Effects of drought and rewatering on growth and transpiration in European beech seedlings late in the growing season

Drought stress is one of the most important environmental factors affecting plant growth and survival. To date, most studies aim at understanding of post-stress physiological and anatomical adaptation to drought stress; however only few studies focus on plant recovery. In the present study, transpiration, shoot water potential, and anatomical and morphological measurements were performed on 4-year-old European beech seedlings with fully developed leaves. The seedlings were exposed to three levels of soil water potential (well-watered, moderate drought stress and severe drought stress) and followed by rewatering under greenhouse conditions. Reduced transpiration rates were observed in the stressed seedlings as a response to drought stress, whereas anatomical and morphological variables remained unchanged. Three days after rewatering, transpiration rates in both moderately and severely stressed seedlings recovered to the levels of those of well-watered seedlings. Drought stress promoted leaf budding, resulting in higher shoot dry mass of stressed seedlings. Our findings indicate that anatomical and morphological adaptations of European beech seedlings to drought stress are visibly limited during late-season growth stages. These results will help us to further understand factors involved in drought adaptation potential of European beech seedlings faced with expected climate-related environmental changes. To complete our findings, further experiments on plant recovery from drought stress should be focused on different periods of growing season.     

Adaptive significance of intermittent shoot growth in Douglas-fir seedlings

We tested the hypothesis that intermittent (lammas) shoot growth in Douglas-fir (Pseudotsuga menziesii var menziesii (Mirb.) Franco) seedlings from dry regions of southwest Oregon is adaptively significant. Seedlings from open-pollinated families (160 total) from two inland (dry) and two coastal (wet) sources were grown under either well-watered or intermittent drought conditions (temporary drought followed by rewatering) for two growing seasons. In the first growing season, the results supported the hypothesis: the frequency of a second flush was genetically controlled (although weakly, h(f) (2) 相似文献   

Morphological and physiological responses of beech (Fagus sylvatica) seedlings to grass-induced below ground competition

We examined morphological and physiological responses of beech (Fagus sylvatica L.) seedlings to grass-induced below ground competition in full-light conditions. Two-year-old beech seedlings were grown during two growing seasons in 160-l containers in bare soil or with a mixture of five grass species widely represented in semi-natural meadows of central France. At the end of the second growing season, beech seedlings in the presence of grass showed significant reductions in diameter and height growth, annual shoot elongation, and stem, root and leaf biomass, but an increase in root to shoot biomass ratio. Grasses greatly reduced soil water availability, which was positively correlated with daily seedling diameter increment. Beech seedlings seemed to respond to water deficit by anticipating stomatal closure. There was evidence of competition for nitrogen (N) by grasses, but its effect on seedling development could not be separated from that of competition for water. By labeling the plants with 15N, we showed that beech seedlings absorbed little N when grasses were present, whereas grasses took up more than 97% of the total N absorbed in the container. We conclude that, even if beech seedlings display morphological and physiological adaptation to below ground competition, their development in full-light conditions may be strongly restricted by competition from grass species.     

Drought tolerance of clonal Malus determined from measurements of stomatal conductance and leaf water potential

We examined tolerance to soil drying in clonally propagated apple (Malus domestica Borkh.) rootstocks used to control shoot growth of grafted scions. We measured leaf conductance to water vapor (g(L)) and leaf water potential (Psi(L)) in a range of potted, greenhouse-grown rootstocks (M9, M26, M27, MM111, AR69-7, AR295-6, AR360-19, AR486-1 and AR628-2) as the water supply was gradually reduced. Irrespective of the amount of available water, rootstocks that promoted scion shoot growth (M26 and MM111) generally had higher g(L) and more negative Psi(L) than rootstocks that restricted scion shoot growth (M27 and M9). After about 37 days of reduced water supply, there were significant decreases in g(L) and Psi(L) in all rootstocks compared with well-watered controls. In all treatments, the slope of the relationship between log (g(L)) and Psi(L) was positive, except for rootstocks AR295-6, AR628-2 and AR486-1 in the severe-drought treatment, where the drought-induced change in the relationship suggests that rapid stomatal closure occurred when leaf water potentials fell below -2.0 MPa. This drought response was associated with increased root biomass production. Rootstock M26 showed little stomatal closure even when its water potential fell below -2.0 MPa, and there was no effect of drought on root biomass production. We conclude that differences among rootstocks in the way that g(L) and Psi(L) respond to drought reflect differences in the mechanisms whereby they tolerate soil drying. We suggest that these differences are related to differences among the rootstocks in their ability to control shoot growth.     

Growth of European beech (Fagus sylvatica L.) saplings is limited by elevated atmospheric vapour pressure deficits

There is an ongoing debate about how European beech might be affected by a future drier climate. While numerous studies have examined the effects of soil drought on beech growth and development, studies investigating the effects of elevated atmospheric water vapour pressure deficit (VPD) are lacking. By increasing VPD in climate and open-top chamber experiments, with moisture in the rooting medium near optimum, we tested the hypothesis that increased VPD negatively affects the growth and development of European beech saplings. In the climate chambers, a reduction in relative air humidity by 40% resulted in a 68% reduction in productivity. Similarly, in the open-top chamber experiment conducted on the forest floor, biomass declined by 30% when relative air humidity was 15% lower. The reduction in biomass was mainly a consequence of a dramatically reduced leaf growth of beech in the elevated VPD treatments. Our results show that growth and development of beech saplings strongly depend not only on soil moisture but also on the prevailing VPD level. We conclude that the vapour pressure deficit is a widely ignored factor which influences the growth and vitality, and possibly also the distribution of European beech. Future forest management schemes under an altered climate should take this factor into account.     

Drought resistance of Ailanthus altissima: root hydraulics and water relations

Drought resistance of Ailanthus altissima (Mill.) Swingle is a major factor underlying the impressively wide expansion of this species in Europe and North America. We studied the specific mechanism used by A. altissima to withstand drought by subjecting potted seedlings to four irrigation regimes. At the end of the 13-week treatment period, soil water potential was -0.05 MPa for well-watered control seedlings (W) and -0.4, -0.8 and -1.7 MPa for drought-stressed seedlings (S) in irrigation regimes S1, S2 and S3, respectively. Root and shoot biomass production did not differ significantly among the four groups. A progressively marked stomatal closure was observed in drought-stressed seedlings, leading to homeostasis of leaf water potential, which was maintained well above the turgor loss point. Root and shoot hydraulics were measured with a high-pressure flow meter. When scaled by leaf surface area, shoot hydraulic conductance did not differ among the treated seedlings, whereas root hydraulic conductance decreased by about 20% in S1 and S2 seedlings and by about 70% in S3 seedlings, with respect to the well-watered control value. Similar differences were observed when root hydraulic conductance was scaled by root surface area, suggesting that roots had become less permeable to water. Anatomical observations of root cross sections revealed that S3 seedlings had shrunken cortical cells and a multilayer endodermal-like tissue that probably impaired soil-to-root stele water transport. We conclude that A. altissima seedlings are able to withstand drought by employing a highly effective water-saving mechanism that involves reduced water loss by leaves and reduced root hydraulic conductance. This water-saving mechanism helps explain how A. altissima successfully competes with native vegetation.     

Effects of experimental drought on the fine root system of mature Norway spruce

Norway spruce (Picea abies (L.) Karst.) is an economically important, but relatively drought-sensitive tree species that might suffer from increasing drought intensities and frequencies, which are predicted to occur in parts of central Europe under future climatic change. In a throughfall exclusion experiment using sub-canopy roofs, we tested the hypothesis that enhanced drought leads to an increase in fine root mortality, and also to a higher, subsequent fine root growth. Fine root production and mortality were assessed using two independent approaches, sequential soil coring (organic layer) and direct observations in minirhizotrons (organic layer plus upper mineral soil). Six weeks of throughfall exclusion resulted in mild drought stress, i.e. a reduction in average soil moisture from 20 to 12 vol.% during the treatment. Based on the sequential coring data, experimental drought did not result in significant changes in fine root biomass during the 6-week treatment period, but caused an increase in fine root mortality by 61% in the 6 weeks following the drought treatment. Remarkably, fine root production showed a synchronous increase in this period, which more than compensated for the loss due to increased mortality. The minirhizotron data confirmed that the drought treatment increased fine root loss in the organic layer. Based on this method, however, root loss occurred during the drought period and was not compensated by increased root production. The mild drought stress was mainly restricted to the organic layer and did not significantly influence fine root dynamics in the mineral soil. We calculated that the drought event resulted in an extra input of about 28 g C m⁻² and 1.1 g N m⁻² to the soil due to increased fine root mortality. We conclude that even periods of mild drought significantly increase fine root mortality and the associated input of root-derived C to the soil organic matter pool in temperate Norway spruce forests.     

Variability in growth, carbon isotope composition, leaf gas exchange and hydraulic traits in the eastern Mediterranean cedars Cedrus libani and C. brevifolia

Four Turkish provenances and five Lebanese provenances of Cedrus libani A. Rich. and one Cypriot provenance of C. brevifolia Henry were compared during the third year of growth in a controlled-climate greenhouse after exposure to a well-watered or moderate-drought treatment. Effects of treatment on CO(2) assimilation (A), stomatal conductance (g(s)), (13)C isotope composition (delta(13)C), growth and biomass were assessed. Hydraulic conductivity and shoot vulnerability to cavitation were measured in well-watered plants only. The Lebanese provenances of C. libani had the highest growth rates, but were the most sensitive to drought. The Turkish provenances of C. libani showed moderate growth rates and moderate drought sensitivity. Cedrus brevifolia had the lowest growth rate and was least sensitive to drought. For each provenance, mean biomass values were positively correlated with delta(13)C and intrinsic water-use efficiency (A/g(s)), and negatively correlated with g(s). Drought reduced growth and favored carbon storage in roots, increasing the ratio of root biomass to aboveground biomass. The drought treatment increased delta(13)C and A/g(s). Specific hydraulic conductivity (K(s)) was similar for the provenance groups, whereas leaf-specific conductivity (K(l)) was lower in the Lebanese provenances than in the other provenances. Within each provenance group, provenances with the highest K(l) were most susceptible to xylem cavitation, but were also the most productive. Growth and drought adaptation were linked with precipitation in each provenance's native range.     

Effects of phenology,water availability and seed source on loblolly pine biomass partitioning and transpiration

One-year-old loblolly pine (Pinus taeda L.) seedlings from four seed sources (Arkansas, Georgia, Texas and Virginia) grown in 1-m-deep sand-filled pits in two water regimes (well-watered and drought) were studied, to gain insight into the process of seedling establishment. Whole-plant transpiration was measured biweekly from July to December. Whole-plant harvests were conducted at 6-week intervals from April to December. Whole-plant transpiration and transpiration per unit leaf and root area were affected by treatment, seedlot and phenology. Seedlings of the Arkansas seedlot maintained significantly higher transpiration rates per unit leaf and root area during drought than seedlings of the Virginia, Georgia or Texas seedlots, but did not accumulate greater biomass. The high transpiration rates of the Arkansas seedlings were attributed to their deep root systems. Allometric relationships indicated that, relative to the whole plant, biomass allocation to needles of drought-treated seedlings was enhanced during the summer (allometric ratio 1.09), whereas allocation to roots was enhanced in the spring and fall (allometric ratios of 1.13 and 1.09, respectively). Relative to the whole plant, biomass allocation to needles of well-watered seedlings was enhanced throughout the experiment (allometric ratio of 1.16 declining to 1.05), whereas the allometric ratio of root to total biomass was 0.89 or less throughout. Allometric relationships also indicated variation in biomass partitioning to roots in three soil layers (0-30, 30-60 and 60-100 cm), which differed among harvests in each soil layer. Root growth in both well-watered and drought-treated seedlings was concentrated in the top soil layer in the spring, shifted to the middle and bottom soil layers in the summer, and then increased in the top soil layer in the fall. Compared with well-watered seedlings, drought-treated seedlings had higher rates of root growth in the bottom soil layer in the fall, a characteristic that would confer tolerance to future periods of limited soil water availability.     

Are marginal beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) provenances a source for drought tolerant ecotypes?

Due to the expected increases of number and intensity of summer droughts in Central Europe the identification of drought tolerant ecotypes becomes more important in future forestry. A common garden experiment with seedlings of Fagus sylvatica provenances from the center (Germany) and eastern margin (Poland) of the species’ distribution range was conducted. Responses of morphological, physiological, chemical and growth parameters to three drought treatments were analyzed. Relative growth rates of the marginal provenance were lower as compared to the central provenance. The marginal seedlings showed a tendency to higher total biomasses because of higher seed masses. In both provenances drought decreased biomass production and root/shoot ratio which was lower in the central provenance. A lower specific root area of the marginal provenance indicated a better adaptation to low xylem water potentials. Under moderate drought, lower leaf δ¹³C signatures may indicate lower stomatal limitation (or a reduced rate of CO₂ assimilation) in the marginal provenance. We conclude that marginal beech provenances may exhibit a better drought adaptation.     

Effects of root trenching of overstorey Norway spruce (Piceaabies) on growth and biomass of underplanted beech (Fagussylvatica) and Douglas fir (Pseudotsugamenziesii) saplings

In Central Europe, the conversion of pure Norway spruce stands (Picea abies [L.] Karst.) into mixed stands with beech (Fagus silvatica L.) and other species like e.g. Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) is accomplished mainly by underplanting of seedlings beneath the canopy of overstorey spruce trees after partial cutting treatments what means exposure to shade and below-ground root competition by the overstorey to the seedlings. Particularly about the second factor, our knowledge is limited. Therefore, we carried out a below-ground competition exclusion experiment by root trenching and investigated the effects on soil resources, growth, and biomass partitioning of underplanted beech and Douglas fir saplings under target diameter and strip cutting treatments. The exclusion of overstorey root competition by trenching increased the soil water potential in the second year that had a fairly dry growing season and led to significantly higher foliar concentrations of most nutrients, particularly in Douglas fir, indicating an amended nutrient supply. Both improvements were accompanied by an increase in length and diameter increment of the underplanted saplings, appearing in both species only after having surpassed a species-specific threshold light value (Douglas fir 16% of above canopy radiation, beech 22%). We also found significant interactions between trenching and light for specific fine root length and further biomass and morphological parameters. Judged by the much steeper increase in height and diameter growth with increasing light after release from below-ground competition, Douglas fir saplings appeared to be more sensitive to root competition than beech saplings what conforms to older findings for beech. According to our results, a strip cutting seems to be more appropriate than a target diameter cutting treatment to replace a pure spruce stand by a mixed stand with beech and Douglas fir.     

Response of cocoa trees (<Emphasis Type="Italic">Theobroma cacao</Emphasis>) to a 13-month desiccation period in Sulawesi,Indonesia

In South-east Asia, ENSO-related droughts represent irregularly occurring hazards for agroforestry systems containing cocoa which are predicted to increase in severity with expected climate warming. To characterize the drought response of mature cocoa trees, we conducted the Sulawesi Throughfall Displacement Experiment in a shaded (Gliricidia sepium) cocoa agroforestry system in Central Sulawesi, Indonesia. Three large sub-canopy roofs were installed to reduce throughfall by about 80% over a 13-month period to test the hypotheses that (i) cocoa trees are sensitive to drought due to their shallow fine root system, and (ii) bean yield is more sensitive to drought than leaf or stem growth. As 83% of fine root (diameter <2 mm) and 86% of coarse root biomass (>2 mm) was located in the upper 40 cm of the soil, the cocoa trees examined had a very shallow root system. Cocoa and Gliricidia differed in their vertical rooting patterns, thereby reducing competition for water. Despite being exposed for several months to soil water contents close to the conventional wilting point, cocoa trees showed no significant decreases in leaf biomass, stem and branch wood production or fine root biomass. Possible causes are active osmotic adjustment in roots, mitigation of drought stress by shading from Gliricidia or other factors. By contrast, production of cocoa beans was significantly reduced in the roof plots, supporting reports of substantial reductions in bean yields during ENSO-related drought events in the region. We conclude that cocoa possesses traits related to drought tolerance which enable it to maintain biomass production during extended dry periods, whereas bean yield appears to be particularly drought sensitive.     

Stomatal conductance,growth and root signaling in Betula pendula seedlings subjected to partial soil drying

Seedlings of Betula pendula Roth were grown with their root systems separated between two soil compartments. Four treatments were imposed: (i) adequate irrigation in both compartments (WW, controls); (ii) adequate irrigation in one compartment and drought in the other compartment (WD); (iii) drought in both compartments (DD); and (iv) half of the root system severed and the remainder kept well-watered (root excision, RE). Predawn leaf water potential, stomatal conductance, soil-to-leaf specific hydraulic conductance, and root and leaf growth decreased in DD-treated seedlings, which also displayed severe leaf shedding (30% loss in leaf area). The DD treatment also resulted in increased concentrations of abscisic acid (ABA) and its glucose ester in the xylem sap of roots and shoots compared to concentrations in control seedlings (about 200 versus 20 nM). Despite the difference in xylem sap concentrations, total ABA flux to the shoots was similar in the two treatments (1-2 pmol ABA m(-2) leaf area s(-1)) as a result of reduced transpiration in the DD-treated seedlings. Compared with root growth in control plants, root growth increased in the RE-treated plants and decreased in the drying compartment of the WD treatment; however, the RE and WD treatments only slightly reduced leaf expansion, and had no detectable effects on shoot water relations or ABA concentrations of the root and shoot xylem sap. We conclude that short-term soil water depletion affecting only 50% of the root system does not cause a measurable stress response in birch shoots, despite root growth cessation in the fraction of drying soil.     

Seasonal dynamics of soil carbon dioxide efflux and simulated rhizosphere respiration in a beech forest

Respiration of the rhizosphere in a beech (Fagus sylvatica L.) forest was calculated by subtracting microbial respiration associated with organic matter decomposition from daily mean soil CO2 efflux. We used a semi-mechanistic soil organic matter model to simulate microbial respiration, which was validated against "no roots" data from trenched subplots. Rhizosphere respiration exhibited pronounced seasonal variation from 0.2 g C m(-2) day(-1) in January to 2.3 g C m(-2) day(-1) in July. Rhizosphere respiration accounted for 30 to 60% of total soil CO2 efflux, with an annual mean of 52%. The high Q10 (3.9) for in situ rhizosphere respiration was ascribed to the confounding effects of temperature and changes in root biomass and root and shoot activities. When data were normalized to the same soil temperature based on a physiologically relevant Q10 value of 2.2, the lowest values of temperature-normalized rhizosphere respiration were observed from January to March, whereas the highest value was observed in early July when fine root growth is thought to be maximal.     

油松蒙古栎混交林细根生物量及养分现存量研究

应用固定样地调查生物量的方法,在辽宁省建平县干旱所南大山试验区进行了油松蒙古栎混交林细根生物量和养分现存量的研究。结果表明,混交林的细根生物量大于油松纯林,两树种混栽后蒙古栎能促进油松的细根生长,提高油松细根生物量在混交林中的比例;混交林细根在各土层中分布得比纯林更均匀、更合理,两树种根系生长协调;混交林细根N、P养分现存量分别是纯林的1.1倍和1.2倍,可见混交林细根具有比纯林更高的养分积累和周转能力;纯林和混交林人工林的各径级活细根和死细根养分现存量都随细根径级的增加而减少,说明≤2 mm径级的细根在细根的养分循环中起主导作用。     

Root distribution of under-planted European beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) below the canopy of a mature Norway spruce stand as a function of light

Aboveground and belowground biomass of 15-year-old under-planted European beech seedlings (Fagus sylvatica L.) in Norway spruce stand were studied along a light gradient in three plots, in the northern part of Slovenia. Differences in soil water content, aboveground and fine root biomass distribution were confirmed between studied plots. Light had significant effect on the total biomass, root-shoot ratio (0.388 ± 0.076 under canopy, 0.549 ± 0.042 in the edge, 0.656 ± 0.047 in the open), specific root length (SRL) of fine beech roots (561.9 ± 42.2 under canopy, 664.3 ± 51.2 in the edge, 618.2 ± 72.8 in the open) and specific leaf area in beech, indicating morphological adjustment to shade. However, SRL of beech fine roots indicated no change between plots. The correlation between total aboveground and root biomass and light below the mature stand canopy was higher in the case of diffuse light intensity. Most fine roots of spruce were concentrated in the top (0–20 cm) soil layer. Beech fine roots under canopy and edge conditions were also concentrated in top (0–20 cm) soil layer and exhibited shift downwards to deeper soil horizons in open plot. Root proportion between beech and spruce changed with light toward beech with increasing light intensity for both fine and coarse roots.     

Provenance-specific growth responses to drought and air warming in three European oak species (Quercus robur, Q. petraea and Q. pubescens)

Provenance-specific growth responses to experimentally applied drought and air warming were studied in saplings of three European oak species: Quercus robur, Quercus petraea and Quercus pubescens. Four provenances of each species were grown in large open-top chambers and subjected to four climates: control, periodic drought, air warming or their combination in 3 subsequent years. Overall growth responses were found among species and provenances, with drought reducing shoot height growth and stem diameter growth and air warming stimulating shoot height growth but reducing stem diameter growth and root length growth. Differential growth responses in shoots, stems and roots resulted in altered allometric growth relations. Root length growth to shoot height growth increased in response to drought but decreased in response to air warming. Stem diameter growth to shoot height growth decreased in response to air warming. The growth responses in shoots and stems were highly variable among provenances indicating provenance-specific sensitivity to drought and air warming, but this response variability did not reflect local adaptation to climate conditions of provenance origin. Shoot height growth was found to be more sensitive to drought in provenances from northern latitudes than in provenances from southern latitudes, suggesting that genetic factors related to the postglacial immigration history of European oaks might have interfered with selective pressure at provenance origins.     

Interacting effects of irradiance and water stress on dry weight and biomass partitioning in Fagus sylvatica seedlings

Abstract

Actual climate models for central Europe predict prolonged summer droughts. Knowledge on how the interaction between light and water availability affects regeneration will hence be of major importance. In an experiment carried out under controlled conditions, newly emerged beech seedlings were grown in pots with sand during 54 days. Three treatments applying three different light levels (2, 9 and 43% relative light intensity) were combined with two soil water treatments (control and drought). At the end of the experiment, seedlings were separated into leaves, stem and root and the seedlings’ dry mass, leaf area and stem length was determined. Low irradiance (2%) had a strong negative effect on dry weights of seedling components, leaf area and specific leaf mass. Drought clearly affected biomass partitioning for seedlings at high irradiance levels (43%). An interaction between irradiance and drought on biomass partitioning in beech seedlings was observed at medium irradiance level (9%). Within a wide range of light levels in the forests, both light and drought may affect biomass partitioning in young seedlings.     

Variation patterns of fine root biomass,production and turnover in Chinese forests

China’s forests cover 208.3 million ha and span a wide range of climates and a large variety of forest types including tropical,temperate,and boreal forests.However the variation patterns of fine root(2 mm in diameter biomass,production,and turnover from the south to the north are unclear.This study summarizes fine root biomass(FRB),production(FRP)and turnover rate(FRT)in Chi na’s forests as reported by 140 case studies published from 1983 to 2014.The results showed that the mean values o FRB,FRP and FRT in China’s forests were 278 gm~(-2)366 gm~(-2)a~(-1),and 1.19 a~(-1),respectively.Compared with other studies at the regional or global scales,FRB in China’s forests was lower,FRP was similar to estimates a the global scale,but FRT was much higher.FRB,FRP,and FRT in China’s forests increased with increasing mean annual precipitation(MAP),indicating that fine root vari ables were likely related to MAP,rather than mean annua temperature or latitude.This is possibly due to the smal variation in temperature but greater variation in precipitation during the growing season.These findings suggest that spatiotemporal variation in precipitation has a more profound impact on fine root dynamics in China’s forests,and this will impact carbon and nutrient cycles driven by root turnover in the future.     

Discrepancy in fine root turnover estimates between diameter-based and branch-order-based approaches: a case study in two temperate tree species

Fine root turnover plays a key role in carbon(C) budgets and nutrients cycles in forest ecosystems.However,the difference between branch-order-based and diameter-based approaches in estimating fine root turnover is still unclear.We studied root biomass turnover based on multiplying root standing biomass by turnover rate(inverse of median root longevity) in two Chinese temperate tree species,Fraxinus mandshurica Rupr.and Larix gmelinii Rupr.The minirhizotron(MR) technique was used to estimate longevities for first and second order roots,and total roots(R total) apparent on the MR tube surface.The corresponding biomass for each root group was estimated by soil monolith.The difference in biomass turnover between R total and the sum of the first and second order roots was used to represent the discrepancy between diameter-and order-based approaches.First order roots had shorter life spans and higher biomass turnover rates than the second order roots in both species.Biomass turnover estimated by the order-based method for F.mandshurica and L.gmelinii were 155.4 g m-2 a-1 and 158.9 g m-2 a-1,respectively,in comparison with 99.5 g m-2 a-1 and 117.7 g m-2 a-1 estimated by the diameter-based method,indicating that the diameter-based approach underestimated biomass turnover.The most probable reason was that the order-based method enhanced separation of the heterogeneous root population into relatively homogenous root groups with varying turnover rates.We conclude that separating fine root pool into different branch orders can improve the accuracy of estimates for fine root turnover,as well as the understanding of the belowground C allocation and nutrient cycling at ecosystem level.