Application of Geostatistics to Characterize Leaf Area Index (LAI) from Flux Tower to Landscape Scales Using a Cyclic Sampling Design

Accurate characterization of leaf area index (LAI) is required to quantify the exchange of energy, water, and carbon between terrestrial ecosystems and the atmosphere. The objective of this study was to use a cyclic sampling design to compare the spatial patterns of LAI of the dominant terrestrial ecosystems that comprised the area around the 447-m WLEF television tower, equipped with an eddy flux system, near Park Falls, Wisconsin, USA. A second objective was to compare the efficiency of cyclic, random, and uniform sampling designs in terms of the precision of spatial information derived per unit sampling effort. The vegetation surrounding the tower was comprised (more than 80%) of four major forest cover types: forested wetlands, upland aspen forests, upland northern hardwood forests, and upland pine forests, and a fifth, nonforested cover type, grass (open meadow). LAI differed significantly among the five cover types and averaged 3.45, 3.57, 3.82, 3.99, and 1.14 for northern hardwoods, aspen, forested wetlands, upland conifers, and grass, respectively. The cyclic sampling design maximized information about the variance of vegetation characteristics of the heterogeneous landscape and decreased by 60% the number of plots needed to obtain the same confidence interval width using a random sampling design. The range of spatial autocorrelation for LAI was 147 m, but it was decreased to 117 m when vegetation cover was included as a covariate. The cyclic sampling design has several important advantages over other sampling designs. The cyclic sampling design increased the sampling efficiency by optimizing the placement of plots so they were distributed more efficiently for geostatistical analyses such as semi-variograms, correlograms, and spatial regression and can incorporate covariates (for example, vegetation cover, soil properties, and so on) that may explain the sources of spatial patterns. The cyclic sampling design was used to derive a spatial map of LAI and the average LAI for the 3 × 2 km area centered on the flux tower was 3.51 ± 0.89 (with a minimum of 0 and a maximum of 6.35). Airborne and satellite reflectance data have also been used to characterize LAI, but in this region, and many other forests of the world, remotely sensed vegetation indexes saturate in forests with an LAI greater than 3-5. The cyclic sampling design also provides a general ecological sampling approach that can be used at multiple scales.     

Seasonal variations of CO2 and water vapour exchange rates over a temperate deciduous forest

Long-term and direct measurements of CO₂ and water vapour exchange are needed over forested ecosystems to determine their net annual fluxes of carbon dioxide and water. Such measurements are also needed to parameterize and test biogeochemical, ecological and hydrological assessment models. Responding to this need, eddy covariance measurements of CO₂ and water vapour were made ever a deciduous forest growing near Oak Ridge, TN, between April 1993 and April 1994. Periodic measurements were made of leaf area index, stomatal resistance, soil moisture and pre-dawn leaf water potential to characterize the gas exchange capacity of the canopy. Four factors had a disproportionate influence on the seasonal variation of CO₂ flux densities. These factors were photon flux densities (during the growing season), temperature (during the dormant season), leaf area index and the occurrence of drought The drought period occurred during the peak of the growing season and caused a significant decline in daily and hourly CO₂ flux densities, relative to observations over the stand when soil moisture was plentiful. The annual net uptake of carbon was calculated by integrating flux measurements and filling missing and spurious data with the relations obtained between measured CO₂ fluxes and environmental forcing variables. The net flux of carbon for the period between April 1993 and April 1994 was -525 g C m^?2 y^?1. This value represents a net flux of carbon from the atmosphere and into the forest. The net annual carbon exchange of this southern temperate broadleaved forest exceeded values measured over a northern temperate forest (which experiences a shorter growing season and has less leaf area) by 200 g C m^?2 y^?1 (cf. Wofsy et al 1993). The seasonal variation of canopy evaporation (latent heat flux) was controlled mostly by changes in leaf area and net radiation. A strong depression in evaporation rates was not observed during the drought Over a broadleaved forest large vapour pressure deficits promote evaporation and trees in a mixed stand are able to tap a variety of deep and shallow water sources.     

The effect of fertilization on sap flux and canopy conductance in a Eucalyptus saligna experimental forest

Land devoted to plantation forestry (50 million ha) has been increasing worldwide and the genus Eucalyptus is a popular plantation species (14 million ha) for its rapid growth and ability to grow well on a wide range of sites. Fertilization is a common silvicultural tool to improve tree growth with potential effects on stand water use, but the relationship between wood growth and water use in response to fertilization remains poorly quantified. Our objectives in this study were to determine the extent, timing and longevity of fertilization effects on water use and wood growth in a non‐water limited Eucalyptus saligna experimental forest near Hilo, HI. We evaluated the short‐ and long‐term effects of fertilization on water use by measuring sap flux per unit sapwood area, canopy conductance, transpiration per unit leaf area and water‐use efficiency in control and fertilized stands. Short‐term effects were assessed by comparing sap flux before and after fertilizer application. Long‐term effects were assessed by comparing control plots and plots that had received nutrient additions for 5 years. For the short‐term response, total water use in fertilized plots increased from 265 to 487 mm yr^?1 during the 5 months following fertilization. The increase was driven by an increase in stand leaf area accompanied by an increase in sap flux per unit sapwood area. Sap flux per unit leaf area and canopy conductance did not differ during the 5 months following fertilizer additions. For the last 2 months of our short‐term measurements, fertilized trees used less water per unit carbon gain (361 compared with 751 kg H₂O kg C^?1 in control stands). Trees with 5 years of fertilization also used significantly more water than controls (401 vs. 302 mm yr^?1) because of greater leaf area in the fertilized stands. Sap flux per unit sapwood area, sap flux per unit leaf area, and canopy conductance did not differ between control and fertilized trees in the long‐term plots. In contrast to the short‐term response, the long‐term response of water use per unit wood growth was not significant. Overall, fertilization of E. saligna at our site increased stand water use by increasing leaf area. Fertilized trees grew more wood and used more water, but fertilization did not change wood growth per unit water use.     

Hydrologic balance in an intact temperate forest ecosystem under ambient and elevated atmospheric CO2 concentration

Abstract Increasing atmospheric CO₂ concentration decreases stomatal conductance in many species, but the savings of water from reduced transpiration may permit the forest to retain greater leaf area index (L). Therefore, the net effect on water use in forest ecosystems under a higher CO₂ atmosphere is difficult to predict. The free air CO₂ enrichment (FACE) facility (n = 3) in a 14‐m tall (in 1996) Pinus taeda L. stand was designed to reduce uncertainties in predicting such responses. Continuous measurements of precipitation, throughfall precipitation, sap flux, and soil moisture were made over 3.5 years under ambient (CO₂^a) and elevated (CO₂^e) ambient + 200 µmol mol^?1). Annual stand transpiration under ambient CO₂ conditions accounted for 84–96% of latent heat flux measured with the eddy‐covariance technique above the canopy. Under CO₂^e, P. taeda transpired less per unit of leaf area only when soil drought was severe. Liquidambar styraciflua, the other major species in the forest, used progressively less water, settling at 25% reduction in sap flux density after 3.5 years under CO₂^e. Because P. taeda dominated the stand, and severe drought periods were of relatively short duration, the direct impact of CO₂^e on water savings in the stand was undetectable. Moreover, the forest used progressively more water under CO₂^e, probably because soil moisture availability progressively increased, probably owing to a reduction in soil evaporation caused by more litter buildup in the CO₂^e plots. The results suggest that, in this forest, the effect of CO₂^e on transpiration was greater indirectly through enhanced litter production than directly through reduced stomatal conductance. In forests composed of species more similar to L. styraciflua, water savings from stomatal closure may dominate the response to CO₂^e.     

Regional atmospheric cooling and wetting effect of permafrost thaw‐induced boreal forest loss

In the sporadic permafrost zone of North America, thaw‐induced boreal forest loss is leading to permafrost‐free wetland expansion. These land cover changes alter landscape‐scale surface properties with potentially large, however, still unknown impacts on regional climates. In this study, we combine nested eddy covariance flux tower measurements with satellite remote sensing to characterize the impacts of boreal forest loss on albedo, eco‐physiological and aerodynamic surface properties, and turbulent energy fluxes of a lowland boreal forest region in the Northwest Territories, Canada. Planetary boundary layer modelling is used to estimate the potential forest loss impact on regional air temperature and atmospheric moisture. We show that thaw‐induced conversion of forests to wetlands increases albedo: and bulk surface conductance for water vapour and decreases aerodynamic surface temperature. At the same time, heat transfer efficiency is reduced. These shifts in land surface properties increase latent at the expense of sensible heat fluxes, thus, drastically reducing Bowen ratios. Due to the lower albedo of forests and their masking effect of highly reflective snow, available energy is lower in wetlands, especially in late winter. Modelling results demonstrate that a conversion of a present‐day boreal forest–wetland to a hypothetical homogeneous wetland landscape could induce a near‐surface cooling effect on regional air temperatures of up to 3–4 °C in late winter and 1–2 °C in summer. An atmospheric wetting effect in summer is indicated by a maximum increase in water vapour mixing ratios of 2 mmol mol^?1. At the same time, maximum boundary layer heights are reduced by about a third of the original height. In fall, simulated air temperature and atmospheric moisture between the two scenarios do not differ. Therefore, permafrost thaw‐induced boreal forest loss may modify regional precipitation patterns and slow down regional warming trends.     

Comparisons of soil‐water content between a Moso bamboo (Phyllostachys pubescens) forest and an evergreen broadleaved forest in western Japan

In Japan, forests of Moso bamboo (Phyllostachys pubescens, an exotic invasive giant bamboo) have naturalized and expanded rapidly, replacing surrounding broadleaved and coniferous forests. To evaluate impacts caused by these forest‐type replacements on the hydrological cycle, soil‐water content and its spatial variability in a Moso bamboo forest were compared with those in an adjacent evergreen broadleaved forest, in a case study of a stand in western Japan (northern Kyushu). The volumetric soil‐water content averaged over depths between 0 and 60 cm was consistently higher in the bamboo stand than that in the broadleaved stand. These results contrast with previous studies comparing the soil‐water content in Moso bamboo forests with that in other forest types. The sum of canopy transpiration and soil evaporation (E) in the bamboo stand tended to be larger than that in the broadleaved stand. Small canopy interception loss was reported in the bamboo forest. Therefore, the large amount of E would counterbalance the small canopy interception loss in the bamboo forest. Differences in soil characteristics between the two stands may be the main factor causing differences in soil‐water content. Spatial variation in soil‐water content in the bamboo stand was larger than that in the broadleaved stand, confirming findings in a previous series of our study. This could happen because the well‐developed root‐system in the bamboo forest enhances preferential flow in the soil. To evaluate the effects of aggressive invasion of alien giant bamboo on the ecosystem functions, we recommend further studies measuring various hydrological components in various Moso bamboo forests.     

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

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

Granier树干液流测定系统在马占相思的水分利用研究中的应用

介绍了Granier热消散探针在树干液流测定中的工作原理,并利用该系统长期监测广东鹤山马占相思林14株样树的液流密度,分析了树木个体内和个体之间液流密度的差异、整树和林段水分利用的量化特征.由于树木边材结构以及周围微环境的差别,树木内和个体间的液流密度差异非常明显,变异系数的平均值分别为15.51%-37.26%、37.46%-50.73%.尽管液流密度的差异较大,但同一株树木不同方位的液流密度之间却呈现明显的线性相关（p＜0.0001）,这是重要的特征值,使得只需测定某一方位的液流密度经尺度外推计算整树和林段蒸腾成为可能.树木液流对环境因子响应的变化规律取决于所参照的时间尺度,日变化主要受光辐射、水汽压差等气候因子的控制,而土壤水份对液流的季节变化影响较大.形态特征明显影响树木的液流,高大树木由于边材较厚、树干粗壮和冠幅较宽而承载较多的辐射能量,因而水分蒸腾较高.对树木液流密度在径向和方位上进行适当的整合,可较准确地计算整树和林段蒸腾.由液流估测的马占相思整树和林段蒸腾的结果显示,该群落的水分利用在时间和空间上均有明显的分化.     

Transpiration of trees and forest stands: short and long-term monitoring using sapflow methods

We show that sapflow is a useful tool for studies of water fluxes in forest ecosystems, because (i) it gives access to the spatial variability within a forest stand, (ii) it can be used even on steep slopes, and (iii) when combined with eddy correlation measurements over forests, it allows separation of individual tree transpiration from the total water loss of the stand. Moreover, sapflow techniques are quite easy to implement. Four sapflow techniques currently coexist, all based on heat diffusion in the xylem. We found a good agreement between three of these techniques. Most results presented here were obtained using the radial flow meter (Granier 1985). Tree sapflow is computed as sap flux density times sapwood area. To scale up from trees to a stand, measurements have to be made on a representative sample of trees. Thus, a number of trees in each circumference class is selected according to the fraction of sapwood they represent in the total sapwood area of the stand. The variability of sap flux density among trees is usually low (CV. 10–15%) in close stands of temperate coniferous or deciduous forests, but is much higher (35–50%) in a tropical rain forest. It also increases after thinning or during a dry spell. A set of 5–10 sapflow sensors usually provides an accurate estimate of stand transpiration. Transpiration measured on two dense spruce stands in the Vosges mountains (France) and one Scot's pine plantation in the Rhine valley (Germany) showed that maximum rate was related to stand LAI and to local climate. Preliminary results comparing the sapflow of a stand of Pinus banksiana to the transpiration of large branches, as part of the BOREAS programme in Saskachewan, Canada showed a similar trend. For modelling purposes, tree canopy conductance (g_c) was calculated from Penman-Monteith equation. In most experiments, calculated canopy conductance was dependent on global radiation (positive effect) and on vapour pressure deficit (negative effect) in the absence of other limiting factors. A comparison of the vapour pressure deficit response curves of g_c for several tree species and sites showed only small differences among spruce, oak and pine forests when including understorey. Tropical rainforests exhibited a similar behaviour.     

Aspen lichens in agricultural and forest landscapes: the importance of habitat quality

The amount of aspen Populus tremula , has declined in the boreal forest landscape. This decline is especially marked in young and intermediate stands due to the lack of regeneration. Aspen regeneration is nowadays mainly restricted to abandoned agricultural land. The decrease of aspen is of particular concern as it has more host-specific species than any other boreal tree species. The main question addressed is whether regenerating aspen stands in agricultural habitats can compensate for the deficiency of young stands in the forest. Data on epiphytic macrolichens show that cyanolichens increased, in number and frequency, with stand age in the forest landscape, and that there was a striking difference in species composition between stands in the two landscapes. Lichens with cyanobacterial and green-algal photobionts dominated in the forest and agricultural stands, respectively. Notably, cyanolichens were not found in stands younger than 50 yr in the forest, and stands younger than 100 yr in the agricultural landscape. This difference between the landscapes cannot be explained by stand age, stand size or isolation. Instead, differences in habitat quality, due to differences in the physical environment associated with the presence of conifers in the older forest stands, appear to be involved. We suggest that in order to conserve cyanolichens that are confined to aspen, active management practices have to be adopted that promote the regeneration of aspen in the forest landscape, and the establishment of conifers in areas where aspen regeneration is confined to the agricultural landscape. In addition, until new aspen stands with appropriate physical environments have been established, these measures must be combined with the preservation of existing old-growth stands, which can provide appropriate source populations.     

Wet- vs. Dry-Season Transpiration in an Amazonian Rain Forest Palm Iriartea deltoidea

The wet and dry seasons in tropical rain forests can differ in precipitation, soil moisture and irradiance more significantly than often assumed. This could potentially affect the water relations of many tree species that may exhibit either increased transpiration in the dry season as a response to the increased irradiance or decreased transpiration as a result of decreases in soil moisture and increases in atmospheric vapor pressure deficit (VPD). Atmospheric data, soil moisture data and sap fluxes in Iriartea deltoidea palms were measured in eastern Ecuador during the wet and dry seasons. There were no differences between total daily sap fluxes in I. deltoidea palms during the wet and dry seasons; however, evaporative demand was significantly higher in the dry season and therefore, transpiration was more restricted by stomatal closure during the dry season than the wet season. This is likely the result of larger atmospheric VPD during the dry season compared with the wet season and possibly the result of reduced soil moisture availability. Additionally, based on published tree abundances in this area, measured sap fluxes in I. deltoidea were scaled up to the hectare level. Transpiration from I. deltoidea palms was estimated to be around 0.03 mm/d, which could represent about 1 percent of total transpiration in this area of the Amazon rain forest. If climate change predictions for more lengthy tropical dry periods are realized, greater stomatal control of dry-season sap flux has the potential to become even more prevalent in tropical species.     

Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

Summary Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day^–1 tree^–1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m² of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s^–1 that is equivalent to 112 mmol m^–2 s^–1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m^–2 s^–1 at the top and 150 mmol m^–2 s^–1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was –27.76±0.27 (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.This paper is dedicated to Prof. Dr. O.L. Lange on the occasion of his 65th birthday     

热消散液流测定系统研究竹子蒸腾的问题和解决思路

旺盛的蒸腾是竹子迅速生长的重要代谢保障,对林区生态系统水循环和水量平衡发挥关键作用,如何准确估测竹林的蒸腾耗水是迫切需要解决的方法难题。目前研究树木水分生理和森林水文广泛采用的热消散液流测定方法(TDP)却少有在竹林中应用,而仅有的几项研究报道由于缺乏严谨的实验验证,结果误差较大而缺少说服力。作者认为,竹壁结构异质性和竹竿中央空腔造成热量的不均匀分布是基于热通量交换的液流测定系统测量竹子蒸腾不准确的主要原因,提出以自行设计的注水变压液流特性测定法和传统的整树容器称重法,验证热消散液流测定系统估测竹子水分利用的适用性。本文还结合竹子的种群结构和无次生生长的特点,提出竹株借助相互连接的地下茎进行水分再分配,异株补偿水力限制,蒸腾主要受年龄而不是竹形大小影响的观点,建议通过分析竹子蒸腾的年龄效应,研究不同种植密度竹林水分利用的变化规律,为竹林生产和集水区水源管理提供有效的林型设计和措施。     

Aspen succession and nitrogen loading: a case for epiphytic lichens as bioindicators in the Rocky Mountains,USA

Question: Can lichen communities be used to assess short‐ and long‐term factors affecting seral quaking aspen (Populus tremuloides) communities at the landscape scale? Location: Bear River Range, within the Rocky Mountains, in northern Utah and southern Idaho, USA. Method: Forty‐seven randomly selected mid‐elevation aspen stands were sampled for lichens and stand conditions. Plots were characterized according to tree species cover, basal area, stand age, bole scarring, tree damage, and presence of lichen species. We also recorded ammonia emissions with passive sensors at 25 urban and agricultural sites throughout an adjacent populated valley upwind of the forest stands. Nonmetric multidimensional scaling (NMS) ordination was used to evaluate an array of 20 variables suspected to influence lichen communities. Results: In NMS, forest succession explained most variance in lichen composition and abundance, although atmospheric nitrogen from local agricultural and urban sources also significantly influenced the lichen communities. Abundance of nitrophilous lichen species decreased with distance from peak ammonia sources and the urban center in all aspen succession classes. One lichen, Phaeophyscia nigricans, was found to be an effective bioindicator of nitrogen loading. Conclusions: Lichen communities in this landscape assessment of aspen forests showed clear responses to long‐term (stand succession) and short‐term (nitrogen deposition) influences. At the same time, several environmental factors (e.g. tree damage and scarring, distance to valley, topography, and stand age) had little influence on these same lichen communities. We recommend further use of epiphytic lichens as bioindicators of dynamic forest conditions.     

Water fluxes within beech stands in complex terrain

We investigated the water balances of two beech stands (Fagus sylvatica L.) on opposite slopes (NE, SW) of a narrow valley near Tuttlingen in the southern Swabian Jura, a low mountain range in Southwest Germany. Our analysis combines results from continuous measurements of forest meteorological variables significant to the forest water balance, stand transpiration (ST) estimates from sap flow measurements, and model simulations of microclimate and water fluxes. Two different forest hydrological models (DNDC and BROOK90) were tested for their suitability to represent the particular sites. The investigation covers the years 2001–2007. Central aims were (1) to evaluate meteorological simulations of variables below the forest canopy, (2) to evaluate ST, (3) to quantify annual water fluxes for both beech stands using the evaluated hydrological models, and (4) to analyse the model simulations with regard to assumptions inherent in the respective model. Overall, both models were very well able to reproduce the observed dynamics of the soil water content in the uppermost 30 cm. However, the degree of fit depended on the year and season. The comparison of experimentally determined ST within the beech stand on the NE-slope during the growing season of 2007 with simulated transpiration did not yield a reliable statistical relationship. The simulation of water fluxes for the beech stand on the NE- and SW-slopes showed similar results for vegetation-related fluxes with both models, but different with respect to runoff and percolation flows. Overall, the higher evaporation demand on the warmer SW-slope did not lead to a significantly increased drought stress for the vegetation but was reflected mainly in decreased water loss from the system. This finding is discussed with regard to potential climate change and its impact on beech growth.     

大连4种城市绿化乔木树种夜间液流活动特征

夜间液流有助于树木物质运输及其体内水分的补充(water recharge), 它不仅对植物的生长发育具有重要的生理生态学意义, 而且对大尺度植物蒸腾耗水的估算可能产生重要影响。2008年6月1日至8月31日, 以热扩散探针(thermal dissipation probe, TDP)技术对大连市劳动公园内的雪松(Cedrus deodara)、大叶榉(Zelkova schneideriana)、丝棉木(Euonymus bungeanus)和水杉(Metasequoia glyptostroboides) 4种乔木树种的不同径阶样木树干边材液流进行了测定, 并结合同步土壤水分与小气候观测结果分析了树木夜间(18:00至次日5:00)液流特征。实验结果表明, 树木普遍存在可感夜间液流, 夜间液流总量占观测期液流总量的比例在样木个体间呈现显著差异, 其变化范围为0.44%-75.96%; 观测期雨天夜间液流波动活跃, 显著高于晴天, 其单日夜间液流总量可持平, 甚至高于日间液流。相关分析表明: 水汽压亏缺(vapor pressure deficit, VPD)和风速的变化与夜间蒸腾显著相关, 它们能够较好地解释液流变化(R² > 0.6); 树木夜间液流主要用于夜间蒸腾和自身水分补充, 夜间液流现象主要发生在前半夜, 后半夜液流平稳且极接近0, 夜间液流量与相应的日间流量(R² = 0.356, p = 0.00)及胸径(R²_Spearman > 0.80)显著相关, 说明植物本身的结构和生理特点也是影响树木夜间液流的重要因子。单株样木夜间液流占全天总蒸腾量的比例低于14.4%, 如不考虑夜间液流的影响, 根据日间液流通过尺度扩展推算的森林生态系统年蒸腾量可能偏低。     

Sapwood to heartwood ratio affects whole-tree water use in dry forest legume and non-legume trees

We investigated vegetation structure, seasonal water use and leaf deciduousness in a seasonally dry forest of Dzibilchaltún, Mexico. Legumes, species which tend to dominate these forests, have an array of water-saving traits. We explored whether legume species had reduced water use under similar growth conditions as other non-legume species of this seasonally dry forest. Sap flux and conductive sapwood area were measured for eight legume and 12 non-legume species. Species abundance, diameter at breast height (DBH), wood density and seasonal leaf cover were characterized in 16, 10 × 10 m² plots. Seasonal stand water use was calculated using the sap flux and ecological data. As predicted, legumes presented lower whole-tree water use compared with sympatric non-legume species. This difference, however, was related to a higher allocation to non-conductive heartwood in legumes and not to differences in sap flux density. Differences in allocation were higher in wider stems (>10 cm DBH); legumes above 25 cm DBH presented nearly half the daily water use of non-legumes of similar size. Wet (July) and dry (March) season stand water use was 629,000 and 156,000 kg ha^?1 month^?1, respectively. During the wet season three non-legume species with high basal area dominated the stand water use, but due to early leaf fall in these species, dry season stand water use was dominated by the legumes.     

马占相思林冠层气孔导度对环境驱动因子的响应

利用Granier热消散探针在2003年10月测定了广东鹤山丘陵地马占相思林14株样树的树干液流,同时监测林冠上方的光合有效辐射、空气湿度和气温,结合树木的形态和林分的结构特征,计算马占相思的整树蒸腾(E)、林分总蒸腾(E_t)以及冠层平均气孔导度(g_c),分析树形特征与整树水分利用的关系、冠层气孔导度对光合有效辐射(PAR)和空气水汽压亏缺(D)的响应.结果表明,整树蒸腾与胸径(P＜0.0001)、边材面积(P＜0.0001)和冠幅(P=0.0007)以自然对数的形式、与树高(P=0.014)以幂函数的形式呈现显著正相关.冠层气孔导度最大值(g_cmax)随D的上升呈对数函数下降(P＜0.0001),对光合有效辐射的响应则呈双曲线函数增加(P＜0.0001).液流测定系统能提供连续和准确的整树和林分蒸腾速率值,经严格数学推导公式计算,最终可求出冠层气孔导度,是研究森林水分利用与环境因子相互关系的有效方法.     

Influence of nutrients,disturbances and site conditions on carbon stocks along a boreal forest transect in central Canada

The interacting influence of disturbances and nutrient dynamics on aboveground biomass, forest floor, and mineral soil C stocks was assessed as part of the Boreal Forest Transect Case Study in central Canada. This transect covers a range of forested biomes–-from transitional grasslands (aspen parkland) in the south, through boreal forests, and into the forested subarctic woodland in the north. The dominant forest vegetation species are aspen, jack pine and spruce. Disturbances influence biomass C stocks in boreal forests by determining its age-class structure, altering nutrient dynamics, and changing the total nutrient reserves of the stand. Nitrogen is generally the limiting nutrient in these systems, and N availability determines biomass C stocks by affecting the forest dynamics (growth rates and site carrying capacity) throughout the life cycle of a forest stand. At a given site, total and available soil N are determined both by biotic factors (such as vegetation type and associated detritus pools) and abiotic factors (such as N deposition, soil texture, and drainage). Increasing clay content, lower temperatures and reduced aeration are expected to lead to reduced N mineralization and, ultimately, lower N availability and reduced forest productivity. Forest floor and mineral soil C stocks vary with changing balances between complex sets of organic carbon inputs and outputs. The changes in forest floor and mineral soil C pools at a given site, however, are strongly related to the historical changes in biomass at that site. Changes in N availability alter the processes regulating both inputs and outputs of carbon to soil stocks. N availability in turn is shaped by past disturbance history, litter fall rate, site characteristics and climatic factors. Thus, understanding the life-cycle dynamics of C and N as determined by age-class structure (disturbances) is essential for quantifying past changes in forest level C stocks and for projecting their future change.     

Up-scaling to stand transpiration of an Asian temperate mixed-deciduous forest from single tree sapflow measurements

Species diversity in mixed forest stands is one of the factors that complicate up-scaling of transpiration from individual trees to stand level, since tree species are architecturally and functionally different. In this study, thermal dissipation probes were used to measure sap flow in five different tree species in a mixed-deciduous mountain forest in South Korea. Easily measurable tree characteristics that could serve to define individual tree water use among the different species were employed to scale up transpiration from single trees to stand level. Tree water use (TWU) was derived from sap flux density (SFD) and sapwood area (SA). Canopy transpiration E was scaled from TWU while canopy conductance (g _c) was computed from E and VPD. SFD, TWU and g _c were correlated with tree diameter at breast height (DBH) for all the five measured species (SFD: R ² = 0.21, P = 0.036; TWU: R ² = 0.83, P < 0.001; g _c: R ² = 0.63, P < 0.001). Maximum stand transpiration (E) during June, before the onset of the Asian monsoon rains, was estimated at 0.97 ± 0.12 mm per day. There was a good (R ² = 0.94, P < 0.0001) agreement between measured and estimated E using the relationship between TWU and DBH. Our study shows that using functional models that employ converging traits among species could help in estimating water use in mixed forest stands. Compared to SA, DBH is a better scalar for water use of mixed forest stands since it is non-destructive and easily obtainable.