基于森林清查资料的江西和浙江森林植被固碳潜力

以我国江西、浙江两省的森林植被为研究对象,基于1999-2003年间第六次全国森林清查数据及收集的1030个亚热带森林样地文献资料,依据林分生长的经验方程,估算了两个地区森林2004-2013年的固碳潜力,并基于455个样点的调查数据研究了不同森林管理措施(纯林间种、间伐、施肥)对森林未来固碳潜力的影响.结果表明:第六次森林清查以来的10年(2004-2013)间,江西森林植被年均自然固碳潜力约11.37 Tg C·a-1(1Tg=1012g),而浙江省森林植被年均自然固碳潜力约4.34 Tg C·a-1.纯林间种对江西、浙江两省森林植被固碳潜力影响最大,其次为间伐抚育,施肥的影响最小,纯林间种、间伐和施肥3种森林管理措施使江西省森林植被固碳潜力分别提高(6.54±3.9)、(3.81±2.02)和(2.35±0.6) Tg C·a-1,浙江省森林植被固碳潜力分别提高(2.64±1.28)、(1.42±0.69)和(1.15±0.29) Tg C·a-1.     

Partitioning direct and indirect human-induced effects on carbon sequestration of managed coniferous forests using model simulations and forest inventories

Temperate forest ecosystems have recently been identified as an important net sink in the global carbon budget. The factors responsible for the strength of the sinks and their permanence, however, are less evident. In this paper, we quantify the present carbon sequestration in Thuringian managed coniferous forests. We quantify the effects of indirect human‐induced environmental changes (increasing temperature, increasing atmospheric CO₂ concentration and nitrogen fertilization), during the last century using BIOME‐BGC, as well as the legacy effect of the current age‐class distribution (forest inventories and BIOME‐BGC). We focused on coniferous forests because these forests represent a large area of central European forests and detailed forest inventories were available. The model indicates that environmental changes induced an increase in biomass C accumulation for all age classes during the last 20 years (1982–2001). Young and old stands had the highest changes in the biomass C accumulation during this period. During the last century mature stands (older than 80 years) turned from being almost carbon neutral to carbon sinks. In high elevations nitrogen deposition explained most of the increase of net ecosystem production (NEP) of forests. CO₂ fertilization was the main factor increasing NEP of forests in the middle and low elevations. According to the model, at present, total biomass C accumulation in coniferous forests of Thuringia was estimated at 1.51 t C ha^?1 yr^?1 with an averaged annual NEP of 1.42 t C ha^?1 yr^?1 and total net biome production of 1.03 t C ha^?1 yr^?1 (accounting for harvest). The annual averaged biomass carbon balance (BCB: biomass accumulation rate‐harvest) was 1.12 t C ha^?1 yr^?1 (not including soil respiration), and was close to BCB from forest inventories (1.15 t C ha^?1 yr^?1). Indirect human impact resulted in 33% increase in modeled biomass carbon accumulation in coniferous forests in Thuringia during the last century. From the forest inventory data we estimated the legacy effect of the age‐class distribution to account for 17% of the inventory‐based sink. Isolating the environmental change effects showed that these effects can be large in a long‐term, managed conifer forest.     

在我国开展林业碳汇项目的利弊分析

因为CO2等温室气体的增加导致的全球气候变暖严重影响了世界各国社会和经济的发展,而森林具有吸收大气中的CO2,减缓气候变暖的作用,因此2001年的《波恩政治协议》和《马拉喀什协定》同意将造林、再造林等林业碳汇项目作为第一承诺期合格的清洁发展机制(CDM)项目。目前,国际上此类活动已相继展开,而我国还未展开此类活动。为了给我国是否可以开展林业碳汇项目提供参考,文章对我国开展林业碳汇项目的利与弊进行了比较详细的分析和比较,认为可以在我国适当开展林业碳汇项目,以促进我国经济和社会的可持续发展,并进一步加强我国在国际上的地位。     

Carbon stock changes and carbon sequestration potential of Flemish cropland soils

Evaluations of soil organic carbon (SOC) stocks are often based on assigning a carbon density to each one of a number of ecosystems or soil classes considered, using data from soil profiles within these categories. A better approach, in which the use of classification methods by which extrapolation of SOC data to larger areas is avoided, can only be used if enough data are available at a sufficiently small scale. Over 190 000 SOC measurements (0–24 cm) have been made in the Flemish cropland (the Northern part of Belgium) in the 1989–2000 period. These SOC data were grouped into 3‐year periods and as means plus standard deviation per (part of) community (polygons). This large dataset was used to calculate SOC stocks and their evolution with time, without data extrapolation. Using a detailed soil map, larger spatial groups of polygons were created based on soil texture and spatial location. Linear regression analysis showed that in the entire study area, SOC stocks had decreased or at best had remained stable. In total, a yearly decrease of 354 kton OC yr^?1 was calculated, which corresponds with a net CO₂ emission of 1238 kton CO₂ yr^?1. Specific regions with a high carbon sequestration potential were identified, based on SOC losses during the 1989–2000 period and the mean 1999 SOC content, compared to the average SOC content of soils in Flanders with a similar soil texture. When restoring the SOC stocks to their 1990 level, we estimated the carbon sequestration potential of the Flemish cropland soils to be some 300 kton CO₂ yr^?1 at best, which corresponds to a 40‐year restoration period. In conclusion, we can say that in regions where agricultural production is very intense, carbon sequestration in the cropland may make only a very modest contribution to a country's effort to reduce greenhouse gas emissions.     

The impact of nitrogen deposition on carbon sequestration in European forests and forest soils

An estimate of net carbon (C) pool changes and long‐term C sequestration in trees and soils was made at more than 100 intensively monitored forest plots (level II plots) and scaled up to Europe based on data for more than 6000 forested plots in a systematic 16 km × 16 km grid (level I plots). C pool changes in trees at the level II plots were based on repeated forest growth surveys At the level I plots, an estimate of the mean annual C pool changes was derived from stand age and available site quality characteristics. C sequestration, being equal to the long‐term C pool changes accounting for CO₂ emissions because of harvest and forest fires, was assumed 33% of the overall C pool changes by growth. C sequestration in the soil were based on calculated nitrogen (N) retention (N deposition minus net N uptake minus N leaching) rates in soils, multiplied by the C/N ratio of the forest soils, using measured data only (level II plots) or a combination of measurements and model calculations (level I plots). Net C sequestration by forests in Europe (both trees and soil) was estimated at 0.117 Gton yr^?1, with the C sequestration in stem wood being approximately four times as high (0.094 Gton yr^?1) as the C sequestration in the soil (0.023 Gton yr^?1). The European average impact of an additional N input on the net C sequestration was estimated at approximately 25 kg C kg^?1 N for both tree wood and soil. The contribution of an average additional N deposition on European forests of 2.8 kg ha^?1 yr^?1 in the period 1960–2000 was estimated at 0.0118 Gton yr^?1, being equal to 10% of the net C sequestration in both trees and soil in that period (0.117 Gton yr^?1). The C sequestration in trees increased from Northern to Central Europe, whereas the C sequestration in soil was high in Central Europe and low in Northern and Southern Europe. The result of this study implies that the impact of forest management on tree growth is most important in explaining the C pool changes in European forests.     

Effect of hydrological conditions on nitrous oxide, methane, and carbon dioxide dynamics in a bottomland hardwood forest and its implication for soil carbon sequestration

This study was conducted at three locations in a bottomland hardwood forest with a distinct elevation and hydrological gradient: ridge (high, dry), transition, and swamp (low, wet). At each location, concentrations of soil greenhouse gases (N₂O, CH₄, and CO₂), their fluxes to the atmosphere, and soil redox potential (Eh) were measured bimonthly, while the water table was monitored every day. Results show that soil Eh was significantly (P < 0.001) correlated with water table: a negative correlation at the ridge and transition locations, but a positive correlation at the permanently flooded swamp location. Both soil gas profile analysis and surface gas flux measurements indicated that the ridge and transition locations could be a sink of atmospheric CH₄, especially in warm seasons, but generally functioned as a minor source of CH₄ in cool seasons. The swamp location was a major source of CH₄, and the emission rate was higher in the warm seasons (mean 28 and median 23 mg m^?2 h^?1) than in the cool seasons (both mean and median 13 mg m^?2 h^?1). Average CO₂ emission rate was 251, 380 and 52 mg m^?2 h^?1 for the ridge, transition and swamp location, respectively. At each location, higher CO₂ emission rates were also found in the warm seasons. The lowest CO₂ emission rate was found at the swamp location, where soil C content was the highest, due to less microbial biomass, less CO₂ production in such an anaerobic environment, and greater difficulty of CO₂ diffusion to the atmosphere. Cumulative global warming potential emission from these three greenhouse gases was in an order of swamp > transition > ridge location. The ratio CO₂/CH₄ production in soil is a critical factor for evaluating the overall benefit of soil C sequestration, which can be greatly offset by CH₄ production and emission.     

Effects of biological invasions on forest carbon sequestration

There has been a rapidly developing literature on the effects of some of the major drivers of global change on carbon (C) sequestration, particularly carbon dioxide (CO₂) enrichment, land use change, nitrogen (N) deposition and climate change. However, remarkably little attention has been given to one major global change driver, namely biological invasions. This is despite growing evidence that invasive species can dramatically alter a range of aboveground and belowground ecosystem processes, including those that affect C sequestration. In this review, we assess the evidence for the impacts of biological invaders on forest C stocks and C sequestration by biological invaders. We first present case studies that highlight a range of invader impacts on C sequestration in forest ecosystems, and draw on examples that involve invasive primary producers, decomposers, herbivores, plant pathogens, mutualists and predators. We then develop a conceptual framework for assessing the effects of invasive species on C sequestration impacts more generally, by identifying the features of biological invaders and invaded ecosystems that are thought to most strongly regulate C in forests. Finally we assess the implications of managing invasive species on C sequestration. An important principle that emerges from this review is that the direct effects of invaders on forest C are often smaller and shorter‐term than their indirect effects caused by altered nutrient availability, primary productivity or species composition, all of which regulate long‐term C pools and fluxes. This review provides a conceptual basis for improving our general understanding of biological invaders on ecosystem C, but also points to a paucity of primary data that are needed to determine the quantitative effects of invaders on ecosystem processes that drive C sequestration.     

Carbon sequestration and ecosystem respiration for 4 years in a Scots pine forest

The net exchange of CO₂ (NEE) between a Scots pine (Pinus sylvestris L.) forest ecosystem in eastern Finland and the atmosphere was measured continuously by the eddy covariance (EC) technique over 4 years (1999–2002). The annual temperature coefficient (Q₁₀) of ecosystem respiration (R) for these years, respectively, was 2.32, 2.66, 2.73 and 2.69. The light‐saturated rate of photosynthesis (A_max) was highest in July or August, with an annual average A_max of 10.9, 14.6, 15.3 and 17.1 μmol m^?2 s^?1 in the 4 years, respectively. There was obvious seasonality in NEE, R and gross primary production (GPP), exhibiting a similar pattern to photosynthetically active radiation (PAR) and air temperature. The integrated daily NEE ranged from 2.59 to ?4.97 g C m^?2 day^?1 in 1999, from 2.70 to ?4.72 in 2000, from 2.61 to ?4.71 in 2001 and from 5.27 to ?4.88 in 2002. The maximum net C uptake occurred in July, with the exception of 2000, when it was in June. The interannual variation in ecosystem C flux was pronounced. The length of the growing season, based on net C uptake, was 179, 170, 175 and 176 days in 1999–2002, respectively, and annual net C sequestration was 152, 101, 172 and 205 g C m^?2 yr^?1. It is estimated that ecosystem respiration contributed 615, 591, 752 and 879 g C m^?2 yr^?1 to the NEE in these years, leading to an annual GPP of ?768, ?692, ?924 and ?1084 g C m^?2 yr^?1. It is concluded that temperature and PAR were the main determinants of the ecosystem CO₂ flux. Interannual variations in net C sequestration are predominantly controlled by average air temperature and integrated radiation in spring and summer. Four years of EC data indicate that boreal Scots pine forest ecosystem in eastern Finland acts as a relatively powerful carbon sink. Carbon sequestration may benefit from warmer climatic conditions.     

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.     

Direct measurement of soil organic carbon content change in the croplands of China

Agricultural soils in China have been estimated to have a large potential for carbon sequestration, and modelling and literature survey studies have yielded contrasting results of soil organic carbon (SOC) stock change, ranging from ?2.0 to +0.6% yr^?1. To assess the validity of earlier estimates, we collected 1394 cropland soil profiles from all over the country and measured SOC contents in 2007–2008, and compared them with those of a previous national soil survey conducted in 1979–1982. The results showed that average SOC content in the 0–20 cm soil increased from 11.95 g kg^?1 in 1979–1982 to 12.67 g kg^?1 in 2007–2008, averaging 0.22% yr^?1. The standard deviation of SOC contents decreased. Four major soil types had statistically significant changes in their mean SOC contents for 0–20 cm. These were: +7.5% for Anthrosols (paddy soils), +18.3% for Eutric Cambisols, +30.5% for Fluvisols, and ?22.3% for Chernozems. The change of SOC contents showed a negative relationship with the average SOC contents of the two sampling campaigns only when soils in the region south of Yangtse River were excluded. SOC contents of the two major soil types in the region south of Yangtse River, i.e., Haplic Alisols/Haplic Acrisols and Anthrosols (paddy soils), changed little or significantly increased, though with a high SOC content. We suggest that the increase of SOC content is mainly attributed to the large increase in crop yields since the 1980s, and the short history as cropland establishment is mainly responsible for the decrease in SOC content for some soil types and regions showing a SOC decline.     

Uncertainties in the relationship between atmospheric nitrogen deposition and forest carbon sequestration

In a recent study, Magnani et al. report how atmospheric nitrogen deposition drives stand-lifetime net ecosystem productivity (NEP_av) for midlatitude forests, with an extremely high C to N response (725 kg C kg⁻¹ wet-deposited N for their European sites). We present here a re-analysis of these data, which suggests a much smaller C : N response for total N inputs. Accounting for dry, as well as wet N deposition reduces the C : N response to 177 : 1. However, if covariance with intersite climatological differences is accounted for, the actual C : N response in this dataset may be <70 : 1. We then use a model analysis of 22 European forest stands to simulate the findings of Magnani et al. Multisite regression of simulated NEP_av vs. total N deposition reproduces a high C : N response (149 : 1). However, once the effects of intersite climatological differences are accounted for, the value is again found to be much smaller, pointing to a real C : N response of about 50–75 : 1.     

呼伦贝尔沙地樟子松人工林乔木层固碳速率及其对气象因子的响应

以呼伦贝尔沙地樟子松人工林为对象,利用树木年轮学方法和解析木法,推算过去41年的樟子松林生物量、碳密度和固碳速率,并分析固碳速率与月均气温、月均最低温、月均最高温、月降水量、月均大气相对湿度等气象因子的关系。结果表明:樟子松林碳密度随着林龄的增加而增加,碳密度从1977年的2.58 t·hm^-2增加到2017年的87.97 t·hm^-2;樟子松林固碳速率的年际差异较大,总体上呈现出先增加后减小,最终趋于稳定的趋势,多年固碳速率为1.37～3.21 t·hm^-2·a^-1,平均为2.13 t·hm^-2·a^-1;樟子松林固碳速率与上一年和当年生长季的降水呈正相关,与非生长季降水呈负相关,特别是与上一年12月和当年3月降水量呈显著相关;与上一年和当年8—9月的月均湿度显著正相关;与上一年及当年生长季的月平均气温、月均最低温和月均最高温呈显著或极显著负相关;樟子松林固碳能力同时受温度和降水的影响,但温度的影响大于降水的影响,在未来该地区气候暖干化的变化趋势下樟子松林...     

Variation of soil and biomass carbon pools in beech forests across a precipitation gradient

Temperate forests have recently been identified as being continuing sinks for carbon even in their mature and senescent stages. However, modeling exercises indicate that a warmer and drier climate as predicted for parts of Central Europe may substantially alter the source/sink function of these economically important ecosystems. In a transect study with 14 mature European beech (Fagus sylvatica L.) forests growing on uniform geological substrate, we analyzed the influence of a large reduction of annual precipitation (970–520 mm yr^?1) on the carbon stocks in fast and slow pools, independent of the well‐known aging effect. We investigated the C storage in the organic L, F, H layers, the mineral soil to 100 cm, and in the biomass (stem, leaves, fine roots), and analyzed the dependence of these pools on precipitation. Soil organic carbon decreased by about 25% from stands with > 900 mm yr^?1 to those with < 600 mm yr^?1; while the carbon storage in beech stems slightly increased. Reduced precipitation affected the biomass C pool in particular in the fine root fraction but much less in the leaf biomass and stem fractions. Fine root turnover increased with a precipitation reduction, even though stand fine root biomass and SOC in the organic L, F, and H layers decreased. According to regression analyses, the C storage in the organic layers was mainly controlled by the size of the fine root C pool suggesting an important role of fine root turnover for the C transfer from tree biomass to the SOC pool. We conclude that the long‐term consequence of a substantial precipitation decrease would be a reduction of the mineral soil and organic layer SOC pools, mainly due to higher decomposition rates. This could turn temperate beech forests into significant carbon sources instead of sinks under global warming.     

Estimation of the carbon sequestration by a heterogeneous forest: night flux corrections, heterogeneity of the site and inter-annual variability

Continuous measurements of the net CO₂ flux exchanged in a mixed forest with the atmosphere were performed over 5 years at the Vielsalm experimental site. The carbon sequestration at the site was deduced by a summation of the measurements. Problems associated with this summation procedure were discussed. The carbon sequestration in the ecosystem was presented and its interannual variability was discussed. An estimation of the night flux correction was given. The correction was applied by replacing measurements made during quiet nights by a parameterization. The impact of the correction was shown to vary between 10 and 20% of the uncorrected flux, according to the year. The need to include the storage flux during turbulent periods was emphasized: its neglect leads to an error which will be greater than the one it tries to correct. It was also shown that the heterogeneity of the site made it necessary to split the data into separate series corresponding to the different vegetation patches and to fill the data gaps by using an algorithm that takes account of the weather conditions. Two series were defined, one corresponding to a beech subplot, the other to a conifer subplot. The uncertainty owing to the data split and the data gap‐filling was about 15–20% annually. The carbon sequestration was then analysed in both the subplots. The length of the growing season was about 210 days in the beech and 240 days in the conifer. The carbon sequestration over 5 years was 2.28 kg C m^2?2 in the beech and 3.58 kg C m^2?2 in the conifer. The main difference between the species appeared in spring, between March and May, when the beeches were leafless. Significant interannual variations were observed in both the subplots. They appeared mainly in summer and were primarily because of the variations in the radiation and air humidity regimes. In addition, an impact of the interannual variation of the vegetation area index (VAI) and of the leaf initiation date was observed in the beech. Finally, a decline of the carbon sequestration efficiency of the ecosystem during the season was observed in both the subplots. It was because of neither the variation in any climatic variables nor VAI variation.     

The impact of co‐occurring tree and grassland species on carbon sequestration and potential biofuel production

We evaluated how three co‐occurring tree and four grassland species influence potentially harvestable biofuel stocks and above‐ and belowground carbon pools. After 5 years, the tree Pinus strobus had 6.5 times the amount of aboveground harvestable biomass as another tree Quercus ellipsoidalis and 10 times that of the grassland species. P. strobus accrued the largest total plant carbon pool (1375 g C m^?2 or 394 g C m^?2 yr), while Schizachyrium scoparium accrued the largest total plant carbon pool among the grassland species (421 g C m^?2 or 137 g C m^?2 yr). Quercus ellipsoidalis accrued 850 g C m^?2, Q. macrocarpa 370 g C m^?2, Poa pratensis 390 g C m^?2, Solidago canadensis 132 g C m^?2, and Lespedeza capitata 283 g C m^?2. Only P. strobus and Q. ellipsoidalis significantly sequestered carbon during the experiment. Species differed in total ecosystem carbon accumulation from ?21.3 to +169.8 g C m^?2 yr compared with the original soil carbon pool. Plant carbon gains with P. strobus were paralleled by a decrease of 16% in soil carbon and a nonsignificant decline of 9% for Q. ellipsoidalis. However, carbon allocation differed among species, with P. strobus allocating most aboveground in a disturbance prone aboveground pool, whereas Q. ellipsoidalis, allocated most carbon in less disturbance sensitive belowground biomass. These differences have strong implications for terrestrial carbon sequestration and potential biofuel production. For P. strobus, aboveground plant carbon harvest for biofuel would result in no net carbon sequestration as declines in soil carbon offset plant carbon gains. Conversely the harvest of Q. ellipsoidalis aboveground biomass would result in net sequestration of carbon belowground due to its high allocation belowground, but would yield lower amounts of aboveground biomass. Our results demonstrate that plant species can differentially impact ecosystem carbon pools and the distribution of carbon above and belowground.     

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].     

Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment

The impact of anthropogenic CO₂ emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO₂ concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO₂ concentrations, the Duke Forest Free‐Air CO₂ Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO₂ concentrations 200 μL L^?1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO₂ conditions, with significantly greater accumulations under the elevated CO₂ treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ～30 g C m^?2 yr^?1 was sequestered in the forest floor of the elevated CO₂ treatment plots relative to the control plots maintained at ambient CO₂ owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO₂ effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO₂ suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO₂ (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO₂.     

Biomass and carbon accumulation in a fire chronosequence of a seasonally dry tropical forest

Seasonally dry tropical forests (SDTF) are a widely distributed vegetation type in the tropics, characterized by seasonal rainfall with several months of drought when they are subject to fire. This study is one of the first attempts to quantify above- and belowground biomass (AGB and BGB) and above- and belowground carbon (AGC and BGC) pools to calculate their recovery after fire, using a chronosequence approach (six forests that ranged form 1 to 29 years after fire and mature forest). We quantified AGB and AGC pools of trees, lianas, palms, and seedlings, and BGB and BGC pools (Oi, Oe, Oa soil horizons, and fine roots). Total AGC ranged from 0.05 to nearly 72 Mg C ha⁻¹, BGC from 21.6 to nearly 85 Mg C ha⁻¹, and total ecosystem carbon from 21.7 to 153.5 Mg C ha⁻¹; all these pools increased with forest age. Nearly 50% of the total ecosystem carbon was stored in the Oa horizon of mature forests, and up to 90% was stored in the Oa-horizon of early successional SDTF stands. The soils were shallow with a depth of <20 cm at the study site. To recover values similar to mature forests, BGC and BGB required <19 years with accumulation rates greater than 20 Mg C ha⁻¹ yr⁻¹, while AGB required 80 years with accumulation rates nearly 2.5 Mg C ha⁻¹ yr⁻¹. Total ecosystem biomass and carbon required 70 and 50 years, respectively, to recover values similar to mature forests. When belowground pools are not included in the calculation of total ecosystem biomass or carbon recovery, we estimated an overestimation of 10 and 30 years, respectively.     

The effect of nitrogen deposition on forest carbon sequestration: a model‐based analysis

The perturbation of the global nitrogen (N) cycle due to the increase in N deposition over the last 150 years will likely have important effects on carbon (C) cycling, particularly via impacts on forest C sequestration. To investigate this effect, and the relative importance of different mechanisms involved, we used the Generic Decomposition And Yield (G'DAY) forest C–N cycling model, introducing some new assumptions which focus on N deposition. Specifically, we (i) considered the effect of forest management, (ii) assumed that belowground C allocation was a function of net primary production, (iii) assumed that foliar litterfall and specific leaf area were functions of leaf N concentration, (iv) assumed that forest canopies can directly take up N, and (v) modified the model such that leaching occurred only for nitrate N. We applied the model with and without each of these modifications to estimate forest C sequestration for different N deposition levels. Our analysis showed that N deposition can have a large effect on forest C storage at ecosystem level. Assumptions (i), (ii) and (iv) were the most important, each giving rise to a markedly higher level of forest C sequestration than in their absence. On the contrary assumptions (iii) and (v) had a negligible effect on simulated net ecosystem production (NEP). With all five model modifications in place, we estimated that the C storage capacity of a generic European forest ecosystem was at most 121 kg C kg^?1 N deposited. This estimate is four times higher than that obtained with the original version of G'DAY (27.8 kg C kg^?1 N). Thus, depending on model assumptions, the G'DAY ecosystem model can reproduce the range of dC : dN_dep values found in the literature. We conclude that effects of historic N deposition must be taken into account when estimating the C storage capacity of a forest ecosystem.