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1.
Seiwa K 《Tree physiology》1999,19(12):793-797
To determine how plants control leaf phenology to maximize annual carbon gain, I examined ontogenetic changes in leaf phenology of Japanese elm, Ulmus davidiana var. japonica Nakai plants of different ages growing in contrasting light environments. Leaf emergence occurred earlier in 1- and 2-year-old seedlings than in current-year seedlings. Although leaf emergence was not affected by light conditions at the sites, it was influenced by plant height. The delay in leaf emergence increased with increasing plant height. These traits indicate that seedlings that received the least light during the summer intercepted light for a long period during the spring; however, the advantage of earlier leaf emergence decreased with increasing plant height. At each site, 1-year-old seedlings had a longer duration of leaf emergence than adults, because of a longer period of favorable light conditions even in the forest understory. Duration of leaf emergence, leaf duration and leaf longevity were usually longer in sun than in shade for both seedlings and adults; however, flexibility in the response to light was greater in seedlings than in adults. The plastic response in leaf phenology during the juvenile stages may contribute to the optimization of light acquisition in habitats with differing light conditions, thereby enhancing seedling survival.  相似文献   

2.
Spatial variation in tree-regeneration density is attributed to the specialization of tree species to light availability for germination and growth. Light availability,in turn, varies across the gap-understorey mosaic. Canopy gaps provide an important habitat for the regeneration of tree species that would otherwise be suppressed in the understory. In subtropical forests, there is still a knowledge-gap relating to how canopy disturbances influence tree-regeneration patterns at local scale, and if they disproportionately favor regeneration of certain species. We aim to analyze whether canopy gaps promote tree regeneration, and tree species are specialized to gaps or understory for germination and growth. We sampled vegetation in 128 plots(0.01 ha), equally distributed in gaps and below canopy, in two subtropical Shorea robusta Gaertn.(Sal) forests in Nepal, recording the number of tree seedlings and saplings in each plot. We compared the regeneration density of seedlings and saplings separately between gaps and the understorey. The mean densities of seedlings and saplings were higher in the gaps at both sites;although there was no difference in the seedling density of the majority of the species between the habitats. No species were confined to either gap or understorey at the seedling stage. We conclude that gaps are not critical for the germination of tree species in Sal forests but these are an important habitat for enabling seedlings to survive into saplings. The classification of trees into regeneration guilds mainly based on germination does not apply to the majority of tree species in subtropical Sal forests. Our results reaffirm that gap creation promotes tree regeneration by favouring seedling survival and growth and can influence forest management for conservation, as well as for plantations.  相似文献   

3.
One-year-old seedlings of Abies balsamea (L.) Mill, Picea glauca (Moench) Voss, Pinus contorta Loudon, Betula papyrifera Marsh., Populus tremuloides Michx. and Populus balsamifera L. were transplanted in the spring, in pots, to the understory of a mixed P. tremuloides-P. balsamifera stand or to an adjacent open site. Growth and leaf characteristics were measured and photosynthetic light response curves determined in mid-August. Overall, the coniferous seedlings showed less photosynthetic plasticity in response to growth conditions than the deciduous species. Abies balsamea, P. glauca and B. papyrifera responded to the understory environment with higher leaf area ratios, and lower photosynthetic light saturation points and area-based leaf respiration relative to values for open-grown seedlings, while they matched or exceeded the height growth of open-grown seedlings. In contrast, seedlings of Pinus contorta, P. tremuloides and P. balsamifera displayed characteristics that were not conducive to survival in the understory. These characteristics included a high light saturation point and leaf dark respiration rate in P. contorta, and lower leaf area variables combined with higher carbon allocation to roots in P. tremuloides and P. balsamifera. By the second growing season, all seedlings of P. tremuloides and P. balsamifera growing in the understory had died.  相似文献   

4.
The effects of canopy gaps on seedling emergence and growth ofCornus controversa andPrunus grayana were studied in a 21-year-oldCryptomeria japonica plantation. The seeds of the two species were sown in December 1995 and their fate was followed until March 2000.P. grayana germinated in 1996, butC. controversa germinated in 1997. In both species studied, more than 70% of the seedlings survived in the forest edge until the end of the experiment, although none survived in the forest understory in the first growing season. In the gap, the survival rate was higher inP. grayana than inC. controversa. In this experiment, some trees were cut to enlarge the area of the gap, in which the growth rate increased markedly forC. controversa seedlings, but not forP. grayana seedlings after the cutting. These two species showed substantial differences in the patterns of seedling emergence, survival and growth in aCr. japonica plantation which had a canopy gap.  相似文献   

5.
Effects of three gaps which are large （118 m^2）, medium （86 m^2） and small （20 m^2）, respectively, and under canopy of Tsuga longibracteata forest on the seedling establishment of T. longibracteata were studied through seed burial experiments from December 2003 to January 2005 in Tianbaoyan National Nature Reserve of Fujian, China. The results showed that the area of gap had an evident effect on the seedling establishment of T. longibracteata. The seedling emergence rates of T. longibracteata in plots of large gap, medium gap, small gap and under canopy were 10%, 10%, 4% and 6%, representing an increasing trend along with the gap size increasing without a significant difference. Rain eroding and insects feeding were two main factors leading to seedling death. The larger the gap size was, the more seedlings were killed by rain erosion and the fewer seedlings were killed by insects feeding. The emergence time of seedlings was almost same in all plots while their death time was different respectively. The gap size had a significant impact on seedling survival rate. The seedling survival rate was highest in the medium gap plot （27.0%） and next to the highest in large gap plot （7.3%）, and seedling in small gap plot and under canopy plot died out after one growing season. Increased light supply in gaps was favorable for the seedlings growth and survival. Increased light supply in the large gap could enhance the growth of seedling leaf and root of T. longibracteata, and the seedling in turn allocated more dry mass to root and leaf, but it has little impact on the growth of stem. This research indicates that T. longibracteata is a pioneer species and its seedling establishment need a medium or large gap （〉50 m^2）.  相似文献   

6.
Studies within and outside the U.S. indicate recurring oak (Quercus spp.) regeneration problems. In deciduous forests of the eastern U.S., a prevailing explanation for this trend is fire suppression leading to high competitor abundance and low understory light. In response, prescribed fire is increasingly used as a management tool to remedy these conditions and encourage future oak establishment and growth. Within eastern Kentucky, we implemented single and repeated (3×) prescribed fires over a 6-yr period (2002–2007). Pre- and post-burn, we quantified canopy cover and oak seedling survival and growth compared to other woody seedlings deemed potential competitors, primarily red maple (Acer rubrum L.) and sassafras (Sassafras albidum (Nutt.) Nees.). Burning temporarily decreased canopy cover 3–10%, but cover rebounded the subsequent growing season. Repeated burning ultimately produced canopy cover about 6% lower than sites unburned and burned once, suggesting a cumulative effect on understory light. Red maple exhibited low survival (∼40%) following single and repeated burns, but growth remained similar to unburned seedlings. Burning had little impact on sassafras survival and led to total height and basal diameters 2× greater than unburned seedlings. A single burn had no impact on red oak (Erythrobalanus spp.) survival and increased height and basal diameters 25–30%, but this positive growth response was driven by seedlings on several plots which experienced high burn temperatures and consequently high overstory mortality. White oaks (Leucobalanus spp.), however, exhibited twice as high mortality compared to those unburned, with no change in growth parameters. Repeated burning negatively impacted survival and growth of both oak groups compared to unburned seedlings. With both burn regimes, oaks with smaller pre-burn basal diameters exhibited the lowest post-burn survival. Thus, despite the ability of prescribed burns to temporarily increase understory light and reduce red maple survival, neither single or repeated burns placed oaks in an improved competitive position. These findings result from a combination of highly variable yet interdependent factors including the (1) life history traits of oaks compared to their co-occurring competitors, (2) pre-burn stature of pre-existing oak seedlings, and (3) variability in fire temperature and effects on understory light.  相似文献   

7.
To examine Quercus robur establishment and growth in low to intermediate light levels, we analysed regeneration in different microsites created by light asymmetry within gaps in two main floodplain forest regions of Slovenia: Dolinsko and Krakovo. Four years after a mast year in 1995, we installed systematic grids of 1×1 m plots in 11 gaps (0.03–0.40 ha) on wet and dry site variants. In 256 plots, seedling species, cover, density, height, height increment, browsing damage, tree architecture, understory vegetation species and cover, and direct and diffuse light were measured. The average seedling density in all gaps was 15/m2, but the highest densities were found in gap positions with low diffuse light levels (10–20%). Competing understory vegetation was more abundant in positions with high diffuse light. In gaps on dry site variants a combination of low diffuse and high direct light was favourable for regeneration. The results indicated that Q. robur can successfully establish in gaps. Here, both light components were sources of within gap resource heterogeneity, therefore knowledge of light asymmetry can improve regeneration success.  相似文献   

8.
Depending on its developmental and morphological characteristics, shrubby or herbaceous understorey vegetation interacts differently with tree seedlings during the regeneration process. In acidic temperate forests, three common understorey plant species??Calluna vulgaris (L.) Hull, Pteridium aquilinum (L.) Kuhn in Kersten, Molinia caerulea (L.) Moench??are known to rapidly colonize forest gaps. Therefore, they often develop at the expense of light-demanding Scots pine (Pinus sylvestris L.) seedlings. An experiment was set up in a nursery in central France to mimic early competition occurring in a newly created gap between Scots pine seedlings and these three common understorey species (young forest-harvested individuals planted at 5 densities from 0 to 57 plants m?2). Pine seedling survival and growth (height, diameter, shoot and root biomass) and a functional trait (leaf mass on an area basis, LMA) were measured for 2 years, and cross-analysed against plant density, plant cover and available light. When understorey plant density increased, pine seedling diameter growth and biomass were negatively affected by all three plant species; height growth only slowed beneath Pteridium. These negative effects were closely linked to competition for light beneath Pteridium and Molinia. The application of the Beer?CLambert law gave an extinction coefficient k that was high for Pteridium, intermediate for Molinia and much lower for Calluna. LMA was confirmed as an effective foliar trait to reflect the degree of stress undergone by pine seedlings.  相似文献   

9.
In a deciduous forest, differences in leaf phenology between juvenile and adult trees could result in juvenile trees avoiding canopy shade for part of the growing season. By expanding leaves earlier or initiating senescence later than canopy trees, juvenile trees would have some period in high light and therefore greater potential carbon gain. We observed leaf phenology of 376 individuals of 13 canopy tree species weekly over 3 years in a deciduous forest in east central Illinois, USA. Our objectives were: (1) to quantify for each species the extent of differences in leaf phenology between juvenile and conspecific adult trees; and (2) to determine the extent of phenological differences between juvenile Aesculus glabra Willd. and Acer saccharum Marsh. trees in understory and gap microhabitats. All species displayed phenological differences between life stages. For 10 species, bud break was significantly earlier, by an average of 8 days, for subcanopy individuals than for canopy individuals. In 11 species, completion of leaf expansion was earlier, by an average of 6 days, for subcanopy individuals than for canopy individuals. In contrast, there were no significant differences between life stages for start of senescence in 10 species and completion of leaf drop in nine species. For eight species, leaf longevity was significantly greater for subcanopy individuals than for canopy individuals by an average of 7 days (range = 4-10 days). Leaf phenology of subcanopy individuals of both Aesculus glabra and Acer saccharum responded to gap conditions. Leaf longevity was 11 days less in the understory than in gaps for Aesculus glabra, but 14 days more in the understory than in gaps for Acer saccharum. Therefore, leaf phenology differed broadly both between life stages and within the juvenile life stage in this community. A vertical gradient in temperature sums is the proposed mechanism explaining the patterns. Temperature sums accumulated more rapidly in the sheltered understory than in an open elevated area, similar to the canopy. Early leaf expansion by juvenile trees may result in a period of disproportionately higher carbon gain, similar to gains made during summer months from use of sun flecks.  相似文献   

10.
For 3 years following a severe, November 1992 tornado, abundance and growth of tree regeneration in intact forest-floor and windthrow-pit microsites were studied in three mixed pine and hardwood stands in the Georgia Piedmont, USA. The research had two objectives: (1) determine if performance of tree regeneration differed between microsite types and between pre- and post-tornado cohorts on intact forest-floor microsites, and (2) determine if variation in light and soil water availability from the disturbance affected performance of eight species artificially seeded into intact forest-floor microsites. Near each of the 42 sample points (12–20 per site) spaced on 15 m grids, species and height of tree seedlings were recorded within a 1 m radius plot of intact forest floor and the nearest windthrow pit. Intact forest-floor microsites were dominated by two late-successional species, Acer rubrum (3.5 pre-tornado stems per m2 and 1.8 post-tornado stems per m2) and Ostrya virginiana (2.6 post-tornado stems per m2), while windthrow pits were dominated by an early successional species, Liriodendron tulipifera (1.7 stems per m2). Although seedling survival did not vary significantly among species or microsite types, first-year height of seedlings in intact forest-floor microsites (24 cm) was significantly greater than those in windthrow-pit microsites (19 cm). Second-year height growth of new seedlings of Cornus florida in intact forest-floor microsites (52 cm) significantly exceeded that of many other combinations of species and microsite type. Species artificially-seeded into intact forest-floor microsites in 1994 and 1995 varied considerably in emergence (<1–24%), survival (<1–16%), and height (5–15 cm), and those with the heaviest seed survived best. From 1994 to 1995, average gap fraction, an index of light availability, decreased 17% from 0.23 to 0.19. Soil water content in 1995 under gap fractions of greater than 0.3 (21%) averaged significantly less than under gap fractions of either 0.1–0.3 (24%) or less than 0.1 (26%). Significant positive correlations were detected for relationships of emergence and survival of several species to soil water content. Overall rankings of seedling performance between microsites (intact forest floor>windthrow pits), between cohorts (pre-tornado>post-tornado origin), and among species (mid- to late->early-successional species) indicate that advanced regeneration and new seedlings of A. rubrum, O. virginiana, and C. florida will be long-term dominants of the understory because of their high abundance, initial growth responses, and shade tolerance.  相似文献   

11.
In a 4-year study, we investigated changes in leaf physiology, crown morphology and whole-tree biomass allocation in seedlings and saplings of shade-tolerant sugar maple (Acer saccharum Marsh.) and intermediate shade-tolerant yellow birch (Betula alleghaniensis Britt.) growing in natural understory light (0.5 to 35% of full sunlight) or in understory light reduced by 50% with shade nets to simulate the effect of gap closure. Leaf physiological parameters were mainly influenced by the light gradient, whereas crown morphological and whole-tree allocational parameters were mainly influenced by tree size. No single physiological, morphological or allocational trait was identified that could explain the difference in shade tolerance between the species. Yellow birch had higher growth rates, biomass allocation to branches and leaf physiological plasticity and lower crown morphological plasticity in unmodified understory light than sugar maple. Sugar maple did not display significant physiological plasticity, but showed variation with tree size in both crown morphology and whole-tree biomass allocation. When sugar maple was small, a greater proportion of whole-tree biomass was allocated to roots. However, physiological differences between the species decreased with decreasing light and most morphological and allocational differences tended to disappear with increasing tree size, suggesting that many species differences in shade-tolerance are expressed mainly during the seedling stage. Understory trees of both species survived for 4 years under shade nets, possibly because of higher plasticity when small and the use of stored reserves when taller.  相似文献   

12.
林隙微生境及更新研究进展   总被引：19，自引：3，他引：19  
宋新章  肖文发 《林业科学》2006,42(5):114-119
系统评述了国内外关于林隙干扰及更新的最新研究进展.林隙是一种经常发生的小规模干扰,是森林群落时空格局变化的驱动力.不同的林隙干扰状况导致了林隙内光照的差异,进一步造成林隙内温度、湿度、土壤理化性质的变化,形成了微生境的时空异质性.林隙干扰通过改变微生境而对幼苗的出土、存活、定居和生长产生重要影响.林隙内种子库的物种丰富度变化很大,是幼苗分布格局的重要影响因子.更新幼苗的存活率和生长量受林隙大小及幼苗在林隙中位置的影响.指出了这一领域今后的研究方向.  相似文献   

13.
Studies of tree seedling physiology and growth under field conditions provide information on the mechanisms underlying inter- and intraspecific differences in growth and survival at a critical period during forest regeneration. I compared photosynthetic physiology, growth and biomass allocation in seedlings of three shade-tolerant tree species, Virola koschynii Warb., Dipteryx panamensis (Pittier) Record & Mell and Brosimum alicastrum Swartz., growing across a light gradient created by a forest-pasture edge (0.5 to 67% diffuse transmittance (%T)). Most growth and physiological traits showed nonlinear responses to light availability, with the greatest changes occurring between 0.5 and 20 %T. Specific leaf area (SLA) and nitrogen per unit leaf mass (N mass) decreased, maximum assimilation per unit leaf area (A area) and area-based leaf N concentration (N area) increased, and maximum assimilation per unit leaf mass (A mass) did not change with increasing irradiance. Plastic responses in SLA were important determinants of leaf N and A area across the gradient. Species differed in magnitude and plasticity of growth; B. alicastrum had the lowest relative growth rates (RGR) and low plasticity. Its final biomass varied only 10-fold across the light gradient. In contrast, the final biomass of D. panamensis and V. koschynii varied by 100- and 50-fold, respectively, and both had higher RGR than B. alicastrum. As light availability increased, all species decreased biomass allocation to leaf tissue (mass and area) and showed a trade-off between allocation to leaf area at a given plant mass (LAR) and net gain in mass per unit leaf area (net assimilation rate, NAR). This trade-off largely reflected declines in SLA with increasing light. Finally, A area was correlated with NAR and both were major determinants of intraspecific variation in RGR. These data indicate the importance of plasticity in photosynthetic physiology and allocation for variation in tree seedling growth among habitats that vary in light availability.  相似文献   

14.
枫香是在观赏、药用、用材方面都有着重要作用且颇具潜力的造林绿化树种,为了加快培育优质的枫香苗木,笔者根据枫香的生长特性,采用不同的轻基质组配、不同的栽种密度和是否进行苗期大小苗分级的方法设置7个处理,对枫香苗木进行容器育苗试验。结果表明：使用LQ基质的处理组在种苗保存率、Ⅰ级苗率、合格苗出圃率、每平方米合格苗出圃数、平均苗高和平均地径等方面均高于使用SX基质或LZ基质的处理组。在轻基质组配和分盘处理相同的情况下,采用143株/m^2的密度培育枫香容器苗,较采用195株/m^2、238株/m^2密度培育枫香苗有着更高的保存率、Ⅰ级苗率和合格苗出圃率,且平均苗高、地径以及苗木整齐度也更优。而进行大小苗分级培养的处理组与未分级培养的处理组在种苗成活率、合格苗出圃率、每平方米合格苗出圃数等指标上差距不大,但进行分级处理后的大苗盘的Ⅰ级苗率远高于未分级组和小苗组。  相似文献   

15.
Regenerating oaks (Quercus L.) on mesic and hydric sites has remained a problem largely because of inadequate density and poor distribution of large oak advance reproduction prior to harvesting. We examined the effect of midstory and understory removal on the establishment and 3-year development of natural and artificial sources of pin oak (Q. palustris Muenchh.) advance reproduction in bottomland forests in southeastern Missouri, USA. Midstory and understory removals increased the photosynthetically-active radiation (PAR) reaching the seedling layer from about 3 to 15%. This increased light did not increase the density of natural pin oak advance reproduction compared to control, but it increased the survival and nominally increased the growth of the natural pin oak advance reproduction. Where the midstory and understory had been removed, underplanted RPM® container stock and bareroot pin oak stock maintained high survival, but of the two only the RPM® stock maintained positive height and diameter growth while bareroot stock suffered some growth reductions. Pin oaks originating from the direct seeding of stratified acorns sown in the spring had low germination and survival, but the survivors had growth rates similar to those of natural seedlings in thinned stands. Applying triclopyr to competitors in the ground flora layer only nominally increased PAR but reduced the percent survival and marginally increased the growth of natural and artificial pin oak. We conclude that artificial reproduction may be used to further increase the probability of achieving adequate numbers of the desired species in the future. Bareroot seedlings may not perform as well as RPM® seedlings and natural seedlings already present. However, bareroot and RPM® seedlings remained significantly larger than the natural seedlings after 3 years.  相似文献   

16.
Leaf functional traits are adaptations that enable plants to live under various environmental conditions. This study aims to determine the differences in leaf functional traits among plants grouped by growth habit, leaf life span, leaf lifestyle, leaf form, and origin. Specific leaf area (SLA) of perennial or evergreen species was lower than that of annual or deciduous species because longer-lived leaves of perennial or evergreen species require more investment in structural integrity and/or defense against disturbances, especially with any resource constraint. SLA of large individuals was lower than that of small individuals. The low SLA in large individuals can improve their response to changing light and water conditions because increasing plant height is advantageous for light competition, but it can also impose a cost in terms of structural support and water transport. Petioles of plants with compound leaves were significantly longer than those of simple leaves because branching is expensive in terms of gaining height. SLA of plants increased with increasing invasiveness accordingly, and SLA of invasive plants was higher than that of their native congeners because invasive plants should invest more biomass on leaf growth rather than leaf structures per unit area to achieve a higher growth rate. Overall, variation in leaf functional traits among different groups may play an adaptive role in the successful survival of plants under diverse environments because leaf functional traits can lead to pronounced effects on leaf function, especially the acquisition and use of light. Plant species with different growth and leaf traits balance resource acquisition and leaf construction to minimize trade-offs and achieve fitness advantages in their natural habitat.  相似文献   

17.
不同光环境下喜树与四川大头茶幼苗的表型可塑性   总被引：2，自引：1，他引：1  
研究不同光环境对常绿阔叶林中喜树和四川大头茶幼苗生长的影响.结果表明:1)落叶阔叶树种喜树幼苗通过提高自遮荫程度、增大叶悬挂角、增加根生物量比和减小比叶面积适应旷地环境,在旷地中具有最大的相对生长速率,而常绿阔叶树种四川大头茶幼苗没有表现出对旷地光环境的良好适应;2)四川大头茶幼苗在林窗中通过增加叶片数、提高单叶面积、比叶面积和叶生物量比接受更多光能,相对于旷地和林下具有较快的高度生长,喜树幼苗比四川大头茶幼苗在林窗中具有更大的单叶面积、叶柄长度、总叶面积和总叶面积增长率;3)在3-8月,喜树在林窗中的株高增加约35.3 cm,约为四川大头茶(约6.6 cm)的5.3倍,喜树在林窗中具有明显的光资源竞争优势,表现出比常绿阔叶树种四川大头茶更高的相对生长速率,这就是喜树存在于常绿阔叶林的一个重要原因;4)郁闭林下的光资源有限,2个树种幼苗都不能在林下良好生长;5)喜树幼苗叶性状在不同光环境的可塑性指数远大于四川大头茶,这是喜树能够在常绿阔叶林中存在的另一重要原因.  相似文献   

18.
Seed limitation of desired indigenous species has constrained the succession of established plantations from an early successional stage, to more natural forests in South China. Because of its low cost and time-saving advantages, direct seeding has been considered a useful method in forest regeneration. To improve our understanding of whether and how direct seeding can lead to the establishment of indigenous tree species, we carried out an experiment in four typical plantations (eucalyptus, mixed-native, mixed-legume, mixed-conifer) and a shrubland in South China. We also tested the effect of understory vegetation and litter removal on seedling establishment and growth. Seeds of three indigenous tree species (Castanopsis chinensis, Cryptocarya chinensis, and Psychotria rubra) were sown with understory vegetation and litter either retained or removed. Seedling emergence, survival, and growth were recorded for 21 months. We found that conditions that favored seedling emergence often differed from those that favored seedling survival. While the removal of understory vegetation and litter did not influence the final seedling establishments for all the three introduced species, growth of Psychotria rubra was influenced at several sites. Seed mass was positively correlated with seedling emergence and subsequent growth. The large-seeded Castanopsis chinensis showed higher emergence and growth than the intermediate-seeded Cryptocarya chinensis or the small-seeded Psychotria rubra. Seedlings of Castanopsis chinensis grew better in plant communities with high understory light penetration and high soil exchangeable potassium. Growth of P. rubra seedlings decreased significantly with soil hydrolyzed nitrogen. In conclusion, direct seeding can be useful for the establishment of indigenous tree species into plantations and shrubland. Optimizing early seedling establishment and growth will require the selection of suitable native species for improving seedling establishment in different plantations. Additionally, understory vegetation and litter and fertilizer should be properly managed to enhance the establishment of certain indigenous tree species.  相似文献   

19.
We experimentally investigated interacting effects of canopy gaps, understory vegetation and leaf litter on recruitment and mortality of tree seedlings at the community level in a 20-year-old lowland forest in Costa Rica, and tested several predictions based on results of previous studies. We predicted that experimental canopy gaps would greatly enhance tree seedling recruitment, and that leaf litter removal would further enhance recruitment of small-seeded, shade-intolerant seedlings in gaps. We created a large (320–540 m2) gap in the center of 5 out of 10 40 m × 40 m experimental plots, and applied the following treatments bimonthly over a 14-month-period in a factorial, split–split plot design: clipping of understory vegetation (cut, uncut), and leaf litter manipulations (removal, addition, control). As expected, experimental gaps dramatically increased tree seedling recruitment, but gap effects varied among litter treatments. Litter addition reduced recruitment in gaps, but enhanced recruitment under intact canopy. Species composition of recruits also differed markedly between gap treatments: several small-seeded pioneer and long-lived pioneer species recruited almost exclusively in gaps. In contrast, a few medium-to-large-seeded shade-tolerant species recruited predominantly under intact canopy. Leaf litter represents a major barrier for seedling emergence and establishment of small-seeded, shade-intolerant species, but enhances emergence and establishment of large-seeded, shade-tolerant species, possibly through increased humidity and reduced detection by predators. Periodic clipping of the understory vegetation marginally reduced tree seedling mortality, but only in experimental gaps, where understory vegetation cover was greatly enhanced compared to intact canopy conditions. Successful regeneration of commercially valuable long-lived pioneer trees that dominate the forest canopy may require clear-cutting, as well as weeding and site preparation (litter removal) treatments in felling clearings. Management systems that mimic natural canopy gaps (reduced-impact selective logging) could favor the regeneration of shade-tolerant tree species, potentially accelerating convergence to old-growth forest composition. In contrast, systems that produce large canopy openings (clear-cutting) may re-initiate succession, potentially leading to less diverse but perhaps more easily managed “natural plantations” of long-lived pioneer tree species.  相似文献   

20.
辽东山区长白落叶松天然更新调查   总被引：11，自引：0，他引：11  
调查辽东山区长白落叶松人工林的长白落叶松天然更新数量、幼苗的存活和生长状况以及其幼树出现的生境.结果表明:在结实的长白落叶松人工林内均有1年生落叶松幼苗,其数量与人工林种子库落叶松种子数量有关,其存活和生长(高生长和叶片数量)状况明显受林分开阔度、地面植被盖度和枯落物的影响.1年生长白落叶松幼苗在林内生长缓慢,一般高生长不超过6 cm,地径生长不超过0.1 cm,叶片数量不超过25针.长白落叶松天然更新幼苗4月末开始出现,6月份幼苗数量最多,9月份林内大部分幼苗死亡.这表明长白落叶松幼苗在林下不能正常存活和生长,而影响其存活和生长的主要因素是光照、草本灌木和地面枯落物.另外,长白落叶松林下很难见到幼树,只有在有种源、光照充足、草本灌木和枯落物少,以及土壤较湿润的生境中能见到其幼树.  相似文献   

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