Flux partitioning in an old-growth forest: seasonal and interannual dynamics

Turbulent fluxes of carbon, water and energy were measured at the Wind River Canopy Crane, Washington, USA from 1999 to 2004 with eddy-covariance instrumentation above (67 m) and below (2.5 m) the forest canopy. Here we present the decomposition of net ecosystem exchange of carbon (NEE) into gross primary productivity (GPP), ecosystem respiration (R(eco)) and tree canopy net CO(2) exchange (DeltaC) for an old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)-western hemlock (Tsuga heterophylla (Raf.) Sarg.) forest. Significant amounts of carbon were recycled within the canopy because carbon flux measured at the below-canopy level was always upward. Maximum fluxes reached 4-6 micromol m(-2) s(-1) of CO(2) into the canopy air space during the summer months, often equaling the net downward fluxes measured at the above-canopy level. Ecosystem respiration rates deviated from the expected exponential relationship with temperature during the summer months. An empirical ecosystem stress term was derived from soil water content and understory flux data and was added to the R(eco) model to account for attenuated respiration during the summer drought. This attenuation term was not needed in 1999, a wet La Ni?a year. Years in which climate approximated the historical mean, were within the normal range in both NEE and R(eco), but enhanced or suppressed R(eco) had a significant influence on the carbon balance of the entire stand. In years with low respiration the forest acts as a strong carbon sink (-217 g C m(-2) year(-1)), whereas years in which respiration is high can turn the ecosystem into a weak to moderate carbon source (+100 g C m(-2) year(-1)).     

Environmental controls on sap flow in a northern hardwood forest

Our objective was to gain a detailed understanding of how photosynthetically active radiation (PAR), vapor pressure deficit (D) and soil water interact to control transpiration in the dominant canopy species of a mixed hardwood forest in northern Lower Michigan. An improved understanding of how these environmental factors affect whole-tree water use in unmanaged ecosystems is necessary in assessing the consequences of climate change on the terrestrial water cycle. We used continuously heated sap flow sensors to measure transpiration in mature trees of four species during two successive drought events. The measurements were scaled to the stand level for comparison with eddy covariance estimates of ecosystem water flux (Fw). Photosynthetically active radiation and D together explained 82% of the daytime hourly variation in plot-level transpiration, and low soil water content generally resulted in increased stomatal sensitivity to increasing D. There were also species-specific responses to drought. Quercus rubra L. showed low water use during both dry and wet conditions, and during periods of high D. Among the study species, Acer rubrum L. showed the greatest degree of stomatal closure in response to low soil water availability. Moderate increases in stomatal sensitivity to D during dry periods were observed in Populus grandidentata Michx. and Betula papyrifera Marsh. Sap flow scaled to the plot level and Fw demonstrated similar temporal patterns of water loss suggesting that the mechanisms controlling sap flow of an individual tree also control ecosystem evapotranspiration. However, the absolute magnitude of scaled sap flow estimates was consistently lower than Fw. We conclude that species-specific responses to PAR, D and soil water content are key elements to understanding current and future water fluxes in this ecosystem.     

Modeling daily gas exchange of a Douglas-fir forest: comparison of three stomatal conductance models with and without a soil water stress function

Modeling stomatal conductance is a key element in predicting tree growth and water use at the stand scale. We compared three commonly used models of stomatal conductance, the Jarvis-Loustau, Ball-Berry and Leuning models, for their suitability for incorporating soil water stress into their formulation, and for their performance in modeling forest ecosystem fluxes. We optimized the parameters of each of the three models with sap flow and soil water content data. The optimized Ball-Berry model showed clear relationships with air temperature and soil water content, whereas the optimized Leuning and Jarvis-Loustau models only showed a relationship with soil water content. We conclude that use of relative humidity instead of vapor pressure deficit, as in the Ball-Berry model, is not suitable for modeling daily gas exchange in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in the Speulderbos forest near the village of Garderen, The Netherlands. Based on the calculated responses to soil water content, we linked a model of forest growth, FORGRO, with a model of soil water, SWIF, to obtain a forest water-balance model that satisfactorily simulated carbon and water (transpiration) fluxes and soil water contents in the Douglas-fir forest for 1995.     

Cacao trees under different shade tree shelter: effects on water use

We asked how shade tree admixture affects cacao water use in agroforests. In Central Sulawesi, Indonesia, cacao and shade tree sap flux was monitored in a monoculture, in a stand with admixed Gliricidia trees and in a mixture with a multi-species tree assemblage, with both mixtures having similar canopy openness. A Jarvis type sap flux model suggested a distinct difference in sap flux response to changes in vapor pressure deficit and radiation among cacao trees in the individual cultivation systems. We argue that differences originate from stomatal control of transpiration in the monoculture and altered radiation conditions and a different degree of uncoupling of the VPD from the bulk atmosphere inside shaded stands. Probably due to high sap flux variability among trees, these differences however did not result in significantly altered average daily cacao water use rates which were 16 L day^?1 in the multi-species assemblage and 22 L day^?1 in the other plots. In shaded stands, water use of single cacao trees increased with decreasing canopy gap fraction in the overstory since shading enhanced vegetative growth of cacao fostering transpiration per unit ground area. Estimated transpiration rates of the cacao tree layer were further controlled by stem density and amounted to 1.2 mm day^?1 in the monoculture, 2.2 mm day^?1 for cacao in the cacao/Gliricidia stand, and 1.1 mm day^?1 in the cacao/multi-species stand. The additional transpiration by the shade trees is estimated at 0.5 mm day^?1 for the Gliricidia and 1 mm day^?1 for the mixed-species cultivation system.     

Modeling annual production and carbon fluxes of a large managed temperate forest using forest inventories, satellite data and field measurements

We evaluated annual productivity and carbon fluxes over the Fontainebleau forest, a large heterogeneous forest region of 17,000 ha, in terms of species composition, canopy structure, stand age, soil type and water and mineral resources. The model is a physiological process-based forest ecosystem model coupled with an allocation model and a soil model. The simulations were done stand by stand, i.e., 2992 forest management units of simulation. Some input parameters that are spatially variable and to which the model is sensitive were calculated for each stand from forest inventory attributes, a network of 8800 soil pits, satellite data and field measurements. These parameters are: (1) vegetation attributes: species, age, height, maximal leaf area index of the year, aboveground biomass and foliar nitrogen content; and (2) soil attributes: available soil water capacity, soil depth and soil carbon content. Main outputs of the simulations are wood production and carbon fluxes on a daily to yearly basis. Results showed that the forest is a carbon sink, with a net ecosystem exchange of 371 g C m(-2) year(-1). Net primary productivity is estimated at 630 g C m(-2) year(-1) over the entire forest. Reasonably good agreement was found between simulated trunk relative growth rate (2.74%) and regional production estimated from the National Forest Inventory (IFN) (2.52%), as well as between simulated and measured annual wood production at the forest scale (about 71,000 and 68,000 m(3) year(-1), respectively). Results are discussed species by species.     

Estimating stand transpiration in a Eucalyptus populnea woodland with the heat pulse method: measurement errors and sampling strategies

Sap flow measurement techniques, such as the heat pulse (compensation) method, are practical means for estimating the water use of individual trees and are often the only reasonable alternative for measuring forest and woodland transpiration in complex heterogeneous terrain. The need to scale estimates of water use from a sample of individual stems to a stand (population) of known area may be satisfied by applying scalars of flux based on tree size or domain. We estimated the aggregate errors in applying the heat pulse technique to the estimation of stand transpiration in a poplar box (Eucalyptus populnea F.J. Muell.) woodland in southeastern Queensland, Australia, by a combination of precision analyses, experimental validation and Monte Carlo simulations of sampling errors. Errors in sap flux density measurements were approximately 13%. The potential error in the flux estimates for individual stems with stratified sampling of sap flux density with depth and bole quadrant based on four sensors was an additional 25%. Conducting wood area, diameter at 1.3 m, leaf area and domain based on Ecological Field Theory all proved excellent scalars of flux at the stand level. With a sample size of six trees stratified by diameter, coefficients of variation in scaling to the stand level were approximately 5% for any of these scalars. The greatest potential source of error in estimating stand transpiration by the heat pulse method was in the measurement of the fluxes of individual stems; scaling these measurements to a homogeneous stand of trees involved less uncertainty.     

How the environment,canopy structure and canopy physiological functioning influence carbon,water and energy fluxes of a temperate broad-leaved deciduous forest--an assessment with the biophysical model CANOAK

This paper focuses on how canopy structure, its physiological functioning and the environment interact to control and drive the exchange of carbon dioxide (CO2) and water vapor between a temperate forest stand and the atmosphere. First, we present an overview of how temporal and spatial variations in canopy structure (e.g., leaf area index, species, leaf inclination angles, leaf clumping) and physiological functioning (e.g., maximal stomatal conductance, photosynthetic capacity) modulate CO2 and water vapor fluxes. Then, with the biophysical model CANOAK, we quantify the effects of leaf dimension and thickness, vertical variations in leaf area and photosynthetic capacity, leaf clumping, leaf inclination angles, photosynthetic capacity, stomatal conductance and weather on the annual sums of CO2, water vapor and sensible heat exchange. Finally, we discuss how much detail is needed in a model to predict fluxes of CO2 and water vapor with acceptable fidelity.     

Can distribution of trees explain variation in nitrous oxide fluxes?

Abstract

The impact of distance to tree stems on nitrous oxide (N₂O) fluxes was examined to determine whether it is possible to improve the accuracy of flux estimates from boreal forest soils. Dark static chambers were placed along transects between pairs of trees within a Norway spruce stand and fluxes of N₂O and carbon dioxide (CO₂) were measured during the period 1999–2003. The groundwater table was measured on every sampling occasion along the transects. In addition, radiation transmission, potential diffusion rate and biomass of forest floor vegetation were measured once at each chamber site along one of the transects and soil samples were collected at three depths, from which pH, denitrification enzyme activity, soil moisture, organic matter, and carbon and nitrogen content were determined. There was a high level of variation in the N₂O fluxes, both spatially and temporally. However, the spatial variation in the N₂O fluxes within the transect could not be explained by differences in any of the measured variables. Sometimes, mainly when no major peaks occurred, N₂O fluxes were significantly correlated with CO₂ release. It is concluded that distance to stems cannot be used to improve the design of sampling schemes or for extrapolating flux levels to larger scales.     

Carbon dioxide and water vapor exchange by young and old ponderosa pine ecosystems during a dry summer

We investigated key factors controlling mass and energy exchange by a young (6-year-old) ponderosa pine (Pinus ponderosa Laws.) plantation on the west side of the Sierra Nevada Mountains and an old-growth ponderosa pine forest (mix of 45- and 250-year-old trees) on the east side of the Cascade Mountains, from June through September 1997. At both sites, we operated eddy covariance systems above the canopy to measure net ecosystem exchange of carbon dioxide and water vapor, and made concurrent meteorological and ecophysiological measurements. Our objective was to understand and compare the controls on ecosystem processes in these two forests. Precipitation is much higher in the young plantation than in the old-growth forest (1660 versus 550 mm year-1), although both forests experienced decreasing soil water availability and increasing vapor pressure deficits (D) as the summer of 1997 progressed. As a result, drought stress increased at both sites during this period, and changes in D strongly influenced ecosystem conductance and net carbon uptake. Ecosystem conductance for a given D was higher in the young pine plantation than in the old-growth forest, but decreased dramatically following several days of high D in late summer, possibly because of xylem cavitation. Net CO2 exchange generally decreased with conductance at both sites, although values were roughly twice as high at the young site. Simulations with the 3-PG model, which included the effect of tree age on fluxes, suggest that, during the fall through spring period, milder temperatures and ample water availability at the young site provide better conditions for photosynthesis than at the old pine site. Thus, over the long-term, the young site can carry more leaf area, and the climatic conditions between fall and spring offset the more severe limitations imposed by summer drought.     

Forest transpiration—targeted through xylem sap flux assessment versus hydrological modeling

The assessment of forest transpiration rates is crucial for determining plant-available soil water consumption and drought risk of trees. Xylem sap flux measurements have been used increasingly to quantify stand transpiration in forest ecosystems. Here, we compare this empirical approach with hydrological modeling on the basis of a stand transpiration dataset of adult beech (Fagus sylvatica), which was acquired across Bavaria, Germany, at eight forest sites. Xylem sap flux sensors were installed in five dominant trees each. Two tree to stand upscaling approaches, related to site-specific (1) sapwood area or (2) to leaf area index, were compared. The outcome was examined each in relation to process-based stand hydrological modeling, using LWF-BROOK90. Distinct relationships between tree diameter at breast height (1.30 m) and sapwood area-weighted sap flux along the radial profile became apparent across the study sites, confirming a generic allometric basis for stand-level upscaling of transpiration. The two upscaling approaches did not differ in outcome, representatively covering stand structure for comparison with modeling. Differential analysis yielded high agreement between the empirical and modeling approaches throughout most of the study period, although LWF-BROOK90 tended to overestimate sap flux measurements under low soil moisture. The two empirical approaches proved reliable for even-aged beech stands, as performance under high stand-structural heterogeneity awaits clarification. Findings advance stand-level hydrological modeling regarding coverage of stomatal behavior during temporary limitation in water availability.     

Influences of environmental factors on the radial profile of sap flux density in Fagus crenata growing at different elevations in the Naeba Mountains, Japan

Sap flux density was measured continuously during the 1999 and 2000 growing seasons by the heat dissipation method in natural Fagus crenata Blume (Japanese beech) forests growing between 550 and 1600 m on the northern slope of the Kagura Peak of the Naeba Mountains, Japan. Sap flux density decreased radially toward the inner xylem and the decrease was best expressed in relation to the number of annual rings from the cambium, or in relation to the relative depth between the cambium and the trunk center, rather than as a function of absolute depth. The relative influences of radiation, vapor pressure deficit and soil water on sap flux density during the growing season were similar for the outer and inner xylem, and at all sites. Measurements of soil water content and water potential at a depth of 0.25 m demonstrated that sap flux density responded similarly and sensitively to water potential changes in this soil layer, despite large differences in rooting depth at different elevations, localizing one important control point in the functioning of this forest ecosystem. Identification of the relative influences of radiation, vapor pressure deficit and drying of the upper soil layer on sap flux density provides a framework for in-depth analysis of the control of transpiration in Japanese beech forests. In addition, the finding that the same general controls are operating on sap flux density despite climate gradients and large differences in overall forest stand structure will enhance understanding of water use by forests along elevation gradients.     

Component carbon fluxes and their contribution to ecosystem carbon exchange in a pine forest: an assessment based on eddy covariance measurements and an integrated model

We used a combination of eddy flux, canopy, soil and environmental measurements with an integrated biophysical model to analyze the seasonality of component carbon (C) fluxes and their contribution to ecosystem C exchange in a 50-year-old Scots pine forest (Pinus sylvestris L.) in eastern Finland (62 degrees 47' N, 30 degrees 58' E) over three climatically contrasting years (2000-2002). Eddy flux measurements showed that the growing Scots pine forest was a sink for CO2, with annual net C uptakes of 131, 210 and 258 g C m-2> year-1 in 2000, 2001 and 2002, respectively. The integrated process model reproduced the annual course of daily C flux above the forest canopy as measured by the eddy covariance method once the site-specific component parameters were estimated. The model explained 72, 66 and 68% of the variation in daily net C flux in 2000, 2001 and 2002, respectively. Modeled annual C loss by respiration was 565, 629 and 640 g C m-2 year-1, accounting for 77, 77 and 65% of annual gross C uptake, respectively. Carbon fluxes from the forest floor were the dominant contributors to forest ecosystem respiration, with the fractions of annual respiration from the forest floor, foliage and wood being 46-62, 27-44 and 9-10%, respectively. The wide range in daily net C uptake during the growing season was largely attributable to day-to-day fluctuations in incident quantum irradiance. During just a few days in early spring and late autumn, ecosystem net C exchange varied between source and sink as a result of large daily changes in temperature. The forest showed a greater reduction in gross C uptake by photosynthesis than in C loss by respiration during the dry summer of 2000, indicating that interannual variability in ecosystem net C uptake at this site was modified mostly by summer rainfall and vapor pressure deficit.     

Soil respiration in a mixed temperate forest and its contribution to total ecosystem respiration

Soil respiration (SR) was measured with an infrared gas analyzer in nine plots representative of the heterogeneous vegetation in a mixed coniferous-deciduous forest in the Belgian Campine region. Selected plots included the two most representative overstory species (Pinus sylvestris L. and Quercus robur L.) in combination with the most representative understory species of the forest. A model that includes temperature and water as the main controlling variables was fitted to the data. We found large spatial variability in SR among plots, with typically lower fluxes under the coniferous overstory than under the deciduous overstory (means of 4.8 +/- 0.4 and 8.8 +/- 0.5 Mg C ha(-1) year(-1), respectively). Total annual soil carbon (C) emissions were estimated by weighting fluxes from different types of vegetation according to their relative contribution to the footprint area of the eddy covariance flux measurement. The relative contribution of the two main tree species to the footprint-weighted total SR varied among seasons with the more abundant coniferous overstory contributing the most to total SR during most of the year. Nonetheless, during summer, the contribution of deciduous plots to total SR was disproportionally high because of the more pronounced seasonality of belowground metabolic activity. Net ecosystem carbon dioxide exchange was measured by eddy covariance, and we estimated total ecosystem respiration (TER) with footprint-constrained nighttime fluxes. Mean total annual SR and TER were 6.1 +/- 0.11 and 9.1 +/- 1.15 Mg C ha(-1) year(-1), respectively. The 95% confidence interval of the ratio of annual SR:TER ranged from 0.58 to 0.76, with a mean of 0.67. The contribution of SR to TER tended to vary seasonally, with minimum contributions during summer (less than 50% of TER) and maximum contributions during winter (about 94% of TER).     

稳定性同位素技术在森林生态系统碳水通量组分区分中的应用

生态系统碳水循环过程中的同位素分馏效应,为区分生态系统净碳交换过程中光合和呼吸通量,蒸散通量中植物蒸腾和土壤蒸发提供了理论依据.本文综述了稳定性同位素技术在这一领域的应用进展.研究结果显示,此方法切实可行,为进一步解释森林生态系统过程提供了广阔的前景.但由于森林生态系统结构复杂,不确定性因素多,国内外研究还处于尝试阶段.该方法在森林生态系统应用中还存在诸如Keeling图技术和涡度相关技术的假设条件难以满足等问题,期待今后在技术和理论上的突破.     

Plant ecophysiology and forest response to global change

There are many ways of studying forest responses to global change. Most current national and international programs focus on net gas exchange of the terrestrial biosphere and are typically interdisciplinary, multi-scale projects. Key objectives of these programs are surprisingly similar to those of classical plant ecophysiology studies, i.e., to explore functional relationships of plant or plant community responses to environmental change. Thus, common research questions that link plant ecophysiology to ecosystem functioning can be identified for both research communities, promising complementarity and synergism for joint research projects. Although some well-established ecophysiological relationships, such as light responses or stomatal limitations of photosynthetic gas exchange, are currently employed in many ecosystem-scale net flux studies for gap-filling or modeling, only 14% (n = 27) of all eddy covariance flux studies in forests (n = 196; published between 1992 and April 2002) include plant ecophysiological measurements (n = 24) or biomass and growth estimates (n = 8). Generally, emphasis is on CO2 exchange measurements at various scales (foliage, shoots, branches; n = 14) and water relations measurements (n = 11). These measurements do not fully support the typical parameterization of stand and regional models, which often need information on canopy architecture and nitrogen nutrition. By means of a complementary research approach, valuable information can be acquired that is unobtainable by means of a single approach. This additional information is important for the identification of underlying biotic and environmental drivers, for the regulation of net ecosystem fluxes and their partitioning, and the independent validation of measured net ecosystem fluxes. Thus, combining micrometeorology and ecophysiology at flux sites is strongly recommended for ecosystem functioning studies.     

中亚热带天然林改造成人工林后土壤呼吸的变化特征

【目的】研究中亚热带常绿阔叶林(天然林)改造成人工林后土壤碳排放量的变化及主要影响因子,为评估森林类型转换对土壤碳排放的影响提供科学依据。【方法】在福建农林大学西芹教学林场的常绿阔叶林及由其改造而来的38年生闽楠人工林与35年生杉木人工林中分别设置4块20 m×20 m样地,利用Li-8100土壤碳通量观测系统于2014年9月—2016年9月进行定点观测,并同期观测土壤温度、含水量、有机碳含量(SOC)、微生物生物量碳含量(MBC)、可溶性有机碳含量(DOC)、0～20 cm土层细根生物量和年凋落物量及凋落物碳氮比(C/N)。【结果】常绿阔叶林改造成闽楠(38年后)和杉木人工林(35年后),年均土壤碳排放通量由16. 22显著降为12. 71和4. 83 tC·hm^-2a^-1,分别减少21. 60%和70. 20%;各林分类型的土壤呼吸温度敏感性Q¹⁰值表现为常绿阔叶林(1. 97)<闽楠人工林(2. 03)<杉木人工林(2. 91),转换为杉木人工林后,Q¹⁰值显著升高(P<0. 05);土壤温度能分别解释常绿阔叶林、闽楠人工林与杉木人工林土壤呼吸速率变化的89. 70%、88. 50%和87. 90%,土壤呼吸速率和土壤含水量相关不显著(P>0. 05);土壤呼吸速率和SOC、MBC、DOC、年凋落物量及0～20 cm土层细根生物量均极显著正相关(P<0. 01);土壤呼吸温度敏感性指数Q¹⁰值和凋落物C/N极显著正相关(P<0. 01),而与年均土壤呼吸速率及MBC极显著负相关(P<0. 01);进一步分析发现土壤MBC和SOC含量是影响土壤呼吸速率的2个最重要因子,而凋落物C/N在影响土壤呼吸温度敏感性中的贡献最大。【结论】中亚热带地区常绿阔叶林改造成闽楠(38年)或杉木(35年)人工林后,土壤碳排放通量显著降低。林分类型转换后树种组成和林分结构发生改变,凋落物数量、质量及细根生物量显著降低,土壤SOC和MBC含量显著下降可共同导致土壤呼吸通量的下降。土壤温度是3种林分类型土壤呼吸季节变化的主导因素,而土壤总有机碳库和土壤微生物量碳库的差异是不同林分之间土壤呼吸差异的主导因素,凋落物C/N对土壤呼吸的Q¹⁰影响最大。为提高模型预测森林类型转换影响土壤碳排放的精度,应综合考虑土壤有机碳库、易变性有机碳库及底物质量的变化。     

Daily and seasonal patterns of carbon and water fluxes above a north Australian savanna

Daily and seasonal fluxes of carbon dioxide and water vapor above a north Australian savanna were recorded over a complete dry season-wet season annual cycle using the eddy covariance technique. Wet season rates of photosynthesis and transpiration were larger than those measured in the dry season and were dominated by the presence of the grassy understory. As the dry season progressed and the grass understory died, ecosystem rates of assimilation and water vapor flux declined substantially. By the end of the dry season, canopy assimilation and evapotranspiration rates were 20-25% of wet season values. Assimilation was light saturated in the dry season but not in the wet season. Stomatal control of transpiration increased between the wet and dry season. This was revealed by the decline in the slope of E with increasing leaf-to-air vapor pressure difference (D) between wet and dry seasons, and also by the significant decrease in the ratio of boundary to canopy conductance observed between the wet and dry seasons. A simple pan-tropical modeling of leaf area index or wet season canopy CO2 flux was undertaken. It was shown that with readily available data for foliar N content and the ratio of rainfall to potential evaporation, leaf index and wet season canopy CO2 flux can be successfully estimated for a number of tropical ecosystems, including north Australian savannas.     

Use of temporal patterns in vapor pressure deficit to explain spatial autocorrelation dynamics in tree transpiration

To quantify the relationship between temporal and spatial variation in tree transpiration, we measured sap flow in 129 trees with constant-heat sap flow sensors in a subalpine forest in southern Wyoming, USA. The forest stand was located along a soil water gradient from a stream side to near the top of a ridge. The stand was dominated by Pinus contorta Dougl. ex Loud. with Picea engelmannii Parry ex Engelm and Abies lasiocarpa (Hook.) Nutt. present near the stream and scattered individuals of Populus tremuloides Michx. throughout the stand. We used a cyclic sampling design that maximized spatial information with a minimum number of samples for semivariogram analyses. All species exhibited previously established responses to environmental variables in which the dominant driver was a saturating response to vapor pressure deficit (D). This response to D is predictable from tree hydraulic theory in which stomatal conductance declines as D increases to prevent excessive cavitation. The degree to which stomatal conductance declines with D is dependent on both species and individual tree physiology and increases the variability in transpiration as D increases. We quantified this variability spatially by calculating the spatial autocorrelation within 0.2-kPa D bins. Across 11 bins of D, spatial autocorrelation in individual tree transpiration was inversely correlated to D and dropped from 45 to 20 m. Spatial autocorrelation was much less for transpiration per unit leaf area and not significant for transpiration per unit sapwood area suggesting that spatial autocorrelation within a particular D bin could be explained by tree size. Future research should focus on the mechanisms underlying tree size spatial variability, and the potentially broad applicability of the inverse relationship between D and spatial autocorrelation in tree transpiration.     

Factors controlling N2O and CH4 fluxes in mixed broad-leaved/Korean pine forest of Changbai Mountain, China

Using the closed chamber technique, thein situ measurements of N₂O and CH₄ fluxes was conducted in a broad-leaved Korean pine mixed forest ecosystem in Changbai Mountain, China, from June 1994 to October 1995. The relationships between fluxes (N₂O and CH₄) and some major environmental factors (temperature, soil water content and soil available nitrogen) were studied. A significant positive correlation between N₂O emission and air/soil temperature was observed, but no significant correlation was found between N₂O emission and soil water content (SWC). This result showed that temperature was an important controlling factor of N₂O flux. There was a significant correlation between CH₄ uptake and SWC, but no significant correlation was found between CH₄ uptake and temperature. This suggested SWC was an important factor controlling CH₄ uptake. The very significant negative correlation between logarithmic N₂O flux and soil nitrate concentration, significant negative correlation between CH₄ flux and soil ammonium content were also found. This project is supported by Chinese Academy of Sciences Responsible editor: Chai Ruihai