共查询到20条相似文献,搜索用时 15 毫秒
1.
Jouni Kilpeläinen Leena Finér Seppo Neuvonen Pekka Niemelä Timo Domisch Anita C. Risch Martin F. Jurgensen Mizue Ohashi Liselotte Sundström 《Forest Ecology and Management》2009
Ant–aphid mutualisms, in which ants tend aphids, which in turn provide honeydew to the ants, are widespread and have been shown to affect plant growth. In boreal forests the effect of ant–aphid mutualism on tree growth can vary with stand age, because forest clear-cutting harms the ecologically most dominant ant partner in such mutualisms, wood ants (Formica rufa group). We studied whether the mutualism between wood ants and Cinara aphids affects the growth of boreal Norway spruces (Picea abies L. Karst.) in stands of different ages. In boreal forests, conifers, unlike deciduous trees, have only few defoliating insects, and therefore we expected the growth loss of conifers due to sap sucking by aphids not to be compensated by reduced insect herbivory due to predation by wood ants. The study was conducted in medium-fertile spruce-dominated stands in eastern Finland. We used stands of four different age classes (5, 30, 60 and 100 years) and selected ten spruces heavily visited and ten spruces lightly visited by ants around five medium-sized ant mounds in each stand age class. The access of ants was blocked on half of the trees in both groups. In the 5-year-old stands, the mean annual height growth of individual heavily visited seedlings was 16.3% greater than in the ones where ant traffic was blocked, but this difference was not significant. In the 30-year-old stands, the mean annual radial growth of the heavily visited spruces was 7.3% smaller than in trees where ant traffic was blocked, and this difference was significant. The mutualism had no significant effect on the radial growth in the 60- and 100-year-old stands. In the 60-year-old stands, however, the spruces that were visited heavily prior to the beginning of the study grew significantly less relative to their past growth than the initially lightly visited trees during the study. This suggests that the ant–aphid mutualism may have long-term effects on tree growth. The ant–aphid mutualism had no significant effect on the growth at the stand level. The results indicate that ant–aphid mutualism can have a significant effect on the growth of individual spruces, but the effect is negligible at ecosystem level. 相似文献
2.
Germination and survival in the initial months represent a critical period for forest species. Many factors have significant influences and can produce a negative effect on regeneration. To prevent this, these factors must be known and their influence must be evaluated. The influence of light and of water availability has been studied in the most important pine species in the Iberian Peninsula, the Pinus pinaster. An experimental seed-sowing was conducted in the Cuéllar experimental site (central Spain), where four different harvest intensities were applied in 2004. Four different watering regimes were performed during two consecutives summers (2006 and 2007) and seedling development was monitored during 18 months. Main findings reveal that, regarding germination and survival in the initial months, light was more important than water availability. Higher germination and survival rates were obtained when the lowest level harvest was applied. However, in terms of biomass and growth, water availability was more influential than harvest intensity. Results improved when there was greater water availability after a few months. The main conclusion of this study is that light shows a higher impact that summer rainfall with respect to germination and survival in the early stages of Mediterranean Maritime Pine. 相似文献
3.
To produce a highly stable wood-based product with increased mechanical properties, phenol formaldehyde (PF) resin impregnation
was combined with the viscoelastic thermal compression (VTC) process. Dimensional stability and bending stiffness were evaluated.
Two PF resins with weight average molecular weights of 172 and 780 were studied at three different concentrations, 5, 10 and
20%. After 24-h room temperature water soak and 2-h boil, both PF treatments at all concentration levels showed high levels
of dimensional stability compared to non-impregnated VTC processed controls. The higher molecular weight PF provided greater
stability with an average thickness swell value of 12% compared to 20 and 37% for the lower molecular weight PF resin treatment
and control, respectively. High anti-swelling efficiency values were recorded for both low and high molecular weight resins,
implying these modifications were effective at reducing the volumetric swelling which occurred in the unmodified control.
PF treatments were also extremely effective at reducing irreversible swelling. The low and high molecular weight resin treatments
had 1/5th and 1/7th the irreversible swelling than the unmodified VTC processed controls, respectively. All dimensional stability
values improved as resin concentrations increased. Both resin types at all concentration levels reduced Young’s modulus. 相似文献
4.
Luis A. Apiolaza Brian Butterfield Shakti S. Chauhan John C. F. Walker 《Annals of Forest Science》2011,68(2):407-414
• Introduction
Genetic testing is the slowest part of a breeding cycle. There is a growing interest in early wood quality screening methodologies. We hypothesized that subjecting 8-month-old radiata pine trees to mechanical perturbance induces reaction wood that permits isolating their likely corewood features. 相似文献5.
Patterns of stand growth and dominance among individual trees change through stand development. We tested a prediction that large, old plantations of Eucalyptus saligna (Sm.) in Hawaii and Brazil would show a pattern of “reverse” growth dominance, with the largest trees contributing more to stand biomass than to current stand increment. This pattern might develop if the largest trees accounted for a large portion of the total resource use of the stand, but were less efficient at utilizing resources to produce stemwood. The 70-year-old plantation in Hawaii had a stem mass of 610 Mg/ha, compared with 325 Mg/ha in the 66-year-old Brazil plantation. Cumulative frequency plots for both sites showed that dominant trees comprised a very large proportion of total stand mass; the largest 20% of the trees accounted for 50% and 30% of the total stand mass in Hawaii and Brazil, respectively. Contrary to expectations, the largest trees continued to produce a disproportionately large amount of total stand growth, with the largest 20% of trees accounting for 60% (Hawaii) and 40% (Brazil) of stand growth. The frequency distributions resulted in positive growth dominance coefficients (similar to Gini coefficients) of 0.14 for Hawaii and 0.09 for Brazil, rather than negative values that would indicate that reverse growth dominance. The failure of these monoculture Eucalyptus stands to develop reverse growth dominance contrasts starkly with a variety of studies with pine species, which tend to show neutral-to-reverse growth dominance. Indeed, continued dominance of large trees suggests the trees might respond well to silvicultural treatments such as thinning or fertilization. Future work that contrasts patterns of growth dominance among species will be important as a foundation for developing generalizations about trends in growth dominance with stand development. 相似文献
6.
《Forest Ecology and Management》2004,201(1):105-117
The importance of different fractions of coarse wood debris (cwd) for species diversity of wood-inhabiting fungi was investigated in near-natural Danish beech stands. Species number per tree increased significantly with increasing tree size, pointing out large trees to be most valuable for fungal diversity if single samples are compared. Rarefaction curves, evaluating the importance of different cwd fractions in a cumulative space, revealed a different and more complex picture. Rarefaction curves based on wood volume showed small trees and branches to host more species per volume unit than larger trees and logs, respectively, while snags were the most species-poor fraction. Surface-based curves showed species density to be rather similar among cwd types, though species density still decreased slightly with tree size. These results are interpreted to reflect a combination of two factors: firstly, small diameter cwd represent a larger surface area per volume and hence more space for fungal sporocarps, than large diameter cwd. This ‘surface area factor’ explains the high degree of similarity of the surface-based rarefaction curves. Secondly, a collection of small diameter cwd involves more separate units than an equal volume of large diameter cwd, and represents thereby more individual cases of fungal infection and, most likely, more variation in environmental conditions. The effect of this ‘number of item factor’ is reflected in the slightly increasing species density per surface area with decreasing tree size. Richness patterns of red-listed and non-red-listed species were found to be strikingly similar across cwd types, and a general preference for large logs among red-listed species was hence absent. An individual look at the most frequent encountered red-listed species revealed substrate preference patterns to occur in three of six species, of which one, the heart-rot agent Ischnoderma resinosum, preferred large logs. Based on these results, it would be obvious to conclude that local fungal species diversity is most efficiently increased in managed forests if small diameter cwd is prioritised for natural decay. However, small diameter wood appear to be unable to support heart-rot agents and other species depending on a long and diverse infection history and thus the integrity of saproxylic communities may be seriously undermined if only small diameter cwd is left for decay. Therefore, we strongly recommend that whole, naturally dead trees, representing the full range of cwd habitats, are prioritised for natural decay in managed forests whenever possible. 相似文献
7.
Eight provenances of 19-year-oldPicea koraiensis Nakai from the provenance trials of Maoershan (45°20′N, 127°30′E), Liangshui (47°10′N, 128°53′E) and Jiagedaqi (50°24′N,
124°07′E) in Northeast China were investigated to analyze the genetic variation in growth characteristics (tree height and
diameter) and wood characteristics (tracheid length, tracheid diameter, tracheid wall thickness, annual ring width as well
as wood density). Great variation in height growth and breast height diameter growth was observed among the provenances, and
along with the increase of tree age, these provenances presented different geographic adaptability. The growth characteristics
ofPicea koraiensis stand at age of 10 in Maoershan and Liangshui provenance trials had a positive correlation with longitude, and with increase
of tree age to 15 and 19, the tree growth of the provenances displayed a significant positive correlation with latitude as
well as altitude. For wood characteristics, great variation was also found among the provenances. There exists a close relation
between growth characteristics and wood properties of the provenance. The height and breast height diameter growth of the
provenance had a positive correlation with tracheid diameter and annual ring width, and a negative correlation with tracheid
wall thickness and wood density. Genetic performance of the provenance in all above characteristics was also investigated
in order to provide more useful information for comprehensive selection of this species for pulpwood and plywood production.
Foundation item: The paper was supported by National Key Project of The State Forestry Administration. (96-011-01-06)
Biography: WANG Qiu-yu (1957-), female, professor in College of Life Sciences, Northeast Forestry University, Harbin 150040, P. R.
China.
Responsible editor: Song Funan 相似文献
8.
This study aims to quantify the contribution of the harvested wood products (HWP) to the carbon removals/emissions by the
agriculture, forestry and land-use sector (hereafter referred to as HWP contribution) using the tier 1 method proposed by
the Intergovernmental Panel on Climate Change (IPCC) in the 2006 Guidelines (hereafter referred to as GL tier 1 method) and
to compare it with the results obtained with the tier 2 method proposed by the IPCC in the Good Practice Guidance for Land
Use, Land-Use Change and Forestry (hereafter referred to as GPG tier 2 method). The HWP contribution was calculated for three
approaches: stock change, production and atmospheric flow. Another objective was to perform a sensitivity analysis in order
to identify the calculation procedures and the input data that have the largest impact on the HWP contribution estimates.
An uncertainty analysis was also carried out with Monte Carlo simulation. The case study of Portugal was analysed in this
study. The HWP contribution obtained with the GL tier 1 method ranged from 150 to 1,240 Gg C year−1, for the period from 1990 to 2004. These results are similar to those obtained with the GPG tier 2 method, except for the
production approach, since the GL tier 1 method underestimated carbon accumulation in solid waste disposal sites (SWDS) under
this approach. The most influential calculation procedures and input data were the algorithm used to calculate the change
in carbon stocks for the pools that follow a first-order decay, the procedure to estimate the input of carbon to the pool
of HWP in SWDS, the procedure to determine the change in carbon stocks of HWP in SWDS for the production approach, the type
of carbon stocks considered in SWDS and the conversion factor for wood-based panels. 相似文献
9.
New Forests - The coppice is a very flexible cultural system producing several calibers adapting to the demands of the market. Small-caliber roundwood from sweet chestnut (Castanea sativa Mill.)... 相似文献
10.
Staffan Jacobson Hagos Lundström Sten Nordlund Ulf Sikström Folke Pettersson 《Scandinavian Journal of Forest Research》2014,29(7):675-685
Increasing use of forest fuels for energy production is generating greater quantities of wood ash. In Sweden, it is recommended that this ash should be returned to the forest to counter soil acidification and avoid potential future nutrient deficiencies, but the effects on tree growth require clarification. Thus, 10 field experiments were established in Sweden in 1990–2006 to study the effects of adding wood ash of various origins, doses and combinations of both ash and nitrogen on stem growth in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst.) stands on mineral soil. Observations after 5–15 years show that growth responses were strongest when N was added, either alone or with wood ash. Growth responses to additions of wood ash without N were small and variable and statistically insignificant at all study sites. However, there were indications that adding wood ash may increase stem-wood growth at fertile sites and decrease it at less fertile sites. Hence, at fertile sites, it may compensate for the growth reductions that normally follow whole-tree harvests, at least temporarily, presumably due to its effects on soil N turnover. At less fertile sites, adding N is probably essential to counteract these growth reductions. 相似文献
11.
Ana I. Leal Ricardo A. Correia Jorge M. Palmeirim José P. Granadeiro 《Agroforestry Systems》2013,87(2):355-363
Semi-natural cork oak woodlands are a biodiversity rich agro-silvo-pastoral system covering large areas in the Mediterranean. Canopies of adult oaks are often pruned, but nothing is known about the consequences of this treatment on biodiversity. We evaluated the impact of pruning on birds that forage in cork oak canopies in an area of southern Portugal. We characterized the use of trees by foraging birds with focal observations, and analysed the effects of pruning on density with point counts on pruned and control areas. As pruning reduces the foraging substrate for foliage gleaners, we predicted that these species would have lower densities in pruned areas. Pruning did not substantially affect overall bird density or species richness. However, as predicted, the density of species that foraged mostly by gleaning in the canopy tended to be lower in pruned areas, especially in winter when differences were statistically significant. In this season the combined density of foliage gleaners in the pruned stations was only half of that in controls. Pruning is also common in other managed Mediterranean woodlands that are important for birds, such as holm oak woodlands and olive groves, and foliage gleaners are likely to be affected in those too. The cumulative effects of pruning on all these habitats need to be assessed, but our results already indicate that pruning has negative consequences that should be properly considered in management decisions. 相似文献
12.
Rainforest deforestation and subsequent reforestation not only alter above-ground vegetation, but also lead to significant changes in the physical and chemical characteristics of soil and in biochemical cycles, which in turn are likely to influence the growth of rainforest plants. However, little research has directly linked soil condition to seedling growth under different forms of rainforest restoration. 相似文献
13.
Jing Wen Min Yi Leiming Dong Lu Zhang Siyu Liu Shenggui Yuan Xiaoxiao Tao Meng Lai 《林业研究》2023,(6):1951-1962
A solid understanding of the efficiency of early selection for fiber dimensions is a prerequisite for breeding slash pine(Pinus elliottii Engelm.) with improved properties for pulp and paper products.Genetic correlations between size of fibers,wood quality and growth properties are also important.To accomplish effective early selection for size of fibers and evaluate the impact for wood quality traits and ring widths,core samples were collected from360 trees of 20 open-pollinated Pinus elliottii... 相似文献
14.
Warren CR 《Tree physiology》2008,28(1):11-19
Internal conductance to CO(2) transfer from intercellular spaces to chloroplasts (g(i)) poses a major limitation to photosynthesis, but only three studies have investigated the temperature dependance of g(i). The aim of this study was to determine whether acclimation to 15 versus 30 degrees C affects the temperature response of photosynthesis and g(i) in seedlings of the evergreen tree species Eucalyptus regnans F. Muell. Six-month-old seedlings were acclimated to 15 or 30 degrees C for 6 weeks before g(i) was estimated by simultaneous measurements of gas exchange and chlorophyll fluorescence (variable J method). There was little evidence for acclimation of photosynthesis to growth temperature. In seedlings acclimated to either 15 or 30 degrees C, the maximum rate of net photosynthesis peaked at around 30 or 35 degrees C. Such lack of temperature acclimation may be related to the constant day and night temperature acclimation regime, which differed from most other studies in which night temperatures were lower than day temperatures. Internal conductance averaged 0.25 mol m(-2) s(-1) at 25 degrees C and increased threefold from 10 to 35 degrees C. There was some evidence that g(i) was greater in seedlings acclimated to 15 than to 30 degrees C, which resulted in seedlings acclimated to 15 degrees C having, if anything, a smaller relative limitation due to g(i) than seedlings acclimated to 30 degrees C. Stomatal limitations were also smaller in seedlings acclimated to 15 degrees C than in seedlings acclimated to 30 degrees C. Based on chloroplast CO(2) concentration, neither maximum rates of carboxylation nor RuBP-limited rate of electron transport peaked between 10 and 35 degrees C. Both were described well by an Arrhenius function and had similar activation energies (57-70 kJ mol(-1)). These findings confirm previous studies showing g(i) to be positively related to measurement temperature. 相似文献
15.
Wei-Xing Zhu 《Forest Ecology and Management》2011,261(3):675-682
Human activities have fundamentally changed global nitrogen (N) cycling, leading to elevated N deposition in most parts of the world. The fate of deposited N, whether being retained to sustain plant growth or causing ecosystem N saturation, is critical to the global carbon (C) cycling and local environment. In a short-term laboratory experiment, we used 15N-labeled NH4+ and NO3− to study the fate of N inputs in forest soils and what regulates N retention. Soils with a wide range of organic matter content and other attributes were collected from a 70-year-old plantation containing monotypic stands of Norway spruce (Picea abies), red pine (Pinus resinosa), sugar maple (Acer saccharum), and red oak (Quercus rubra), and separated into 0-5 cm and 5-15 cm layers. Nitrogen added to the soil was either immediately extracted (Time 0: T0) with K2SO4 solution, or incubated for 7 d (T7) and then extracted. Retention of 15N into the non-extractable soil pool at T0 was limited; but after the 7-d incubation, between 20 and 70% of the 15NH4+ was retained. Nitrification transformed on average 50% of the 15NH4+ into 15NO3− during the incubation while retention of 15NO3− at T7 remained low (7.40 ± 1.08%). Retention of 15NH4+ into non-extractable soil at T7 was positively correlated to the percentage of soil organic matter (SOM) (r2 = 0.323, P < 0.001), and was significantly higher (P < 0.001) in the high-SOM 0-5 cm layer than in the low-SOM 5-15 cm layer. Conversion of 15NH4+ to 15NO3− during incubation significantly reduced the 15NH4+ retention (P < 0.001). Our results suggest that the variations of SOM and other soil attributes play strong roles in the retention of newly deposited inorganic N and could affect forest ecosystem responses to chronic N deposition. 相似文献
16.
Garel Makouanzi Gilles Chaix Sophie Nourissier Philippe Vigneron 《Southern Forests》2018,80(2):151-158
Thirty-four Eucalyptus urophylla × Eucalyptus grandis hybrids were evaluated with a view to selecting for improved growth and wood-quality traits for plantations in the Congo. Height, circumference at breast height and volume were measured at 12, 27, 37, 49 and 60 months. Lignin content, the syringyl/guaiacyl ratio and total extractives content were predicted by near-infrared spectroscopy using wood powder samples collected from trees at breast height. While wood chemical properties were stable and under strong genetic control, growth traits were not. The genetic correlation between lignin content and growth was weak and negative, whereas the environmental correlation was also weak but positive. The genetic improvement of E. urophylla × E. grandis clones, based on growth features, leads to a limited decrease in lignin content and syringyl content and to a limited increase in extractives content. 相似文献
17.
Litterfall abundance and composition of organic and inorganic nitrogen and phosphorus forms were studied for 34 months in two different forest ecosystems (Vaccinio uliginosi–Betuletum pubescentis (Vu–Bp) and Empetro nigri–Pinetum (En–P)) in S?owiński National Park (northern Poland). Including broadleaves, litterfall abundance in Vu–Bp was above 25% higher than in En–P. In Vu–Bp the mean litterfall abundance was equal to 4050 kg ha−1 year−1 with needles (31%) and broadleaves (32%) dominating while in En–P the mean litterfall abundance was equal to 2828 kg ha−1 year−1 and needless made up 59% of total. Almost all litterfall fractions in both forest ecosystems followed a clear, coherent seasonal pattern with maximal values in Autumn and minimal values in the growing season. In general, inter-annual fall variation of needles, branches, broadleaves and seeds was not significant in both ecosystems, however, for the fall of needles and branches a decreasing tendency was observed. In Vu–Bp and in En–P total litterfall (Vu–Bp: T-N 97.5%, Norg 96.8%, En–P: T-N 95.6%, Norg 97.9%) as well as the needless (Vu–Bp: T-N 31.9%, Norg 46.4%; En–P: T-N 53.5%; Norg 53.6%) were the most abundant with nitrogen with the supremacy of organic forms. Besides needles fall, the highest nitrogen return in Vu–Bp was contributed by broadleaves, seeds and branches fall, while in En–P with branches and bark fall. The lowest nitrogen return was contributed by inflorescences and flower buds fall. Similar to organic nitrogen, organic phosphorus was dominant in Vu–Bp (91%) and in En–P (77%) ecosystems. In Vu–Bp total phosphorus contributed by broadleaves was comparable to deposited with needles. In the fallen needless phosphorus contribution was significantly higher in En–P than in Vu–Bp. Nearly 40% of total phosphorus return was contributed by branches, seeds and other litterfall material in Vu–Bp. Litterfall mass in Vu–Bp showed positive correlation with air humidity, total nitrogen content in the rooting zone, and negative with underground water level and air temperature indicating lower resistance towards climatic stress. At both forest ecosystems total litterfall mass, total nitrogen and organic nitrogen were positively correlated with air humidity, while neither total litterfall, nor total nitrogen (T-N) and total phosphorus (T-P) at both forest ecosystems were correlated directly with precipitation volume. This is probably because air humidity is partially related to precipitation (rSp = 0.31), however it could be also an effect of sea sprays contribution. Homogeneous Pinus sylvestris stand indicates higher resistance towards unfavorable climatic conditions than Betula pubescens, and this is why it should be preferably used in afforestation campaigns in seashore. 相似文献
18.
The effect of harvest residue management options on biomass and nutrient accumulation in understory vegetation, as well as the contribution of understory to nutrient cycling, were assessed during the early rotation stage of a Eucalyptus globulus Labill. plantation in Central Portugal. The effects of residue management options on early tree growth were also evaluated. Treatments established at the time of plantation and replicated four times in a simple completely randomised design included removal of harvest residues (R), incorporation of residues into the soil by harrowing (I) and maintenance of residues on the soil surface (S). Understory biomass was sampled in the spring between 2002 and 2006, and every 2 months between March 2006 and March 2007. The latter samples were stratified into biomass, standing dead mass and litter for net above ground primary production (NAPP) assessment. Samples were oven dried, weighed and analysed for nutrient contents. Results showed that understory standing biomass strongly increased from the first to the third year and that quantities of nutrients accumulated in ground vegetation followed similar patterns between the three treatments. Nutrient accumulation in ground vegetation was greater than in tree biomass until at least the second spring after plantation. Bimonthly sampling revealed treatment R to have the largest amounts of standing biomass, standing dead mass, litter and nutrient immobilisation, while treatment S exhibited the lowest values. NAPP (4th–5th year) was 639, 511 and 362 g m−2 year−1, respectively in R, I and S, corresponding the standing biomass increase to 277, 183 and 143 g m−2 year−1. These values are comparable to those observed for litter fall in similar stands (age and tree density) in the same area. The contribution of ground vegetation to nutrient accumulation in the system was unaffected by harvest residue management methods, but further research is necessary in order to establish whether slash management options influence long term tree growth and vegetation dynamics. 相似文献
19.
We assessed growth traits and wood properties of DH 32-29, a clone of Eucalyptus urophylla × E. grandis, at age of two to six years in Guangdong in China. Analysis of variance of studied traits showed that there were significant differences (1% level) on all studied traits among ages except for wood basic density. Analysis of age trends of growth traits and wood properties revealed that rotation length of DH32-29 should be more than six years or longer. Phenotypic correlations among traits at individual ages indicated that correlations between growth traits were strongly positive. There was significant change in relationship between growth and wood basic density with increasing age, ranging from -0.03 to -0.54 at 2 and 5 year and 0.003 to 0.3 at 3, 4 and 6 year.Correlations between Pilodyn pin penetration and basic density measuredon increment cores showed that Pilodyn could rank or group genotypesor sites into density classes, but failure to predict individual tree and individual clone. 相似文献
20.
Vicenç Bros 《Forest Ecology and Management》2011,261(3):611-619
In fire-prone regions, understanding the response of species to fire is a major goal in order to predict the effects on biodiversity. Furthermore, postfire management can also model this response through the manipulation of environmental characteristics of the burnt habitat. We have examined the taxonomic and functional response to fire and postfire management of a Mediterranean snail community affected by a summer fire in 2003. After the fire, the area was logged, leaving wood debris on the ground, and three alternative practices were implemented in several plots within the burnt area: subsoiling, removal of trunks having branches, total removal of trunks and branches, as well as one area not logged. Our results indicated that fire exerted a major impact on the snail community, strongly reducing diversity and species richness, particularly for forest species living in the humus and having European distribution ranges. By contrast, we found slight differences within the postfire practices, presumably because of the strong initial impact of fire and subsequent xerophilous postfire conditions. However, the area with only trunk removal showed a positive response of generalist snail species, probably due to moist microhabitats provided by the accumulation of wood debris on the ground. The effects of postfire management should be further explored due to the expected increase of fire risk associated with climate change and land-use histories. 相似文献