广西马山岩溶次生林群落生物量和碳储量

岩溶植被在岩溶生态系统碳循环和全球碳平衡中具有重要的作用。通过对马山县岩溶次生林年龄序列(幼龄林、中龄林和老龄林)3个演替阶段9个样地(20 m×50 m)的系统取样调查,研究了停止人为干扰后岩溶次生林生物量和碳储量的变化。结果表明:沿幼林、中林和老林群落的顺向演替发展,群落生物量显著增加(P0.05),从幼林群落的48.17 t/hm2、到中林群落113.47 t/hm2,再到老林群落242.59 t/hm2。老林生态系统的碳储量较高,平均为236.69 t/hm2,中林和幼林较低且非常相近,分别为225.17 t/hm2和224.76 t/hm2,各次生林生态系统的碳储量差异不显著(P0.05)。土壤碳储量的大小顺序为幼林(198.44 t/hm2)中林(167.39 t/hm2)老林(113.43 t/hm2)。沿群落正向演替,各次生林生态系统中植物碳储量和土壤碳储量的比例发生明显的变化。幼林的土壤碳储量占生态系统碳储量的88.29%,植物碳储量只占11.71%;中林相应为74.34%和25.66%;而老林为47.92%和52.08%。可见,随着岩溶植被的正向演替,土壤碳转变为植物碳的趋势十分明显,这是岩溶森林不同于酸性土森林的一个显著特征。     

Large Sample Area and Size Are Needed for Forest Soil Seed Bank Studies to Ensure Low Discrepancy with Standing Vegetation

A large number of small-sized samples invariably shows that woody species are absent from forest soil seed banks, leading to a large discrepancy with the seedling bank on the forest floor. We ask: 1) Does this conventional sampling strategy limit the detection of seeds of woody species? 2) Are large sample areas and sample sizes needed for higher recovery of seeds of woody species? We collected 100 samples that were 10 cm (length) ×10 cm (width) ×10 cm (depth), referred to as larger number of small-sized samples (LNSS) in a 1 ha forest plot, and placed them to germinate in a greenhouse, and collected 30 samples that were 1 m×1 m×10 cm, referred to as small number of large-sized samples (SNLS) and placed them (10 each) in a nearby secondary forest, shrub land and grass land. Only 15.7% of woody plant species of the forest stand were detected by the 100 LNSS, contrasting with 22.9%, 37.3% and 20.5% woody plant species being detected by SNLS in the secondary forest, shrub land and grassland, respectively. The increased number of species vs. sampled areas confirmed power-law relationships for forest stand, the LNSS and SNLS at all three recipient sites. Our results, although based on one forest, indicate that conventional LNSS did not yield a high percentage of detection for woody species, but SNLS strategy yielded a higher percentage of detection for woody species in the seed bank if samples were exposed to a better field germination environment. A 4 m² minimum sample area derived from power equations is larger than the sampled area in most studies in the literature. Increased sample size also is needed to obtain an increased sample area if the number of samples is to remain relatively low.     

The viability of seeds of rainforest species after experimental soil burials under tropical wet lowland forest in north-eastern Australia

Seed samples of 50 rainforest species representative of all successional stages from tropical lowland forests in north-eastern Australia were buried in nylon material bags under rainforest for periods up to 2 years. The seed samples were exhumed after 6 weeks, 6 months, 1 year and 2 years, and their viabilities assessed. Substantial portions of the samples of 31 species of pioneer and early and late secondary species retained viability for 2 years in the buried samples, and germinated immediately they were exposed to greenhouse conditions. In approximately one-third of these species, the dormancy was completely enforced by the burial conditions since fresh, mature seed germinated immediately in the greenhouse. The remainder of the long-lived seeds showed some evidence of variable initial periods of innate dormancy produced by their hard, impermeable coats. There was no indication that the burial conditions enforced or induced any dormancy in the seeds of primary forest trees. The mean viability of the primary forest seeds examined was 10% after 6 months burial. The soft-coated seeds were obligate immediate germinators; they germinated or died. The remainder showed variably delayed germination, and this was interpreted as a mechanical dormancy produced by their leathery, fibrous, or stony endocarps. The group of late secondary species sampled contained both obligate immediate germinators and seeds with well developed dormancy mechanisms. Many of the buried 'seedlings’ of primary forest species remained alive for a year or more attached to seeds, demonstrating the ability of the larger primary seeds to sustain seedlings under conditions adverse to photosynthesis and growth. Some seedlings of Castanospermum australe were alive and were transplanted successfully after 2 years burial. This enables primary forest species to stock seedlings on the forest floor in contrast to the secondary species which store seed in the soil.     

A comparison of seed banks across a sand dune successional gradient at Lake Michigan dunes (Indiana,USA)

In habitats where disturbance is frequent, seed banks are important for the regeneration of vegetation. Sand dune systems are dynamic habitats in which sand movement provides intermittent disturbance. As succession proceeds from bare sand to forest, the disturbance decreases. At Indiana Dunes National Lakeshore, we examined the seed banks of three habitat types across a successional gradient: foredunes, secondary dunes, and oak savanna. There were differences among the types of species that germinated from each of the habitats. The mean seed bank density increased across the successional gradient by habitat, from 376 to 433 to 968 seeds m⁻², but with foredune and secondary dune seed bank densities being significantly lower than the savanna seed bank density. The number of seeds germinated was significantly correlated with soil organic carbon, demonstrating for this primary successional sequence that seed density increases with stage and age. The seed bank had much lower species richness than that of the aboveground vegetation across all habitats. Among sites within a habitat type, the similarity of species germinated from the seed banks was very low, illustrating the variability of the seed bank even in similar habitat types. These results suggest that restoration of these habitats cannot rely on seed banks alone.     

亚热带常绿阔叶林不同演替阶段土壤种子库的初步研究

 本文把四川缙云山亚热带常绿阔叶林的演替系列分成6个阶段：(1)弃耕地;(2)灌草丛;(3)马尾松林;(4)以马尾松为优势的常绿针、阔叶混交林;(5)以常绿阔叶树为优势的针、阔叶混交林;(6)常绿阔叶林。分别从每个阶段分3层采集0.5m2,l0cm厚度的5个小样地的土样,带回实验室,通过萌发法研究土壤种子库。得到如下结论：A、种子数量和物种多样性在演替早期阶段(从弃耕地到马尾松林)一般随演替发展而减少,种子数目随土层加深而降低;后期阶段(马尾松林到常绿阔叶林)种子数则大致随演替发展而增加;同一阶段各土层种子数差异不大。B、各演替阶段土壤种子库的种类组成均以草本植物为主。C、在湿度和光照比较一致的情况下,萌发种子数与温度成正相关。大多数种类的种子80%以上在实验的头六个星期内萌发。D、演替早期阶段的地上植物种类组成与土壤种子库中种类组成基本一致,演替后期阶段相关甚少。本文还讨论了土壤种子库研究的取样体积问题,以及土壤种子库在植被动态中的作用。     

岷江冷杉林皆伐后次生群落结构和物种多样性的演替动态

为阐明岷江冷杉林皆伐后次生群落的演替过程,采用空间代替时间的方法,在川西米亚罗林区海拔3100—3600 m的阴坡选择岷江冷杉林皆伐后次生演替10、20、30、40和50a阶段的次生群落作为研究对象,对其群落结构和物种多样性的动态进行了研究。不同演替阶段的树木均呈显著聚集分布。按群落中优势种的重要值将该演替序列划分为3个类型:悬钩子-蔷薇灌丛、白桦阔叶林和桦木-岷江冷杉针阔混交林。随次生演替,乔木和灌木种的物种丰富度趋于增加,而草本种的物种丰富度趋于减少;乔木和灌木种的Shannon-Wiener多样性指数趋于增大,而草本的Shannon-Wiener多样性指数趋于减小;乔木、灌木和草本层的Pielou均匀度指数均趋于增大;乔木层的Simpson优势度指数趋于减小,灌木和草本层的Simpson优势度指数在演替0—40年阶段趋于增大,而在演替50a阶段趋于减小。在该演替序列中,乔木、灌木和草本层的物种组成均呈耐荫种替代非耐荫种的趋势。     

滇东南岩溶山地次生林土壤种子库储量与优势成分

采用样线取样与温室萌发实验相结合的方法 ,调查了滇东南典型岩溶山地次生林的土壤种子库储量及其优势成分。结果发现 ,岩溶山地次生林下贮存有丰富的植物种子 ,在厚度为 1 0cm的表层土壤中的种子储量变动于 493 0～ 1 493 0粒 /m2 之间 ,高于对照成熟林的 3 780粒 /m2 。在 1 0cm的垂直剖面上 ,单位体积的种子储量在厚度为 0～ 2、2～ 5、5～ 1 0cm的三个层面间逐渐下降 ,但 5～ 1 0cm土层内仍然储藏了大量的种子。三种次生林的乔、灌、草、藤的比例组成与对照成熟林的差异不大 ,无论是种子个体数量还是物种种类数都以草本为主 (平均为 85 .5 %和 69.9% ) ,灌木次之 (平均为 1 2 .8%和 1 9.4% ) ,藤本 (平均为 1 .6%和 4.2 % )和乔木 (平均为 0 .6%和 6.5 % )稀少。在所研究的三个次生林样地内种子数量处于前三位的种子都来自菊科的紫茎泽兰 (Eupatoriumadenophorum) ,胜红蓟 (Ageratumconyzoides) ,劲直白酒草 (Conyzastricta) ,一点红(Emiliasonchifolia)等四个物种中的三个种 ,与成熟林下的土壤种子库中前三位物种基本相同。土壤种子库与地表植被间的相似性较小。     

Rodent-favored cache sites do not favor seedling establishment of shade-intolerant wild apricot (Prunus armeniaca Linn.) in northern China

Seed dispersal by scatter hoarders is widely assumed to involve directed dispersal, in which microhabitats selected for caching also benefit seedling establishment and growth. However, in many systems, this may not be true if animal-favored cache sites do not match the safe sites for plants, or if cache sites benefit the plant in one life stage and not another. Here, we investigated whether cache sites selected by rodents are favorable for seedling establishment and growth of shade-intolerant wild apricot (Prunus armeniaca Linn.) in northern China. We tracked tagged seeds and compared the germination and growth of seedlings from rodent-cached seeds with that of naturally established seedlings in a secondary forest and shrubland stand. Rodents preferred to cache seeds under shrubs with medium canopy cover (31–60 %) in litter substrate in the secondary forest, and under shrubs with high canopy cover (>60 %) in soil or litter substrate in the shrubland stand, neither of which conveyed an advantage for seedling establishment. Although fewer caches were made along shrub edges, or under low canopy cover (≤30 %) in the secondary forest, or along shrub edges, open areas of grass, or under low canopy cover in the shrubland stand, these cache sites consistently contributed to higher survival rates. The microhabitats of grass, soil, or low canopy cover significantly promoted the emergence, survival, and growth of naturally established seedlings. Our results are best explained by the conflicting demands of rodents for caching seeds in more secure sites and P. armeniaca’s high-light requirements for seedling recruitment. We argue that the relationship between favored cache sites for seed hoarders and safe sites for plants will often not match but may still allow a reasonable rate of establishment and regeneration.     

中亚热带常绿阔叶林不同演替阶段土壤活性有机碳含量及季节动态

为探明不同演替阶段土壤碳吸存潜力,选取演替时间为15a(演替初期)、47a(演替中期)、110a(演替后期)3个中亚热带常绿阔叶林,分析了各演替阶段的土壤有机碳(SOC)含量以及土壤微生物量碳(MBC)、可溶性碳(DOC)和微生物熵(SMQ)的季节变化。结果表明:演替中、后期不同土层的土壤SOC、MBC、DOC含量和SMQ均显著高于演替初期(P<0.05);与演替中期相比,演替后期土壤MBC、DOC含量有所降低,SOC含量和SMQ无显著差异。土壤SOC、MBC和DOC含量随土层加深而显著性降低(演替初、中期DOC除外),并随演替进行逐渐向腐殖质层富集。不同演替阶段MBC、DOC和SMQ均有显著季节变化,最低值出现在秋季,最高值随演替进程由冬季逐步转向夏季。相关分析表明,不同演替阶段土壤活性有机碳含量与土壤有机碳含量极显著相关(P<0.01),且土壤活性有机碳(MBC、DOC)和SMQ对土壤碳库变化更为敏感。     

The seed bank in an estonian calcareous grassland: Comparison of different successional stages

Seed bank species-composition and seed-density were determined in a successional calcareous (alvar) grassland in western Estonia. Three similar study areas were chosen to compare two different successional stages: open alvar grassland and overgrown areas with young pine forest 30 to 40 years old. In both successional stages, the centre and the edge of a relatively uniform stand were examined. Fifteen soil samples (7 cm in diameter, 5 cm deep) were taken from each of twelve sampling sites. The seedling emergence method was used to estimate seeds in the soil samples. A total of 69 species were detected in the seed bank, of which 18 did not occur in the vegetation. Eighty-nine taxa were recorded in the vegetation and of these 38 were not detected in the seed bank. Fifty-one species occurred both in the seed bank and in the vegetation. The three most abundant taxa in the seed bank wereCarex tomentosa, Linum catharticum andPlantago media, which together made up 49% of the seedlings recorded. Differences in the species compositions of seed bank samples from grassland and forest sites were negligible, although the species richness per area of the above-ground vegetation was significantly higher in the open grassland. The only species tending to be lost from forest site vegetation but still occurring in the forest soil seed bank wereArenaria serpylifolia, Cerastium fontanum andLinum catharticum. About half of all the emerged species from all samples belonged to the transient or short-term persistent seed bank. In the grassland sites there were more species which belonged to the transient seed bank than in the forest sites, where the seed bank contained more short-term persistent type seeds. The seed density was significantly higher in forest sites and lower in grassland sites, which may be explained by the better germination conditions in well-illuminated communities. On the basis of the current study it might be assumed that the soil seed banks of overgrown alvar grasslands which include young pine forests can play a certain role in grassland restoration management.     

Patterns of restoration of soil physciochemical properties and microbial biomass in different landslide sites in the sal forest ecosystem of Nepal Himalaya

The pattern of natural restoration in soil components and processes was documented in five landslide-damaged (1–58-year-old) sites in the moist tropical sal (Shorea robusta) forest ecosystem of Nepal Himalaya. Comparisons were made with an undisturbed forest site in the same region. Concentrations of soil organic C, total N, total P and extractable nutrients (Ca, Mg and K) increased with the age of sites. The 58-year-old site showed concentrations of soil organic C, total N and total P that were 75–89% of concentrations in the undisturbed sal forest. The soil microbial biomass, the active fraction of soil organic matter, showed similar seasonal variations at all sites. The amount of mean microbial biomass (expressed as C, N and P contents) increased 4–5 times at the 58-year-old site relative to the 1-year-old site, and the bulk increase occurred within the initial 15 year. The increase in the C/N ratio of soil microbial biomass with age (9.4–11.6 years) reflected change in its composition. Although the net N-mineralization rate increased consistently until 58 years of age, the proportion of nitrification rate relative to ammonification rate distinctly decreased beyond 40 years. On the other hand, the soil available-N (both NO₃⁻ and NH₄⁺) concentrations increased from 1 to 40 year and then declined; with age the proportion of NH₄⁺ increased, however. Rates of restoration in soil properties were faster in the early successional stages (1–15 year) than late stages. Among different soil properties the restoration of soil microbial biomass (C and N) was faster than soil organic C and total N. Best fit power function models showed that the estimated times for the 58-year-old site to reach the level of the undisturbed, mature sal forest would be about 30–35 year for microbial biomass (C and N) and about 100–150 year for organic C and total N. Higher accumulation of soil microbial biomass and high N-mineralization rate at late successional stages indicated the re-establishment of enriched soil and restitution of nutrient cycling during the course of ecosystem restoration.     

啮齿动物捕食压力下生境类型和覆盖处理对辽东栎种子命运的影响

在六盘山区的退化辽东栎灌丛和辽东栎次生林 (以下分别称“灌丛样地“和“次生林样地”),研究了群落生境和清除凋落物、凋落物覆盖及土壤覆盖等处理对啮齿动物取食和搬运/贮藏辽东栎种子的影响。结果表明：(1) 凋落物和土壤覆盖处理在种子释放的前期阶段有利于种子留存,但释放40天后,种子在释放点的最终留存率在灌丛样地显著高于次生林样地 (Z = -2.333, P = 0.020)。灌丛样地的自然状态的种子最终留存率显著高于凋落物覆盖处理 (Z = -0.674,P = 0.05),但其它处理间无显著差异;次生林样地的凋落物覆盖和清除凋落物处理种子的最终留存率为0%,自然状态和土壤覆盖处理均不足1%,各处理间无显著差异。(2) 种子被啮齿动物的就地取食率在不同群落生境间差异显著 (Z = -2.333,P = 0.020);在灌丛样地,凋落物覆盖处理种子被啮齿动物就地取食率最高 (45.56%),而次生林样地的土壤覆盖处理最高 (64.81%),种子被啮齿动物就地取食率在灌丛样地和次生林样地均为清除凋落物处理最低 (分别为23.70%和40.00%);生境类型和覆盖处理对种子搬运后的取食率均无显著影响。(3) 次生林样地种子被啮齿动物搬运后的埋藏率显著高于灌丛样地 (Z = -2.88,P = 0.004);在灌丛样地,土壤覆盖处理种子搬运后的埋藏率最高仅5.56%,而在次生林样地,清除凋落物处理最高达9.26%。(4) 种子被啮齿动物搬运后取食的距离在灌丛样地和次生林样地分别为3.01 m和2.13 m,差异显著 (Z = -2.080,P = 0.038),而种子搬运后的埋藏距离前者小于后者 (分别为1.35 m和2.10 m),两者间差异不显著。     

Seed dynamics during forest succession in Costa Rica

Soil seed banks and current seed inputs each play a role in tropical succession. We compared the abundance and floristic composition of seeds from these two sources at a Costa Rican site by germinating seeds from the soil, measuring seed inputs for 3 yr, and monitoring the earliest colonists in a forest clearing.There were an estimated 6800 viable seeds/m² in the soil of 3.3-yr-old vegetation, 9500 seeds/m² in 11-yr-old vegetation, and 7000 seeds/m² in a 75-yr-old forest. An estimated 10100 seeds/m² fell on the soil surface of the young successional vegetation during 3 yr and 3700 seeds/m² fell during that same time in the forest.Locally produced seeds accounted for about 75% of the seed input to the soil surface early in succession. Seeds dispersed out of young successional vegetation increased the quantity and species richness of the seed input and storage in an adjacent forest. Much of the species richness of the young successional vegetation resulted from seeds dispersed there from other communities by animals.Deforestation stimulated germination of most seeds in the surface soil of the old forest, including seeds of the dominant canopy tree. The recruitment of seedlings from the soil seed bank numerically overwhelmed that from post-disturbance seed rain and sprouts.We evaluated patterns of soil seed storage during succession and predicted the ability of vegetation of differing ages to respond to disturbance. Immediately after disturbance the number of seeds in the soil plummeted due to mortality, low inputs, and germination. As the vegetation regrew, the soil seed bank increased to a peak after 4 to 7 yr, then gradually decreased to its pre-disturbance size. High-frequency pulses of disturbance should result in reduced species richness, dominance by species with long-lived seeds, and fast recovery by seedling recruitment from the soil seed bank.Journal series number 6459 from the Institute of Food and Agricultural Sciences, University of Florida, Gainesville, Florida 32611, USA.Reprint requests to J. J. E. at Florida.     

滇东南岩溶山地退化植被土壤种子库的储量与组成

 在云南省东南部3个典型岩溶地点的土壤种子库储量与组成的研究表明,不同地点代表不同退化与恢复阶段的次生林、灌丛与退化草地的土壤内均储藏有丰富的植物种子,其密度值变动于4 090～14 930粒·m－2之间;物种数23～45种,其中草本物种及其种子储量最丰, 灌木次之,乔木和藤本及其种子稀少。在0～10 cm土层内,随着深度的加深,种子密度由0～2 cm,2～5 cm,5～10 cm逐渐减少,次生林的降低幅度相对较小。相对而言,种子密度值、物种数都以次生林为高,灌丛与退化草地之间的差异较小。同时次生林种子库中的乔、灌木的种子数量相对较多,而草本相对较少。从物种组成来看,少数物种的种子在库中的储量十分丰富,且各种子库间在物种组成上有很大的相似性,同一地点的3种植被类型间的两两相似性系数在0.6～0.7之间,不同地点的任一两类型间的相似性系数在0.4～0.6之间。9.3%的物种在所有样地内都出现。每一种子库中储量最丰的前3位都来自菊科的胜红蓟（Ageratum conyzoides）、劲直白酒草(Conyza stricta)、紫茎泽兰(Eupatorium adenophorum)、一点红（Emilia sonchifolia）,唇形科的鸡骨柴（Elsholtzia fruticosa）,蔷薇科的长毛绣线菊(Spiraea martinii var. pubescens)。     

不同生态系统对湘西洛塔降雨量及岩溶效应的影响

采用雨量器、滤膜法、CO2气体监测管法、标准试片对湘西洛塔不同生态系统降雨量、泥沙量、土壤CO2浓度、溶蚀速度进行了研究。结果表明:森林生态系统降雨量高于灌丛生态系统,观测第一年高6.84mm,第二年高218.08mm,且时间趋向均匀;灌丛生态系统水土流失严重,其泥沙含量是森林生态系统的1.53倍,土壤侵蚀为强度,而森林生态系统为微度;不同深度土壤CO2浓度在森林和灌丛生态系统中具有相同的变化趋势,在春季、秋季、冬季随土层加深而增大,在夏季,随土层加深先减少后增大,但季节变化动态略有差异,森林生态系统不同深度、不同季节土壤CO2浓度均大于灌丛生态系统;从全年土壤CO2浓度对比来看,10、20、40cm处,森林生态系统分别是灌丛生态系统的1.66、1.59和1.90倍;不同深度上,溶蚀速度在森林和灌丛生态系统具有相同的变化趋势,15cm处均大于地面,从同一深度来看,森林生态系统溶蚀速度均大于灌丛生态系统,地面、15cm处溶蚀速度分别是灌丛生态系统的1.11倍、2.57倍,说明森林生态系统具有较好的岩溶效应。     

Vkgetation analysis along a successional gradient from heath to oak forest

At Hjelm Hede in Denmark where an oak shrub is invading into a heathland the successional process has been studied on poor sandy outwashed plain. The study used a transect through a successional gradient from open heath to oak shrub where vegetation, major topsoil parameters and tree characteristics were analysed. The study enables us with spatial variation of key parameters to combine numerous detailed studies from the same site into an overall picture of changing vegetation and soil parameters reflecting the succession, which has been documented previously. The data were analysed by the use of Detrended Correspondence Analysis (DCA). Along the gradient from heath to forest the thick mor layer of the heathland soil decomposed, the CM ratio decreased from 25 to 15, pH increased and ammonium became available for the plants during a time span of approximately 120 years. The vegetation on the open heath was typical Danish inland heath vegetation dominated by Calluna vulgaris, Empetrum nigrum and Deschampsiaflexuosa. Under the young oak shrub the vegetation was dominated by Deschampsia Jexuosa, Trientalis europaea , and Maianthemum bijiolium. In the oldest and most forest like parts of the shrub a vegetation dominated by Holcus mollis, Anemone nemomsa and Stellaria holostea was found. Woody chamaephytes were replaced first by hemicryptophytes and later by geophytes. The vegetation's Ellenberg values indicated an increase in nitrogen availability and a decrease in acidity and light availability through secondary succession.     

The size and composition of soil seed-banks in remnant patches of three structural rainforest types in North Queensland

The size and species composition of the soil seed-bank in a remnant patch of each of three structurally and floristically distinct rainforests (Complex Mesophyll Vine Forest, Complex Notophyll Vine Forest and Semi-Evergreen Vine Thicket) were assessed. Seeds of 94 species germinated from 12 surface soil samples collected from each site. All three seed-banks were composed mostly of herbs characteristic of roadsides and agricultural land, and pioneer rainforest trees and shrubs. Agglomerative classifications indicated that the seed-bank samples from each rainforest remnant had a characteristic species composition and could be distinguished reliably from seed-bank samples drawn from other sites. Seeds of species present in the standing forest were poorly represented in the seed-banks except for one long-lived pioneer tree, Dendrocnide photinophylla, at one site. The seed-bank from the seasonally dry vine thicket was significantly larger (4000 seeds m^-2) than those from the two moister sites (400–600 seeds m^-2, contained more seeds of roadside and agricultural herbs, and fewer seeds of rainforest pioneer and secondary shrubs and trees. We suggest three explanations for the different seed-bank structure observed in the seasonally dry forest site. First, with increased deciduousness in rainforests, seed-banks are increasingly subject to invasion and domination by seeds of rapidly maturing herbs. Second, long-lived seeds that germinate in canopy gaps would be less likely to accumulate under deciduous forests because they would he exposed annually to conditions suitable for germination. Third, chronic disturbance by cattle and pigs produces sites suitable for the establishment of rapidly maturing herbs, and possibly disperses their seeds into the forest.     

Disturbance Effects on the Seed Bank of Mexican Cloud Forest Fragments1

The density and floristic composition of the soil seed bank was assessed in six cloud forest fragments with different levels of human disturbance in central Veracruz, Mexico. A total of 8416 seeds germinated in 60 soil samples, at 5‐cm depth, corresponding to 107 species, 85 genera, and 48 families. Significant differences were found among study sites in seed densities with values ranging from 873 to 3632/m². Tree species contributed 20 percent of the total soil seed bank in four sites and herbs accounted for the majority of the species in each site. Among tree species, Trema micrantha displayed the highest seed density, accounting for 84 percent of the germinated seeds. In general, the tree species composition of the soil seed bank did not closely reflect the composition of the tree community. Results suggest that disturbance produced by human activities (trail use, selective cutting of trees, livestock) may influence the size and composition of the soil seed bank in forest fragments. Sites where human activity has been reduced showed the highest proportion of dormant seeds.     

浙江天童地区常绿阔叶林退化对土壤养分库和碳库的影响

为了解常绿阔叶林退化对土壤碳库和养分库的影响,采用空间代替时间的研究方法,以常绿阔叶林顶级群落为参照,选择了次生常绿阔叶幼年林、次生针阔混交林、次生针叶林、灌丛和灌草丛代表不同的退化类型,分别对其土壤氮磷养分库、碳库进行了调查和分析。结果表明：土壤氮库贮量从大到小依次为,成熟常绿阔叶林、次生常绿阔叶幼年林、灌丛、次生针叶林、灌草丛和次生针阔混交林;土壤总磷含量也是在成熟林最高,次生针阔混交林和次生针叶林的总磷含量显著高于次生常绿阔叶幼年林和灌丛;土壤有机碳含量从高到低依次为：成熟常绿阔叶林,次生针叶林、次生常绿阔叶幼年林、灌丛、灌草丛和次生针阔混交林;土壤铵态氮在成熟林、灌丛和灌草丛的库容量最大,其次分别为次生幼年常绿阔叶林、次生针阔混交林,最小的为次生针叶林;硝态氮则在灌草丛的库容量最大,其次分别为次生针叶林、次生针阔混交林和成熟林针叶林,最小的为次生常绿阔叶幼年林和灌丛。统计显示,常绿阔叶林退化不仅导致土壤有机碳库含量的显著下降,也使得土壤氮磷养分库含量显著下降。可以认为,砍伐导致的大量生物量输出和森林管理措施的影响,植物种类组成的改变,土壤物理性质的改变以及养分和有机碳的主要生物化学转化环节发生改变是导致此类变化的主要因素,常绿阔叶林顶极群落土壤是该地区土壤的最大养分库和碳库。     

Growth of Valley Oak (<Emphasis Type="Italic">Quercus Lobata Nee</Emphasis>) in Four Floodplain Environments in the Central Valley of California

Although there have been numerous studies of California upland oak regeneration and growth there has been no research investigating oak sapling growth in riparian environments. This study looks at the growth response of young valley oak (Quercus lobata Nee), a dominant late successional riparian species in California, across environmental variation on a floodplain. Growth was measured over the course of three years at four different sites. The sites were chosen to represent the range of successional stages and surface age. Growth was significantly higher on younger, unforested sites. There was no difference in growth rate in the two forested sites (early successional cottonwood willow forest and late successional mature oak forest). Herbivory was highest in the cottonwood willow forest, where density of young oaks was also highest. The impact of flooding was measured on the youngest floodplain surface, an open floodplain restoration site where acorns were planted the year our study began. There was a significant negative impact of flooding on sapling growth in all but the first year of growth. Taken together these results suggest that existing forest trees and flooding both inhibit the growth of valley oaks on the floodplain, and that restoration in open sites may be more successful than restoration under an existing canopy.