Coexistence mechanisms of evergreen,deciduous and coniferous trees in a mid-montane mixed forest on Mt. Emei,Sichuan, China

Structure and regeneration of a mid-montane (2200 m a.s.l.)mixed forest codominated by evergreen (Lithocarpuscleistocarpus), deciduous (Acer flabellatum)andconiferous (Tsuga chinensis, Abies fabri, andTaxus chinensis) trees were analyzed in a 40m× 60 m plot on Mt. Emei, Sichuan, China. Plant communitystructure and composition varied depending on topographic micro-habitat withinthe plot. Four topographic communities (topo-communities) were distinguishedwith dominant species corresponding to topography: (1)Abies – valley bank, (2) Acer– lower steep slope, (3) Lithocarpus – uppergentle slope, and (4) Tsuga – ridge. The coexistencemechanisms of the evergreen, deciduous, and coniferous trees were determined byidentifying the regeneration process characteristic of each dominant species,asrepresented by their seedling dispersion patterns and seedling establishmentalong the topographic gradients. The saplings and seedlings of the dominantswere distributed differently according to the topography:Lithocarpus under the canopy of parent trees and in gapsofthe upper gentle slope and ridge, Acer mainly in bothwell-lit and shady sites on the lower steep slope and on fallen logs,Abies on the valley bank but only in well-lit sites,Tsuga on the ridge, Taxus mostly onrock and well-lit sites on the slopes. Distribution of surviving saplings andseedlings was also related to the species of nearby canopy trees. We suggestthat Lithocarpus, Tsuga and Acer tendto be self-replacing in their own topographic habitats, andAbies survives as a fugitive by occupying occasionalsuitable gaps. The variation in soil conditions, particularly nutrients, withtopography affected seedling establishment and the growth of trees. Thesuccessional change of quantitative species composition, as predicted by theMarkovian model, shows the mixed forest to be in a sustained climax stage.     

Seedling establishment of five evergreen tree species in relation to topography,sika deer (<Emphasis Type="Italic">Cervus nippon yakushimae</Emphasis>) and soil surface environments

We investigated the seedling survival of five evergreen tree species over 3 years inside and outside deer-exclusion fences in a warm temperate evergreen broad-leaved forest on Yakushima Island, Japan. Seedling survival was examined in relation to topography, herbivory by sika deer, and the soil surface environment (i.e., soil surface wetness, light conditions, slope inclination, and soil disturbance). The study species included Myrsine seguinii Lév., Syzygium buxifolium Hook. et Arn. (Group A: species distributed on the upper slope of the study site), Litsea accuminata (Bl.) Kurata, Schefflera octophylla (Lour.) Harms (Group B: species distributed on the lower slope), and Cleyera japonica Thunb. p.p. emend. Sieb. et Zucc. (Group C: species distributed on both slopes). The soil surface environment on the upper slope was drier, lighter, and more easily disturbed than the lower slope. Generalized linear model analyses indicated that seedling survival in fenced and unfenced quadrats was greater on the upper slope than on the lower slope for Group A and B species but not for Group C species. A micro-spatial scale analysis revealed that seedling survival was correlated with soil wetness, ground light conditions, and soil disturbance but not slope inclination. These results indicate that seedling survival was correlated with topography, sika deer herbivory, and the micro-spatial scale environment. Topography-related differences in seedling survival appear to adequately reflect the observed adult plant distributions for Group A and C species but not for Group B species.     

Coastal dune topography as a determinant of abiotic conditions and biological community restoration in northern KwaZulu-Natal,South Africa

Topography is rarely considered as an independent goal of restoration. However, topography determines microenvironmental conditions and hence living conditions for species. Restoring topography may therefore be an important first step in ecological restoration. We aimed at establishing the relative importance of topography where coastal dunes destroyed by mining are rebuilt as part of a rehabilitation program. We assessed the response of (1) microclimatic and soil conditions, and (2) woody plant and millipede species richness and density, to location-specific topographic profiles. We enumerated the topographic profile using variables of dune morphology (aspect, elevation, and gradient) as well as relative position on a dune (crest, slope, and valley). Temperature, relative humidity, and light intensity varied with aspect, elevation, gradient, and position. However, regeneration age was a better predictor of soil nutrient availability than these topographic variables. Age also interacted with topographic variables to explain tree canopy density and species richness, as well as millipede species richness. The density of keeled millipedes (forest specialists) was best explained by topographic variables alone. The transient nature of these new-growth coastal dune forests likely masks topography-related effects on communities because age-related succession (increasing structural complexity) drives the establishment and persistence of biological communities, not habitat conditions modulated by topography. However, our study has shown that the microhabitats associated with topographic variability influence specialist species more than generalists.     

Multivariate path analysis of the relationships between seedling regeneration and environmental factors beneath a dwarf bamboo understory

Seedling emergence and establishment are fragile processes that determine the direction and structure of forest succession and regeneration. However, seedling emergence and establishment are easily affected by biotic and abiotic (environmental) factors. A dense and expanding understory of dwarf bamboo is one such important factor that can seriously hinder the seedling regeneration. We conducted a field experiment to investigate the emergence and establishment of canopy tree seedlings under artificially controlled densities of dwarf bamboo. We found that understory dwarf bamboo obstructed seedling emergence but reduced the death of seedlings. Although understory dwarf bamboo reduced the median retention time of seedlings, dense bamboo increased the mean survival time of seedlings. Our results suggest that understory dwarf bamboo has multiple selectivities for tree seedling emergence and establishment: high‐density dwarf bamboo was beneficial to evergreen species but lower‐density of bamboo was conducive to the survival of deciduous species, it means the dwarf bamboo potentially alters successional trajectories of forest communities. Path analysis revealed that the most important factors affecting tree seedling emergence and death were the abundance of seeds in the seed bank and the density of emerged seedlings, and that the soil temperature promoted seedling emergence but increased seedling death, the thickness of litter limited seedling emergence, and the leaf area index of the bamboo canopy limited seedling death. The present study suggests that dwarf bamboo can directly alter the microenvironment, significantly reducing light levels and soil temperature but increasing the thickness of litter and soil humus, thereby indirectly impacting the regeneration of tree seedlings. Our results indicate that various factors affected seedling emergence, and there were complex indirect relationships among these factors. In general, biological factors had a stronger influence on tree seedling regeneration than environmental factors.     

Effects of topographic and edaphic conditions on alpine plant species distribution along a slope gradient on Mount Norikura,central Japan

This study examined the effects of topographic and edaphic conditions on alpine plant species distribution along a slope gradient on Mt. Norikura (3026 m a.s.l.), central Japan. Topographic and edaphic factors investigated at 40 plots were: slope inclination, ground surface texture, soil water content and soil inorganic nitrogen concentration (NO₃-N, NH₄-N). The topographic and edaphic factors changed with slope positions: slope inclination was steeper, soil texture was coarser, and soil water and inorganic nitrogen concentration decreased with increasing slope position. Five vegetation types were located along the slope gradient and related to two factor-groups: (1) changes in soil water, NH₄-N, slope inclination along the slope gradient, and (2) ground surface texture. A tall herbaceous plant community developed at the low slope position, near tarns, with fine soil surface texture, high soil water and NH₄-N, while Dicentra peregrina dominated on an unstable rubble slope near the ridge top. The distribution of each species was predictable from the two factor-groups. Although the five vegetation types were related to the two factor-groups, responses to the two factor-groups differed among species, even within the same vegetation type. Therefore, this study showed that the topography of the terrain largely regulated alpine plant distribution by affecting edaphic conditions, and that global warming may alter species composition by changing edaphic conditions.     

Effects of vegetation canopy and climate on seedling establishment in Mediterranean shrubland

Abstract. Question: Does the influence of plant canopy on seedling establishment interact with climate conditions, and particularly, do intensified drought conditions, enhance a positive effect of the vegetation canopy on seedlings in Mediterranean‐type ecosystems. Location: Mediterranean shrubland near Barcelona, Spain at 210 m a.s.l. Methods: Over the course of four years we recorded seedling emergence and survival in open areas and below vegetation under control, drier and warmer experimental climatic conditions. Results: Seedling emergence is more sensitive to climate conditions than later stages of growth. When considering the whole set of species, the total number of established seedlings at the end of the experiment was lower in the drought and warming stands than in control ones, and vegetation canopy increased the number of these seedlings in the drought stands. Drought reduced seedling emergence but not warming, while the interaction between climate treatments and vegetation canopy was not significant. Seedling survival was lower in the warming treatment than in the control. Under drought conditions, vegetation canopy increased seedling emergence of the dominant Globularia alypum. In control stands, vegetation canopy reduced their survival. Vegetation canopy increased the survival of the dominant Erica multiflora in warming stands, and it reduced the survival of G alypum in drought stands. No significant effects of drought and warming were observed in the seed rain of these two species. Conclusions: The balance of the facilitation‐competition interactions between vegetation canopy and seedling establishment in Mediterranean‐type ecosystems determined by water availability, and drought conditions enhance the positive effect of vegetation canopy. This interaction is species‐specific and shows important between‐year variability.     

Impacts of streaked shearwater (Calonectris leucomelas) on tree seedling regeneration in a warm-temperate evergreen forest on Kanmurijima Island,Japan

The ground vegetation of an evergreen broad-leaved Persea thunbergii-dominated forest on Kanmurijima Island has been heavily damaged by a ground-burrowing seabird, the streaked shearwater (Calonectris leucomelas). To clarify the effects of seabird trampling and burrowing on the recruitment of tree seedlings, 22 paired quadrats, protected and unprotected against seabirds, were laid out under various degrees of canopy coverage, ranging from heavy- to light-shade. Protection from seabird activities resulted in an increase in species richness of tree seedlings. Seabird activities had a significant effect on tree seedling diversity, while canopy coverage was shown to be important for herbaceous species diversity. Though seedling emergence of Persea thunbergii, the evergreen canopy dominant, was not affected by seabird activities and canopy coverage, that of Mallotus japonicus, a deciduous pioneer tree, was negatively affected by the both factors. Seabird activities and evergreen heavy-shade canopy negatively affected seedling survivorship of both species. Low survivorship in seedlings of the canopy species may doom the present-day warm-temperate evergreen forest of Persea thunbergii on the island. Mechanical impacts of trampling and burrowing and the resulting soil erosion may play an important role in forest regeneration and dynamics.     

Interspecific variation in tree seedling establishment in canopy gaps in relation to tree density

Abstract. We tested whether interspecific variation in tree seedling establishment in canopy gaps was significantly related to interspecific variation in tree density, for seven deciduous forest tree species (Quercus alba, Hamamelis virginiana, Acer rubrum, Sassafras albidum, Quercus rubra, Prunus serotina, Ostrya virginiana). For each species, seedling establishment was calculated as the difference in seedling density before experimental gap creation versus three years after gap creation. In each of the six experimentally-created gap types (33 % or 66 % removal of tree basal area from 0.01-ha, 0.05-ha or 0.20-ha patches), differences in seedling establishment among species were significantly related to differences in their density in the tree canopy. A regression model with log_e tree density as the independent variable accounted for between 93 % and 98 % of interspecific variation in seedling establishment. Our results provide empirical support for models of tree dynamics in gaps that assume seedling establishment depends on canopy tree density.     

Fire,canopy cover and seedling dynamics in Mediterranean shrubland of northeastern Spain

Abstract. Seedling establishment and survival of the main woody species of early-successional shrubland in northeastern Spain were studied from 1992 to 1995 with emphasis on the importance of vegetation cover (existence of open areas and the situation beneath the plant canopy) and microhabitat (occurrence of stones, litter and bare soil). In the absence of fire, vegetation cover (measured at a scale of 30cm × 30 cm) was not correlated with seedling emergence of most species, nor with growth and survival of seedlings up to one year old. Seedlings older than one year showed a similar pattern: their density was not significantly different in both kinds of habitats. The emergence of seedlings was mainly associated with the presence of mature plants – which can provide seeds – and with the absence of unsuitable microhabitats (large stones, deep litter and bare soil). In April 1994 a wildfire burned the study area. This enabled a study of the pattern of post-fire establishment. After the wildfire, seedling emergence of several species increased and most species produced seedlings with higher survival and growth rates than in the period before the fire. Pre-fire cover, however, did not show significant effects on post-fire seedling dynamics in most cases.     

地形异质性对云南普洱季风常绿阔叶林物种多样性的影响

地形异质性通过调控树木生长所需的养分、水分和光照等而成为亚热带森林结构与物种组成的重要驱动因子。但是, 地形异质性对季风常绿阔叶林物种多样性及其分布影响的研究还相对较少。该文基于云南普洱30 hm²森林动态监测样地(大样地) 750个20 m × 20 m的样方调查数据, 以海拔、坡度、凹凸度和坡向4个地形因子为变量, 采用C均值模糊聚类分析大样地的地形类型, 进而分析不同地形条件下的群落物种组成及群落物种多样性; 采用Torus转换检验法, 探讨物种与地形关联性, 为季风常绿阔叶林生物多样性保护提供科学依据。研究结果表明, 大样地可分为山脊、陡坡、缓坡、高谷和沟谷等5类地形, 地形面积分别是8.00、6.04、7.68、2.76和5.52 hm²。大样地中胸径(DBH) ≥ 1 cm的木本植物个体153 418株, 分属79科179属271种。5类地形中, 物种丰富度、不同径级的植株密度和比例明显不同, 多样性及优势物种多度分布具有较大差异。种-面积曲线表明, 同等面积条件下, 随着取样面积增加, 山脊的物种丰富度始终最小, 高谷次之, 沟谷的物种丰富度始终最大。种-个体数累积曲线表明, 随着个体数增加, 山脊物种丰富度的累积速率最小, 种丰富度增加缓慢, 高谷次之。在被检验的123个物种中, 与地形相关的物种有83个, 高达67.5%的物种与至少一类地形存在显著相关关系。山脊和缓坡中与地形具有显著负相关关系的物种数超过显著正相关的物种数; 而与陡坡、高谷和沟谷显著正相关的物种数高于显著负相关的物种数。普洱大样地地形异质性对物种多样性维持的贡献率为7.8%。     

Topographical pattern of the forest vegetation on a river basin in a warm-temperate hilly region,central Japan

The relationship between tree distribution and topography was examined in a small river basin (3.4 ha) comprising a complex mosaic of topographical units at 10² to 10³ m² order, each of which had a shallow valley bordered by small ridges or breaks of slopes. Twenty-five major woody species were divided into two groups (groups A and B) based on a cluster analysis using the distribution data in the basin. Group A, which mainly consisted of early-successional species, was distributed around the valley sites of the topographical units, while group B, which mainly consisted of late-successional species, was distributed around the ridge sites of the topographical units. This vegetation pattern coincided with erosional condition in the basin. That is, the valley sites were eroded more actively than the ridge sites, as soil depth tended to be thin in the valley sites and thick in the ridge sites, and because large (canopy) trees were restricted in the ridge sites. There was no tendency that group B was replacing group A, and hence it was suggested that repeated disturbance by slope failures or small-scale shallow landslides have prevented compositional change from the early-successional (group A) to the late-successional (group B) species by preventing the invasion of the latter into valley sites.     

Variation of carbon and nitrogen cycling processes along a topographic gradient in a central Amazonian forest

It is well recognized in the literature that topography can influence soil nutrient stocks and dynamics in temperate regions, but for tropical forests, this source of variation has sometimes been ignored. The nature of such variations may depend upon the soil type, which in turn, is closely linked to local or regional topography. This study characterizes the soil and describes the status of carbon and nitrogen in vegetation, litterfall, litter‐layer and soil upper layers along the main positions of a topographic gradient (plateau, slope and valley), 60 km north of Manaus, on Cuieiras Reserve watershed. Nitrogen concentrations in living leaves, fresh litterfall, litter‐layer and soil upper layers were lower in the valley than in both slope and plateau plots. Carbon concentrations in plant material were not significantly different among the three topographic positions, resulting in higher C : N ratios in valley plots. Local topography (plateau, slope and valley) clearly was an influential factor in the nutrient distribution along the study locations. Lower rates of N cycling processes in the valley are probably related to its sandy soil texture and seasonal flooding.     

水土流失治理措施对小流域土壤有机碳和全氮的影响

明确综合治理条件下小流域土壤有机碳(Soil organic carbon,SOC)和全氮(Total nitrogen,TN)的空间分布特征及其影响因素,对科学评价水土流失区土壤固碳潜力具有重要意义。以黄土高原丘陵沟壑区典型小流域(砖窑沟流域)为对象,基于流域内3种典型地貌类型(梁峁坡、沟坡、沟谷)和3种典型水土流失治理措施(水平梯田、林地和草地措施,坡耕地为对照),采集土壤样品737个,研究地貌类型和水土流失治理措施对小流域SOC和TN变化的影响。结果表明,同一地貌类型上,水平梯田、林地和草地措施的SOC和TN(0—10 cm土层)含量均显著高于坡耕地(P<0.1)。梁峁坡上,水平梯田、林地和草地措施条件下的SOC和TN含量较坡耕地依次提高了18%和24%、70%和59%、25%和21%;沟坡上,林地和草地措施的SOC和TN较坡耕地依次提高了76%和54%、25%和27%。同一治理措施在不同地貌类型间对0—10 cm土层SOC和TN的影响存在显著差异(P<0.1)。水平梯田条件下,沟谷的SOC和TN含量比峁坡提高了46%和43%;林地措施条件下,沟坡的SOC和TN含量比峁坡提高了18%和6%;草地措施条件下,沟坡的SOC和TN含量比峁坡提高了14%和18%。0—100 cm土层的SOC或TN在不同地貌类型或不同治理措施间的差异与土壤水分含量(Soil moisture,SM)的变化趋势基本一致,并且SOC或TN与SM呈指数关系y=aebx(y为SOC或TN,x为SM)。     

Allelopathic potential of the neotropical dry-forest tree Acacia pennatula Benth.: inhibition of seedling establishment exceeds facilitation under tree canopies

Secondary succession after land abandonment in tropical dry forests has been suggested to be favoured by the facilitation effects for seedling establishment exerted by pioneer trees isolated in these savannah-like landscapes. However, it has also been noticed that these pioneer species may sometimes have an encroaching effect and arrest succession for several decades. We investigated in this study whether allelopathy can play a role in limiting seedling establishment of co-occurring tree species under the canopy of Acacia pennatula by means of lab bioassays and field experiments in north-west Nicaragua. Leaf extracts of A. pennatula did not affect seed germination but reduced the general growth and especially the development of the root compartment in seedlings, shifting their biomass allocation model to a reduced root/shoot ratio. Survival of planted seedlings under the canopy of A. pennatula was about 20–30% lower than outside, and this reduction was particularly pronounced as the dry season progressed, despite the milder conditions (e.g. higher soil moisture) being experienced in the inner positions under the canopy. Altogether, our results suggest that, rather than facilitating, A. pennatula may inhibit the establishment of seedlings under its canopy probably by means of an allelopathic interference in the development of the root system with critical negative consequences for young seedlings in terms of overcoming the dry season. This article warns about overemphasizing the nucleation effect that remnant and isolated trees may have to facilitate secondary succession in these highly disturbed savannah-like tropical dry forests.     

Variation in nutrient characteristics of surface soils from the Luquillo Experimental Forest of Puerto Rico: A multivariate perspective

We assessed the effects of landscape features (vegetation type and topography), season, and spatial hierarchy on the nutrient content of surface soils in the Luquillo Experimental Forest (LEF) of Puerto Rico. Considerable spatial variation characterized the soils of the LEF, and differences between replicate sites within each combination of vegetation type (tabonuco vs. palo colorado vs. dwarf vs. pasture) and topographic position (ridge vs valley) accounted for 11–60% of the total variation in soil properties. Nevertheless, mean soil properties differed significantly among vegetation types, between topographic positions, and between seasons (wet vs dry). Differences among vegetation types reflected soil properties (e.g., bulk density, soil moisture, Na, P, C, N, S) that typically are related to biological processes and inputs of water. In forests, differences between topographic positions reflected elements (e.g., Ca, Mg, K, and Al) that typically are associated with geochemical processes; however, the nutrients and elements responsible for topographic differences in dwarf forest were different from those in other forest types. In pastures, differences between topographic positions were associated with the same soil properties responsible for differences among the other vegetation types. Pastures also had reduced N levels and different soil characteristics compared to undisturbed tabonuco forest. The only soil parameter that differed significantly between seasons was soil moisture. Soils of the LEF do not support the contention that N becomes limiting with an increase in elevation, and suggest that absolute pool sizes of N and P are not responsible for the reduction in productivity with elevation.     

Impacts of secondary seed dispersal and herbivory on seedling survival in Aesculus turbinata

Abstract. Aesculus turbinata is a tree species with large seeds (6.2 g mean dry weight). We studied the demography of its seeds and seedlings in a temperate deciduous forest in northern Japan to elucidate the ecological significance of large seeds with special reference to herbivory and secondary dispersal. Both seed and seedling stages suffered greatly from herbivores. Seedling herbivory was important judged from experiments with shoot clipping and hypogeal cotyledon removal. However, some seedlings survived through re-sprouting after herbivory. Survival rate and percentage resprouting seedlings were lower than those with remaining cotyledons, though seedling size was not affected. This suggests that stored resources in hypogeal cotyledons are working as a kind of ‘risk hedge’ against severe aboveground shoot clipping experienced by A. turbinata. The spatial distribution of seedlings was expanded via seed scatter-hoarding by rodents. Seedling survival rate was higher within canopy gaps than under closed canopy, indicating that canopy gaps are safe sites for establishment, and was negatively correlated with seedling density. Therefore, secondary seed dispersal in this species seems to be effective in ‘finding’ safe sites and in ‘escaping’ density-dependent mortality. The large seeds and seedlings of A. turbinata are attractive to herbivores, but the high resistance of seedlings to herbivory due to large reserves and the effective secondary dispersal appear to mitigate these disadvantages.     

Habitat degradation,topography and rainfall variability interact to determine seed distribution and recruitment in a sand dune grassland

Question: Understanding the mechanisms underlying how habitat degradation, topography and rainfall variability interactively affect seed distribution and seedling recruitment is crucial for explaining plant community patterns and dynamics. Interactions between these major factors were studied together in a semiarid sand dune grassland. Location: Eastern Inner Mongolia, China. Methods: The study system used four sites of fixed, semifixed, semishifting and shifting sand dune grasslands, representing a gradient of habitat degradation. We investigated the density of germinable seeds deposited in the top 5 cm of soil and in situ seedling emergence (number of seedlings emerging early in the growing season) and establishment (number of plants recruited at the end of the growing season) at three topographic positions (dune top, windward and leeward sides) within each site over 2 years that differed in rainfall. Habitat characteristics (i.e. vegetation cover, plant species composition and diversity, soil moisture and nutrient availability and soil erodibility) of the four sites were also measured. Results: Habitat degradation (i.e. decreased vegetation cover and enhanced wind erosion rate) significantly reduced the size of the germinable soil seed bank. On average, germinable seed number from the high‐vegetation cover fixed dune was 36‐fold larger than the low‐vegetation cover shifting dune, and eight‐ and two‐fold larger, respectively, than the semishifting and semifixed dunes with intermediate vegetation cover. We observed within‐habitat variability in seed distribution, but among‐topographic position variation differed among habitats. Seedling recruitment showed large between‐year, and among‐ and within‐habitat variability, but these variations varied significantly depending on the response variables evaluated (i.e. initial seedling density, final plant density, emergence rate and recruitment rate). Path analysis revealed complex density‐dependent positive and negative, direct and indirect effects of germinable seed density and initial seedling density on recruitment, but the relative importance of these density‐dependent effects varied depending on habitat type and rainfall availability. Conclusion: Our results suggest that habitat degradation, microtopography and rainfall availability interact in shaping sand dune seed bank and plant community recruitment patterns and dynamics. Their effects were mainly mediated through changes in both the biotic and abiotic environment during the process of habitat deterioration.     

Linking seed dispersal,germination and seedling recruitment in the invasive species <Emphasis Type="Italic">Berberis darwinii</Emphasis> (Darwin’s barberry)

Seedling recruitment is a multi-phased process involving seed production, dispersal, germination, seedling establishment and subsequent survival. Understanding the factors that determine success at each stage of this process is of particular interest to scientists and managers seeking to understand how invasive species spread and persist, and identify critical stages for management. To understand the factors and processes influencing recruitment of the invasive species Berberis darwinii Hook. (Darwin’s barberry), temporal and spatial patterns of seed dispersal, germination and seedling establishment were examined. Seed dispersal from a large source population was measured over two fruiting seasons, and subsequent patterns of seedling emergence and survival within each cohort were measured. Seed longevity was tested under both natural and artificial conditions. Seeds were widely dispersed by birds, up to 450 m from the source population. Dispersal was essential to seedling establishment, as few seedlings survived beneath the parent canopy. Seeds were relatively short-lived in the soil under both field and glasshouse conditions, with few surviving for more than 1 year. Patterns of newly emerged seedlings largely reflected patterns of seed rain, but seedling survival was significantly affected by distance from source population, seedling density and light environment. These results suggest that recruitment of B. darwinii is dependent on dispersal of seeds to favourable microsites. Management priorities should include the removal of fruiting plants, and seedling control in highlight areas.     

北京东灵山区两种生境条件下辽东栎幼苗补充与建立的比较

 北京东灵山区的人工油松（Pinus tabulaeformis）林中常常混生有萌生的辽东栎（Quercus wutaishanica）种群。局部地带辽东栎与油松形成混交林,在山脊及山坡上部一些特殊生境辽东栎甚至取代油松林成为优势种。为了探讨北京东灵山区辽东栎林的天然更新机制及辽东栎幼苗在人工油松林中的天然更新,在油松人工林的林下和林缘两种生境条件下将辽东栎种子播种于2 cm深的土中,对辽东栎幼苗的补充和建立进行了对照试验。结果显示,在辽东栎结实丰年,两种生境条件下播种后辽东栎种子丢失差异并不显著,而辽东栎幼苗补充和建立方面的差异极显著。由于森林动物特别是啮齿类动物的活动对辽东栎幼苗建立的影响具有二重性：消耗种子与促进萌发,本试验特别关注辽东栎幼苗子叶丢失现象及其丢失后果。由于辽东栎种子萌根较早,在幼苗出土前其粗大的主根长达10～20 cm,并已初步形成根系,表明已有相当比例的营养物质从子叶转移到根部。因此当幼苗出土后如果仅仅只是子叶的丢失对辽东栎幼苗的成活和生长均无明显的影响。通过研究发现辽东栎幼苗期子叶丢失有3种形式：1）仅丢失子叶,幼苗其它部位未受伤害,多发生在林缘生境中,占发芽坚果的29.69%;2）整个幼苗连根被拖出地面,多发生在林下生境,占62.43%;3）在子叶与根颈的连接处主干被咬断,林缘为50.88%,显著高于林下的8.41%。但只有后两种形式才导致幼苗死亡。导致这些现象的原因是由于林缘土壤腐殖质含量低,比较干燥,土质较坚实,以及幼苗出土前坚果主根生长得较长等缘故,当幼苗遭受动物捕食时不会轻易地被拖出地面,拖走的往往仅仅是子叶。至生长季节结束,林缘样方辽东栎幼苗的成活率接近20%,茎干平均高度7.94 cm,芽数11.24·株-1,叶片平均干重为81.14 mg·株-1,且几乎全部由丢失子叶的幼苗所构成;而林下样方辽东栎幼苗的成活率不足2%,茎干平均高度4.74 cm,芽数7.52·株-1,叶片平均干重仅为42.27 mg·株-1,以未丢失子叶的幼苗为主。试验结果表明,林缘及类似林缘的环境条件更有利于辽东栎的实生更新。     

Forest encroachment into a Californian grassland: examining the simultaneous effects of facilitation and competition on tree seedling recruitment

Competition and facilitation are both considered major factors affecting the structure of plant assemblages, yet few studies have quantified positive, negative, and net effects simultaneously. In this study, we investigated the positive, negative, and net effects of tree saplings on the encroachment of two tree species, Douglas fir (Pseudotsuga menziesii) and tanoak (Lithocarpus densiflora), into a coastal California grassland. The study involved three components: sampling the spatial distributions of P. menziesii and L. densiflora in the grasslands, a field experiment examining seedling survival in different grassland environments, and a greenhouse experiment examining the effects of soil moisture on early seedling performance. The field experiment was conducted over a 2-year period, using Pseudotsuga in 2002 and both species in 2003. Seedlings were separated into four treatment groups: those planted in open grassland, in shaded grassland, under artificial (plastic) conifer saplings, and under natural Pseudotsuga saplings. Air temperature, relative humidity, soil moisture, incident radiation levels and fog water inputs were measured for each treatment group in 2003. In the greenhouse experiment, Pseudotsuga and Lithocarpus seedlings were grown for 13 weeks in watering treatments simulating the summer soil moisture conditions of the open grasslands and under Pseudotsuga saplings. Surveys of naturally established seedlings found that Lithocarpus occurred only under Pseudotsuga saplings, while most Pseudotsuga seedlings were located near but not directly under conspecific saplings. In the field experiment, positive effects of tree saplings were much larger than negative effects, resulting in strong net facilitation of seedling establishment. Survival for both species was always higher under the plastic and live trees than in the open or shade plots. The primary mechanism facilitating seedling survival appeared to be increased soil moisture caused by input of fog precipitation coupled with reduced microsite evaporation. The greenhouse experiment further showed that soil moisture strongly affected seedling performance, with both species having much higher photosynthetic rates in the higher moisture treatment. In the lower moisture treatment, Pseudotsuga seedlings had higher photosynthetic rates and stomatal conductance than Lithocarpus, suggesting they may be able to better tolerate the environmental conditions found in the open grasslands. Our combined results suggest that rate and patterning of woody plant encroachment can be strongly influenced by facilitation and that fog precipitation may play a key role in plant interactions.