常绿阔叶林林冠环境对栲幼苗建成的影响

植物幼苗建成阶段是决定种群自然更新的关键生活史阶段。研究林冠环境对常绿阔叶林优势种幼苗建成阶段的影响对该类森林的恢复和管理具有重要意义。2014-2016年, 该研究在重庆市缙云山国家级自然保护区的常绿阔叶林的不同林冠环境(大林窗: >150 m ², 中林窗: 100-150 m ², 小林窗: 50-100 m ², 对照: 林下)下进行栲(Castanopsis fargesii)种子野外播种实验, 并对栲幼苗命运和生长情况进行了3年的持续监测。结果表明: (1)栲幼苗出土时间从7月持续到12月, 出苗时间较长, 大林窗对幼苗出土具有延迟作用; (2)栲种子野外平均萌发率为(62.8 ± 2.0)%, 第3个生长季(2016年)末幼苗平均存活率为(65.1 ± 2.2)%, 枯萎是栲幼苗死亡的主要原因; (3)林冠环境对栲种子萌发率及第1个生长季(2014年)末的幼苗存活率无显著影响, 对第2个(2015年)和第3个生长季末的幼苗存活率具有显著影响; (4)林冠环境在第1个生长季对幼苗生长无明显影响, 但在第2个和第3个生长季具有显著影响, 大、中林窗中幼苗总生物量、株高、基径、根长和叶片数显著高于林下, 比叶面积显著低于林下; (5) 3个生长季内, 4类林冠条件下栲幼苗的叶质量比和茎质量比升高, 根质量比和根冠比降低, 并且从第2个生长季开始大林窗中栲幼苗的叶质量比显著高于林下, 根质量比和根冠比显著低于林下。栲幼苗早期的存活和生长依赖种子储存的能量, 受林冠条件影响较弱, 后期则依赖光合作用, 受林冠条件影响较强, 从整个幼苗建成过程看, 大、中林窗更有利于栲幼苗定居。     

常绿阔叶林林冠环境对栲幼苗建成的影响

植物幼苗建成阶段是决定种群自然更新的关键生活史阶段。研究林冠环境对常绿阔叶林优势种幼苗建成阶段的影响对该类森林的恢复和管理具有重要意义。2014–2016年,该研究在重庆市缙云山国家级自然保护区的常绿阔叶林的不同林冠环境(大林窗:150 m~2,中林窗:100–150 m~2,小林窗:50–100 m~2,对照:林下)下进行栲(Castanopsis fargesii)种子野外播种实验,并对栲幼苗命运和生长情况进行了3年的持续监测。结果表明:(1)栲幼苗出土时间从7月持续到12月,出苗时间较长,大林窗对幼苗出土具有延迟作用;(2)栲种子野外平均萌发率为(62.8±2.0)%,第3个生长季(2016年)末幼苗平均存活率为(65.1±2.2)%,枯萎是栲幼苗死亡的主要原因;(3)林冠环境对栲种子萌发率及第1个生长季(2014年)末的幼苗存活率无显著影响,对第2个(2015年)和第3个生长季末的幼苗存活率具有显著影响;(4)林冠环境在第1个生长季对幼苗生长无明显影响,但在第2个和第3个生长季具有显著影响,大、中林窗中幼苗总生物量、株高、基径、根长和叶片数显著高于林下,比叶面积显著低于林下;(5) 3个生长季内, 4类林冠条件下栲幼苗的叶质量比和茎质量比升高,根质量比和根冠比降低,并且从第2个生长季开始大林窗中栲幼苗的叶质量比显著高于林下,根质量比和根冠比显著低于林下。栲幼苗早期的存活和生长依赖种子储存的能量,受林冠条件影响较弱,后期则依赖光合作用,受林冠条件影响较强,从整个幼苗建成过程看,大、中林窗更有利于栲幼苗定居。     

杉木和木荷种子萌发及幼苗生长对光梯度的响应

光是影响种子萌发和幼苗生长的关键因素.为理解不同树种种子萌发及幼苗生长对光梯度变化的响应机制,本文研究了不同光照强度(分别为自然光强的100%、60%、40%、15%和5%)对杉木和木荷种子萌发及幼苗生长的影响,探讨了两树种种子萌发和幼苗生长对光照响应的差异性.结果表明: 光照强度对两树种的种子萌发和幼苗生长均具有显著影响. 随着光照强度的减弱, 杉木种子萌发率增大,萌发指数增大,木荷种子萌发率和萌发指数则先增大后减小,在40%光照强度下达到最大值.两树种幼苗存活率在全光照(100%光照)下均为0,在5%～60%光照处理下则随着光照强度的减弱而显著降低.两树种幼苗根长、地径和株高对光梯度变化的响应趋势一致,随着光照强度的减弱,根长显著减小,地径和株高则先增大后减小,在5%光照强度下达到最小.随着光照强度的减弱,杉木幼苗根、茎、叶及总生物量降低,木荷幼苗生物量积累在15%～60%光照强度下较高, 5%光照强度下最小,且相同光照强度下,木荷幼苗各部分生物量均大于杉木.两树种幼苗应对低光环境时,表现出较大的茎和叶的生物量分配比,而根生物量比和根冠比降低.表明杉木苗期生长不耐阴,需要相对较强的光照,而木荷苗期具有较强的耐阴性,对弱光环境的适应性更强,能够在郁闭的林冠下定植和正常生长.     

山地落叶阔叶林优势树种米心水青冈幼苗的定居

在郁闭的林冠下、模拟林窗和林间空旷地3种光照环境中,研究了施肥和未施肥的米心水青冈幼苗生存和生长发育过程．结果表明,幼苗在林下的发育受到光照不足的严重抑制,生长在林下的幼苗比生长在林窗和空旷地幼苗死亡快,死亡率高,幼苗在郁闭林下能够生存的时间不超过12周．施肥处理的幼苗比对照的幼苗死亡快,死亡率高．幼苗死亡很可能与土壤中的病原生物有关．虽然由林下光照强度增加到模拟林窗的光照强度后,幼苗发育有明显改善,但幼苗在模拟林窗环境和空旷地的生长没有显著差异．结果说明,米心水青冈新出土的幼苗在郁闭林冠下易受伤害,更新立苗阶段需要有比较好的光照条件,肥沃土壤可能也无助于改善郁闭林冠下幼苗的定居．     

三峡地区柏木种子萌发和幼苗更新的研究

对三峡地区柏木(Cupressus funebris)林柏木种子的萌发、幼苗存活、幼苗生长进行了研究.结果表明,(1)柏木种子的萌动期约为10 d,播种35-45 d后进入萌发高峰期,柏木种子的平均萌发率为47.7%.土壤基质对种子萌发的影响是极显著的,光照对萌发则没有显著的影响.(2)幼苗平均死亡率为52.6%.土壤类型和光照强度对幼苗的存活都有显著影响,在林下土和80%光照条件下幼苗的存活率最高.(3)柏木幼苗生长初期茎的生长动态呈"S"型,而叶片数量的增长动态近于直线型.幼苗生长初期生物量形成主要受光照条件的影响,土壤类型的影响不显著.土壤和光照条件对柏木林柏木幼苗的更新都有显著影响,但是土壤条件主要影响种子萌发和幼苗的存活,而光照条件则主要对幼苗的生长和存活产生影响.     

鸟类对山黄麻种子的传播及其生态作用

在云南西双版纳的勐宋和勐仑地区观察和研究了传统山黄麻种子的鸟类。在野外持网捕获摄食山黄麻果实的鸟类,称量后分别放在布袋内,让其自然排泄,然后标志释放,观察和网捕共记录到摄食山黄麻果实的鸟类36种,其中26种挂网捕获并从排泄物得到种子。用鸟类排泄物中的山黄麻种子、山黄麻树上采摘的成熟果实及人工分离山黄麻种子播种,进行萌发对比实验,实验结果表明,直接播种成熟果实,在阳光充足的空旷地、林冠下或用树叶覆盖遮光等条件下都未萌发,种子容易霉烂或被虫蚀;从山黄麻果肉中分离出来的种子具有萌发能力;而经鸟类摄食、消化后从果肉中分离的种子,具有较好的萌发活力。在不同条件下山黄麻种子萌发情况差异明显,林冠下条件对种子有抑制作用,阳光作用下种子显示出很好的萌发及成活率。鸟类排泄物种子放在林上或用树叶覆盖遮光,经过较长时间便可萌发,而人工分离种子不能萌发。鸟类摄食及消化过程对山黄麻种子的处理表现出很好的生态互惠关系,观察萌发后的山黄麻幼苗的生长情况还表明,幼苗生长也需要光照和较开阔的生境条件,鸟类取食山黄麻果实及其将种子远距离传播适宜生境,正好具有这样的生态作用。     

不同光照对望天树种子萌发和幼苗早期生长的影响

在不同光照梯度的人工遮荫和森林生境中,研究了西双版纳季节雨林标志树种望天树的种子萌发和幼苗早期生长特征.结果表明,裸地上的强光照和深度遮荫均不利于望天树种子的萌发,中等程度的遮荫有利于种子萌发.望天树种子萌发率在林窗中央最大,而且萌发迅速,林窗边缘和林下生境不利于种子萌发.幼苗株高、基径和单株叶面积等生长指标均在部分遮荫处理条件下最大;幼苗根冠比在裸地上最高,且随遮荫程度的增加而降低;幼苗比叶面积在一定光照强度范围内随遮荫程度的增加而增大,在3层遮荫最大.除幼苗根冠比以外的其它生长参数均在林窗中央最大.讨论了环境因子(主要是光照强度和光质)对望天树种子萌发和幼苗生长的影响.     

光照对绒毛番龙眼种子萌发的影响

在实验室和自然条件下,研究了光照强度和光照时间对西双版纳季节雨林冠层树种绒毛番龙眼(Pometia tomentosa)种子萌发的影响.结果表明:不论是在持续光照还是在周期性光照条件下,3种不同强度光照处理的绒毛番龙眼种子萌发率都很高,均与对照(全黑暗)无显著差异;光照可加快种子萌发速度(降低萌发速率系数),提高萌发指数和幼苗活力指数.人工遮荫有利于绒毛番龙眼种子萌发率的提高,在100%自然全光照(natural sunlight, NS)下萌发率最低(27.8%),而在0.6% NS处理萌发率最高(78.9%);萌发速率系数在全光照下最大,随遮荫程度的增加而降低,萌发指数和幼苗活力指数则随遮荫程度的增加而增大.在遮荫萌发实验中,绒毛番龙眼种子的萌发可能主要受种子脱水敏感性的影响,即强光下剧烈的水分蒸发引起的暂时水分亏缺导致种子脱水死亡或萌发延迟;已经"萌发"的种子也可能因短期水分胁迫而不能生长到"第一对真叶出现"这一萌发标准就死亡,从而降低种子萌发率.在绒毛番龙眼种质资源保护工作中,既要注意避免强光下温度升高引起的种子过度脱水死亡,又要保证提供适宜光照条件以提高萌发质量和幼苗成活率.     

环境因子对黄顶菊种子萌发的影响

黄顶菊是一种入侵性极强的外来杂草,其种子对环境的适应是其成功入侵的前提.通过室内条件下黄顶菊种子的物理特性、环境因子对其种子萌发及幼苗生长影响等方面的研究,可为探讨黄顶菊快速蔓延的原因与揭示黄顶菊成功入侵机制提供理论依据.结果表明,黄顶菊种子极多而轻与黄顶菊大面积扩散密切相关,温度、光照和种子埋藏深度是决定黄顶菊种子萌发的关键因素.黄顶菊种子萌发的温度范围为5～40℃,25℃最适于种子的萌发,25～35℃条件下有利于幼苗的生长;黄顶菊种子是光敏感型种子,在土壤表面的萌发率最高(74.44%),大于3cm的深层土壤中则不能萌发;pH 对黄顶菊种子的萌发影响不明显,但其幼苗在酸性条件下生长较好;黄顶菊种子的萌发能忍受一定的盐胁迫和干旱胁迫,但随着胁迫程度的加重,根长和苗长逐渐降低,50%的土壤含水量对黄顶菊种子萌发和幼苗生长比较适宜.     

不同光照处理下青藏高原克隆植物黄帚橐吾(Ligulariavirgaurea)种子大小对其幼苗生长的影响

以高寒草甸克隆植物黄帚橐吾为实验材料,通过遮荫网模拟植被遮荫,研究种子大小与萌发及幼苗生长能力的关系和幼苗对光照条件的反应。结果表明：（1）在自然光照下,黄帚橐吾种子大小对种子萌发的影响显著,大种子的萌发率高于小种子。遮荫生境下,大、小种子萌发率有所降低,但遮荫对小种子萌发的影响比大种子显著。小种子的萌发率下降了近1/8,而大种子的萌发率仅下降了1/11。（2）黄帚橐吾种子大小对幼苗生物量积累影响显著,大种子幼苗总生物量（TB）大于小种子幼苗的。但生物量的分配与播种时间相关,播种后60 d,在自然光照条件下,大种子幼苗对根生物量的分配大于小种子幼苗,而对叶生物量的分配则正好相反。在遮荫环境中,大、小种子幼苗普遍对根的生物量分配增加,大种子幼苗根冠比（R/S）大于小种子幼苗。（3）黄帚橐吾种子大小对幼苗的生长也有明显影响。在自然光照下,小种子幼苗的相对生长速率（RGR）较大于大种子幼苗,但叶面积比率（LAR）、叶面积干质量比（SLA）、叶干质量（LWR）差别不明显。在遮荫条件下,幼苗的LAR、SLA、LWR显著增加,但大、小种子幼苗间差异不显著,幼苗的RGR减小,小种子幼苗的减小趋势大于大种子幼苗。     

Fine-scale spatial heterogeneity and seed size determine early seedling survival in a desert perennial shrub (<Emphasis Type="Italic">Ericameria nauseosa:</Emphasis> Asteraceae)

In heterogenous landscapes, seeds settle in some microsites more readily than others, independently of whether those microsites are suitable for germination and subsequent growth, and survival of seedlings. Wild seeds and seedlings of Ericameria nauseosa var. oreophila in the sand-dune ecosystem near Mono Lake, California show both concordance and conflict in where seeds are retained and where seedlings flourish. Using a field experiment, we followed performance of seeds and seedlings (i.e., the rate of germination and successful emergence, initial seedling size, seedling growth and survival over first growing season), by planting seeds of known sizes in pre-identified microsites (windward and lee aspects of interspaces between shrubs, under shrub canopies, and under snag canopies, respectively). Heavier seeds were more likely to germinate and emerge successfully and resulted in initially larger seedlings than lighter seeds. These initially larger seedlings subsequently remained larger over the growing season and lived longer than initially smaller seedlings. Independent of seed and initial seedling size, seedlings that germinated under the canopy of adult shrubs grew larger and survived longer than seedlings that germinated in interspaces (open space with little or no vegetation) or under snags (dead adult shrubs). Seedlings grown under the canopy of adult shrubs experienced significantly less solar radiation and wind-deposited sand than seedlings grown in interspace or snag microsites. Sand burial more than 1 cm was lethal for seedlings. The two variables of seed size and microsite type had by far greater impact on eventual growth and survival of seedlings than did aspect, and the effects of the former two variables were independent of each other. This study amplifies the body of work on E. nauseosa at this site demonstrating that the earliest events in the life history of this long-lived perennial shrub appear to persist through time despite the harsh and variable environment.     

Promotion by 5-Aminolevulinic Acid of Germination of Pakchoi （Brassica campestris ssp. chinensis var. communis Tsen et Lee） Seeds Under Salt Stress

The seed germination and seedling growth of pakchoi （Brassica campestris ssp. chinensis var. communis Tsen et Lee cv. Hanxiao） were not significantly inhibited until the concentration of NaCl was increased to 150 mmol/L. Treatment of pakchoi seeds with exogenous 5-aminolevulinic acid （ALA）, at concentrations ranging from 0.01 to 10.00 mg/L, promoted seed germination when seeds were stressed by salinity, whereas levulinic acid （LA）, an inhibitor of ALA dehydrase, significantly inhibited seed germination and seedling growth, suggesting that metabolism of ALA into porphyrin compounds was necessary for seed germination and seedling growth. Determination of respiratory rate during seed germination showed that ALA increased seed respiration under both normal conditions and salt stress. Furthermore, salt stress decreased levels of endogenous ALA, as well as heme, in etiolated seedlings. More salt-tolerant cultivars of pakchoi contained higher relative levels of endogenous ALA and heme under conditions of salt stress. These results indicate that salt stress may inhibit the biosynthesis of endogenous ALA and then heme, which is necessary for seed germination, and treatment of seeds with exogenous ALA prior to germination may be associated with the biosynthesis of heme.     

R.M. Cowling,D.M. Richardson & S.M. Pierce (eds.) 1977. Vegetation of Southern Africa.

We determined the role of bird dispersal in seed and seedling dynamics of the tree Kalopanax pictus from 7 years of observing seed rain and seedling emergence in a broad-leaved deciduous forest in central Japan. We also performed an experiment on the influence of seed pulp on germination of seeds of K. pictus. Seeds of this species can lie dormant for several years, and this causes rather constant yearly seedling emergence in spite of irregular seed production. The spatial distribution of the seedlings that emerged each year (maximum distance from nearest conspecific seed-bearing tree of 90 m) was wider than that of gravity-dispersed seeds (max. distance of 37 m), suggesting seed dispersal by birds in winter. Emerged seedling densities at sites over 20 m from the nearest conspecific seed-bearing tree were highest in the spring of 1991, about half a year after the largest seed fall of the observation period. However, emerged seedling densities within 20 m from seed-bearing trees were highest in 1992, 1.5 years after the largest seed fall. These field observations may be explained by the experimental results on the effects of seed pulp on germination. Intact seeds germinate slowly at low germination rates, while seeds without seed pulp germinate quickly at high germination rates. Fallen seeds with seed pulp thus appear to form a seed bank near seed sources (temporal dispersal), while seeds scattered by birds appear to increase the possibility of reaching the present safe sites in distant areas with quick germination (spatial dispersal).     

A test of the escape and colonization hypotheses for zoochorous tree species in a Western Amazonian forest

In order to assess the importance of seed dispersal (escape and colonization hypotheses), I used transplant experiments for seeds and seedlings of 5–11 plant species with fleshy fruits in a lowland tropical forest (Tinigua National Park, Colombia). I controlled seed density, distance to parental tree, and habitat type. I monitored seed removal, seedling survival, and seedling growth during the first year of development for an average of 554 seeds and 169 seedlings for each species. I supplemented the experimental results with measurements of natural recruitment. I found little support for the escape hypothesis during the seed and seedling stages. For six species that showed differences in seed removal associated with distance, five showed highest removal away from, than close to parent trees, suggesting predator satiation. Seedling survival during the first year was not consistently associated with low densities and long distances from parent trees. For the majority of species, seedlings did not survive flooding in low basins, and there was growth advantage for most plant species in canopy gaps. These differences imply advantages for seed dispersal to adequate habitats, as predicted by the colonization hypothesis. In contrast to experiments, strong negative distance-dependent effects were evident when analyzing natural recruitment patterns. The ratio between saplings and seedlings was higher away from parent trees for the species with enough recruitment to be analyzed and this suggests that a negative distance-dependent effect may also occur after seedling establishment. This pattern is suspected for several other species, but an analysis with some of the other most common trees showed a variety of negative, neutral, and positive distance dependent effects. This study emphasizes the importance of long-term studies to asses the role of seed dispersal.     

Seedling Establishment of a Canopy Tree Species in Malaysian Tropical Rain Forests

Abstract Unlike many other canopy tree species in tropical rain forests, Dryobalanops aromatica (Dipterocar-paceae, local name: Kapur) establishes monospecific dominant (monodominant) forests in Peninsular Malaysia. In natural conditions, monodominance of Kapur does not necessarily mean low species diversity of the Kapur forests. While the emergent canopy layer is occupied by Kapur, many other plant species, which are common to lowland dipterocarp primary forests in the same region, are found in lower canopy layer and understory.
To understand the ecological implications of the monodominance of Kapur, we monitored post dispersal survival and seedling establishment in a pure stand of Kapur in a plantation in Kepong, near Kuala Lumpur. Immediately after seed fall, seeds and cotyledon-stage seedlings suffered high predation by vertebrates such as rodents. The predation pressure was higher in a more general fruiting year (1991) than in a sporadic fruiting year (1992). In contrast to the high mortality of seeds and newly emerged seedlings, seedlings surviving to the six-leaf stage showed low mortality, which allowed the establishment of a sapling bank.
The occurrence of saplings of Kapur with a wide range of size classes in natural forests indicates that this species is more shade tolerant than other dipterocarp species such as the Shorea group and that it could well respond to enhanced light conditions caused by canopy opening. These characteristics may partly contribute to maintaining monodominance of Kapur.     

An interactive effect of simultaneous death of dwarf bamboo, canopy gap, and predatory rodents on beech regeneration

To clarify the interactive effect of the simultaneous death of dwarf bamboo (Sasa kurilensis), forest canopy gap formation, and seed predators on beech (Fagus crenata) regeneration, we analyzed beech demography from seed fall until the end of the first growing season of seedlings in an old-growth forest near Lake Towada, northern Japan. The simultaneous death of S. kurilensis took place in 1995. We established four types of sampling site differing in forest canopy conditions (closed or gap) and Sasa status (dead or alive). Beech seed survival and emergence ratio were both highest in gaps with dead Sasa (gap-dead), because rate of predation was lowest. Seedling survival during the first growing season was also highest in the gap-dead treatment, because of less predation and less damping off. As a result, even though density of seed fall was lowest in the gap-dead treatment, the living seedling density there was highest at the end of the first growing season. Predation, which caused the greatest mortality during the seed and seedling stages, was significantly lower at both sites in gaps and sites with dead Sasa. This was probably due to changes in the behavior of rodents in response to the structure of the forest canopy and undergrowth. Both the death of Sasa and canopy gap formation allowed seedlings to avoid damping off because of the high light availability. The indirect effect of the simultaneous death of Sasa and canopy gap formation in reducing predation contributed more to beech regeneration than their direct effect in increasing light for the seedlings.     

Seed and seedling characteristics of the Himalayan hornbeam (Carpinus viminea Wall.), India

To study of seed and seedling characteristics, seeds of the Himalayan Hornbeam (Carpinus viminea Wall.) were collected from low and high-elevation sites during the peak maturation period. Paired t-tests indicated that seed weight, percentage of sound seeds, and germination were significantly greater for high-elevation seeds whereas seed moisture (%) was low. Seedling growth performance was better for high-elevation seeds than for those from low elevation. This indicates that environmental conditions at high elevation may favour germination and seedling growth of the species.     

Seed rain and advanced regeneration in a tropical rain forest

By comparing seed rain, seedling and sapling community structures we assessed the possible role played by vertebrate seed dispersal as a structuring factor in advanced regeneration of closed-canopied sites in the tropical rain forest of Los Tuxtlas, Mexico. Seed weight, initial morphology of seedlings and species abundance were also analyzed to determine if these traits influenced the probability of establishment in the shade.About half of the seed species falling in five closed forest sites (25×25 m) during one year came from fruiting trees growing within the sites (local seeds) and half from fruiting trees found outside the sites (immigrant seeds). Seeds of liana and upper-canopy species were over-represented among immigrant seeds compared with seeds of understory tree species. This probably reflects the activity of frugivorous arboreal mammals, bats, and birds. Species with immigrant seeds had both a lower abundance and a narrower spatial distribution than locally produced seeds. Therefore, immigrant seeds showed higher diversity values than locally produced seeds.Average seed size and the proportion of epigeous seedlings were similar in local and immigrant species. Under closed-canopied sites, factors affecting community organization seem to operate selectively, favoring the establishment of large-seeded, local abundant species in the advanced regeneration. However, the fact that some saplings of immigrant species were found in the plots suggest that a slow species infiltration may be occurring leading to a slow shift in the advanced regeneration species composition. We propose that the influence of seed dispersal on advanced regeneration structure depends on the disturbance history of the patches where seeds land.     

Scatter-and clump-dispersal and seedling demography: hypothesis and implications

Summary Fruit-eating animals deposit viable seeds in patterns that determine the conditions under which seeds and seedlings live or die. Many tree species are scatter-dispersed by birds, bats, or other small frugivores that regurgitate, defecate, or drop seeds singly or in pairs. These scatterdispersed plant species normally recruit as isolated individuals, and are unlikely to evolve exceptional resistance to herbivores, pathogens, or to other sources of density-dependent seed or seedling mortality. Other tree species are clump-dispersed by larger terrestrial or arboreal frugivores that defecate seeds in masses which produce bouquets of seedlings. Because their seeds invariably germinate in close proximity to other seedlings, clump-dispersed species necessarily evolve chemical or mechanical defenses against seed predators, pathogens, and herbivores that act in a densitydependent manner.Population and genetic attributes should reflect this basic dichotomy in the conditions of seedling recruitment. I predict that seedlings of scatter-dispersed species rarely survive near parents or in dense aggregations under frugivore roosts. Seed dispersal should be mandatory, often to light gaps or other special habitats. Outbred adults and juveniles are expected to exist at low densities in loose aggregations or random distributions. Seedlings of clump-dispersed trees are pre-adapted for survival in dense aggregations near parents, as well as in fecal clumps. Substantial recruitment of juveniles and young adults should occur from undispersed seeds under and near parent trees. Such species should be common, highly aggregated, and show strong genetic family structure. Because recruitment requires dispersal, scatter-dispersed plant species should be especially vulnerable to loss of dispersal agents. Because offspring consistently recruit near parents, clump-dispersed plants should be less vulnerable to temporary loss of dispersal agents.     

Germination and emergence of annual species and burial depth: Implications for restoration ecology

Due to the high content of viable seeds, topsoil is usually spread on ground left bare during railway and motorway construction to facilitate the regeneration of vegetation cover. However, during handling of the topsoil, seeds are often buried deeply and they cannot germinate or the seedlings cannot emerge from depth. This study experimentally explores the predictive value of seed mass for seed germination, mortality and seedling emergence at different burial depths for 13 common annual species in semiarid Mediterranean environments. We separate the effect of burial depth on germination and emergence by means of two experiments. In the germination experiment, five replicates of 20 seeds for each species were buried at depths ranging from 0 to 4 cm under greenhouse conditions. Germinated and empty or rotten seeds were counted after 8 weeks. In the emergence experiment, five replicates of four newly-germinated seeds per species were buried at the same depths under controlled conditions and emergence was recorded after 3 weeks. The effect of burial depth on percentage of germination and seedling emergence was dependent on seed size. Although all species showed a decrease in germination with burial depth, this decrease was greater for small-than large-seeded species. Percentage of emergence was positively related to seed mass but negatively related to burial depth. Seed mortality was higher for small-than large-seeded species, but there was no general effect of burial depth on this variable. Thus, the current practice of spreading 30 cm deep layers of topsoil in post-construction restoration projects is unadvisable. In this restoration scenario, thinner layers of topsoil should be used to achieve the maximum potential of the topsoil for germination and seedling establishment.