Regional atmospheric cooling and wetting effect of permafrost thaw‐induced boreal forest loss

In the sporadic permafrost zone of North America, thaw‐induced boreal forest loss is leading to permafrost‐free wetland expansion. These land cover changes alter landscape‐scale surface properties with potentially large, however, still unknown impacts on regional climates. In this study, we combine nested eddy covariance flux tower measurements with satellite remote sensing to characterize the impacts of boreal forest loss on albedo, eco‐physiological and aerodynamic surface properties, and turbulent energy fluxes of a lowland boreal forest region in the Northwest Territories, Canada. Planetary boundary layer modelling is used to estimate the potential forest loss impact on regional air temperature and atmospheric moisture. We show that thaw‐induced conversion of forests to wetlands increases albedo: and bulk surface conductance for water vapour and decreases aerodynamic surface temperature. At the same time, heat transfer efficiency is reduced. These shifts in land surface properties increase latent at the expense of sensible heat fluxes, thus, drastically reducing Bowen ratios. Due to the lower albedo of forests and their masking effect of highly reflective snow, available energy is lower in wetlands, especially in late winter. Modelling results demonstrate that a conversion of a present‐day boreal forest–wetland to a hypothetical homogeneous wetland landscape could induce a near‐surface cooling effect on regional air temperatures of up to 3–4 °C in late winter and 1–2 °C in summer. An atmospheric wetting effect in summer is indicated by a maximum increase in water vapour mixing ratios of 2 mmol mol^?1. At the same time, maximum boundary layer heights are reduced by about a third of the original height. In fall, simulated air temperature and atmospheric moisture between the two scenarios do not differ. Therefore, permafrost thaw‐induced boreal forest loss may modify regional precipitation patterns and slow down regional warming trends.     

Carbon dioxide exchange of a Russian boreal forest after disturbance by wind throw

The exchange of carbon dioxide (CO₂) between the atmosphere and a forest after disturbance by wind throw in the western Russian taiga was investigated between July and October 1998 using the eddy covariance technique. The research area was a regenerating forest (400 m × 1000 m), in which all trees of the preceding generation were uplifted during a storm in 1996. All deadwood had remained on site after the storm and had not been extracted for commercial purposes. Because of the heterogeneity of the terrain, several micrometeorological quality tests were applied. In addition to the eddy covariance measurements, carbon pools of decaying wood in a chronosequence of three different wind throw areas were analysed and the decay rate of coarse woody debris was derived. During daytime, the average CO₂ uptake flux was ?3 µmol m^?2s^?1, whereas during night‐time characterised by a well‐mixed atmosphere the rates of release were typically about 6 µmol m^?2s^?1. Suppression of turbulent fluxes was only observed under conditions with very low friction velocity (u* ≤ 0.08 ms^?1). On average, 164 mmol CO₂ m^?2d^?1 was released from the wind throw to the atmosphere, giving a total of 14.9 mol CO₂ m^?2 (180 g CO₂ m^?2) released during the 3‐month study period. The chronosequence of dead woody debris on three different wind throw areas suggested exponential decay with a decay coefficient of ?0.04 yr^?1. From the magnitude of the carbon pools and the decay rate, it is estimated that the decomposition of coarse woody debris accounted for about a third of the total ecosystem respiration at the measurement site. Hence, coarse woody debris had a long‐term influence on the net ecosystem exchange of this wind throw area. From the analysis performed in this work, a conclusion is drawn that it is necessary to include into flux networks the ecosystems that are subject to natural disturbances and that have been widely omitted into considerations of the global carbon budget. The half‐life time of about 17 years for deadwood in the wind throw suggests a fairly long storage of carbon in the ecosystem, and indicates a very different long‐term carbon budget for naturally disturbed vs. commercially managed forests.     

Annual cycles of water vapour and carbon dioxide fluxes in and above a boreal aspen forest

Water vapour and CO₂ fluxes were measured using the eddy correlation method above and below the overstorey of a 21-m tall aspen stand in the boreal forest of central Saskatchewan as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Measurements were made at the 39.5-m and 4-m heights using 3-dimensional sonic anemometers (Kaijo-Denki and Solent, respectively) and closed-path gas analysers (LI-COR 6262) with 6-m and 4.7-m long heated sampling tubing, respectively. Continuous measurements were made from early October to mid-November 1993 and from early February to late-September 1994. Soil CO₂ flux (respiration) was measured using a LI-COR 6000-09 soil chamber and soil evaporation was measured using Iysimetry. The leaf area index of the aspen and hazelnut understorey reached 1.8 and 3.3, respectively. The maximum daily evapotranspiration (E) rate was 5–6 mm d^?1. Following leaf-out the hazelnut and soil accounted for 22% of the forest E. The estimated total E was 403 mm for 1994. About 88% of the precipitation in 1994 was lost as evapotranspiration. During the growing season, the magnitude of half-hourly eddy fluxes of CO₂ from the atmosphere into the forest reached 1.2 mg CO₂ m^?2 s^?1 (33 μmol C m^?2 s^?1) during the daytime. Downward eddy fluxes at the 4-m height were observed when the hazelnut was growing rapidly in June and July. Under well-ventilated night-time conditions, the eddy fluxes of CO₂ above the aspen and hazelnut, corrected for canopy storage, increased exponentially with soil temperature at the 2-cm depth. Estimates of daytime respiration rates using these relationships agreed well with soil chamber measurements. During the 1994 growing season, the cumulative net ecosystem exchange (NEE) was -3.5 t C ha^?1 y^?1 (a net gain by the system). For 1994, cumulative NEE, ecosystem respiration (R) and gross ecosystem photosynthesis (GEP = R - NEE) were estimated to be -1.3, 8.9 and 10.2 t C ha^?1 y^?1 respectively. Gross photosynthesis of the hazelnut was 32% of GEP.     

The Net Landscape Carbon Balance—Integrating terrestrial and aquatic carbon fluxes in a managed boreal forest landscape in Sweden

The boreal biome exchanges large amounts of carbon (C) and greenhouse gases (GHGs) with the atmosphere and thus significantly affects the global climate. A managed boreal landscape consists of various sinks and sources of carbon dioxide (CO₂), methane (CH₄), and dissolved organic and inorganic carbon (DOC and DIC) across forests, mires, lakes, and streams. Due to the spatial heterogeneity, large uncertainties exist regarding the net landscape carbon balance (NLCB). In this study, we compiled terrestrial and aquatic fluxes of CO₂, CH₄, DOC, DIC, and harvested C obtained from tall‐tower eddy covariance measurements, stream monitoring, and remote sensing of biomass stocks for an entire boreal catchment (~68 km²) in Sweden to estimate the NLCB across the land–water–atmosphere continuum. Our results showed that this managed boreal forest landscape was a net C sink (NLCB = 39 g C m^?2 year^?1) with the landscape–atmosphere CO₂ exchange being the dominant component, followed by the C export via harvest and streams. Accounting for the global warming potential of CH₄, the landscape was a GHG sink of 237 g CO₂‐eq m^?2 year^?1, thus providing a climate‐cooling effect. The CH₄ flux contribution to the annual GHG budget increased from 0.6% during spring to 3.2% during winter. The aquatic C loss was most significant during spring contributing 8% to the annual NLCB. We further found that abiotic controls (e.g., air temperature and incoming radiation) regulated the temporal variability of the NLCB whereas land cover types (e.g., mire vs. forest) and management practices (e.g., clear‐cutting) determined their spatial variability. Our study advocates the need for integrating terrestrial and aquatic fluxes at the landscape scale based on tall‐tower eddy covariance measurements combined with biomass stock and stream monitoring to develop a holistic understanding of the NLCB of managed boreal forest landscapes and to better evaluate their potential for mitigating climate change.     

Effects of climatic variability on the annual carbon sequestration by a boreal aspen forest

To evaluate the carbon budget of a boreal deciduous forest, we measured CO₂ fluxes using the eddy covariance technique above an old aspen (OA) forest in Prince Albert National Park, Saskatchewan, Canada, in 1994 and 1996 as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). We found that the OA forest is a strong carbon sink sequestering 200 ± 30 and 130 ± 30 g C m^–2 y^–1 in 1994 and 1996, respectively. These measurements were 16–45% lower than an inventory result that the mean carbon increment was about 240 g C m^–2 y^–1 between 1919 and 1994, mainly due to the advanced age of the stand at the time of eddy covariance measurements. Assuming these rates to be representative of Canadian boreal deciduous forests (area ≈ 3 × 10⁵ km²), it is likely they can sequester 40–60 Tg C y^–1, which is 2–3% of the missing global carbon sink. The difference in carbon sequestration by the OA forest between 1994 and 1996 was mainly caused by the difference in leaf emergence date. The monthly mean air temperature during March–May 1994, was 4.8 °C higher than in 1996, resulting in leaf emergence being 18–24 days earlier in 1994 than 1996. The warm spring and early leaf emergence in 1994 enabled the aspen forest to exploit the long days and high solar irradiance of mid-to-late spring. In contrast, the 1996 OA growing season included only 32 days before the summer solstice. The earlier leaf emergence in 1994 resulted 16% more absorbed photosynthetically active radiation and a 90 g C m^–2 y^–1 increase in photosynthesis than 1996. The concomitant increase in respiration in the warmer year (1994) was only 20 g C m^–2 y^–1. These results show that an important control on carbon sequestration by boreal deciduous forests is spring temperature, via the influence of air temperature on the timing of leaf emergence.     

仪器的加热效应校正对生态系统碳水通量估算的影响

涡度相关技术的广泛应用为获取生态系统碳、水通量提供了可能,但在开路式涡度相关系统中,仪器的加热效应增大了观测数据的不确定性。为了衡量仪器的加热效应,以ChinaFLUX3个典型生态系统(长白山温带针阔混交林(CBS)、海北灌丛草甸(HBGC)、鼎湖山亚热带常绿阔叶林(DHS))为研究对象,就仪器的加热效应校正对碳、水通量估算的影响进行分析。结果表明:加热校正没有改变生态系统的能量闭合特征,也没有对水汽通量的估算产生影响,但显著减小了CBS和HBGC非生长季的净生态系统生产力(NEP),进而减少了NEP的年总量,对DHS没有显著影响。NEP减小幅度受到温度的强烈影响,CBS为7.7%～10.4%,远小于HBGC的76.6%～85.2%,HBGC的NEP大幅降低主要是由夜间NEP的改变导致生态系统呼吸(RE)的增大而引起。因而,在温带生态系统中,充分考虑加热校正对于准确估算生态系统的碳收支具有重要作用。     

Nonlinear responses of ecosystem carbon fluxes to nitrogen deposition in an old-growth boreal forest

Nitrogen (N) deposition is known to increase carbon (C) sequestration in N-limited boreal forests. However, the long-term effects of N deposition on ecosystem carbon fluxes have been rarely investigated in old-growth boreal forests. Here we show that decade-long experimental N additions significantly stimulated net primary production (NPP) but the effect decreased with increasing N loads. The effect on soil heterotrophic respiration (Rh) shifted from a stimulation at low-level N additions to an inhibition at higher levels of N additions. Consequently, low-level N additions resulted in a neutral effect on net ecosystem productivity (NEP), due to a comparable stimulating effect on NPP and Rh, while NEP was increased by high-level N additions. Moreover, we found nonlinear temporal responses of NPP, Rh and NEP to low-level N additions. Our findings imply that actual N deposition in boreal forests likely exerts a minor contribution to their soil C storage.     

Drivers of postfire soil organic carbon accumulation in the boreal forest

The accumulation of soil carbon (C) is regulated by a complex interplay between abiotic and biotic factors. Our study aimed to identify the main drivers of soil C accumulation in the boreal forest of eastern North America. Ecosystem C pools were measured in 72 sites of fire origin that burned 2–314 years ago over a vast region with a range of ? mean annual temperature of 3°C and one of ? 500 mm total precipitation. We used a set of multivariate a priori causal hypotheses to test the influence of time since fire (TSF), climate, soil physico‐chemistry and bryophyte dominance on forest soil organic C accumulation. Integrating the direct and indirect effects among abiotic and biotic variables explained as much as 50% of the full model variability. The main direct drivers of soil C stocks were: TSF >bryophyte dominance of the FH layer and metal oxide content >pH of the mineral soil. Only climate parameters related to water availability contributed significantly to explaining soil C stock variation. Importantly, climate was found to affect FH layer and mineral soil C stocks indirectly through its effects on bryophyte dominance and organo‐metal complexation, respectively. Soil texture had no influence on soil C stocks. Soil C stocks increased both in the FH layer and mineral soil with TSF and this effect was linked to a decrease in pH with TSF in mineral soil. TSF thus appears to be an important factor of soil development and of C sequestration in mineral soil through its influence on soil chemistry. Overall, this work highlights that integrating the complex interplay between the main drivers of soil C stocks into mechanistic models of C dynamics could improve our ability to assess C stocks and better anticipate the response of the boreal forest to global change.     

东北阔叶红松林能量平衡特征

采用涡度相关法,结合小气候观测,对东北阔叶红松林的能量平衡特征进行了研究。结果表明,森林全年获得的辐射能量为2.3×109J/m2,平均净辐射(Rn)强度为72.1 W/m2,12月最小,平均为5.8W/m2,6月最大,平均为127 W/m2。除了受太阳高度角的支配,Rn对中小尺度天气变化响应显著。非生长季,森林主要能量支出项为感热通量(H),约占Rn的72%,H最大值出现在5月份;生长季,主要能量支出项为潜热通量(LE),约占Rn的60%,LE最大值出现在7月份。全年因蒸散消耗的能量为1.2×109J/m2,占净辐射的52%,森林蒸散的水量为493mm,占降水量的88%。波文比β近似呈U字型变化,其值受森林物候变化影响显著,在非生长季平均值约为3.0,生长季为0.5左右。土壤热通量(G)在非生长季表现为能量平衡方程的收入项,约占有效能量的5.0%;生长季表现为支出项,约占有效能量的4.0%,其变化过程与土壤温度梯度及叶面积指数密切相关。长白山通量观测站能量平衡收支闭合度为86%,不闭合的原因有待于进一步的研究。     

Age-dependent response of boreal forest to temperature and rainfall variability

We used measurements of CO₂ exchange by six Canadian boreal forest stands to determine how sites of contrasting age respond to interannual temperature and precipitation variation. The stands ranged in age (time since last burn) from 4 to 155 years. The study included years that were anomalously cool and moist, warm and dry, cool and dry, and warm and moist. Warmer than average springs accelerated the onset of older stand (> 20 years) photosynthesis by as much as 37 days and younger stand (< 20 years) photosynthesis by as much as 25 days. The warm–dry year resulted in a marked reduction of mid‐summer CO₂ uptake by the younger, but not older, stands. The mid‐summer decline in young stand photosynthesis reflected the combination of warmth and drought; similar declines were not observed during the cool–dry or warm–moist years. The annual carbon gain by the oldest stands was greatest during the warm–dry year as a result of the expanded growing season. The annual carbon gain by the youngest stands was consistent from year to year, largely as a result of offsets between increased spring photosynthesis and reduced mid‐summer photosynthesis during the warm–dry year. Night‐time CO₂ efflux increased by 2–29% during the warm–moist year relative to the warm–dry year. This increase was not systematically related to age. We conclude that the major source of interannual CO₂ exchange variation at the landscape scale is the ability of older, evergreen canopies to respond to warm springs by advancing the onset of photosynthesis. Drought‐related reductions in photosynthesis, moisture‐driven respiratory losses, and the effects of spring warmth on young‐stand photosynthesis are of secondary importance. The advantage conferred on older, evergreen stands during warmer years carries implications for the possible effects of climate change on boreal forest succession. Warmer temperatures may increase the competitive advantage of late successional species relative to early successional species, and also the incidence of fire, thereby accelerating plant succession and shortening the fire‐return interval.     

中国东北地区阔叶红松林与兴安落叶松林的碳通量特征及其影响因子比较

北半球中高纬度的森林生态系统在全球碳循环过程中扮演着非常重要的角色。基于中国东北地区阔叶红松林与兴安落叶松林2007年和2008年2a生长季的涡度相关通量资料及气象观测资料,比较分析了两类生态系统的碳通量特征及其环境控制因子。结果表明:研究期间,阔叶红松林与兴安落叶松林都表现为碳吸收,强度分别为199gCm-2(阔叶红松林2a生长季平均值)与49gCm-2(兴安落叶松林2008年生长季);阔叶红松林碳吸收强度在生长季的大部分时段都大于兴安落叶松林。半小时尺度上,两类生态系统的呼吸作用均与10cm土壤温度呈显著的指数相关,兴安落叶松林生态系统呼吸的温度敏感性(Q10=3.44)显著大于阔叶红松林(Q10=1.90);日尺度上,阔叶红松林与兴安落叶松林碳释放/吸收的转变临界温度为10℃左右。研究期间,兴安落叶松林生态系统的水分利用效率高于阔叶红松林生态系统。     

The use of eddy covariance to infer the net carbon dioxide uptake of Brazilian rain forest

• 1 Eddy covariance measurements of CO₂ flux, based on four and six week campaigns in Rondôdnia, Brazil, have been used in conjunction with a model to scale up data to a whole year, and thus estimate the carbon balance of the tropical forest ecosystem, and the changes in carbon balance expected from small interannual variations in climatological conditions.
• 2 One possible source of error in this estimation arises from the difficulty in measuring fluxes under stably stratified meteorological conditions, such as occur frequently at night. Flux may be ‘lost’ because of low velocity advection, caused by nocturnal radiative cooling at sites on raised ground. Such effects may be detected by plotting the net ecosystem flux of CO₂, F_eco is a function of wind speed. If flux is ‘lost’ then F_eco is expected to decline with wind speed. In the present data set, this did not occur, and F_eco was similar to the nocturnal flux estimated independently from chamber measurements.
• 3 The model suggests that in 1992/3, the Gross Primary Productivity (GPP) was 203.3 mol C m^?2 y^?1 and ecosystem respiration was 194.8 mol C m^?2 y^?1, giving an ecosystem carbon balance of 8.5 mol C m^?2 y^?1, equivalent to a sink of 1.0 ton C ha^?1 y^?1. However, the sign and magnitude of this figure is very sensitive to temperature, because of the strong influence of temperature on respiration.
• 4 The model also suggests that the effect of temperature on the net carbon balance is strongly dependent on the partial pressure of CO₂.

     

Linking forest fires to lake metabolism and carbon dioxide emissions in the boreal region of Northern Québec

Natural fires annually decimate up to 1% of the forested area in the boreal region of Québec, and represent a major structuring force in the region, creating a mosaic of watersheds characterized by large variations in vegetation structure and composition. Here, we investigate the possible connections between this fire‐induced watershed heterogeneity and lake metabolism and CO₂ dynamics. Plankton respiration, and water–air CO₂ fluxes were measured in the epilimnia of 50 lakes, selected to lie within distinct watershed types in terms of postfire terrestrial succession in the boreal region of Northern Québec. Plankton respiration varied widely among lakes (from 21 to 211 μg C L^?1 day^?1), was negatively related to lake area, and positively related to dissolved organic carbon (DOC). All lakes were supersaturated in CO₂ and the resulting carbon (C) flux to the atmosphere (150 to over 3000 mg C m² day^?1) was negatively related to lake area and positively to DOC concentration. CO₂ fluxes were positively related to integrated water column respiration, suggesting a biological component in this flux. Both respiration and CO₂ fluxes were strongly negatively related to years after the last fire in the basin, such that lakes in recently burnt basins had significantly higher C emissions, even after the influence of lake size was removed. No significant differences were found in nutrients, chlorophyll, and DOC between lakes in different basin types, suggesting that the fire‐induced watershed features influence other, more subtle aspects, such as the quality of the organic C reaching lakes. The fire‐induced enhancement of lake organic C mineralization and C emissions represents a long‐term impact that increases the overall C loss from the landscape as the result of fire, but which has never been included in current regional C budgets and future projections. The need to account for this additional fire‐induced C loss becomes critical in the face of predictions of increasing incidence of fire in the circumboreal landscape.     

长白山阔叶红松林能量平衡和蒸散

利用开路涡动相关系统的连续观测结果,分析了长白山阔叶红松林2008年能量平衡各分量和蒸散量的特征,并对生长季和非生长季能量各分量和蒸散量的差异进行了比较.结果表明:该观测系统能量闭合度为72%,处于国际同类观测的中等水平;能量各分量日、季差异显著,生长季森林生态系统最主要的能量支出项为潜热通量,约占可用能量的66%,非生长季最主要的能量支出项为感热通量,约占可用能量的63%.长白山阔叶红松林2008年蒸散量为484.7 mm,占同期降水量(558.9 mm)的87%,证实森林蒸散耗水是我国北方温带森林最主要的水分支出项.     

The propagation of errors in long-term measurements of land-atmosphere fluxes of carbon and water

For surface fluxes of carbon dioxide, the net daily flux is the sum of daytime and nighttime fluxes of approximately the same magnitude and opposite direction. The net flux is therefore significantly smaller than the individual flux measurements and error assessment is critical in determining whether a surface is a net source or sink of carbon dioxide. For carbon dioxide flux measurements, it is an occasional misconception that the net flux is measured as the difference between the net upward and downward fluxes (i.e. a small difference between large terms). This is not the case. The net flux is the sum of individual (half-hourly or hourly) flux measurements, each with an associated error term. The question of errors and uncertainties in long-term flux measurements of carbon and water is addressed by first considering the potential for errors in flux measuring systems in general and thus errors which are relevant to a wide range of timescales of measurement. We also focus exclusively on flux measurements made by the micrometeorological method of eddy covariance. Errors can loosely be divided into random errors and systematic errors, although in reality any particular error may be a combination of both types. Systematic errors can be fully systematic errors (errors that apply on all of the daily cycle) or selectively systematic errors (errors that apply to only part of the daily cycle), which have very different effects. Random errors may also be full or selective, but these do not differ substantially in their properties. We describe an error analysis in which these three different types of error are applied to a long-term dataset to discover how errors may propagate through long-term data and which can be used to estimate the range of uncertainty in the reported sink strength of the particular ecosystem studied.     

Direct and indirect climate change effects on carbon dioxide fluxes in a thawing boreal forest–wetland landscape

In the sporadic permafrost zone of northwestern Canada, boreal forest carbon dioxide (CO₂) fluxes will be altered directly by climate change through changing meteorological forcing and indirectly through changes in landscape functioning associated with thaw‐induced collapse‐scar bog (‘wetland’) expansion. However, their combined effect on landscape‐scale net ecosystem CO₂ exchange (NEE_LAND), resulting from changing gross primary productivity (GPP) and ecosystem respiration (ER), remains unknown. Here, we quantify indirect land cover change impacts on NEE_LAND and direct climate change impacts on modeled temperature‐ and light‐limited NEE_LAND of a boreal forest–wetland landscape. Using nested eddy covariance flux towers, we find both GPP and ER to be larger at the landscape compared to the wetland level. However, annual NEE_LAND (?20 g C m^?2) and wetland NEE (?24 g C m^?2) were similar, suggesting negligible wetland expansion effects on NEE_LAND. In contrast, we find non‐negligible direct climate change impacts when modeling NEE_LAND using projected air temperature and incoming shortwave radiation. At the end of the 21st century, modeled GPP mainly increases in spring and fall due to reduced temperature limitation, but becomes more frequently light‐limited in fall. In a warmer climate, ER increases year‐round in the absence of moisture stress resulting in net CO₂ uptake increases in the shoulder seasons and decreases during the summer. Annually, landscape net CO₂ uptake is projected to decline by 25 ± 14 g C m^?2 for a moderate and 103 ± 38 g C m^?2 for a high warming scenario, potentially reversing recently observed positive net CO₂ uptake trends across the boreal biome. Thus, even without moisture stress, net CO₂ uptake of boreal forest–wetland landscapes may decline, and ultimately, these landscapes may turn into net CO₂ sources under continued anthropogenic CO₂ emissions. We conclude that NEE_LAND changes are more likely to be driven by direct climate change rather than by indirect land cover change impacts.     

Comparison of carbon dioxide fluxes over three boreal black spruce forests in Canada

Although mature black spruce forests are a dominant cover type in the boreal forest of North America, it is not clear how their carbon (C) budgets vary across the continent. The installation of an eddy covariance flux tower on an Old Black Spruce (OBS) site in eastern Canada (EOBS, Québec) provided a first opportunity to compare and contrast its annual (2004) and seasonal C exchange with two other pre-existing OBS flux sites from different climatic regions located in Saskatchewan [Southern OBS (SOBS)] and Manitoba [Northern OBS (NOBS)]. Although there was a relatively uniform seasonal pattern of net ecosystem productivity (NEP) among sites, EOBS had a lower total annual NEP than the other two sites. This was primarily because warmer soil under a thicker snowpack at EOBS appeared to increase winter C losses and low light suppressed both NEP and gross ecosystem productivity (GEP) in June. Across sites, greater total annual GEP and ecosystem respiration ( R ) were associated with greater mean annual air temperatures and an earlier beginning of the growing season. Also, GEP at all three sites showed a stronger relationship with air temperature in spring and early summer compared with later in the growing season, highlighting the importance of springtime conditions to the C budget of these boreal ecosystems. The three sites had different parameter estimates describing the responses of R and GEP at the half hour time scale to near surface temperature and light, respectively. On the other hand, the responses of both R and GEP to temperature at the monthly scale did not differ among sites. These results suggest that a general parameterization could be sufficient at coarse time resolutions to model the response of C exchange to environmental factors of mature black spruce forests from different climatic regions.     

改进Biome-BGC模型模拟哈佛森林地区水、碳通量

Biome-BGC模型通过耦合植被、土壤与大气间的水分与CO2交换过程,实现植被生产力的模拟,但土壤水平衡模块的不够完善,导致在长时间无降水情况下植被生产力模拟存在较大误差.针对这一问题,本文对Biome-BGC模型中土壤水分胁迫气孔导度方程、蒸散计算公式及土壤水分流失过程等3方面进行了改进和调整,利用改进的Biome-BGC模型模拟美国哈佛森林地区蒸散、植被生产力,并与地面通量观测值进行了比较.结果表明,改进后模拟精度有明显的提高,蒸散、植被生态系统生产力(NEE)与观测值间的决定系数分别由0.483和0.658提高到0.617和O.813,蒸散逐年均方根误差平均下降了48.7％,NEE逐年误差平方和平均下降了39.8％.改进后的模型模拟结果更接近观测值.     

Strategies for measuring and modelling carbon dioxide and water vapour fluxes over terrestrial ecosystems

Continuous and direct measurements of ecosystem carbon dioxide and water vapour fluxes can improve our ability to close regional and global carbon and hydrological budgets. On this behalf, an international and multidisciplinary group of scientists (micrometeorologists, ecophysiologists and biogeochemists) assembled at La Thuile, Italy to convene a workshop on ‘Strategies for Monitoring and Modelling CO₂ and Water Vapour Fluxes over Terrestrial Ecosystems’. Over the course of the week talks and discussions focused on: (i) the results from recent field studies on the annual cycle of carbon dioxide and water vapour fluxes over terrestrial ecosystems; (ii) the problems and pitfalls associated with making long-term flux measurements; (iii) alternative methods for assessing ecosystem carbon dioxide and water vapour fluxes; (iv) how direct and continuous carbon dioxide and water vapour flux measurements could be used by the ecological and biogeochemical modelling communities; and (v) if, how and where to proceed with establishing a network of long-term flux measurement sites. This report discusses the purpose of the meeting and summarizes the conclusions drawn from the discussions by the attending scientists. There was a consensus that recent advances in instrumentation and software make possible long-term measurements of carbon dioxide and water vapour fluxes over terrestrial ecosystems. At this writing, eight research teams have conducted long-term carbon dioxide and water vapour flux experiments and more long-term studies are anticipated. The participants advocated an experimental design that would make long-term flux measurement valuable to a wider community of modelers, biogeochemists and ecologists. A network of carbon dioxide and water vapour flux measurement stations should include ancillary measurements of meteorological, ecological and biological variables. To assess spatial representativeness of the long term and tower-based flux measurements, periodic aircraft-based flux experiments and satellite-based assessments of land cover were recommended. Occasional cuvette-based measurements of leaf-level carbon dioxide and water vapour fluxes were endorsed to provide information on the biological control of surface fluxes. They can also provide data to parameterize ecophysiological models. Flask sampling of stable carbon isotopes was advocated to extend the flux measurements to the global scale.