Seedling and Sapling Dynamics of Treefall Pits in Puerto Rico1

Seedling and sapling dynamics in a Puerto Rican rain forest were compared between forest understory and soil pits created by the uprooting of 27 trees during Hurricane Hugo. Soil N and P, organic matter, and soil moisture were lower and bulk densities were higher in the disturbed mineral soils of the pits than in undisturbed forest soils ten months after the hurricane. No differences in N and P levels were found in pit or forest soils under two trees with N–fixing symbionts (Inga laurina and Ormosia krugii) compared to soils under a tree species without N–fixing sym–bionts (Casearia arborea), but other soil variables (Al, Fe, K) did vary by tree species. Forest plots had greater species richness of seedlings (<10 cm tall) and saplings (10–100 cm tall) than plots in the soil pits (and greater sapling densities), but seedling densities were similar between plot types. Species richness and seedling densities did not vary among plots associated with the three tree species, but some saplings were more abundant under trees of the same species. Pit size did not affect species richness or seedling and sapling densities. Recruitment of young Cecropia schreberiana trees (>5 m tall) 45 months after the hurricane was entirely from the soil pits, with no tree recruitment from forest plots. Larger soil pits had more tree recruitment than smaller pits. Defoliation of the forest by the hurricane created a large but temporary increase in light availability. Recruitment of C. schreberiana to the canopy occurred in gaps created by the treefall pits that had lower soil nutrients but provided a longer–term increase in light availability. Treefall pits also significantly altered the recruitment and mortality of many understory species in the Puerto Rican rain forest but did not alter species richness.     

What saplings can tell us about forest expansion over natural grasslands

Questions: 1. Do the species composition, richness and diversity of sapling communities vary significantly in differently sized patches? 2. Do forest patches of different sizes differ in woody plant colonization patterns? Location: São Francisco de Paula, Rio Grande do Sul, Brazil, 29°28'S,50°13'W. Methods: Three woody vegetation types, differing in structural development (patch size) and recovering for 10 years from cattle and burning disturbances, were sampled on grassland. We analysed the composition and complexity of the woody sapling communities, through relative abundance, richness and diversity patterns. We also evaluated recruitment status (residents vs. colonizers) of species in communities occurring in different forest patch size classes. Results : 1. There is a compositional gradient in sapling communities strongly associated with forest patch area. 2. Richness and diversity are positively correlated to patch area, but only in poorly structured patches; large patches present richness and diversity values similar to small patches. 3. Resident to colonizer abundance ratio increases from nurse plants to large patches. The species number proportion between residents and colonizers is similar in small and large patches and did not differ between these patch types. 4. Large patches presented a high number of exclusive species, while nurse plants and small patches did not. Conclusions: Woody plant communities in Araucaria forest patches are associated with patch structure development. Richness and diversity patterns are linked to patch colonization patterns. Generalist species colonize the understorey of nurse plants and small patches; resident species cannot recruit many new individuals. In large patches, sapling recruitment by resident adults precludes the immigration of new species into the patches, limiting richness and diversity.     

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.     

Continental rain forest fragments in Singapore resist invasion by exotic plants

Abstract Aim In general, the plant communities of oceanic islands suffer more from exotic plant invasions than their continental equivalents. At least part of this difference may be contributed by differences in non‐biological factors, such as the antiquity and intensity of human impacts and the absence of internal barriers to dispersal, rather than differences in inherent invasibility. We tested the resistance of species‐rich continental rain forests to plant invasion on a small, continental island that has been subject to prolonged and intensive human impact. Location Singapore is a 683‐km² equatorial island <1 km from the Asian mainland and with a population of 4 million people. It has a continental biota but has been subject to human impacts as intense as on any oceanic island. Methods We sampled twenty‐nine sites in seven vegetation types, ranging from urban wasteland to fragments of primary lowland rain forest. In each sample plot, all plant species were identified, exotic cover was estimated, and a range of environmental variables measured. Additional qualitative surveys for exotic invasion were made in other forest areas in Singapore. The data were analysed by Spearman's rank correlation coefficient. Results The number of exotic species recorded at a site was unrelated to the number of native species. Across all sites, percentage canopy opening had the highest correlation with the number of exotic species, while soil pH (which largely reflects the incorporation of calcareous construction wastes) had the highest correlation if the mangrove sites were excluded. There were no exotics in mangrove forest and only a tropical American, bird‐dispersed shrub, Clidemia hirta (L.) D. Don (Melastomataceae: Koster's Curse), in primary and tall secondary forest patches. The species‐poor early stages of woody plant succession on highly degraded soils were also very resistant to exotic plant invasion. Main conclusions Long‐isolated rain forest fragments in an exotic‐dominated continental island landscape resist invasion by exotic plants, suggesting that the problems on oceanic islands may reflect an inherently greater invasibility. This study also adds to the increasing evidence that the floras of tropical rain forest fragments in South‐east Asia are remarkably resilient on a time‐scale of decades to a century or more.     

Monsoon rain forest seedling dynamics, northern Australia: contrasts with regeneration in eucalypt-dominated savannas

Aim To explore: (1) the relative influences of site conditions, especially moisture relations, on pathways and rates of monsoon rain forest seedling and sapling regeneration, especially of canopy dominants, in northern Australia; and (2) contrasts between regeneration syndromes of dominant woody taxa in savannas and monsoon rain forest. Location Four monsoon rain forest sites, representative of regional major habitat and vegetation types, in Kakadu National Park, northern Australia. Methods A decadal study involved: (1) initial assessment over 2.5 years to explore within‐year variability in seed rain, dormant seed banks and seedling (< 50 cm height) dynamics; and (2) thereafter, monitoring of seedling and sapling (50 cm height to 5 cm d.b.h.) dynamics undertaken annually in the late dry season. On the basis of observations from this and other studies, regeneration syndromes of dominant monsoon rain forest taxa are contrasted with comparable information for dominant woody savanna taxa, Eucalyptus and Corymbia especially. Results Key observations from the monsoon rain forest regeneration dynamics study component are that: (1) peak seed rain inputs of rain forest taxa were observed in the wet season at perennially moist sites, whereas inputs at seasonally dry sites extended into, or peaked in, the dry season; (2) dormant soil seed banks of woody rain forest taxa were dominated by pioneer taxa, especially figs; (3) longevity of dormant seed banks of woody monsoon rain forest taxa, including figs, was expended within 3 years; (4) seedling recruitment of monsoon rain forest woody taxa was derived mostly from wet season seed rain with limited inputs from soil seed banks; (5) at all sites rain forest seedling mortality occurred mostly in the dry season; (6) rain forest seedling and sapling densities were consistently greater at moist sites; (7) recruitment from clonal reproduction was negligible, even following unplanned low intensity fires. Main conclusions By comparison with dominant savanna eucalypts, dominant monsoon rain forest taxa recruit substantially greater stocks of seedlings, but exhibit slower aerial growth and development of resprouting capacity in early years, lack lignotubers in mesic species, and lack capacity for clonal reproduction. The reliance on sexual as opposed to vegetative reproduction places monsoon rain forest taxa at significant disadvantage, especially slower growing species on seasonally dry sites, given annual–biennial fires in many north Australian savannas.     

Patterns in woody species diversity, richness and partitioning of diversity in forest communities of tropical deciduous forest biome

The regressive succession model hypothesizes tropical savanna-woodlands to be a degraded stage of primary deciduous forests. Species diversity, richness and evenness of woody species in savanna-woodlands, secondary deciduous forests and mature deciduous forests of central India were compared to test if the regressive succession explained pattern in species richness, diversity, functional diversity and basal area. At the plot scale (0.1 ha) secondary deciduous forests and savanna-woodlands had similar species diversity, a pattern not consistent with the regressive model of deciduous forest succession, and mature deciduous forests had greater species diversity and richness (p<0.05). When examined at a larger scale or community scale by pooling all plots within a community type, the trend in diversity persisted even with greater effort allocated to sampling of secondary deciduous forests. Species richness at the community scale was greatest in secondary deciduous forest as expected from species area relationship. The communities shared 28 woody species but the species composition was significantly different between the communities. I suggest that conservation of tropical deciduous forests based on regressive succession model is problematic.     

Nutrient Addition Differentially Affects Soil Carbon Sequestration in Secondary Tropical Dry Forests: Early‐ versus Late‐Succession Stages

There is considerable interest in the potential use of soils to sequester carbon for climate change mitigation. As such, there is a need to evaluate the potential for carbon accumulation in tropical regions. We compared the effects of three annual additions of nitrogen and/or phosphorus on soil carbon and nitrogen contents and pools (bulk soil, macro‐, meso‐, and microaggregates) of two regenerating secondary tropical dry forest differing in nutrient status and succession stage (10‐year‐old early‐succession stage and approximately 60‐year‐old late‐succession stage). The selected forest sites were located on a shallow calcareous soil in the Yucatán Peninsula (Mexico). The primary production is limited by nitrogen and phosphorus in early‐succession stage and by phosphorus in late‐succession stage. In each forest site, four independent plots (12 × 12 m²) were established, the treatments being: controls and plots fertilized during three consecutive years with nitrogen, phosphorus, or nitrogen plus phosphorus. In both forests, soil carbon and nitrogen contents were consistently high, with soil carbon:nitrogen ratios generally greater than 10. Results indicate that usually there are no significant increases of soil carbon stock associated to late succession but can be increased to 3.7 Mg·ha^?1·yr^?1 with adoption of fertilizer practices. The potential soil carbon sequestration in early‐succession forest was estimated to be 2.7 Mg·ha^?1·yr^?1, and there is no indication that fertilization improves carbon sequestration. In short, results suggest that the soil potential for carbon sequestration in these ecosystems is high and depends on the specific nutrient status of the site.     

Lowland boreal forests characterization in Algonquin Provincial Park relative to beaver (Castor canadensis) foraging and edaphic factors

Beaver (Caster canadensis) foraging and edaphic conditions can modify the vegetational characteristics of woody plant community in lowland boreal forests. Effective management of these areas requires an understanding of the relative contribution of these factors in shaping the woody plant community structure. Our objective was to quantify the effects of herbivory by beavers and edaphic conditions on woody plant community organization of lowland boreal forests surrounding beaver ponds. Woody vegetation and soils were sampled at 15 ponds occupied by beavers and one other pond abandoned by them in southern Algonquin Park, Ontario. We measured spatial variation in plant diversity, foraging rates and sapling recruitment of trees and shrubs along gradients of beaver foraging intensity and soil moisture, P, K, Mg, and pH. Beavers fed preferentially on a small number of deciduous species and the number of cut stems declined sharply with increasing distance from ponds. Conifers increased in relative dominance to deciduous species in the presence of beavers. Plant species richness and stem and basal area diversity peaked at intermediate distances (about 25 m) from ponds. Sapling recruitment by non-preferred species was positively related to foraging intensity. Total stem abundance and basal area and sapling recruitment by four preferred species (Populus tremuloides, Acer rubrum, Acer saccharum and Corylus cornuta) were negatively related to foraging intensity. However, by including Alnus rugosa and Salix bebbiana (also preferred by beavers) these patterns changed, becoming positively related to foraging intensity. There was also a pronounced gradient in soil moisture, which also decreased with distance from ponds. The other measured edaphic variables did not vary consistently with distance from ponds. Sapling recruitment in mesic versus xeric species varied consistently with hydrid conditions along the moisture gradient, such that variation in moisture also could produce the observed pattern of plant diversity. Diversity patterns changed three years after beaver abandonment of a pond, though sapling recruitment patterns in preferred and non-preferred species around the abandoned pond were similar to the occupied ponds. These observations suggest spatial variation in woody plant richness and diversity could be determined by combined effects of both herbivory (disturbance by beavers) and variable responses of different species to edaphic conditions.     

Potential tree species extinction,colonization and recruitment in Afromontane forest relicts

Tree species regeneration determines future forest structure and composition, but is often severely hampered in small forest relicts. To study succession, long-term field observations or simulation models are used but data, knowledge or resources to run such models are often scarce in tropical areas. We propose and implement a species accounting equation, which includes the co-occurring events extinction, colonization and recruitment and which can be solved by using data from a single inventory. We solved this species accounting equation for the 12 remaining Afromontane cloud forest relicts in Taita Hills, Kenya by comparing the tree species present among the seedling, sapling and mature tree layer in 82 plots. A simultaneous ordination of the seedling, sapling and mature tree layer data revealed that potential species extinctions, colonizations and recruitments may induce future species shifts. On landscape level, the potential extinction debt amounted to 9% (7 species) of the regional species pool. On forest relict level, the smallest relicts harbored an important proportion of the tree species diversity in the regeneration layer. The average potential recruitment credit, defined as species only present as seedling or sapling, was 3 and 6 species for large and small forest relicts, while the average potential extinction debt was 12 and 4 species, respectively. In total, both large and small relicts are expected to lose approximately 20% of their current local tree species pool. The species accounting equations provide a time and resource effective tool and give an improved understanding of the conservation status and possible future succession dynamics of forest relicts, which can be particularly useful in a context of participatory monitoring.     

The effects of herbivory by a mega- and mesoherbivore on tree recruitment in sand forest, South Africa

Herbivory by megaherbivores on woody vegetation in general is well documented; however studies focusing on the individual browsing effects of both mega- and mesoherbivore species on recruitment are scarce. We determined these effects for elephant Loxodonta africana and nyala Tragelaphus angasii in the critically endangered Sand Forest, which is restricted to east southern Africa, and is conserved mainly in small reserves with high herbivore densities. Replicated experimental treatments (400 m(2)) in a single forest patch were used to exclude elephant, or both elephant and nyala. In each treatment, all woody individuals were identified to species and number of stems, diameter and height were recorded. Results of changes after two years are presented. Individual tree and stem densities had increased in absence of nyala and elephant. Seedling recruitment (based on height and diameter) was inhibited by nyala, and by elephant and nyala in combination, thereby preventing recruitment into the sapling stage. Neither nyala or elephant significantly reduced sapling densities. Excluding both elephant and nyala in combination enhanced recruitment of woody species, as seedling densities increased, indicating that forest regeneration is impacted by both mega- and mesoherbivores. The Sand Forest tree community approached an inverse J-shaped curve, with the highest abundance in the smaller size classes. However, the larger characteristic tree species in particular, such as Newtonia hildebrandtii, were missing cohorts in the middle size classes. When setting management goals to conserve habitats of key importance, conservation management plans need to consider the total herbivore assemblage present and the resulting browsing effects on vegetation. Especially in Africa, where the broadest suite of megaherbivores still persists, and which is currently dealing with the 'elephant problem', the individual effects of different herbivore species on recruitment and dynamics of forests and woodlands are important issues which need conclusive answers.     

Spatial structure of the abiotic environment and its association with sapling community structure and dynamics in a cloud forest

Analyzing the relationship between the spatial structures of environmental variables and of the associated seedling and sapling communities is crucial to understanding the regeneration processes in forest communities. The degree of spatial structuring (i.e., spatial autocorrelation) of environmental and sapling community variables in the cloud forest of Teipan, S Mexico, were analyzed at a 1-ha scale using geostatistical analysis; after fitting semivariogram models for each set of variables, the association between the two sets was examined through cross-variograms. Kriging maps of the sapling community variables (density, cover, species richness, and mortality and recruitment rates) were obtained through conditional simulation method. Canopy openness, total solar radiation, litter depth, soil temperature and soil moisture were spatially structured, as were sapling density, species richness and sapling mortality rate. Mean range in semivariograms for environmental and sapling community variables were 13.14 ± 3.67 and 12.68 ± 5.71 m (±SE), respectively. The spatial structure of litter depth was negatively associated with the spatial structures of sapling density, species richness, and sapling community cover; in turn, the spatial structure of soil moisture was positively associated with the spatial structure of recruitment rate. These associations of the spatial structures of abiotic and sapling community variables suggest that the regeneration processes in this cloud forest is driven by the existence of different microsites, largely characterized by litter depth variations, across which saplings of tree species encounter a range of opportunities for successful establishment and survival.     

Structure and dynamics of a tropical dry forest in Ghana

Forest under low rainfall (averaging 745 mm yr^-1) on the Shai Hills in S.E. Ghana has redeveloped following cessation of farming in the 1890s. Forest stature is low, with a canopy at about 11 m, principally of three species, Diospyros abyssinica, D. mespiliformis and Millettia thonningii. Drypetes parvifolia and Vepris heterophylla are common understorey trees. Twelve species of woody liane were recorded. Species of thicket vegetation in the area were also present at low density. Most species are evergreen.Tree mortality averaged 2.3% yr^-1 and exceeded recruitment (1.5% yr^-1). Differences between species in mortality and recruitment were pronounced: canopy species showed a small decline in density; understorey species increased markedly and the thicket species declined. Seed production was very variable, but seedling establishment was very poor for all species. Seedling mortality was high (11% yr^-1) especially for small seedlings. These population trends probably represent the latter stages of succession of forest regrowth after farming about 100 years ago.Compared with tropical rain forest, Shai Hills forest has similar relative tree diameter growth (1–3.5% yr^-1), mortality and recruitment rates, and small-litter fall (5.52 t ha^-1 yr^-1).Shai Hills forest differs from rain forest by its short stature, relatively few (evergreen) tree species, poor regeneration from seed, high soil nutrient status and low rainfall. Similar forests have been reported in east Africa and in parts of New Guinea.Abbreviations dbh diameter at breast height (1.3 m) - gbh girth at breast height died May 1984     

Response of recruitment to light availability across a tropical lowland rain forest community

1.  Many hypotheses about species coexistence involve differential resource use and trade-offs in species' life-history traits. Quantifying resource use across most species in diverse communities, although, has seldom been attempted.
2.  We use a hierarchical Bayesian approach to quantify the light dependence of recruitment in 263 woody species in a 50-ha long-term forest census plot in Panama. Data on sapling recruitment were obtained using the 1985–1990 and 1990–1995 census intervals. Available light was estimated for each recruit from yearly censuses of canopy density.
3.  We use a power function (linear log–log relationship) to model the light effect on recruitment. Different responses of recruitment to light are expressed by the light effect parameter b . The distribution of b had a central mode at 0.8, suggesting that recruitment of many species responds nearly linearly to increasing light.
4.  Nearly every species showed increases in recruitment with increasing light. Just nine species (3%) had recruitment declining with light, while 198 species (75%) showed increasing recruitment in both census intervals. Most of the increases in recruitment were decelerating, i.e. the increase was less at higher light ( b  < 1). In the remaining species, the response to light varied between census intervals (24 species) or species did not have recruits in both intervals (41 species).
5.   Synthesis. Nearly all species regenerate better in higher light, and recruitment responses to light are spread along a continuum ranging from modest increase with light to a rather strict requirement for high light. These results support the hypothesis that spatio-temporal variation in light availability may contribute to the diversity of tropical tree species by providing opportunities for niche differentiation with respect to light requirements for regeneration.     

Recovery of plant species richness and composition after slash-and-burn agriculture in a tropical rainforest in Madagascar

Slash-and-burn agriculture is an important driver of deforestation and ecosystem degradation, with large effects on biodiversity and carbon sequestration. This study was conducted in a forest in Madagascar, which consists of fragments of slash-and-burn patches, within a matrix of secondary and primary forest. By recording species richness, abundance, and composition of trees, shrubs, and herbs in fallows of various age and slash-and-burn history, and in the secondary and primary forest, we show how slash-and-burn intensity (number of cycles, duration of abandonment), years since last abandonment, and environmental factors (distance to primary forest and topography) affect the natural succession and recovery of the forest ecosystem. We used ordination analyses to examine how the species composition varied between the different successions stages, and to examine tree recruitment. Our results show shrub dominance the first years after abandonment. Thereafter, a subsequent increase in species richness and abundance of tree seedlings and saplings suggests a succession towards the diversity and composition of the secondary and primary forest, although a big gap between the oldest fallows and the secondary forest shows that this will take much more than 30 years. A high number and frequency of slash-and-burn cycles decreased tree seedling and sapling richness and abundance, suggesting that reducing slash-and-burn intensity will increase the speed of tree recruitment and fallow recovery. Trees can be planted into fallows to speed up vegetation and soil recovery, such that fallows can be usable within needed time and thus the extension of cultivated areas reduced. We recommend further testing of six potential species for restoration based on their early colonization of the fallows and their survival through vegetation succession.     

生境片段化对千岛湖马尾松林内土壤种子库的影响

土壤种子库是森林群落更新的主要来源之一,对森林的演替和恢复等具有重要意义。生境片段化现象正日益严重地影响着森林群落,并可影响森林土壤种子库。研究了千岛湖地区的大陆及岛屿次生马尾松林内土壤种子库的组成及其影响因素(e.g.,岛屿面积,形状指数,隔离度和距岛屿边缘距离等)。根据大陆和岛屿的面积及边缘梯度,采用大数量小样方法,分别在土壤种子库最大化(初冬,2015年12月)和最小化(晚春,2016年4月)时期对马尾松林内土壤进行了机械取样。对土壤样品进行萌发实验,检测了两个时期的土壤种子库上层(0—2 cm)和下层(2—5 cm)种子组成,并通过广义线性混合效应模型等手段分析其影响因素。结果显示:(1)所有316个土壤样本中,萌发出幼苗1422株,隶属于29科、40属、41种。其中,木本植物幼苗占13种1024株,草本占28种398株。(2)Jaccard指数和相关性分析均显示初冬、晚春时期的土壤种子库组成具有很高的相似性;土壤种子库上、下层组成的相似性也很高。(3)广义线性混合效应模型分析显示,在大陆和岛屿上,土壤种子库下层种子含量低于上层;而大陆样地土壤种子库中的木本植物种子数较岛屿样地高。岛屿上,土壤种子库中的种子数随土层的加深而降低;随边缘梯度升高也下降,尤其是草本植物的种子。对于岛屿上的木本植物,不耐阴种的种子数量远大于耐阴种,尤其是土壤下层。表明千岛湖地区马尾松林内土壤种子库组成受到生境片段化的影响,进而可能作用于该类型森林群落的演替。     

Regeneration of monsoon rain forest in northern Australia: the dormant seed bank

Abstract. As part of a wider study examining regeneration pathways in monsoon rain forest vegetation in northern Australia, the dormant component of the soil seed bank was assessed by storing soil samples for over six dry season months, before watering in shade-house trials. Six soil samples were collected from each of 34 sites broadly representative of the range of regional monsoon rain forest vegetation. Four floristic seed bank groups were derived through TWINSPAN classification. Mean group densities of germinants ranged from 25–144/m². Dormant seed banks were least dense, and most sparsely distributed, in sandy soils. Seed bank samples were dominated by woody pioneer monsoon rain forest species, especially figs; exotic weeds and savanna taxa (e.g. Poaceae) were relatively more common at seasonally dry sites. Dormant seed banks comprised species mostly present in the standing vegetation, although a small number of germinants represented species not growing at half the sites. Regeneration of woody pioneers from dormant seed banks is least likely to be of importance on infertile, seasonally dry sites.     

The role of remnant forest trees in tropical secondary succession

This paper emphasizes the contribution of remnant trees to the establishment of woody species during succession on abandoned fields and pastures in the Mexican rain forest area, Los Tuxtlas, Veracruz. Remnant trees, original large forest trees left in the clearings by traditional farmers will become natural perching sites for both passing and resident birds. Frugivorous birds drop or regurgitate seeds and fruits which fall under the canopies of remnant trees during their stay, thus contributing to an accumulation of species, which make these remnant trees into ‘regeneration nuclei’. The species transported into these sites belong chiefly to older stages of successional development and reach these otherwise isolated areas, counteracting the depauperization of tropical land, brought about by both intensive and extensive clearing. In a study of seven remnant trees, 29 woody species and two climbers were found, 86% of which are bird dispersed. The total number of species per tree varied from 6 to 15 and was higher under remnant trees with fruits attractive to birds. Floristic variations of the understorey as detected by detrended correspondence analysis was correlated with the relative amount of shade-tolerant primary and late secondary trees versus light dependent pioneers and early successional trees.     

木论喀斯特常绿落叶阔叶林木本植物萌生特征

萌生更新是植物进行自我更新的重要方式之一。为了阐明喀斯特常绿落叶阔叶林植物的萌生特征,基于木论25 hm²动态监测样地的调查数据,分析了木本植物萌生更新数量特征、不同生活型植物萌生能力的差异、萌生能力与地形因子和萌生能力与物种多样性的关系。研究结果表明：样地具有萌生现象的木本植物共有197种,隶属59个科137个属,分别占样地植物科属种的86.8%、93.7%、91.3%。萌生现象在样地内各物种中普遍存在,滇丁香、长管越南茜、火棘、香叶树等物种的萌生能力较强。不同生活型的植物的萌生能力存在显著差异,常绿树种的萌生能力显著高于落叶树种（P<0.001）。萌生物种丰富度比例及萌生物种个体比例都与群落物种多样性呈显著正相关。萌生能力与土层厚度呈显著负相关,与凹凸度呈显著正相关,此外萌生物种丰富度比例还与海拔呈正相关关系,而萌生物种个体比例与岩石出露率和土壤坡度呈正相关关系。由此可见,作为喀斯特森林群落更新中一种占优势的更新方式,萌生更新在一定程度上能够增加群落物种多样性,萌生能力与地形因子存在一定关联。     

Succession stage variation in population size in an early-successional herb in a peri-urban forest

Urban and peri-urban forests incur high anthropogenic pressures (e.g. recreational activities, artificialization, and eutrophication). Plant species from early-successional, transient, forest habitats, often characterized by a short life span and a persistent seed bank in the soil may differ from late-successional species in key-factors for population persistence. This study investigated variation in population size and seedling recruitment for different forest succession stages and three consecutive years in Centaurium erythraea, an early-successional biennial herb, occurring in a peri-urban forest of Brussels urban zone (Belgium). Forest succession stage had a significant impact on C. erythraea population size and on its temporal fluctuation. Populations in closing vegetation (evolving to late-succession stages) showed small population sizes and a low number of recruits compared to populations from stable early-succession vegetation and clearcuts. The number of recruits was the highest after clearcutting, which can be related to the expression of the soil seed bank. Populations showed year-to-year variation in size (flowering individuals and recruits), even in stable (over three years) early-succession forest vegetation. In the absence of disturbance changing succession stage, population size is expected to depend on seed set of the previous years and subsequent seedling recruitment, which can be affected by environmental stochasticity. Opening gaps in the herbaceous vegetation may stimulate seedling recruitment, also in unoccupied patches where “cryptic” seed populations are present in the soil. Forest path and road verges, despite their potential negative impact on forests, can constitute refuge habitats for early-successional forest plant species. Their management should involve the preservation of these species.     

亚热带次级森林演替过程中模拟氮磷沉降对土壤微生物生物量及土壤养分的影响

氮（N）沉降深刻影响着森林生态系统的生物多样性、生产力和稳定性。亚热带地区森林土壤磷（P）的有效性较低,N沉降将更突显P的限制作用。N、P输入对亚热带次级森林土壤的影响是否依赖于森林演替阶段知之甚少。选取两种不同演替年龄阶段（年轻林：<40 a;老年林：>85 a）的亚热带常绿阔叶林,设置模拟N和/或P沉降（10 g m^-2 a^-1）4个处理（Ctrl、N、P、NP）,连续处理4.5年后采集表层、次表层和下底层（0-15、15-30、30-60 cm）土壤样品,综合分析了土壤微生物生物量碳（MBC）氮（MBN）和多种土壤养分含量。结果表明,MBC、MBN及土壤养分含量均随土壤深度增加而降低。N添加对两种演替阶段森林土壤中MBC和MBN均无显著影响。施P相关处理（P和NP）对年轻林表层土壤MBC和MBN无显著影响,但显著增加了老年林表层土壤MBC和MBN（P<0.05）,表明老年林可能比年轻林更易受P限制。N添加显著增加了两种演替森林表层土壤可溶性有机氮（DON）、氨态氮（NH₄⁺-N）和硝态氮（NO₃^--N）的含量（P<0.05）;P相关处理（P和NP）显著增加两种演替阶段表层和次表层土壤速效磷（AP）以及表层土壤全磷（TP）的含量（P<0.05）。土壤MBC和MBN与土壤中各养分指标（可溶性有机碳DOC、DON、NH₄⁺-N、NO₃^--N、AP、全碳TC、全氮TN和TP）呈显著正相关关系,土壤TC、TN和DOC是影响土壤微生物生物量的主要因子。研究可为评估和揭示未来全球环境变化背景下不同演替林龄亚热带森林的土肥潜力及土壤质量的演变提供一定的科学理论依据。