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     

Rock-eating mycorrhizas: their role in plant nutrition and biogeochemical cycles

Rates of water uptake by individual trees in a native Australian forest were measured on the Liverpool Plains, New South Wales, Australia, using sapflow sensors. These rates were up-scaled to stand transpiration rate (expressed per unit ground area) using sapwood area as the scalar, and these estimates were compared with modelled stand transpiration. A modified Jarvis-Stewart modelling approach (Jarvis 1976), previously used to calculate canopy conductance, was used to calculate stand transpiration rate. Three environmental variables, namely solar radiation, vapour pressure deficit and soil moisture content, plus leaf area index, were used to calculate stand transpiration, using measured rates of tree water use to parameterise the model. Functional forms for the model were derived by use of a weighted non-linear least squares fitting procedure. The model was able to give comparable estimates of stand transpiration to those derived from a second set of sapflow measurements. It is suggested that short-term, intensive field campaigns where sapflow, weather and soil water content variables are measured could be used to estimate annual patterns of stand transpiration using daily variation in these three environmental variables. Such a methodology will find application in the forestry, mining and water resource management industries where long-term intensive data sets are frequently unavailable.     

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.     

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.     

Canopy transpiration of mountain mixed forest as a function of environmental conditions in boundary layer

The canopy transpiration of mountain mixed forest was investigated during summer 2006 at research plot Smrekovec in Tatra National Park in Slovakia after heavy windstorm in November 2004 on the area of 12,000 hectares. The research plot is situated in untouched forest at altitude 1249 m on the southern-east oriented slope. The forest is mixed with 80% of spruce trees and 20% of larch trees with rich under storey vegetation, 120 years old, 7th altitudinal vegetation stage. Whole tree sap flow based on up to dated stem tissue heat balance method was continuously measured in nine 120-years old larch and spruce trees. Stem basal area was used for tree-canopy up-scaling. Meteorological parameters were measured on the top of eddy stuff tower above investigated forest. Two virtual monocultures were assumed for characterizing of both species. The sap flow and tree transpiration were calculated for the whole measured period for both spruce and larch virtual forests.     

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.     

Canopy transpiration in a chronosequence of Central Siberian pine forests

Tree transpiration was measured in 28, 67, 204 and 383‐y‐old uniform stands and in a multicohort stand (140–430 y) of Pinus sylvestris ssp. sibirica Lebed. in Central Siberia during August 1995. In addition transpiration of three codominant trees was monitored for two years in a 130‐y‐old stand. All stands established after fire. Leaf area index (LAI) ranged between 0.6 (28‐y‐old stand) and 1.6 for stands older than 67‐y. Stand xylem area at 1.3 m height increased from 4 cm² m^?2 (28‐y) to 11.5 cm² m^?2 (67‐y) and decreased again to 7 cm² m^?2 in old stands. Above‐ground living biomass increased from 1.5 kg dry weight m^?2 (28‐y) to 14 kg dry weight m^?2 (383‐y). Day‐to‐day variation of tree transpiration in summer was dependent on net radiation, vapour pressure deficit, and soil water stress. Tree‐to‐tree variation of xylem flux was small and increased with heterogeneity in canopy structure. Maximum rates of xylem flux density followed the course of net radiation from mid April when a constant level of maximum rates was reached until mid September when low temperatures and light strongly reduced flux density. Maximum sap flux density (60 g m^?2 s^?1) and canopy transpiration (1.5 mm d^?1) were reached in the 67‐y stand. Average canopy transpiration of all age classes was 0.72 ± 0.3 mm d^?1. Canopy transpiration (E) was not correlated with LAI but related to stand sapwood area SA (E = ? 0.02 + 1.15SA R²) which was determined by stand density and tree sapwood area.     

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.     

Time constant for water transport in loblolly pine trees estimated from time series of evaporative demand and stem sapflow

 The use of stem sap flow data to estimate diurnal whole-tree transpiration and canopy stomatal conductance depends critically upon knowledge of the time lag between transpiration and water flux through the stem. In this study, the time constant for water movement in stems of 12-year-old Pinus taeda L. individuals was estimated from analysis of time series data of stem water flux and canopy transpiration computed from mean daytime canopy conductance, and diurnal vapor pressure deficit and solar radiation measurements. Water uptake through stems was measured using a constant-heat sapflow probe. Canopy transpiration was correlated to stem uptake using a resistance-capacitance equation that incorporates a time constant parameter. A least-squares auto-regression determined the parameters of the resistance-capacitance equation. The time constants for ten loblolly pine trees averaged 48.0 (SE = 2.0) min and the time lag for the diurnal frequency averaged 47.0 (SE = 2.0) min. A direct-cross correlation analysis between canopy transpiration and sap flow time series showed maximum correlation at an approximately 30 min lag. Residuals (model-predicted minus actual stem flow data) increased with increasing soil moisture depletion. While the time constants did not vary significantly within the range of tree sizes studied, hydraulic resistance and capacitance terms were individually dependent on stem cross-sectional area: capacitance increased and resistance decreased with stem volume. This result may indicate an inverse adjustment of resistance and capacitance to maintain a similar time constant over the range of tree sizes studied.     

不同树高对荷木液流密度、整树蒸腾量和时滞的效应

人工林面积不断增大,这不仅能解决由于森林砍伐引起的一系列社会问题,而且还对解决水土保持、二氧化碳减排等环境问题起到重要作用。了解人工林的生长特性和蒸腾效率,对植被生长、恢复和管理有着重要意义。为此,该研究连续监测了华南地区12棵不同高度荷木人工林的液流密度,对样树以高度划分等级,采取错位相关法分析不同高度等级胸高处液流与冠层蒸腾的时滞效应。结果表明:气候环境相同时,所有样树胸高处液流日格型相似;荷木林蒸腾量优势木中间木劣势木,所有树木湿季月蒸腾量大于干季月蒸腾量;不同高度等级之间时滞差异显著,劣势木时滞50min,优势木和中间木时滞20min;所有样树干湿季时滞差异不显著,同一高度级两季节时滞差少于10min。这些说明:在干季华南地区土壤水分仍然相对较充足,植物输水阻力没有受到土壤水分降低和长距离水分传导的影响;中间木和优势木时滞短,水力阻力小,蒸腾量大并占据着林段的有利资源;劣势木长势低矮,时滞长,导管阻力大,蒸腾量少,光合作用需要的水热资源少,所以回馈根部的营养物质少,不均衡的营养循环使得林段分化愈明显,劣势木将逐渐从林段中被淘汰。该文指出在荷木人工林生长后期,对于长势低矮,生命力极弱的劣势木应定期砍伐,这样能增加优势木和中间木对光照及水分等有利资源的分配,提高林分质量,增加林地生产力。     

Transpiration of Pinus rotundata on a wooded peat bog in central Europe

Transpiration of a central European endemic tree species, Pinus rotundata Link, growing on a wooded peat bog in the Třeboň Basin, Czech Republic, was studied in 1999–2000. Transpiration was measured by sap flow techniques (heat field deformation method) on individual trees and scaled up to stand level. The radial patterns of sap flow density showed narrow peaks in the outer part of the xylem, sapwood accounted for 47–60% of the xylem radius and 72–84% of the xylem basal area. Adult trees tolerated well both short-term flooding during the growing season and drawdown of the water table to a depth of 60 cm below ground level. The maximum and mean daily transpiration rates were 3.0 and 1.8 mm per day, and were thus similar to published data for Scots pine. The seasonal total transpiration (25 April–20 October 2000, 180 days) amounted to 322 mm, or 62% of the potential evapotranspiration over this period. This canopy transpiration was compensated by 319 mm of precipitation. The difference between the accumulated precipitation and the accumulated transpiration (derived from seasonal sap flow measurements) closely mimicked the seasonal course of the water table.     

Sap flux of five co-occurring tree species in a temperate broad-leaved forest during seasonal soil drought

In an old-growth forest in Central Germany, sap flux was studied in five broad-leaved tree species that were assumed to differ in drought sensitivity. Under moist soil conditions, average daily sap flux density (J _s) in the outermost xylem varied by a factor of 2.3 among the species (67–152 g cm⁻² per day, n=5 trees per species), and declined in the sequence Fagus sylvatica > Acer pseudoplatanus > Tilia cordata > Carpinus betulus > Fraxinus excelsior. Decreasing soil moisture content (Θ) resulted in linearly reduced J _s in four of the species. During a dry period, J _s was reduced by 44% in T. cordata, 39% in F. sylvatica, 37% in A. pseudoplatanus and 31% in C. betulus compared to sap flux at equal vapour pressure deficit (D) in the wet period. F. excelsior, the only ring-porous species studied, lacked a significant response in J _s to D and Θ. The relative reduction in water use during the dry period was not related to the assumed drought sensitivity of the species as inferred from their abundance in natural woodlands. J _s was positively correlated with tree diameter at breast height (DBH) in three species but decreased with DBH in two species. Dyeing experiments revealed that DBH accounted for 94% of the variation in sapwood area found in a bulk sample of all diffuse-porous trees. This suggests that DBH is a reliable estimator of sapwood area of temperate diffuse-porous species irrespective of species identity. In contrast, sap flux density was found to be greatly dependent on tree species. The estimated whole-plant water use for diffuse-porous trees of a given diameter (49 cm) ranged between 74 and 168 kg per day per species under moist soil conditions. Thus, in temperate mixed forests, species-specific differences in water use can result in a considerable spatial heterogeneity of canopy transpiration.     

Effects of stand age and tree species on canopy transpiration and average stomatal conductance of boreal forests

We quantified the effect of stand age and tree species composition on canopy transpiration (E_C) by analysing transpiration per unit leaf area (E_L) and canopy stomatal conductance (G_S) for boreal trees comprising a five stand wildfire chronosequence. A total of 196 sap flux sensors were used on 90 trees consisting of Betula papyrifera Marsh (paper birch; present in the youngest stand), Populus tremuloides Michx (quaking aspen), Pinus banksiana Lamb. (jack pine), and Picea mariana (Mill.) (black spruce). While fine roots were positively correlated with stand E_C; leaf area index, basal area, and sapwood area were not. Stands less than 70 years old were dominated by Populus tremuloides and Pinus banksiana and stands greater than 70 years old were composed almost entirely of Picea mariana. As Populus tremuloides and Pinus banksiana increased in size and age, they displayed an increasing sapwood to leaf area ratio (A_S : A_L), a constant minimum leaf water potential (Ψ_L), and a constant proportionality between G_S at low vapour pressure deficit (Dj G_Sref) and the sensitivity of G_S to D (–δ). In contrast, A_S : A_L, minimum Ψ_L, and the proportionally between –δ and G_Sref decreased with height and age in Picea mariana. A G_S model that included the effects of D, A_S : A_L, tree height, and for Picea mariana an increasing soil to leaf water potential gradient with stand age, was able to capture the effects of contrasting hydraulic properties of Picea mariana, Populus tremuloides and Pinus banksiana during stand development after wildfire.     

Regulation of water flux through trunks, branches, and leaves in trees of a lowland tropical forest

We studied regulation of whole-tree water use in individuals of five diverse canopy tree species growing in a Panamanian seasonal forest. A construction crane equipped with a gondola was used to access the upper crowns and points along the branches and trunks of the study trees for making concurrent measurements of sap flow at the whole-tree and branch levels, and vapor phase conductances and water status at the leaf level. These measurements were integrated to assess physiological regulation of water use from the whole-tree to the single-leaf scale. Whole-tree water use ranged from 379 kg day⁻¹ in a 35 m-tall Anacardium excelsum tree to 46 kg day⁻¹ in an 18 m-tall Cecropia longipes tree. The dependence of whole-tree and branch sap velocity and sap flow on sapwood area was essentially identical in the five trees studied. However, large differences in transpiration per unit leaf area (E) among individuals and among branches on the same individual were observed. These differences were substantially reduced when E was normalized by the corresponding branch leaf area:sapwood area ratio (LA/SA). Variation in stomatal conductance (g _s) and crown conductance (g _c), a total vapor phase conductance that includes stomatal and boundary layer components, was closely associated with variation in the leaf area-specific total hydraulic conductance of the soil/leaf pathway (G _t). Vapor phase conductance in all five trees responded similarly to variation in G _t. Large diurnal variations in G _t were associated with diurnal variation in exchange of water between the transpiration stream and internal stem storage compartments. Differences in stomatal regulation of transpiration on a leaf area basis appeared to be governed largely by tree size and hydraulic architectural features rather than physiological differences in the responsiveness of stomata. We suggest that reliance on measurements gathered at a single scale or inadequate range of scale may result in misleading conclusions concerning physiological differences in regulation of transpiration. Received: 1 October 1997 / Accepted: 6 March 1998     

A comparison of sap flux and water relations of leaves of various isolated trees with special reference to foundation movement in clay soil

Diurnal variation in sap flux (S) through stems of six trees, two each of Ulmus procera SALISB., Melaleuca styphelioides SM. and Prunus cerasifera EHRH. ‘Nigra’ (referred to hereafter by their generic names), were estimated from measurements of heat pulse velocities. Leaf water potential (ψ), stomatal conductance (g _s ) and transpiration from leaves (T) of all replicate trees were measured at 1300–1500h, once during the summer. On two separate occasions measurements were made of S, ψ, (g _s ) and T for one each of Ulmus and Melaleuca trees to study diurnal variations in these parameters. A 12×12 m² area around each tree was kept covered to simulate the condition of trees growing on pavements adjacent to residential properties. Sap flux for these tree species was in the order Melaleuca>Ulmus>Prunus. It is suggested that the smaller canopy and sapwood area in Prunus compared to the other two species is responsible for lower water potential and lower transpiration rate than the other species. Detailed analysis of the diurnal variation in sap flux and water relation of leaves of Melaleuca and Ulmus indicated sap flux of Melaleuca to be greater than that of Ulmus at the same transpiration rate per unit leaf area although the sapwood area of the two species was marginally different. This may have been due either to the difference in canopy conductance or in leaf area between the two species. With the assumption that sap flux closely resembles the rate of soil water extraction for both species, results indicate that Melaleuca is likely to extract soil water at a higher rate than Ulmus and hence is capable of causing greater shrinkage and soil movement than Ulmus.     

Estimating transpiration from 6-year-old Eucalyptus grandis trees: development of a canopy conductance model and comparison with independent sap flux measurements

Daily patterns of stomatal conductance (g_s), xylem pressure potential (P) and canopy microclimatic variables were recorded on 11 sample days as part of a one-year study of the water use of Eucalyptus grandis Hill ex Maiden in the eastern Transvaal, South Africa. Measured g_s was found to be largely controlled by quantum flux density (Q) and ambient vapour pressure deficit (D). Canopy conductance (g_c) was determined for hourly intervals using g_s measurements and leaf areas in four different canopy levels. A simple model was constructed to allow the prediction of g_c and transpiration from Q, D and season of year. The model was used to estimate transpiration rates from 10 trees in a later study of similarly-aged E. grandis trees, in which sap flow in each tree was measured using the heat pulse velocity (HPV) technique. Five of the trees were monitored on a summer day and five on a winter day. Correspondence between HPV sap flow and modelled transpiration was good for the summertime comparisons, but measured winter-time sap flow rates were underestimated by the model, especially under conditions of high sap flow. The discrepancy is believed to result from having insufficient data from the conductance study to describe the response of g_s to relatively high D in winter. Marked variation in transpiration per unit leaf area indicates that a relatively large number of trees must be sampled for the HPV technique to be used to obtain a mean rate for an entire stand in winter.     

Succession May Maintain High Leaf Area: Sapwood Ratios and Productivity in OldSubalpine Forests

Old forests are generally believed to exhibit low net primary productivity (NPP) and therefore to be insignificant carbon sinks. This relationship between age and NPP is based, in part, on the hypothesis that the biomass of respiratory tissues such as sapwood increases with age to a point where all photosynthate is required just to maintain existing tissue. However, this theoretical connection between respiration:assimilation ratios and forest productivity is based on age-dependent trends in the sapwood:leaf ratios of individual trees and even-aged stands; it does not take into account such processes in natural forests as disproportional increases in shade-tolerant species over time and multiple-age cohorts. Ignoring succession and structural complexity may lead to large underestimates of the productivity of old forests and inaccurate estimates of the ages at which forest productivity declines. To address this problem, we compared biomass allocation and productivity between whitebark pine, a shade-intolerant, early-successional tree species, and subalpine fir, a shade-tolerant, late-successional species, by harvesting 14 whitebark pines and nine subalpine firs that varied widely in dbh and calculating regression models for dbh vs annual productivity and biomass allocation to leaves, sapwood, and heartwood. Late-successional subalpine fir allocated almost twice as much biomass to leaves as early-successional whitebark pine. Subalpine firs also had a much lower allocation to sapwood and higher growth rates across all tree sizes. We then modeled biomass allocation and productivity for 12 natural stands in western Montana that were dominated by subalpine fir and whitebark pine varying in age from 67 to 458 years by applying the regressions to all trees in each stand. Whole-stand sapwood:leaf ratios and stand productivity increased asymptotically with age. Sapwood:leaf ratios and productivity of whitebark pine in these stands increased for approximately 200–300 years and then decreased slowly over the next 200 years. In contrast, sapwood:leaf ratios of all sizes of subalpine fir were lower than those of pine and productivity was higher. As stands shifted in dominance from pine to fir with age, subalpine fir appeared to maintain gradually increasing rates of whole-forest productivity until stands were approximately 400 years old. These results suggest that forests such as these may continue to sequester carbon for centuries. If shade-tolerant species that predominate late in succession maintain high assimilation-to-respiration ratios in other forests, we may be underestimating production in old forests, and current models may underestimate the importance of mature forests as carbon sinks for atmospheric CO₂ in the global carbon cycle. Received 16 February 1999; accepted 24 November 1999.     

宁夏罗山自然保护区3种典型林分凋落物和土壤层水源涵养能力综合评估

森林凋落物层和土壤层是森林生态水文效应中的主要贡献层，对森林生态系统水土保持功能和水源涵养能力有重要影响。对比宁夏罗山自然保护区3种典型林分类型凋落物和土壤层水文效应的变化规律和水源涵养能力大小，为该地区的森林生态水文、水土保持和森林管理提供科学依据。以该自然保护区青海云杉纯林、油松纯林和青海云杉油松混交林为研究对象，运用称量、室内浸泡、环刀法和回归分析法对凋落物和土壤层的水文效应进行测定和拟合，并使用熵权法对二者的水源涵养能力进行评估。结果表明：（1）青海云杉油松混交林凋落物的总厚度和总储量显著高于青海云杉纯林和油松纯林（P<0.05），3种林分类型的半分解层的厚度和储量高于未分解层。（2）凋落物层最大持水量范围为63.29-95.08t/hm²，最大持水率范围为335.97%-353.85%，有效拦蓄量范围为34.09-63.92t/hm²，三者均为青海云杉油松混交林>云杉纯林>油松纯林。（3）3种林分类型的凋落物持水量（Q）与浸泡时间（t）呈对数函数关系，吸水速率（V）与浸泡时间（t）呈幂函数关系。（4）从3种林分类型的土壤物理性质和持水特性得出，3种林分类型的土壤层水文效应的等级排序为云杉油松混交林>油松纯林>青海云杉纯林。（5）凋落物层和土壤层的水源涵养能力大小为青海云杉油松混交林（0.43）>油松林（0.3）>青海云杉林（0.27）。综合来看，青海云杉油松混交林的凋落物层和土壤层的水源涵养能力最优，其次是纯林，说明混交林在水土保持和水源涵养方面比纯林更具优势。     

Stability in the patterns of long‐term development and growth of the Canadian spruce–moss forest

Aim The spruce–moss forest is the main forest ecosystem of the North American boreal forest. We used stand structure and fire data to examine the long‐term development and growth of the spruce–moss ecosystem. We evaluate the stability of the forest with time and the conditions needed for the continuing regeneration, growth and re‐establishment of black spruce (Picea mariana) trees. Location The study area occurs in Québec, Canada, and extends from 70°00′ to 72°00′ W and 47°30′ to 56°00′ N. Methods A spatial inventory of spruce–moss forest stands was performed along 34 transects. Nineteen spruce–moss forests were selected. A 500 m² quadrat at each site was used for radiocarbon and tree‐ring dating of time since last fire (TSLF). Size structure and tree regeneration in each stand were described based on diameter distribution of the dominant and co‐dominant tree species [black spruce and balsam fir (Abies balsamea)]. Results The TSLF of the studied forests ranges from 118 to 4870 cal. yr bp . Forests < 325 cal. yr bp are dominated by trees of the first post‐fire cohort and are not yet at equilibrium, whereas older forests show a reverse‐J diameter distribution typical of mature, old‐growth stands. The younger forests display faster height and radial growth‐rate patterns than the older forests, due to factors associated with long‐term forest development. Each of the stands examined established after severe fires that consumed all the soil organic material. Main conclusions Spruce–moss forests are able to self‐regenerate after fires that consume the organic layer, thus allowing seed regeneration at the soil surface. In the absence of fire the forests can remain in an equilibrium state. Once the forests mature, tree productivity eventually levels off and becomes stable. Further proof of the enduring stability of these forests, in between fire periods, lies in the ages of the stands. Stands with a TSLF of 325–4870 cal. yr bp all exhibited the same stand structure, tree growth rates and species characteristics. In the absence of fire, the spruce–moss forests are able to maintain themselves for thousands of years with no apparent degradation or change in forest type.     

Transpirational Water Loss in Invaded and Restored Semiarid Riparian Forests

The invasive tree, Tamarix sp., was introduced to the United States in the 1800s to stabilize stream banks. The riparian ecosystem adjacent to the middle Rio Grande River in central New Mexico consists of mature cottonwood (Populus fremontii) gallery forests with a dense Tamarix understory. We hypothesized that Populus would compensate for reduced competition by increasing its water consumption in restored riparian plots following selective Tamarix removal, resulting in similar transpiration (T) among stands. The northern study site included a Populus stand invaded by Tamarix (INV_N) and a restored Populus‐only stand (RES_N), as did a southern site (INV_S and RES_S) approximately 80 miles south. At each site, 20 × 20–m plots were established where up to 16 stems were monitored throughout the 2004 growing season using thermal dissipation sapflow sensors. Populus sapflux rates were greater in restored stands, suggesting those trees compensated for understory removal by using more water. Sapflow was scaled to estimate stand‐level T based on a quantitative assessment of sapwood basal area (A_sw) by species. Although exotic species represented 85 and 91% of the total stems in the invaded stands, it amounted to only 3% (INV_S) and 4% (INV_N) of the total A_sw, contributing proportionately less to T compared to Populus. Our results indicate that removing Tamarix from the Populus understory in this riparian forest had a minimal impact on stand water balance. Riparian restoration of the type discussed herein should focus primarily on enhancing riparian health rather than generating water.