Carbon allocation in forest ecosystems

Carbon allocation plays a critical role in forest ecosystem carbon cycling. We reviewed existing literature and compiled annual carbon budgets for forest ecosystems to test a series of hypotheses addressing the patterns, plasticity, and limits of three components of allocation: biomass, the amount of material present; flux, the flow of carbon to a component per unit time; and partitioning, the fraction of gross primary productivity (GPP) used by a component. Can annual carbon flux and partitioning be inferred from biomass? Our survey revealed that biomass was poorly related to carbon flux and to partitioning of photosynthetically derived carbon, and should not be used to infer either. Are component fluxes correlated? Carbon fluxes to foliage, wood, and belowground production and respiration all increased linearly with increasing GPP (a rising tide lifts all boats). Autotrophic respiration was strongly linked to production for foliage, wood and roots, and aboveground net primary productivity and total belowground carbon flux (TBCF) were positively correlated across a broad productivity gradient. How does carbon partitioning respond to variability in resources and environment? Within sites, partitioning to aboveground wood production and TBCF responded to changes in stand age and resource availability, but not to competition (tree density). Increasing resource supply and stand age, with one exception, resulted in increased partitioning to aboveground wood production and decreased partitioning to TBCF. Partitioning to foliage production was much less sensitive to changes in resources and environment. Overall, changes in partitioning within a site in response to resource supply and age were small (<15% of GPP), but much greater than those inferred from global relationships. Across all sites, foliage production plus respiration, and total autotrophic respiration appear to use relatively constant fractions of GPP – partitioning to both was conservative across a broad range of GPP – but values did vary across sites. Partitioning to aboveground wood production and to TBCF were the most variable – conditions that favored high GPP increased partitioning to aboveground wood production and decreased partitioning to TBCF. Do priorities exist for the products of photosynthesis? The available data do not support the concept of priorities for the products of photosynthesis, because increasing GPP increased all fluxes. All facets of carbon allocation are important to understanding carbon cycling in forest ecosystems. Terrestrial ecosystem models require information on partitioning, yet we found few studies that measured all components of the carbon budget to allow estimation of partitioning coefficients. Future studies that measure complete annual carbon budgets contribute the most to understanding carbon allocation.     

Primary production and carbon allocation in relation to nutrient supply in a tropical experimental forest

Nutrient supply commonly limits aboveground plant productivity in forests, but the effects of an altered nutrient supply on gross primary production (GPP) and patterns of carbon (C) allocation remain poorly characterized. Increased nutrient supply may lead to a higher aboveground net primary production (ANPP), but a lower total belowground carbon allocation (TBCA), with little change in either aboveground plant respiration (APR) or GPP. Alternatively, increases in nutrient supply may increase GPP, with the quantity of GPP allocated aboveground increasing more steeply than the quantity of GPP allocated belowground. To examine the effects of an elevated nutrient supply on the C allocation patterns in forests, we determined whole‐ecosystem C budgets in unfertilized plots of Eucalyptus saligna and in adjacent plots receiving regular additions of 65 kg N ha^?1, 31 kg P ha^?1, 46 kg K ha^?1, and macro‐ and micronutrients. We measured the absolute flux of C allocated to the components of GPP (ANPP, TBCA and APR), as well as the fraction of GPP allocated to these components. Fertilization dramatically increased GPP. Averaged over 3 years, GPP in the fertilized plots was 34% higher than that in the unfertilized controls (3.95 vs. 2.95 kg C m^?2 yr^?1). Fertilization‐related increases in GPP were allocated entirely aboveground – ANPP was 85% higher and APR was 57% higher in the fertilized than in the control plots, while TBCA did not differ significantly between treatments. Carbon use efficiency (NPP/GPP) was slightly higher in the fertilized (0.53) compared with the control plots (0.51). Overall, fertilization increased ANPP and APR, and these increases were related to a greater GPP and an increase in the fraction of GPP allocated aboveground.     

小兴安岭谷地云冷杉林的碳密度与生产力

采用样地清查和异速生长方程法,量化了处于衰退状态的小兴安岭谷地云冷杉林的森林碳密度和生产力.结果表明： 2011年森林碳密度总量为268.14 t C·hm^-2,其中植被碳密度、碎屑碳密度和土壤碳密度分别为74.25、16.86和177.03 t C·hm^-2.2006—2011年,乔木层碳密度从80.86 t C·hm^-2减少到71.73 t C·hm^-2,主要树种冷杉、白桦、云杉和兴安落叶松的碳密度年均减少比例分别为0.5%、1.2%、2.7%和3.7%,毛赤杨、红松和花楷槭的碳密度年均增加比例分别为2.9%、3.9%和7.2%.森林净初级生产力(NPP)为4.69 t C·hm^-2·a^-1,地下部和地上部NPP比值为0.56,凋落物损失部分是总NPP的最大组分,所占比例为34.5%.森林生态系统中2个主要碳输出途径异养呼吸和粗木质残体分解的年通量分别为293.67和119.29 g C·m^-2·a^-1.森林净生态系统生产力(NEP)为55.90 g C·m^-2·a^-1.研究结果表明,处于衰退状态的谷地云冷杉林仍具有一定的碳汇功能.     

Effects of logging on carbon dynamics of a jack pine forest in Saskatchewan,Canada

We calculated carbon budgets for a chronosequence of harvested jack pine (Pinus banksiana Lamb.) stands (0‐, 5‐, 10‐, and～29‐year‐old) and a～79‐year‐old stand that originated after wildfire. We measured total ecosystem C content (TEC), above‐, and belowground net primary productivity (NPP) for each stand. All values are reported in order for the 0‐, 5‐, 10‐, 29‐, and 79‐year‐old stands, respectively, for May 1999 through April 2000. Total annual NPP (NPP_T) for the stands (Mg C ha^?1 yr^?1±1 SD) was 0.9±0.3, 1.3±0.1, 2.7±0.6, 3.5±0.3, and 1.7±0.4. We correlated periodic soil surface CO₂ fluxes (R_S) with soil temperature to model annual R_S for the stands (Mg C ha^?1 yr^?1±1 SD) as 4.4±0.1, 2.4±0.0, 3.3±0.1, 5.7±0.3, and 3.2±0.2. We estimated net ecosystem productivity (NEP) as NPP_T minus R_H (where R_H was calculated using a Monte Carlo approach as coarse woody debris respiration plus 30–70% of total annual R_S). Excluding C losses during wood processing, NEP (Mg C ha^?1 yr^?1±1 SD) for the stands was estimated to be ?1.9±0.7, ?0.4±0.6, 0.4±0.9, 0.4±1.0, and ?0.2±0.7 (negative values indicate net sources to the atmosphere.) We also calculated NEP values from the changes in TEC among stands. Only the 0‐year‐old stand showed significantly different NEP between the two methods, suggesting a possible mismatch for the chronosequence. The spatial and methodological uncertainties allow us to say little for certain except that the stand becomes a source of C to the atmosphere following logging.     

Carbon dioxide exchange dynamics over a mature switchgrass stand

Switchgrass (Panicum virgatum L.) has gained importance as feedstock for bioenergy over the last decades due to its high productivity for up to 20 years, low input requirements, and potential for carbon sequestration. However, data on the dynamics of CO₂ exchange of mature switchgrass stands (>5 years) are limited. The objective of this study was to determine net ecosystem exchange (NEE), ecosystem respiration (Re), and gross primary production (GPP) for a commercially managed switchgrass field in its sixth (2012) and seventh (2013) year in southern Ontario, Canada, using the eddy covariance method. Average NEE flux over two growing seasons (emergence to harvest) was ?10.4 μmol m^?2 s^?1 and reached a maximum uptake of ?42.4 μmol m^?2 s^?1. Total annual NEE was ?380 ± 25 and ?430 ± 30 g C m^?2 in 2012 and 2013, respectively. GPP reached ?1354 ± 23 g C m^?2 in 2012 and ?1430 ± 50g C m^?2 in 2013. Annual Re in 2012 was 974 ± 20 g C m^?2 and 1000 ± 35 g C m^?2 in 2013. GPP during the dry year of 2012 was significantly lower than that during the normal year of 2013, but yield was significantly higher in 2012 with 1090 g  m^?2, compared to 790 g m^?2 in 2013. If considering the carbon removed at harvest, the net ecosystem carbon balance came to 106 ± 45 g C  m^?2 in 2012, indicating a source of carbon, and to ?59 ± 45 g C m^?2 in 2013, indicating a sink of carbon. Our results confirm that switchgrass can switch between being a sink and a source of carbon on an annual basis. More studies are needed which investigate this interannual variability of the carbon budget of mature switchgrass stands.     

Respiratory carbon use and carbon storage in mid‐rotation loblolly pine (Pinus taeda L.) plantations: the effect of site resources on the stand carbon balance

We used estimates of autotrophic respiration (R_A), net primary productivity (NPP) and soil CO₂ evolution (S_ff), to develop component carbon budgets for 12‐year‐old loblolly pine plantations during the fifth year of a fertilization and irrigation experiment. Annual carbon use in R_A was 7.5, 9.0, 15.0, and 15.1 Mg C ha^?1 in control (C), irrigated (I), fertilized (F) and irrigated and fertilized (IF) treatments, respectively. Foliage, fine root and perennial woody tissue (stem, branch, coarse and taproot) respiration accounted for, respectively, 37%, 24%, and 39% of R_A in C and I treatments and 38%, 12% and 50% of R_A in F and IF treatments. Annual gross primary production (GPP=NPP+R_A) ranged from 13.1 to 26.6 Mg C ha^?1. The I, F, and IF treatments resulted in a 21, 94, and 103% increase in GPP, respectively, compared to the C treatment. Despite large treatment differences in NPP, R_A, and carbon allocation, carbon use efficiency (CUE=NPP/GPP) averaged 0.42 and was unaffected by manipulating site resources. Ecosystem respiration (R_E), the sum of S_ff, and above ground R_A, ranged from 12.8 to 20.2 Mg C ha^?1 yr^?1. S_ff contributed the largest proportion of R_E, but the relative importance of S_ff decreased from 0.63 in C treatments to 0.47 in IF treatments because of increased aboveground R_A. Aboveground woody tissue R_A was 15% of R_E in C and I treatments compared to 25% of R_E in F and IF treatments. Net ecosystem productivity (NEP=GPP‐R_E) was roughly 0 in the C and I treatments and 6.4 Mg C ha^?1 yr^?1 in F and IF treatments, indicating that non‐fertilized treatments were neither a source nor a sink for atmospheric carbon while fertilized treatments were carbon sinks. In these young stands, NEP is tightly linked to NPP; increased ecosystem carbon storage results mainly from an increase in foliage and perennial woody biomass.     

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.     

Asymmetry in above‐ and belowground productivity responses to N addition in a semi‐arid temperate steppe

Nitrogen (N) enrichment often increases aboveground net primary productivity (ANPP) of the ecosystem, but it is unclear if belowground net primary productivity (BNPP) track responses of ANPP. Moreover, the frequency of N inputs may affect primary productivity but is rarely studied. To assess the response patterns of above‐ and belowground productivity to rates of N addition under different addition frequencies, we manipulated the rate (0–50 g N m^?2 year^?1) and frequency (twice vs. monthly additions per year) of NH₄NO₃ inputs for six consecutive years in a temperate grassland in northern China and measured ANPP and BNPP from 2012 to 2014. In the low range of N addition rates, BNPP showed the greatest negative response and ANPP showed the greatest positive responses with increases in N addition (<10 g N m^?2 year^?1). As N addition increased beyond 10 g N m^?2 year^?1, increases in ANPP dampened and decreases in BNPP ceased altogether. The response pattern of net primary productivity (combined above‐ and belowground; NPP) corresponded more closely to ANPP than to BNPP. The N effects on BNPP and BNPP/NPP (f_BNPP) were not dependent on N addition frequency in the range of N additions typically associated with N deposition. BNPP was more sensitive to N addition frequency than ANPP, especially at low rates of N addition. Our findings provide new insights into how plants regulate carbon allocation to different organs with increasing N rates and changing addition frequencies. These root response patterns, if incorporated into Earth system models, may improve the predictive power of C dynamics in dryland ecosystems in the face of global atmospheric N deposition.     

森林生态系统碳循环的基本概念和野外测定方法评述

全球气候变化与森林生态系统碳循环息息相关,定量评估森林碳收支是生态系统与全球变化研究的重要任务。30年来森林生态系统碳循环研究已经取得了长足的进展,但全球和区域森林碳收支仍然存在很大的不确定性。这一方面与森林生态系统本身的复杂性有关,另一方面也与具体研究方法有关。评述了森林生态系统碳循环的基本概念和主要野外测定方法,为我国森林生态系统碳循环研究提供可参考的方法论。从生态系统碳浓度、密度、通量、分配和周转5个方面回顾了碳循环相关概念,指出碳浓度和碳储量是对碳库的静态描述,而碳通量和碳周转是对碳库的动态描述。净初级生产力是测量最普遍的碳通量组分,但大多数情况下因忽略了一些细节而被系统低估。普遍使用的净生态系统生产力,由于没有包含非CO2形式的水文、气象和干扰过程产生的碳通量,通常情况下高于生态系统净碳累积速率。在详细介绍碳通量组分的基础上,改进了森林生态系统碳循环的概念模型。重点讨论了碳通量的3种地面实测方法:测树学方法、箱法和涡度协方差法,并指出了其注意事项和不确定性来源。针对当前碳循环研究的突出问题,建议从4个方面减小碳循环测定的不确定性:(1)恰当运用生物量方程估算乔木生物量;(2)尽可能全面测定生态系统碳组分;(3)给出碳通量估算值的不确定性;(4)多种途径交互验证。     

Pinus taeda forest growth predictions in the 21st century vary with site mean annual temperature and site quality

Climate projections from 20 downscaled global climate models (GCMs) were used with the 3‐PG model to predict the future productivity and water use of planted loblolly pine (Pinus taeda) growing across the southeastern United States. Predictions were made using Representative Concentration Pathways (RCP) 4.5 and 8.5. These represent scenarios in which total radiative forcing stabilizes before 2100 (RCP 4.5) or continues increasing throughout the century (RCP 8.5). Thirty‐six sites evenly distributed across the native range of the species were used in the analysis. These sites represent a range in current mean annual temperature (14.9–21.6°C) and precipitation (1,120–1,680 mm/year). The site index of each site, which is a measure of growth potential, was varied to represent different levels of management. The 3‐PG model predicted that aboveground biomass growth and net primary productivity will increase by 10%–40% in many parts of the region in the future. At cooler sites, the relative growth increase was greater than at warmer sites. By running the model with the baseline [CO₂] or the anticipated elevated [CO₂], the effect of CO₂ on growth was separated from that of other climate factors. The growth increase at warmer sites was due almost entirely to elevated [CO₂]. The growth increase at cooler sites was due to a combination of elevated [CO₂] and increased air temperature. Low site index stands had a greater relative increase in growth under the climate change scenarios than those with a high site index. Water use increased in proportion to increases in leaf area and productivity but precipitation was still adequate, based on the downscaled GCM climate projections. We conclude that an increase in productivity can be expected for a large majority of the planted loblolly pine stands in the southeastern United States during this century.     

广西不同林龄喀斯特森林生态系统碳储量及其分配格局

基于广西喀斯特地区45块1000 m²样地的调查,研究幼龄林、中龄林、近熟林、成熟林、过熟林5个林龄阶段喀斯特森林植被与土壤碳储量的分配格局.结果表明: 广西不同林龄喀斯特森林总碳储量表现为幼龄林(86.03 t·hm^-2)＜近熟林(110.63 t·hm^-2)＜中龄林(112.11 t·hm^-2)＜成熟林(149.1 t·hm^-2)＜过熟林(244.38 t·hm^-2);各林龄阶段植被不同层碳储量分配均不同,乔木层所占比例占绝对优势,达到92.3%～98.7%,随林龄的增加而增长,灌木层、草本层、凋落物层所占比例分别为0.3%～1.9%、0.3%～1.2%和0.3%～2.5%,细根所占比例为0.3%～3.3%.土壤有机碳密度随土层深度的增加而递减,土壤层碳储量为51.75～81.21 t·hm^-2,所占生态系统比例为33.2%～66.2%,其随林龄的增大呈减小趋势.生态系统地上、地下部分碳储量分别为22.80～141.72和62.30～102.66 t·hm^-2,除过熟林外均为地下部分＞地上部分,地上碳储量随林龄的增大呈逐渐增加的趋势,地下碳储量的变化规律与土壤碳储量变化趋势一致.土壤层和乔木层为生态系统的主要碳库,二者所占比例达到了96%以上.     

The annual carbon budget of a French pine forest (Pinus pinaster) following harvest

Eddy covariance measurements of net ecosystem exchange (NEE) of carbon dioxide and sensible and latent heat have operated since clear felling of a 50‐year old maritime pine stand in Les Landes, in Southwestern France. Turbulent fluxes from the closed‐path system are computed via different methodologies, including those recommended from EUROFLUX (Adv. Ecol. Res. 30 (2000) 113; Agric. Forest Meteorol. 107 (2001a, b) 43 and 71), and sensitivity analysis demonstrates the merit of post‐processing for accurate flux calculation. Footprint modeling, energy balance closure, and empirical modeling corroborate the eddy flux measurements, indicating best reliability in the daytime. The ecosystem, a net source of atmospheric CO₂, is capable of fixing carbon during fair weather during any season due to the abundance of re‐growing species (mostly grass), formerly from the understorey. Annual carbon loss of 200–340 g m^?2 depends on the period chosen, with inter‐annual variability evident during the 18‐month measurement period and apparently related to available light. Empirical models, with weekly photosynthetic parameters corresponding to seasonal vegetation and respiration depending on soil temperature, fit the data well and allow partitioning of annual NEE into GPP and TER components. Comparison with a similar nearby mature forest (Agric. Forest Meteorol. 108 (2001) 183) indicates that clear‐cutting reduces GPP by two thirds but TER by only one third, transforming a strong forest sink into a source of CO₂. Likewise, the loss of 50% of evapotranspiration (by the trees) leads to increased temperatures and thus reduced net radiation (by one third), and a 50% increase in sensible heat loss by the clear cut.     

Carbon balance in the tundra, boreal forest and humid tropical forest during climate change: scaling up from leaf physiology and soil carbon dynamics

Carbon exchange by the terrestrial biosphere is thought to have changed since pre-industrial times in response to increasing concentrations of atmospheric CO₂ and variations (anomalies) in inter-annual air temperatures. However, the magnitude of this response, particularly that of various ecosystem types (biomes), is uncertain. Terrestrial carbon models can be used to estimate the direction and size of the terrestrial responses expected, providing that these models have a reasonable theoretical base. We formulated a general model of ecosystem carbon fluxes by linking a process-based canopy photosynthesis model to the Rothamsted soil carbon model for biomes that are not significantly affected by water limitation. The difference between net primary production (NPP) and heterotrophic soil respiration (R_h) represents net ecosystem production (NEP). The model includes (i) multiple compartments for carbon storage in vegetation and soil organic matter, (ii) the effects of seasonal changes in environmental parameters on annual NEP, and (iii) the effects of inter-annual temperature variations on annual NEP. Past, present and projected changes in atmospheric CO₂ concentration and surface air temperature (at different latitudes) were analysed for their effects on annual NEP in tundra, boreal forest and humid tropical forest biomes. In all three biomes, annual NEP was predicted to increase with CO₂ concentration but to decrease with warming. As CO₂ concentrations and temperatures rise, the positive carbon gains through increased NPP are often outweighed by losses through increased R_h, particularly at high latitudes where global warming has been (and is expected to be) most severe. We calculated that, several times during the past 140 years, both the tundra and boreal forest biomes have switched between being carbon sources (annual NEP negative) and being carbon sinks (annual NEP positive). Most recently, significant warming at high latitudes during 1988 and 1990 caused the tundra and boreal forests to be net carbon sources. Humid tropical forests generally have been a carbon sink since 1960. These modelled responses of the various biomes are in agreement with other estimates from either field measurements or geochemical models. Under projected CO₂ and temperature increases, the tundra and boreal forests will emit increasingly more carbon to the atmosphere while the humid tropical forest will continue to store carbon. Our analyses also indicate that the relative increase in the seasonal amplitude of the accumulated NEP within a year is about 0–14% year^?1 for boreal forests and 0–23% year^?1 in the tundra between 1960 and 1990.     

Estimating carbon carrying capacity in natural forest ecosystems across heterogeneous landscapes: addressing sources of error

Evaluating contributions of forest ecosystems to climate change mitigation requires well‐calibrated carbon cycle models with quantified baseline carbon stocks. An appropriate baseline for carbon accounting of natural forests at landscape scales is carbon carrying capacity (CCC); defined as the mass of carbon stored in an ecosystem under prevailing environmental conditions and natural disturbance regimes but excluding anthropogenic disturbance. Carbon models require empirical measurements for input and calibration, such as net primary production (NPP) and total ecosystem carbon stock (equivalent to CCC at equilibrium). We sought to improve model calibration by addressing three sources of errors that cause uncertainty in carbon accounting across heterogeneous landscapes: (1) data‐model representation, (2) data‐object representation, (3) up‐scaling. We derived spatially explicit empirical models based on environmental variables across landscape scales to estimate NPP (based on a synthesis of global site data of NPP and gross primary productivity, n=27), and CCC (based on site data of carbon stocks in natural eucalypt forests of southeast Australia, n=284). The models significantly improved predictions, each accounting for 51% of the variance. Our methods to reduce uncertainty in baseline carbon stocks, such as using appropriate calibration data from sites with minimal human disturbance, measurements of large trees and incorporating environmental variability across the landscape, have generic application to other regions and ecosystem types. These analyses resulted in forest CCC in southeast Australia (mean total biomass of 360 t C ha^?1, with cool moist temperate forests up to 1000 t C ha^?1) that are larger than estimates from other national and international (average biome 202 t C ha^?1) carbon accounting systems. Reducing uncertainty in estimates of carbon stocks in natural forests is important to allow accurate accounting for losses of carbon due to human activities and sequestration of carbon by forest growth.     

Temperate forest responses to carbon dioxide, temperature and nitrogen: a model analysis

The ITE Edinburgh Forest Model, which describes diurnal and seasonal changes in the pools and fluxes of C, N and water in a fully coupled forest–soil system, was parametrized to simulate a managed conifer plantation in upland Britain. The model was used to examine (i) the transient effects on forest growth of an IS92a scenario of increasing [CO₂] and temperature over two future rotations, and (ii) the equilibrium (sustainable) effects of all combinations of increases in [CO₂] from 350 to 550 and 750 μmol mol^?1, mean annual temperature from 7.5 to 8.5 and 9.5°C and annual inputs of 20 or 40 kg N ha^?1. Changes in underlying processes represented in the model were then used to explain the responses. Eight conclusions were supported by the model for this forest type and climate.

• 1 Increasing temperatures above 3°C alone may cause forest decline owing to water stress.
• 2 Elevated [CO₂] can protect trees from water stress that they may otherwise suffer in response to increased temperature.
• 3 In N-limiting conditions, elevated [CO₂] can increase allocation to roots with little increase in leaf area, whereas in N-rich conditions elevated [CO₂] may not increase allocation to roots and generally increases leaf area.
• 4 Elevated [CO₂] can decrease water use by forests in N-limited conditions and increase water use in N-rich conditions.
• 5 Elevated [CO₂] can increase forest productivity even in N-limiting conditions owing to increased N acquisition and use efficiency.
• 6 Rising temperatures (along with rising [CO₂]) may increase or decrease forest productivity depending on the supply of N and changes in water stress.
• 7 Gaseous losses of N from the soil can increase or decrease in response to elevated [CO₂] and temperature.
• 8 Projected increases in [CO₂] and temperature (IS92a) are likely to increase net ecosystem productivity and hence C sequestration in temperate forests.

     

Carbon, nitrogen and phosphorus dynamics in creek water of a southeast Asian mangrove forest

Water column dynamics of carbon, nitrogen and phosphorus in the main creek of the Bangrong mangrove forest, Phuket Island, Thailand, were examined during the dry season. Water sampled from the upper and lower reaches of the creek throughout entire neap and spring tide periods was incubated under saturated irradiation and in the dark. The activity of microbial primary producers and heterotrophs were evaluated from changes in O₂, TCO₂, DOC, DIN, DON and PO₄ ^3- concentrations. Gross primary production was moderate (1.6–2.4 M C h^–1) with no pronounced spatial and temporal variations. A large fraction of the assimilated inorganic carbon and nitrogen was released in the form of DOC (50–90%) and DON (50–60%) indicating that primary producers were under stress or nutrient depleted. PO₄ ^3- and occasionally DIN appeared to be the limiting nutrients. The pelagic heterotrophic community was supported by DOC (50–100%) and DON (40–90%) excreted by primary producers when exposed to light. However, rapid light attenuation in the turbid creek water rendered the entire water column strongly heterotrophic at all times (1.1–2.8 M C h^–1). The microbial heterotrophs are therefore largely supported by particulate and dissolved substrates derived from tidal resuspension, mangrove root exudates and leachates from fallen leaves on the forest floor. The relatively high concentrations of metabolites (e.g. CO₂, NH₄ ⁺) in creek water were primarily supplied by microbial mineralization. Water seeping from creek banks, which was only enriched in TCO₂ (2 times) and PO₄ ^3- (20–100 times) compared with creek water, is not considered an important source of solutes in the waterways of the Bangrong mangrove forest. Although the results obtained here are only strictly representative for the dry season, water column processes in the wet season are not expected to differ much due to the limited seasonal variations with respect to physical, chemical and biological parameters in the Bangrong area.     

Carbon pools in a boreal mixedwood logging chronosequence

Mixedwood forests are an ecologically and economically important forest type in central Canada, but the ecology of these forests is not as well studied as that of single-species dominated stands in the boreal forest. Northern boreal mixedwood forests have only recently been harvested and the effects of harvesting on carbon content in these stands are unknown. We quantified the carbon content and aboveground net primary production (NPP) for four different-aged mixedwood boreal forest stands in northern Manitoba, Canada. The stands included 11-, 18-, and 30-year-old stands that originated from harvesting and a 65-year-old fire-originated stand that typifies the origin of all northern boreal mixed-wood forests that are coming under management. Trees included black spruce (Picea mariana (Mill.) B.S.P.), jack pine (Pinus banksiana Lamb.), balsam poplar (Populus balsamifera L.), and quaking aspen (Populus tremuloides Michx.). Overstory biomass was estimated using species-specific allometric models that generally explained greater than 95% of the observed variation in biomass. Carbon content of the overstory vegetation was greatest in the 65-year-old stand and was 74% larger than the 11-year-old stand and showed a positive relationship with stand age (F_{1, 2}=122.62, P=0.0081 R²=0.99). The slope of mineral soil carbon did not differ significantly among stands (F_{1, 2}=0.39, P=0.5956, R²=0.16). Coarse woody debris carbon content followed a U-shaped pattern among stands. Aboveground NPP differed by 24% between the youngest and oldest stand. Mean annual carbon accumulation and aboveground NPP rates of the mixedwood forests were on average two times greater than nearby relatively pure stands studied during the BOREAS (BOReal Ecosystem Atmospheric Study) project. The trends in the results, along with other field studies, suggest that harvesting does not significantly affect the total soil carbon content. The results of this study suggest that scientists should be cautious about extrapolating results from BOREAS stands to a broader region until more data on other forest types and regions are available.     

氮添加对内蒙古温带典型草原生态系统碳交换的影响

生态系统碳交换(NEE)是评估碳循环及平衡的重要指标,由生态系统总初级生产力(GPP)和生态系统呼吸(ER)共同决定。以往研究表明,N添加能显著促进草地生态系统植物的生长进而提高生态系统的生产力,但N添加如何影响生态系统碳交换的结论仍不明确。同时,对于不同剂量的N添加对生态系统碳交换影响有何差异也不清楚。于2012和2013年在内蒙古草原开展N添加控制实验,设置中等剂量(10 g N m~(-2)a~(-1),N10)和高等剂量(40 g N m~(-2)a~(-1),N40)两个N添加处理,并采用生态系统原位观测箱系统监测不同N处理条件下的NEE动态。结果表明:2年中等剂量N添加处理(N10)下GPP较对照分别增加了15.6%和20%,而ER的变化不显著,该处理下NEE较对照显著降低了230%和337%(即固碳能力增强)。与中等剂量N添加处理结果不同,高等剂量N添加处理下GPP和ER均有不显著的降低趋势,同时,尽管该处理下NEE有升高的趋势(即固碳能力降低),但并不显著。土壤水分改善、土壤温度下降以及叶片N浓度增加可能是中等剂量氮添加促进该生态系统固碳能力的重要机制,而土壤酸化和物种组成改变可能是导致高等剂量N添加下生态系统固碳能力低于中等剂量的重要原因。研究结果表明,不同剂量N添加对生态系统生产力与呼吸的作用机制存在差异,导致生态系统固碳能力有着明显区别。     

长白山原始针叶林沼泽湿地生态系统碳储量

采用年轮分析及相对生长方程法与碳/氮分析仪测定法,测定温带长白山沿湿地过渡带环境梯度依次分布的5种典型原始沼泽类型(草丛沼泽-C、灌丛沼泽-G、落叶松泥炭藓沼泽-LN、落叶松藓类沼泽-LX和落叶松苔草沼泽-LT)生态系统碳储量(植被和土壤)、植被净初级生产力与年净固碳量,定量评价温带森林湿地固碳能力及其长期碳汇作用,并揭示其沿过渡带水分环境梯度的空间分异规律。结果表明:①5种天然沼泽类型的植被碳储量(3.18±1.70)—(112.2±18.3) tC/hm~2沿过渡带环境梯度总体上呈递增趋势,针叶林沼泽显著高于C和G 12.2—34.3倍,G高于C 0.6倍,且LX和LT显著高于LN 0.3—0.6倍;②土壤碳储量(296.3±42.2)—(824.50±50.79) tC/hm~2沿过渡带环境梯度总体上呈递减趋势,C显著高于G和针叶林沼泽30.8%—178.3%(P0.05),G显著高于针叶林沼泽38.7%—112.8%,且LN和LT显著高于LX 32.8%—53.4%;③生态系统碳储量(408.42±57.53)—(827.52±50.96) tC/hm~2沿过渡带环境梯度总体上也呈递减趋势,C显著高于G和针叶林沼泽30.2%—102.7%,G显著高于针叶林沼泽21.5%—55.6%,且LN和LT显著高于LX 18.8%—28.0%;④5种沼泽类型的植被净初级生产力与年净固碳量分布在(5.74±0.08)—(10.98±1.67) t hm~(-2) a~(-1)和(2.44±0.03)—(5.17±0.83)tC hm~(-2) a~(-1),其中,LX和LT的植被净初级生产力显著高于C、G和LN 61.2%—91.3%和34.5%—59.6%;而在植被年净固碳量方面,3种针叶林沼泽类型均显著高于C和G 28.7%—111.9%和19.4%—96.6%。故长白山5种天然沼泽类型的植被净初级生产力与年净固碳量沿湿地过渡带环境梯度总体上呈现出阶梯式递增趋势,且仅有LX和LT达到了中国陆地植被及全球陆地植被平均固碳水平。因此,温带长白山沼草丛沼泽和灌丛沼泽长期碳汇作用强于森林沼泽,湿地碳汇管理实践中应重视草丛沼泽和灌丛沼泽的保护与恢复。