The European carbon balance. Part 3: forests

We present a new synthesis, based on a suite of complementary approaches, of the primary production and carbon sink in forests of the 25 member states of the European Union (EU‐25) during 1990–2005. Upscaled terrestrial observations and model‐based approaches agree within 25% on the mean net primary production (NPP) of forests, i.e. 520±75 g C m^?2 yr^?1 over a forest area of 1.32 × 10⁶ km² to 1.55 × 10⁶ km² (EU‐25). New estimates of the mean long‐term carbon forest sink (net biome production, NBP) of EU‐25 forests amounts 75±20 g C m^?2 yr^?1. The ratio of NBP to NPP is 0.15±0.05. Estimates of the fate of the carbon inputs via NPP in wood harvests, forest fires, losses to lakes and rivers and heterotrophic respiration remain uncertain, which explains the considerable uncertainty of NBP. Inventory‐based assessments and assumptions suggest that 29±15% of the NBP (i.e., 22 g C m^?2 yr^?1) is sequestered in the forest soil, but large uncertainty remains concerning the drivers and future of the soil organic carbon. The remaining 71±15% of the NBP (i.e., 53 g C m^?2 yr^?1) is realized as woody biomass increments. In the EU‐25, the relatively large forest NBP is thought to be the result of a sustained difference between NPP, which increased during the past decades, and carbon losses primarily by harvest and heterotrophic respiration, which increased less over the same period.     

Carbon balance of different aged Scots pine forests in Southern Finland

We estimated annual net ecosystem exchange (NEE) of a chronosequence of four Scots pine stands in southern Finland during years 2000–2002 using eddy covariance (EC). Net ecosystem productivity (NEP) was estimated using growth measurements and modelled mass losses of woody debris. The stands were 4, 12, 40 and 75 years old. The 4‐year‐old clearcut was a source of carbon throughout the year combining a low gross primary productivity (GPP) with a total ecosystem respiration (TER) similar to the forest stands. The annual NEE of the clearcut, measured by EC, was 386 g C m^?2. Tree growth was negligible and the estimated NEP was ?262 g C m^?2 a^?1. The annual GPPs at the other sites were close to each other (928?1072 g C m^?2 a^?1), but TER differed markedly, being greatest at the 12‐year‐old site (905 g C m^?2 a^?1) and smallest in the 75‐year‐old stand (616 g C m^?2 a^?1). Measurements of soil CO₂ efflux showed that different rates of soil respiration largely explained the differences in TER. The NEE and NEP of the 12‐year‐old stand were close to zero. The forested stands were sinks of carbon. They had similar annual patterns of carbon exchange and half‐hourly eddy fluxes were highly correlated, indicating similar responses to the environment. The NEE in the 40‐year‐old stand varied between ?179 and –192 g C m^?2 a^?1, while NEP was between 214 and 242 g C m^?2 a^?1. The annual NEE of the 75‐year‐old stand was 323 g C m^?2 and NEP was 252 g C m^?2. This indicates that there was no reduction in carbon sink strength with stand age.     

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.     

Paired-tower measurements of carbon and energy fluxes following disturbance in the boreal forest

Disturbances by fire and harvesting are thought to regulate the carbon balance of the Canadian boreal forest over scales of several decades. However, there are few direct measurements of carbon fluxes following disturbances to provide data needed to refine mathematical models. The eddy covariance technique was used with paired towers to measure fluxes simultaneously at disturbed and undisturbed sites over periods of about one week during the growing season in 1998 and 1999. Comparisons were conducted at three sites: a 1‐y‐old burned jackpine stand subjected to an intense crown fire at the International Crown Fire Modelling Experiment site near Fort Providence, North‐west Territories; a 1‐y‐old clearcut aspen area at the EMEND project near Peace River, Alberta; and a 10‐y‐old burned, mixed forest near Prince Albert National Park, Saskatchewan. Nearby mature forest stands of the same types were also measured as controls. The harvested site had lower net radiation (R_n), sensible (H) and latent (LE) heat fluxes, and greater ground heat fluxes (G) than the mature forest. Daytime CO₂ fluxes were much reduced, but night‐time CO₂ fluxes were identical to that of the mature aspen forest. It is hypothesized that the aspen roots remained alive following harvesting, and dominated soil respiration. The overall effect was that the harvested site was a carbon source of about 1.6 gC m^?2 day^?1, while the mature site was a sink of about ?3.8 gC m^?2 day^?1. The one‐year‐old burn had lower R_n, H and LE than the mature jackpine forest, and had a continuous CO₂ efflux of about 0.8 gC m^–2 day^?1 compared to the mature forest sink of ? 0.5 g C m^?2 day^?1. The carbon source was likely caused by decomposition of fire‐killed vegetation. The 10‐y‐old burned site had similar H, LE, and G to the mature mixed forest site. Although the diurnal amplitude of the CO₂ fluxes were slightly lower at the 10‐y‐old site, there was no significant difference between the daily integrals (? 1.3 gC m^?2 day^?1 at both sites). It appears that most of the change in carbon flux occurs within the first 10 years following disturbance, but more data are needed on other forest and disturbance types for the first 20 years following the disturbance event.     

Carbon sequestration potential of tropical pasture compared with afforestation in Panama

Tropical forest ecosystems play an important role in regulating the global climate, yet deforestation and land‐use change mean that the tropical carbon sink is increasingly influenced by agroecosystems and pastures. Despite this, it is not yet fully understood how carbon cycling in the tropics responds to land‐use change, particularly for pasture and afforestation. Thus, the objectives of our study were: (1) to elucidate the environmental controls and the impact of management on gross primary production (GPP), total ecosystem respiration (TER) and net ecosystem CO₂ exchange (NEE); (2) to estimate the carbon sequestration potential of tropical pasture compared with afforestation; and (3) to compare eddy covariance‐derived carbon budgets with biomass and soil inventory data. We performed comparative measurements of NEE in a tropical C₄ pasture and an adjacent afforestation with native tree species in Sardinilla (Panama) from 2007 to 2009. Pronounced seasonal variation in GPP, TER and NEE were closely related to radiation, soil moisture, and C₃ vs. C₄ plant physiology. The shallow rooting depth of grasses compared with trees resulted in a higher sensitivity of the pasture ecosystem to water limitation and seasonal drought. During 2008, substantial amounts of carbon were sequestered by the afforestation (–442 g C m^–2, negative values denote ecosystem carbon uptake), which was in agreement with biometric observations (–450 g C m^–2). In contrast, the pasture ecosystem was a strong carbon source in 2008 and 2009 (261 g C m^–2), associated with seasonal drought and overgrazing. In addition, soil carbon isotope data indicated rapid carbon turnover after conversion from C₄ pasture to C₃ afforestation. Our results clearly show the potential for considerable carbon sequestration of tropical afforestation and highlight the risk of carbon losses from pasture ecosystems in a seasonal tropical climate.     

An initial intercomparison of micrometeorological and ecological inventory estimates of carbon exchange in a mid-latitude deciduous forest

The role of mid‐latitude forests in the sequestration of carbon (C) is of interest to an increasing number of scientists and policy‐makers alike. Net CO₂ exchange can be estimated on an annual basis, using eddy‐covariance techniques or from ecological inventories of C fluxes to and from a forest. Here we present an intercomparison of annual estimates of C exchange in a mixed hardwood forest in the Morgan‐Monroe State Forest, Indiana, USA for two years, 1998 and 1999. Based on eddy‐covariance measurements made at 1.8 times canopy height from a tower, C uptake by the forest was 237 and 287 g C m^?2 y^?1 for 1998 and 1999, respectively. For the same time period, biometric and ecophysiological measures and modelled estimates of all significant carbon fluxes within deciduous forests were made, including: change in living biomass, aboveground and belowground detritus production, foliage consumption, and forest floor and soil respiration. Using this ecological inventory method for these same two time periods, C uptake was estimated to be 271 and 377 g C m^?2 y^?1, which are 14.3% and 31.4% larger, respectively, than the tower‐based values. The relative change between this method's annual estimates is consistent with that of the eddy‐covariance based values. Our results indicate that the difference in annual C exchange rates was due to reduced heterotrophic soil respiration in 1999.     

Impact of afforestation-associated management changes on the carbon balance of grassland

Afforestations can be considerable carbon (C) sources due to C losses from the soil after site preparation for tree planting and decreased primary production. In this study, the transition from grassland to afforestation was investigated using two eddy flux towers, which were operated in parallel for 3 years, one on a young afforestation and one on an adjacent grassland. Differences between the fluxes at the two sites were attributable to the management of the sites, without confounding influences of meteorological variability. Site preparation with deep ploughing of the planting rows destroyed 30% of the grassland vegetation at the afforestation site and reduced gross primary productivity by 41% in the first year. At the afforestation site 38 g m^?2 less C was sequestered compared with the nonafforested grassland during the first year. In the following years, the C sink at the afforestation site was higher than at the grassland indicating that soil C loss due to site preparation and land use change on the afforestation occurred only during the first year. Metrological conditions, especially summer drought, caused a high interannual variability of the C balance: both sites were small C sources in 2005 (67 g C m^?2 a^?1 at the grassland and 19 g C g^?1 a^?1 at the afforestation site) and small C sinks in 2004 and 2006 (?72.5 and ?16 g C m^?2 a^?1 at the grassland and ?34 and ?61 g C g^?1 a^?1 at the afforestation). Sheep grazing and mowing affected the short‐term dynamics of the C balance and sheep grazing accelerated the C turnover on the grassland site. The investigated afforestation site did not provide any short‐term way of sequestering additional C even though soil C losses during the first 3 years were relatively small.     

森林经营与管理下的温室气体排放、碳泄漏和净固碳量研究进展

森林在减缓全球气候变化和大气CO₂浓度升高上具有重要作用.森林经营与管理下的新造林和森林保护具有显著的固碳功能,其中,新造林和森林保护的固碳速率分别为0.04～7.52、0.33～5.20 t C·hm^-2·a^-1.同时,营造林过程中物资的生产和运输导致边界内产生温室气体排放;营造林导致的活动转移、市场效应和生态环境变化导致边界外产生碳泄漏.本文综述了国内外森林经营与管理活动边界内温室气体排放源的界定、计量方法、温室气体排放量与排放速率;边界外碳泄漏的类型、计量方法与碳泄漏量;净固碳量以及温室气体排放和碳泄漏对固碳的抵消强度.边界内温室气体排放对固碳的抵消强度为0.01%～19.3%,进一步考虑碳泄漏时可增至95%.若仅考虑森林经营与管理在边界内直接产生的温室气体排放与可测量的活动转移碳泄漏,森林经营与管理具有较好的净固碳效益,且相比于农田固碳措施在温室气体净减排方面具有更好的应用前景.随着我国各项重大生态工程新一期的开展和对工程固碳效益的关注,为增加重大生态工程对温室气体的净减排量,有必要在工程开展前进行合理规划、在工程开展过程中加强控制和监测以减少工程实施导致的边界内温室气体排放和边界外碳泄漏.     

An inventory‐based analysis of Canada's managed forest carbon dynamics, 1990 to 2008

Canada's forests play an important role in the global carbon (C) cycle because of their large and dynamic C stocks. Detailed monitoring of C exchange between forests and the atmosphere and improved understanding of the processes that affect the net ecosystem exchange of C are needed to improve our understanding of the terrestrial C budget. We estimated the C budget of Canada's 2.3 × 10⁶ km² managed forests from 1990 to 2008 using an empirical modelling approach driven by detailed forestry datasets. We estimated that average net primary production (NPP) during this period was 809 ± 5 Tg C yr^?1 (352 g C m^?2 yr^?1) and net ecosystem production (NEP) was 71 ± 9 Tg C yr^?1 (31 g C m^?2 yr^?1). Harvesting transferred 45 ± 4 Tg C yr^?1 out of the ecosystem and 45 ± 4 Tg C yr^?1 within the ecosystem (from living biomass to dead organic matter pools). Fires released 23 ± 16 Tg C yr^?1 directly to the atmosphere, and fires, insects and other natural disturbances transferred 52 ± 41 Tg C yr^?1 from biomass to dead organic matter pools, from where C will gradually be released through decomposition. Net biome production (NBP) was only 2 ± 20 Tg C yr^?1 (1 g C m^?2 yr^?1); the low C sequestration ratio (NBP/NPP=0.3%) is attributed to the high average age of Canada's managed forests and the impact of natural disturbances. Although net losses of ecosystem C occurred during several years due to large fires and widespread bark beetle outbreak, Canada's managed forests were a sink for atmospheric CO₂ in all years, with an uptake of 50 ± 18 Tg C yr^?1 [net ecosystem exchange (NEE) of CO₂=?22 g C m^?2 yr^?1].     

缺苞箭竹(Fargesia denudata)和拐棍竹(F.robusta)的饲用品质差异

缺苞箭竹(Fargesia denudata)和拐棍竹(F.robusta)分别是岷山和邛崃山山系野生大熊猫取食频率最高的竹种。本文以四川王朗自然保护区的缺苞箭竹和卧龙自然保护区的拐棍竹为研究对象,通过比较其营养成分、单宁和总酚含量,明确二者的饲用品质差异,以期为野生大熊猫潜在栖息地、廊道以及人工纯林等建设的竹种选择提供依据。结果表明:(1)缺苞箭竹的粗蛋白和粗纤维含量显著优于拐棍竹(P<0.05);(2)拐棍竹的笋、茎的钙和笋、叶磷含量显著优于缺苞箭竹(P<0.05);(3)氨基酸总量、单宁和总酚含量,拐棍竹的叶均更优(P<0.05),但缺苞箭竹的笋、茎单宁含量更优(P<0.05),且缺苞箭竹的笋总酚含量、茎氨基酸总量含量均更优(P<0.05);(4)粗脂肪含量,缺苞箭竹的茎、拐棍竹的笋和叶的含量稍优,但不显著(P>0.05)。两竹种在营养成分、单宁和总酚含量上的饲用价值各有优劣,配合食用,粗蛋白、氨基酸总量、钙和磷可形成互补,并减少单宁、总酚的摄入。鉴于两竹为大熊猫喜食竹种,可作为野生大熊猫潜在栖息地、廊道和人工纯林建设的竹种,在岷山和邛崃山山系的相关区域混合栽种。     

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.     

Radon fluxes in tropical forest ecosystems of Brazilian Amazonia: night-time CO2 net ecosystem exchange derived from radon and eddy covariance methods

Radon‐222 (Rn‐222) is used as a transport tracer of forest canopy–atmosphere CO₂ exchange in an old‐growth, tropical rain forest site near km 67 of the Tapajós National Forest, Pará, Brazil. Initial results, from month‐long periods at the end of the wet season (June–July) and the end of the dry season (November–December) in 2001, demonstrate the potential of new Rn measurement instruments and methods to quantify mass transport processes between forest canopies and the atmosphere. Gas exchange rates yield mean canopy air residence times ranging from minutes during turbulent daytime hours to greater than 12 h during calm nights. Rn is an effective tracer for net ecosystem exchange of CO₂ (CO₂ NEE) during calm, night‐time hours when eddy covariance‐based NEE measurements are less certain because of low atmospheric turbulence. Rn‐derived night‐time CO₂ NEE (9.00±0.99 μmol m^?2 s^?1 in the wet season, 6.39±0.59 in the dry season) was significantly higher than raw uncorrected, eddy covariance‐derived CO₂ NEE (5.96±0.51 wet season, 5.57±0.53 dry season), but agrees with corrected eddy covariance results (8.65±1.07 wet season, 6.56±0.73 dry season) derived by filtering out lower NEE values obtained during calm periods using independent meteorological criteria. The Rn CO₂ results suggest that uncorrected eddy covariance values underestimate night‐time CO₂ loss at this site. If generalizable to other sites, these observations indicate that previous reports of strong net CO₂ uptake in Amazonian terra firme forest may be overestimated.