Dependence of the aboveground CO2 exchange rate on tree size in field-grown hinoki cypress (Chamaecyparis obtusa)

The CO₂ exchange of the aboveground parts for five different-sized 17-year-old (as of 1991) hinoki cypress (Chamaecyparis obtusa) trees growing in the field was non-destructively measured over one year, using an open CO₂ exchange system. The CO₂ exchange of individual trees decreased with decreasing tree sizes, such as aboveground phytomass, leaf mass and leaf area. However, the CO₂ exchange abruptly decreased near the smallest-suppressed sample tree. The size dependence was well described by a generalized power function. The annual gross photosynthesis of individual trees was proportional to the square root of leaf mass or leaf area. The dependence of CO₂ exchange on annual phytomass increment was described by a simple power function with an exponent value less than unity, suggesting that CO₂ exchange per unit of phytomass increment was lower in larger-sized trees than in smaller-sized trees. The mean photosynthetic activity of a tree, i.e., gross photosynthesis per unit of leaf area, slightly increased to its highest value with decreasing leaf area and then decreased abruptly near the smallest sample tree. The maximum value of mean photosynthetic activity was estimated to be 2.85 kg CO₂ m⁻² year⁻¹ for a leaf area of 1.56 m² tree⁻¹. The ratio of mean photosynthetic activity to the maximum photosynthetic activity was the highest in an intermediate tree and decreased gradually toward larger-sized trees by ca. 60% and also decreased toward the smallest suppressed tree by ca. 35%.     

Temperature effect on maintenance and growth respiration coefficients of young, field-grown hinoki cypress (Chamaecyparis obtusa)

Respiration measurements were made on the entire aboveground parts of young, field-grown hinoki cypress (Chamaecyparis obtusa) trees at monthly intervals over a 5-year period, to examine the effect of temperature on maintenance and growth respiration coefficients. The respiration rate of the trees was grouped on a monthly basis and then partitioned into maintenance and growth components. The maintenance respiration coefficient increased exponentially with air temperature. The maintenance respiration coefficient at a temperature of 0°C and itsQ ₁₀ value were 0.205 mmol CO₂ g⁻¹ d.w. month⁻¹ and 1.81, respectively. The growth respiration coefficient, which was virtually independent of temperature, had a mean value of 38.06±1.95 (SE) mmol CO₂g⁻¹ d.w. The growth rate increased exponentially with increasing temperature up to a peak at around 18°C, and thereafter declined, thereby resulting in the growth respiration rate being increasingly less sensitive to increasing air temperature. The reported decreases in theQ ₁₀ value of total respiration with increasing air temperature is due to the way in which the growth component of respiration responds to temperature.     

Effect of fruiting on carbon budgets of apple tree canopies

Summary Carbon budgets were calculated from net photosynthesis and dark respiration measurements for canopies of field-grown, 3-year-old apple trees (Malus domestica Borkh.) with maximum leaf areas of 5.4 m² in a temperature-controlled Perspex tree chamber, measured in situ over 2 years (July 1988 to October 1990) by computerized infrared gas analysis using a dedicated interface and software. Net photosynthesis (Pn) and carbon assimilation per leaf area peaked at respectively 8.3 and 7.7 mol CO₂ m^–2 s^–1 in April. Net photosynthesis (Pn) and dark respiration (Rd) per tree peaked at 3.6 g CO₂ tree^–1 h^–1 (Pn) and 1.2 g CO₂ tree^–1 h^–1 (Rd), equivalent to 4.2 mol CO₂ (Pn) and 1.4 mol CO₂ (Rd) m^–2 s^–1 with maximum carbon gain per tree in August and maximum dark respiration per tree in October 1988 and 1989. In May 1990, a tree was deblossomed. Pn (per tree) of the fruiting apple tree canopy exceeded that of the non-fruiting tree by 2–2.5 fold from June to August 1990, attributed to reduced photorespiration (RI), and resulting in a 2-fold carbon gain of the fruiting over the non-fruiting tree. Dark respiration of the fruiting tree canopy progressively exceeded, with increasing sink strength of the fruit, by 51% (June–August), 1.4-fold (September) and 2-fold (October) that of the non-fruiting tree due to leaf (i. e. not fruit) respiration to provide energy (a) to produce and maintain the fruit on the tree and (b) thereafter to facilitate the later carbohydrate translocation into the woody perennial parts of the tree. The fruiting tree reached its optium carbon budget 2–4 weeks earlier (August) then the non-fruiting tree (September 1990). In the winter, the trunk respired 2–100 g CO₂ month^–1 tree^–1. These data represent the first long-term examination of the effect of fruiting without fruit removal which shows increased dark respiration and with the increase progressing as the fruit developed.     

Soil respiration and FDA hydrolysis following conversion of abandoned agricultural lands to natural vegetation in Central Korea

Soil respiration and the hydrolysis of fluorescein diacetate (FDA) as a measure of total microbial activity were investigated in central Korea, at three sites that had been changed from abandoned agricultural lands to natural vegetation: rice field conversion to forest (RF), crop field conversion to shrub (CS), and indigenous forest (IF). Seasonal variations in soil respiration were affected by soil temperature and, to a lesser extent, by photosynthetically active radiation (PAR) and soil moisture. The mean annual rate of soil respiration (g CO₂ m^-2 hr^-1) was highest at CS (0.36), followed by IF (0.29) and RF (0.28), whereas the total annual soil respiration (kg CO₂ m^-2 yr^-1) was 2.82 for CS, 2.46 for IF, and 2.40 for RF. Mean annual FDA hydrolysis (μg FDA min^-1 g^-1 dry soil) was higher at RS (4.56) and IF (4.61) than at CS (3.65). At all three land-use change sites, soil respiration was only very weakly correlated with FDA hydrolysis.     

The contribution of crassulacean acid metabolism to the annual productivity of two aquatic vascular plants

Summary Net annual productivity and annual carbon budgets were determined for populations of Littorella uniflora var. americana and Isoetes macrospora in a mesotrophic and oligotrophic lake in northern Wisconsin, to assess the contribution of Crassulacean Acid Metabolism (CAM) to annual productivity of the species in their natural environment. Nocturnal carbon accumulation (CAM), daytime uptake of external CO₂ via the C₃ mechanism, and refixation of endogenously generated CO₂ from daytime respiration were the sources of carbon income. CAM activity as diurnal acid rhythms reached maxima of 89 to 182 eq·g^-1 leaf fresh weight for the various populations.Maximum rates of daytime ¹⁴C uptake ranged from 0.56 to 1.46 mg C·g^-1 leaf dry wt.·h^-1 for the study populations. Refixation of daytime respired CO₂ averaged 37% for the four populations. Carbon loss was due largely to dark respiration, during the day and night. Nocturnal carbon accumulation, daytime CO₂ uptake and 24-h dark respiration were of similar magnitude, indicating dark respiration was equivalent to 50% of gross photosynthesis.Net annual production was measured for each population by following leaf turnover. Turnover rates for the Littorella populations were 1.56 and 1.72·yr^-1, and for the Isoetes populations, 0.85 and 1.00·yr^-1. Measured net annual productivity and calculated net annual productivity (based on carbon exchange) agreed within an average of 12% for the four populations. While CAM activity was greater for the more productive population of each species, the results suggest that the contribution of CAM to annual productivity is greater for the less productive population of each species. CAM contributed 45 to 55% of the annual carbon gain for the study populations.     

Long-term Respiratory Cost of Maintenance and Growth of Field-grown Young Hinoki Cypress (Chamaecyparis obtusa)

Respiration rate of the entire above-ground parts of field-grown8-year-old hinoki cypress [Chamaecyparis obtusa(Sieb. et Zucc.)Endl.] was measured at monthly intervals over a 5-year period,to evaluate the trend in proportion of maintenance and growthcomponents of respiration with stand development. Representativesample trees were selected for respiration measurements. Theannual respiration rates of individual sample trees were combinedand partitioned into maintenance and growth components by regressingspecific respiration rate on mean relative growth rate. Maintenanceand growth respiration coefficients generated in this way were5.2 mol CO₂kg^-1year^-1and 39 mol  CO₂kg^-1, which are equivalentto 14.3 mg C kg^-1C h^-1(at mean annual air temperature of 15.1°C) and 0.94 kg C kg^-1C. Considering the maintenance andgrowth respiration coefficients, and phytomass and phytomassincrement of individual trees in the stand, the maintenanceand growth respiration rates of the stand were estimated. Theproportion of the maintenance respiration increased, whereasthat of the growth respiration decreased with stand development,due to decreasing relative growth rate.Copyright 1997 Annalsof Botany Company Chamaecyparis obtusa; growth respiration coefficient; hinoki; maintenance respiration coefficient; stand respiration     

土壤细菌呼吸对西双版纳热带森林恢复的响应

揭示不同恢复阶段热带森林土壤细菌呼吸季节变化及其主控因素,对于探明土壤细菌呼吸对热带森林恢复的响应机制具有重要的科学意义。以西双版纳不同恢复阶段热带森林(白背桐群落、崖豆藤群落和高檐蒲桃群落)为研究对象,运用真菌呼吸抑制法及高通量宏基因组测序技术分别测定土壤细菌呼吸速率和细菌多样性,并采用回归分析及结构方程模型揭示热带森林恢复过程中土壤细菌多样性、pH、土壤碳氮组分变化对土壤细菌呼吸速率的影响特征。结果表明：1)不同恢复阶段热带森林土壤细菌呼吸速率表现为：高檐蒲桃群落((1.51±0.62)CO₂ mg g^-1 h^-1)显著高于崖豆藤群落((1.16±0.56)CO₂ mg g^-1 h^-1)和白背桐群落((0.82±0.60)CO₂ mg g^-1 h^-1)(P<0.05)。2)不同恢复阶段土壤细菌呼吸速率呈显著的单峰型季节变化(P<0.05),最大值均出现在9月：高檐蒲桃群落((...     

Ecological studies on the timberline of Mt. Fuji

An ecological study of dry matter production was made in a dwarf forest dominated byAlnus maximowiczii at the timberline of Mt. Fuji. Annual gross production was estimated by two methods, namely the summation method using stem analysis and total photosynthesis calculated from leaf area and photosynthetic rate per leaf area. Seasonal changes in relative light intensity and in leaf area were measured in a quadrat. Photosynthesis and respiration rates of samples were measured in temperature-regulated assimilation chambers. The phytomass was 2,989 g d.w.m^?2, and those of stems and branches, leaves, and roots were 1,672 g, 293 g, and 1,024 g respectively. The growing period of this plant was about four months and this plant expanded leaves quickly. The maximum gross photosynthetic rate was 21 mg CO₂dm^?2 h^?1 on September 1. Annual net production estimated by examining the annual rings was 922 g d.w.m^?2 year^?1 and annual respiration was 735 g. Annual gross production estimated from photosynthetic rates was 1,747 g d.w.m^?2 year^?1. The sum of annual net production by stem analysis and respiration agree closely with gross production estimated from photosynthetic rate. Gross production of this dwarf forest is comparable to the beech forest of the upper cool temperate zone owing to the high photosynthetic rate ofAlnus maximowiczii.     

Woody-tissue respiration for Simarouba amara and Minquartia guianensis,two tropical wet forest trees with different growth habits

We measured CO₂ efflux from stems of two tropical wet forest trees, both found in the canopy, but with very different growth habits. The species were Simarouba amara, a fast-growing species associated with gaps in old-growth forest and abundant in secondary forest, and Minquartia guianensis, a slow-growing species tolerant of low-light conditions in old-growth forest. Per unit of bole surface, CO₂ efflux averaged 1.24 mol m^–2 s^–1 for Simarouba and 0.83 mol m^–2s^–1 for Minquartia. CO₂ efflux was highly correlated with annual wood production (r ²=0.65), but only weakly correlated with stem diameter (r ²=0.22). We also partitioned the CO₂ efflux into the functional components of construction and maintenance respiration. Construction respiration was estimated from annual stem dry matter production and maintenance respiration by subtracting construction respiration from the instantaneous CO₂ flux. Estimated maintenance respiration was linearly related to sapwood volume (39.6 mol m^–3s^–1 at 24.6° C, r ²=0.58), with no difference in the rate for the two species. Maintenance respiration per unit of sapwood volume for these tropical wet forest trees was roughly twice that of temperate conifers. A model combining construction and maintenance respiration estimated CO₂ very well for these species (r ²=0.85). For our sample, maintenance respiration was 54% of the total CO₂ efflux for Simarouba and 82% for Minquartia. For our sample, sapwood volume averaged 23% of stem volume when weighted by tree size, or 40% with no size weighting. Using these fractions, and a published estimate of aboveground dry-matter production, we estimate the annual cost of woody tissue respiration for primary forest at La Selva to be 220 or 350 g C m^–2 year^–1, depending on the assumed sapwood volume. These costs are estimated to be less than 13% of the gross production for the forest.     

Elevated CO2 increases belowground respiration in California grasslands

This study was designed to identify potential effects of elevated CO₂ on belowground respiration (the sum of root and heterotrophic respiration) in field and microcosm ecosystems and on the annual carbon budget. We made three sets of respiration measurements in two CO₂ treatments, i.e., (1) monthly in the sandstone grassland and in microcosms from November 1993 to June 1994; (2) at the annual peak of live biomass (March and April) in the serpentine and sandstone grasslands in 1993 and 1994; and (3) at peak biomass in the microcosms with monocultures of seven species in 1993. To help understand ecosystem carbon cycling, we also made supplementary measurements of belowground respiration monthly in sandstone and serpentine grasslands located within 500 m of the CO₂ experiment site. The seasonal average respiration rate in the sandstone grassland was 2.12 mol m^-2 s^-1 in elevated CO₂, which was 42% higher than the 1.49 mol m^-2 s^-1 measured in ambient CO₂ (P=0.007). Studies of seven individual species in the microcosms indicated that respiration was positively correlated with plant biomass and increased, on average, by 70% with CO₂. Monthly measurements revealed a strong seasonality in belowground respiration, being low (0–0.5 mol CO₂ m^-2 s^-1 in the two grasslands adjacent to the CO₂ site) in the summer dry season and high (2–4 mol CO₂ m^-2 s^-1 in the sandstone grassland and 2–7 mol CO₂ m^-2 s^-1 in the microcosms) during the growing season from the onset of fall rains in November to early spring in April and May. Estimated annual carbon effluxes from the soil were 323 and 440 g C m^-2 year^-1 for the sandstone grasslands in ambient and elevated CO₂. That CO₂-stimulated increase in annual soil carbon efflux is more than twice as big as the increase in aboveground net primary productivity (NPP_a) and approximately 60% of NPP_a in this grassland in the current CO₂ environment. The results of this study suggest that below-ground respiration can dissipate most of the increase in photosynthesis stimulated by elevated CO₂.CIWDPB Publication # 1271     

Measurement of net ecosystem exchange,productivity and respiration in three spruce forests in Sweden shows unexpectedly large soil carbon losses

Measurement of net ecosystem exchange was made using the eddy covariance method above three forests along a north-south climatic gradient in Sweden: Flakaliden in the north, Knottåsen in central and Asa in south Sweden. Data were obtained for 2 years at Flakaliden and Knottåsen and for one year at Asa. The net fluxes (N_ep) were separated into their main components, total ecosystem respiration (R_t) and gross primary productivity (P_g). The maximum half-hourly net uptake during the heart of the growing season was highest in the southernmost site with ?0.787 mg CO₂ m^?2 s^?1 followed by Knottåsen with ?0.631 mg CO₂ m^?2 s^?1 and Flakaliden with ?0.429 mg CO₂ m^?2 s^?1. The maximum respiration rates during the summer were highest in Knottåsen with 0.245 mg CO₂ m^?2 s^?1 while it was similar at the two other sites with 0.183 mg CO₂ m^?2 s^?1. The annual N_ep ranged between uptake of ?304 g C m^?2 year^?1 (Asa) and emission of 84 g C m^?2 year^?1 (Knottåsen). The annual R_t and P_g ranged between 793 to 1253 g C m^?2 year^?1 and ?875 to ?1317 g C m^?2 year^?1, respectively. Biomass increment measurements in the footprint area of the towers in combination with the measured net ecosystem productivity were used to estimate the changes in soil carbon and it was found that the soils were losing on average 96–125 g C m^?2 year^?1. The most plausible explanation for these losses was that the studied years were much warmer than normal causing larger respiratory losses. The comparison of net primary productivity and P_g showed that ca 60% of P_g was utilized for autotrophic respiration.     

Spatial–temporal variation in soil respiration in an oak–grass savanna ecosystem in California and its partitioning into autotrophic and heterotrophic components

The spatial upscaling of soil respiration from field measurements to ecosystem levels will be biased without studying its spatial variation. We took advantage of the unique spatial gradients of an oak–grass savanna ecosystem in California, with widely spaced oak trees overlying a grass layer, to study the spatial variation in soil respiration and to use these natural gradients to partition soil respiration according to its autotrophic and heterotrophic components. We measured soil respiration along a 42.5 m transect between two oak trees in 2001 and 2002, and found that soil respiration under tree canopies decreased with distance from its base. In the open area, tree roots have no influence on soil respiration. Seasonally, soil respiration increased in spring until late April, and decreased in summer following the decrease in soil moisture content, despite the further increase in soil temperature. Soil respiration significantly increased following the rain events in autumn. During the grass growing season between November and mid-May, the average of CO₂ efflux under trees was 2.29 μmol m⁻² s⁻¹, while CO₂ efflux from the open area was 1.40 μmol m⁻² s⁻¹. We deduced that oak root respiration averaged as 0.89 μmol m⁻² s⁻¹, accounting for 39% of total soil respiration (oak root + grass root + microbes). During the dry season between mid-May and October, the average of CO₂ efflux under trees was 0.87 μmol m⁻² s⁻¹, while CO₂ efflux from the open areas was 0.51 μmol m⁻² s⁻¹. Oak root respiration was 0.36 μmol m⁻² s⁻¹, accounting for 41% of total soil respiration (oak root + microbes). The seasonal pattern of soil CO₂ efflux under trees and in open areas was simulated by a bi-variable model driven by soil temperature and moisture. The diurnal pattern was influenced by tree physiology as well. Based on the spatial gradient of soil respiration, spatial analysis of crown closure and the simulation model, we spatially and temporally upscaled chamber measurements to the ecosystem scale. We estimated that the cumulative soil respiration in 2002 was 394 gC m⁻² year⁻¹ in the open area and 616 gC m⁻² year⁻¹ under trees with a site-average of 488 gC m⁻² year⁻¹.     

川西贡嘎山峨眉冷杉成熟林生态系统CO2通量特征

成熟森林的碳收支对陆地生态系统碳循环研究具有重要意义。目前,我国关于西南亚高山暗针叶林成熟林碳通量的研究还相对较少,尚不明确对碳循环的作用。以涡度相关技术为基础,对川西贡嘎山东坡峨眉冷杉成熟林生态系统尺度的CO_2通量进行长期定位观测。利用2015年6月至2016年5月观测数据,分析了峨眉冷杉成熟林净生态系统CO_2交换量(NEE)、生态系统呼吸(Re)和总生态系统生产力(GPP)的季节变异特征及其源汇状况,并结合环境因子,分析CO_2通量的主要控制因子。结果表明:(1)峨眉冷杉成熟林NEE具有明显的日变化特征,呈现"U"形变化,白天为负值,夜间为正值,中午前后CO_2通量达到最大;各月间日平均NEE变化差异显著,NEE峰值最大出现在2015年6月(-0.64 mg CO_2m~(-2)s~(-1)),峰值最小出现在2016年1月(-0.08 mg CO_2m~(-2)s~(-1));日平均NEE由正值变为负值的时间夏季最早,冬季最晚,NEE由负值变为正值的时间冬季最早,夏季最晚。(2)峨眉冷杉成熟林NEE、Re和GPP具有明显的月变化。2015年6月和12月NEE分别达到最大值(-46.02 g C m~(-2)月~(-1))和最小值(-1.42 g C m~(-2)月~(-1));Re呈现单峰变化,最大和最小值分别出现在2015年6月(84.78 g C m~(-2)月~(-1))和2016年1月(12.82 g C m~(-2)月~(-1));GPP最大值和最小值分别出现在2015年6月(130.81 g C m~(-2)月~(-1))与2016年1月(16.15 g C m~(-2)月~(-1))。(3)空气温度(T_a)、5 cm土壤温度(T_(s5))和光合有效辐射(PAR)是影响峨眉冷杉成熟林CO_2通量的主要环境因子。T_a与CO_2通量呈指数相关(R~2=0.5283,P0.01);白天CO_2通量与PAR显著相关(R~2=0.4373,P0.01);夜晚CO_2通量与T_(s5)显著相关(R~2=0.4717,P0.01)。(4)全年NEE、Re和GPP分别为-241.87、564.81 g C m~(-2)和806.68 g C m~(-2),表明川西贡嘎山峨眉冷杉成熟林具有较强的碳汇功能。     

A meta-analysis of leaf gas exchange and nitrogen in trees grown under elevated carbon dioxide

The response of trees to rising atmospheric CO₂ concentration ([CO₂]) is of concern to forest ecologists and global carbon modellers and is the focus of an increasing body of research work. I review studies published up to May 1994, and several unpublished works, which reported at least one of the following: net CO₂ assimilation (A), stomatal conductance (g_s), leaf dark respiration (R_d) leaf nitrogen or specific leaf area (SLA) in woody plants grown at <400 μmol mol^?1 CO₂ or at 600–800 μmol mol^?1 CO₂. The resulting data from 41 species were categorized according to growth conditions (unstressed versus stressed), length of CO₂ exposure, pot size and exposure facility [growth chamber (GC), greenhouse (GH), or open-top chamber (OTC)] and interpreted using meta-analytic methods. Overall, A showed a large and significant increase at elevated [CO₂] but length of CO₂ exposure and the exposure facility were important modifiers of this response. Plants exposed for < 50 d had a significantly greater response, and those from GCs had a significantly lower response than plants from longer exposures or from OTC studies. Negative acclimation of A was significant and general among stressed plants, but in unstressed plants was influenced by length of CO₂ exposure, the exposure facility and/or pot size. Growth at elevated [CO₂] resulted in moderate reductions in gs in unstressed plants, but there was no significant effect of CO₂ on gs in stressed plants. Leaf dark respiration (mass or area basis) was reduced strongly by growth at high [CO₂] > while leaf N was reduced only when expressed on a mass basis. This review is the first meta-analysis of elevated CO₂ studies and provides statistical confirmation of several general responses of trees to elevated [CO₂]. It also highlights important areas of continued uncertainty in our understanding of these responses.     

Photosynthetic and stomatal responses of two tropical and two temperate trees to atmospheric humidity

The effects of leaf to air vapour pressure differences (ΔW) on net photosynthetic rate (P_N) and stomatal conductance (g_s) were examined in the leaves of two tropical rain forest trees, Eugenia grandis and Pongamia pinnata, and two temperate evergreen trees, Viburnum awabuki and Daphniphyllum macropodum. A single leaf was set inside a small chamber and ΔW was varied from 7 to 24 mmol mol^-1 at 25 and 500 μmol m^-2 s^-1 of photon flux density. P_N and g_s of the two tropical rain forest trees decreased with increasing ΔW, while the two temperate evergreen trees were not highly responsive to ΔW. P. pinnata was more sensitive to ΔW in its stomatal response, and had a higher stomatal density and higher stomatal index than did the two temperate trees and another tropical tree. Significant reductions i n g_s and intercellular CO₂ concentrations in the two tropical trees at high ΔW suggest that the decline of P_N was due to the decrease in g_s. The responses of P_N and g_s indicated that the tropical trees were more sensitive to ΔW than were the temperate ones. This revised version was published online in September 2006 with corrections to the Cover Date.     

Partitioning of respiration from soil,litter and plants in a mixed-grassland ecosystem

Summary Carbon dioxide effluxes from plants, litter and soil were measured in two mixed-grassland sites in Saskatchewan, Canada. Ecosystems at both locations were dominated by Agropyron dasystachyum (Hook.) Scribn. Respiration rates of intact and experimentally-modified systems were measured in field chambers using alkali-absorption. Removal of green leaves, dead leaves, and litter from a wet sward reduced respiration to as low as 58% of the rate in an intact system. In a dry sward green shoots were the only significant above-ground source of CO₂.Carbon dioxide effluxes from different parts of A. dasystachyum plants, and from soil samples were measured in laboratory vessels at 20° using alkali-absorption. Respiration of green leaves (1.46 mg CO₂ g^-1 h^-1) was significantly higher than microbial respiration in moist, dead leaf samples (0.79 mg CO₂ g^-1 h^-1) or litter (0.75 mg CO₂ g^-1 h^-1). Microbial respiration in air-dried, dead plant material was very low. Average repiration rates of roots separated from soil cores (0.24 mg CO₂ g^-1 h^-1) were lower than many values reported in the literature, probably because the root population sampled included inactive, suberized and senescent roots. Root respiration was estimated to be 17–26% of total CO₂ efflux from intact cores.Laboratory data and field measurements of environmental conditions and plant biomass were combined in order to reconstruct the CO₂ efflux from the shoot-root-soil system. Reconstructed rates were 1.3 to 2.3 times as large as field measured rates, apparently because of stimulation to respiration caused by the experimental manipulations. The standing dead and litter fractions contributed 26% and 23% of the total CO₂ efflux in a wet sward. Both field-measured and reconstructed repiration values suggest that in situ decomposition of standing dead material under moist conditions can be a significant part of carbon balance in mixed grassland.     

Energetic Costs of Tissue Construction in Yellow-poplar and White Oak Trees Exposed to Long-term CO2Enrichment

Two methods were used to estimate construction costs for leaves,stems, branches and woody roots of yellow-poplar (LiriodendrontulipiferaL.) trees grown at ambient (35 Pa) and elevated (65Pa) CO₂for 2.7 years and trees of white oak (Quercus albaL.)grown at these same CO₂partial pressures for 4 years. Samplecombustion in a bomb calorimeter combined with measurementsof ash and nitrogen content provided the primary method of estimatingtissue construction costs (W_G; g glucose g^-1dry mass). Thesevalues were compared with a second, simpler method in whichcost estimates were derived from tissue ash, carbon and nitrogencontent (V_G). Estimates of W_Gwere lower for leaves, branchesand roots of yellow-poplar and for leaves of white oak grownat elevated compared with ambient CO₂partial pressures. TheseCO₂-induced differences in W_Granged from 3.7% in yellow-poplarroots to 2.1% in white oak leaves. Only in the case of yellow-poplarleaves, however, were differences in V_Gobserved between CO₂treatments.Leaf V_Gwas 1.46 g glucose g^-1dry mass in ambient-grown treescompared with 1.41 g glucose g^-1dry mass for CO₂-enriched trees.Although paired-estimates of W_Gand V_Gclustered about a 1:1 linefor leaves and branches, estimates of V_Gwere consistently lowerthan W_Gfor stems and roots. Construction costs per unit leafarea were 95 g glucose m^-2for yellow-poplar trees grown at ambientCO₂and 106 g glucose m^-2for trees grown at elevated CO₂partialpressures. No differences in area-based construction costs wereobserved for white oak. Whole-plant energy content was 1220g glucose per tree in ambient-grown white oak compared with2840 g glucose per tree for those grown at elevated CO₂partialpressures. These differences were driven largely by CO₂-inducedchanges in total biomass. We conclude that while constructioncosts were lower at elevated CO₂partial pressures, the magnitudeof this response argues against an increased efficiency of carbonuse in the growth processes of trees exposed to CO₂enrichment. Bomb calorimeter; construction costs; elevated CO₂; energy allocation; global change; growth respiration; heat of combustion; respiration; Liriodendron tulipifera; Quercus alba     

Carbon balance and allocation of assimilated CO2 in Scots pine, Norway spruce, and Silver birch seedlings determined with gas exchange measurements and 14C pulse labelling

Carbon dioxide is released from the soil to the atmosphere in heterotrophic respiration when the dead organic matter is used for substrates for soil micro-organisms and soil animals. Respiration of roots and mycorrhiza is another major source of carbon dioxide in soil CO₂ efflux. The partitioning of these two fluxes is essential for understanding the carbon balance of forest ecosystems and for modelling the carbon cycle within these ecosystems. In this study, we determined the carbon balance of three common tree species in boreal forest zone, Scots pine, Norway spruce, and Silver birch with gas exchange measurements conducted in laboratory in controlled temperature and light conditions. We also studied the allocation pattern of assimilated carbon with ¹⁴C pulse labelling experiment. The photosynthetic light responses of the tree species were substantially different. The maximum photosynthetic capacity (P _max) was 2.21 μg CO₂ s⁻¹ g⁻¹ in Scots pine, 1.22 μg CO₂ s⁻¹ g⁻¹ in Norway spruce and 3.01 μg CO₂ s⁻¹ g⁻¹ in Silver birch seedlings. According to the pulse labelling experiments, 43–75% of the assimilated carbon remained in the aboveground parts of the seedlings. The amount of carbon allocated to root and rhizosphere respiration was about 9–26%, and the amount of carbon allocated to root and ectomycorrhizal biomass about 13–21% of the total assimilated CO₂. The ¹⁴CO₂ pulse reached the root system within few hours after the labelling and most of the pulse had passed the root system after 48 h. The transport rate of carbon from shoot to roots was fastest in Silver birch seedlings.     

Seasonal soil‐surface carbon fluxes from the root systems of young Pinus radiata trees growing at ambient and elevated CO2 concentration

Elevated atmospheric CO₂ concentration may result in increased below‐ground carbon allocation by trees, thereby altering soil carbon cycling. Seasonal estimates of soil surface carbon flux were made to determine whether carbon losses from Pinus radiata trees growing at elevated CO₂ concentration were higher than those at ambient CO₂ concentration, and whether this was related to increased fine root growth. Monthly soil surface carbon flux density (f) measurements were made on plots with trees growing at ambient (350) and elevated (650 μmol mol^?1) CO₂ concentration in large open‐top chambers. Prior to planting the soil carbon concentration (0.1%) and f (0.28 μmol m^?2 s^?1 at 15 °C) were low. A function describing the radial pattern of f with distance from tree stems was used to estimate the annual carbon flux from tree plots. Seasonal estimates of fine root production were made from minirhizotrons and the radial distribution of roots compared with radial measurements of f. A one‐dimensional gas diffusion model was used to estimate f from soil CO₂ concentrations at four depths. For the second year of growth, the annual carbon flux from the plots was 1671 g y^?1 and 1895 g y^?1 at ambient and elevated CO₂ concentrations, respectively, although this was not a significant difference. Higher f at elevated CO₂ concentration was largely explained by increased fine root biomass. Fine root biomass and stem production were both positively related to f. Both root length density and f declined exponentially with distance from the stem, and had similar length scales. Diurnal changes in f were largely explained by changes in soil temperature at a depth of 0.05 m. Ignoring the change of f with increasing distance from tree stems when scaling to a unit ground area basis from measurements with individual trees could result in under‐ or overestimates of soil‐surface carbon fluxes, especially in young stands when fine roots are unevenly distributed.     

Thirty years of in situ tree growth under elevated CO2: a model for future forest responses?

Rising concentrations of atmospheric carbon dioxide have been predicted to stimulate the growth of forest trees. However, long-term effects on trees growing to maturity and to canopy closure while exposed to elevated CO₂ have never been examined. We compared tree ring chronologies of Mediterranean Quercus ilex which have been continuously exposed to elevated CO₂ (around 650 μmol mol^–1) since they were seedlings, near two separate natural CO₂ springs with those from trees at nearby ambient-CO₂‘control’ sites. Trees grown under high CO₂ for 30 years (1964–93) showed a 12% greater final radial stem width than those growing at the ambient-CO₂ control sites. However, this stimulation was largely due to responses when trees were young. By the time trees were 25–30 y old the annual difference in tree ring width between low and high CO₂ grown trees had disappeared. At any given tree age, elevated CO₂ had a relatively greater positive effect on tree ring width in years with a dry spring compared to years with more rainfall between April and May. This indicates a beneficial effect of elevated CO₂ on tree water relations under drought stress. Our data suggest that the early regeneration phase of forest stands can be accelerated in CO₂-enriched atmospheres and that maximum biomass per land area may be reached sooner than under lower CO₂ concentrations. In our study, high CO₂ grown Q. ilex trees reached the same stem basal area at the age of 26 y as control trees at 29 y, i.e. three years earlier (faster turnover of carbon?). Reliable predictions of the future development of forests need to account for the variable responses of trees over their entire lifetime. Such responses to elevated CO₂ can presently only be assessed at such unique field sites.