake was much higher and in others much lower than tree growth. However this relationship was much stronger and linear (r 2 = 0.93) on a weekly to monthly time-scale from budburst to the date of radial growth cessation, indicating a strong link between net carbon uptake and tree growth, while carbon losses by respiration occurring after this date upset this relationship. • Despite the lack of correlation between annual data, the NEE and BI cumulated over the 10 years of observations were very close. • On the annual time-scale, net primary productivity calculated from eddy fluxes and from biological measurements showed a good correlation.