Testing heterogeneity–diversity relationships in tropical forest restoration

Restoring small-scale habitat heterogeneity in highly diverse systems, like tropical forests, is a conservation challenge and offers an excellent opportunity to test factors affecting community assembly. We investigated whether (1) the applied nucleation restoration strategy (planting tree islands) resulted in higher habitat heterogeneity than more homogeneous forest restoration approaches, (2) increased heterogeneity resulted in more diverse tree recruitment, and (3) the mean or coefficient of variation of habitat variables best explained tree recruitment. We measured soil nutrients, overstory and understory vegetation structure, and tree recruitment at six sites with three 5- to 7-year-old restoration treatments: control (no planting), planted tree islands, and conventional, mixed-species tree plantations. Canopy openness and soil base saturation were more variable in island treatments than in controls and plantations, whereas most soil nutrients had similar coefficients of variation across treatments, and bare ground was more variable in control plots. Seedling and sapling species density were equivalent in plantations and islands, and were substantially higher than in controls. Species spatial turnover, diversity, and richness were similar in island and plantation treatments. Mean canopy openness, rather than heterogeneity, explained the largest proportion of variance in species density. Our results show that, whereas canopy openness and soil base saturation are more heterogeneous with the applied nucleation restoration strategy, this pattern does not translate into greater tree diversity. The lack of a heterogeneity–diversity relationship is likely due to the fact that recruits respond more strongly to mean resource gradients than variability at this early stage in succession, and that seed dispersal limitation likely reduces the available species pool. Results show that planting tree islands facilitates tree recruitment to a similar degree as intensive plantation-style restoration strategies.     

Understory Colonization of Eucalyptus Plantations in Hawaii in Relation to Light and Nutrient Levels

Exotic tree plantations may serve as catalysts for native forest regeneration in agriculturally degraded landscapes. In 2001, we evaluated plant species regeneration in the understory of a 7‐year‐old experimental Eucalyptus saligna forest in Hawaii approximately 1 year after the cessation of 5 years of herbicide. These forests were organized in a 2 × 2–factorial design of planting density (1 × 1– or 3 × 3–m spacing) and fertilization (unfertilized control and regular fertilization), which resulted in varying resource availabilities. We found that understory biomass was highest under high light conditions, regardless of fertilization treatment, whereas species richness was lowest under fertilized 1 × 1–m plots. The understory was dominated by species exotic to Hawaii. The most common tree species, the noxious weed Citharexylum caudatum, was particularly successful because high light–saturated photosynthesis rates and a low light compensation point allowed for high growth and survival under both light conditions. To assess longer‐term recruitment patterns, we resurveyed a portion of this site in 2006 and also surveyed five Eucalyptus plantations in this region of Hawaii that differed in age (5–23 years), species (E. saligna, E. grandis, E. cloeziana, E. microcorys), and management (experimental, industrial, nonindustrial stewardship); all were established on previous agricultural sites within approximately 3 km of native‐dominated forest. Again, very few native species were present in any of the stands, indicating that within certain landscapes and for native species with certain life history traits, exotic plantations may be ineffective nursery ecosystems for the regeneration of native species.     

Influence of overstory composition on understory colonization by native species in plantations on a degraded tropical site

Abstract. Patterns of understory colonization by native and naturalized trees and shrubs were evaluated in 4.5-year-old plantations of three exotic tree species, Casuarina equisetifolia, Eucalyptus robusta, and Leucaena leucocephala, on a degraded coastal grassland site with reference to overstory composition and understory environmental conditions. 19 secondary forest species were established in the plantation understories (with a total area of 0.52 ha), while no natural regeneration occurred in unplanted, though protected, control areas. The majority of these species (90 %) and the total seedling population (97 %) were zoochorous, indicating the importance of frugivorous bats and particularly birds as facilitators of secondary forest species colonization. Understory species richness and seedling densities were affected significantly by overstory composition, the most abundant regeneration occurring beneath Leucaena and least under Casuarina. Understory colonization rates within mixed-species stands were intermediate between those of single-species stands of the trees comprising their overstories. Significant negative correlations were found between understory species richness and seedling density, and forest floor depth and dry mass, especially for small-seeded ornithochorous species. Higher colonization rates near the peripheries of plantation plots relative to plot interiors were due in part to roosting site preferences by frugivores, particularly bats. The study results indicate that overstory species selection can exert a significant influence on subsequent patterns of colonization by secondary forest species and is an important consideration in the design of plantations for ‘catalyzing’ succession on deforested, degraded sites.     

Cultivation of Non‐timber Forest Products Alters Understory Light Availability in a Humid Tropical Forest in Mexico1

The planting of non‐timber forest products (NTFPs) in the understory of tropical forests is promoted in many regions as a strategy to conserve forested lands and meet the economic needs of rural communities. While the forest canopy is left intact in most understory plantations, much of the midstory and understory vegetation is removed in order to increase light availability for cultivated species. We assessed the extent to which the removal of vegetation in understory plantations of Chamaedorea hooperiana Hodel (Arecaceae) alters understory light conditions. We also examined how any changes in light availability may be reflected by changes in the composition of canopy tree seedlings regenerating in understory plantations. We employed a blocked design consisting of four C. hooperiana plantation sites; each site was paired with an adjacent, unmanaged forest site. Hemispherical canopy photographs were taken and canopy tree seedlings were identified and measured within 12 3 × 2 m randomly placed plots in each site for a total of 96 plots (4 blocks × 2 sites × 12 plots). Plantation management did not affect canopy openness or direct light availability but understory plantations had a higher frequency of plots with greater total and diffuse light availability than unmanaged forest. Comparisons of canopy tree seedling composition between understory plantations and unmanaged forest sites were less conclusive but suggest that management practices have the potential to increase the proportion of shade‐intolerant species of tree seedlings establishing in plantations. Given the importance of advanced regeneration in gap‐phase forest dynamics, these changes may have implications for future patterns of succession in the areas of forest where NTFPs are cultivated.     

Age‐related Crown Thinning in Tropical Forest Trees

Gap dynamics theory proposes that treefall gaps provide high light levels needed for regeneration in the understory, and by increasing heterogeneity in the light environment allow light‐demanding tree species to persist in the community. Recent studies have demonstrated age‐related declines in leaf area index of individual temperate trees, highlighting a mechanism for gradual changes in the forest canopy that may also be an important, but less obvious, driver of forest dynamics. We assessed the prevalence of age‐related crown thinning among 12 tropical canopy tree species sampled in lowland forests in Panama and Puerto Rico (total N = 881). Canopy gap fraction of individual canopy tree crowns was positively related to stem diameter at 1.3 m (diameter at breast height) in a pooled analysis, with 10 of 12 species showing a positive trend. Considered individually, a positive correlation between stem diameter and canopy gap fraction was statistically significant in 4 of 12 species, all of which were large‐statured canopy to emergent species: Beilschmiedia pendula, Ceiba pentandra, Jacaranda copaia, and Prioria copaifera. Pooled analyses also showed a negative relationship between liana abundance and canopy gap fraction, suggesting that lianas could be partially obscuring age‐related crown thinning. We conclude that age‐related crown thinning occurs in tropical forests, and could thus influence patterns of tree regeneration and tropical forest community dynamics.     

Demographic spatial genetic structure of the Neotropical tree, Jacaranda copaia

We used genotypes from six microsatellite loci and demographic data from a large mapped forest plot to study changes in spatial genetic structure across demographic stages, from seed rain to seedlings, juveniles, and adult diameter classes in the Neotropical tree, Jacaranda copaia. In pairwise comparisons of genetic differentiation among demographic classes, only seedlings were significantly differentiated from the other diameter classes; F(ST) values ranged from 0.006 to 0.009. Furthermore, only seedlings showed homozygote excess suggesting biparental inbreeding in the large diameter reproductive adults. We found very low levels of relatedness in the first distance class of trees, 1-26 cm diameter (F(ij) = 0.011). However, there was a 5- to 10-fold rise in relatedness in the smallest distance class, from the smallest to the largest tree diameter classes (F(ij) = 0.110 for individuals > 56 cm diameter). A variety of non-mutually exclusive mechanisms have been invoked perviously to explain such a pattern, including natural selection, history, or nonequilibrium population dynamics. The long-term demographic data available for this species allow us to evaluate these mechanisms. Jacaranda is a fast-growing, light-demanding species with low recruitment rates and high mortality rates in the smaller diameter classes. It successfully regenerates only in large light gaps, which occur infrequently and stochastically in space and time. These factors contribute to the nonequilibrium population dynamics and observed low genetic structure in the small size classes. We conclude that the pattern of spatial genetic transitions in Jacaranda is consistent with overlapping related generations and strong but infrequent periods of high recruitment, followed by long periods of population decline.     

Tree leaf litter composition and nonnative earthworms influence plant invasion in experimental forest floor mesocosms

Dominant tree species influence community and ecosystem components through the quantity and quality of their litter. Effects of litter may be modified by activity of ecosystem engineers such as earthworms. We examined the interacting effects of forest litter type and earthworm presence on invasibility of plants into forest floor environments using a greenhouse mesocosm experiment. We crossed five litter treatments mimicking historic and predicted changes in dominant tree composition with a treatment of either the absence or presence of nonnative earthworms. We measured mass loss of each litter type and growth of a model nonnative plant species (Festuca arundinacea, fescue) sown into each mesocosm. Mass loss was greater for litter of tree species characterized by lower C:N ratios. Earthworms enhanced litter mass loss, but only for species with lower C:N, leading to a significant litter × earthworm interaction. Fescue biomass was significantly greater in treatments with litter of low C:N and greater mass loss, suggesting that rapid decomposition of forest litter may be more favorable to understory plant invasions. Earthworms were expected to enhance invasion by increasing mass loss and removing the physical barrier of litter. However, earthworms typically reduced invasion success but not under invasive tree litter where the presence of earthworms facilitated invasion success compared to other litter treatments where earthworms were present. We conclude that past and predicted future shifts in dominant tree species may influence forest understory invasibility. The presence of nonnative earthworms may either suppress of facilitate invasibility depending on the species of dominant overstory tree species and the litter layers they produce.     

百山祖常绿阔叶林灌草层物种组成和 分布的10年动态

森林群落的动态受多方面因素的影响。林下的草本植物和木本植物幼苗幼树对于森林的更新及动态具有重要作用。为了解百山祖常绿阔叶林群落下层植物物种组成和分布的动态变化, 我们分析了2003-2013年间百山祖5 ha森林动态样地灌草层物种(包括DBH < 1 cm的木本植物和所有草本植物)在物种重要值、α多样性以及物种分布等方面的变化。结果表明: (1) 10年间, 群落灌草层植物个体数减少了28.7%, 其中草质藤本植物个体数下降比例最大(76.9%), 而乔木的幼苗和幼树下降比例最小(14.4%); (2)整个灌草层的物种数明显下降, 同时乔木的幼苗和幼树及直立灌木的均匀度均下降, 而草本植物均匀度有所上升; (3)除短尾柯(Lithocarpus brevicaudatus)、叶萼山矾(Symplocos phyllocalyx)、显脉野木瓜(Stauntonia conspicua)和福建悬钩子(Rubus fujianensis)等植物重要值逆势上扬外, 直立灌木物种重要值10年间的变化值与其在2003年时的重要值呈显著正相关, 而其余各类生活型植物都显示, 初始重要值越高的物种在10年后下降幅度越大; (4)物种分布与生境类型的相关性变化复杂, 除与山谷生境正相关的物种数量增加外, 研究期间与生境存在显著相关性的物种数都有所减少; (5) 10年来仅陡坡处物种多样性指数没有显著下降。总体来说, 百山祖常绿阔叶林灌草层群落仍未达到顶极状态, 2003-2013年间正缓慢向顶极群落演替。群落内部的种间和种内竞争应是当前群落动态的主导因素。     

Influence of woody invader control methods and seed availability on native and invasive species establishment in a Hawaiian forest

When invasive woody plants become dominant, they present an extreme challenge for restoration of native plant communities. Invasive Morella faya (fire tree) forms extensive, nearly monospecific stands in wet and mesic forests on the Island of Hawai’i. We used logging, girdling, and selective girdling over time (incremental girdling) to kill stands of M. faya at different rates, with the objective of identifying a method that best promotes native forest re-establishment. We hypothesized that rapid canopy opening by logging would lead to establishment of fast-growing, non-native invaders, but that slower death of M. faya by girdling or incremental girdling would increase the establishment by native plants adapted to partial shade conditions. After applying the M. faya treatments, seed banks, seed rain, and plant recruitment were monitored over 3 years. Different plant communities developed in response to the treatments. Increased light and nitrogen availability in the logged treatment were associated with invasion by non-native species. Native species, including the dominant native forest tree, (Metrosideros polymorpha) and tree fern (Cibotium glaucum), established most frequently in the girdle and incremental girdle treatments, but short-lived non-native species were more abundant than native species. A diverse native forest is unlikely to develop following any of the treatments due to seed limitation for many native species, but girdling and incremental girdling promoted natural establishment of major components of native Hawaiian forest. Girdling may be an effective general strategy for reestablishing native vegetation in areas dominated by woody plant invaders.     

桉树人工林更新方式对林下植物功能群的影响

该研究综合运用野外调查和室内分析的方法,评估桉树人工林林下植物功能群的组成、分布及更新方式和相关环境因子之间的关系。结果表明:林分更新5 a后,除了非禾本科杂草功能群外,其他林下植物功能群的物种丰富度均呈现不同程度的增加,但与对照组(砍伐迹地)相比,其差异程度均不显著(P0.05);与对照组相比,藤本和蕨类功能群的相对多度也出现增加趋势,但禾本科草本功能群显著减少(P0.05);木本、藤本和蕨类功能群的相对盖度也呈现增加趋势,但禾本科草本功能群与对照组相比显著减少(P0.05);主成分分析(PCA)发现萌芽更新或植苗更新林的林下植物功能群组成和分布与对照组相比均发生了显著的变异,但不同更新方式(萌芽和植苗)下其林下植物功能群组成和分布差异不明显;通过冗余分析(RDA)确定了冠层透光系数、土壤孔隙度、坡向和土壤氮磷比是影响该林地林下植物功能群的主要因子,它们的叠加效应能解释大于75%的林下植物功能群的变异,最终模型通过排序得到冠层透光系数是影响该林地林下植物功能群的最主要因子。短期的研究发现萌芽和植苗这两种不同的更新方式对桉树林下植物功能群的影响有限,这可能与这两种更新方式形成的林冠结构和土壤理化性质差异性较小有关。     

Fire frequency and tree canopy structure influence plant species diversity in a forest-grassland ecotone

Disturbances and environmental heterogeneity are two factors thought to influence plant species diversity, but their effects are still poorly understood in many ecosystems. We surveyed understory vegetation and measured tree canopy cover on permanent plots spanning an experimental fire frequency gradient to test fire frequency and tree canopy effects on plant species richness and community heterogeneity within a mosaic of grassland, oak savanna, oak woodland, and forest communities. Species richness was assessed for all vascular plant species and for three plant functional groups: grasses, forbs, and woody plants. Understory species richness and community heterogeneity were maximized at biennial fire frequencies, consistent with predictions of the intermediate disturbance hypothesis. However, overstory tree species richness was highest in unburned units and declined with increasing fire frequency. Maximum species richness was observed in unburned units for woody species, with biennial fires for forbs, and with near-annual fires for grasses. Savannas and woodlands with intermediate and spatially variable tree canopy cover had greater species richness and community heterogeneity than old-field grasslands or closed-canopy forests. Functional group species richness was positively correlated with functional group cover. Our results suggest that annual to biennial fire frequencies prevent shrubs and trees from competitively excluding grasses and prairie forbs, while spatially variable shading from overstory trees reduces grass dominance and provides a wider range of habitat conditions. Hence, high species richness in savannas is due to both high sample point species richness and high community heterogeneity among sample points, which are maintained by intermediate fire frequencies and variable tree canopy cover.     

Strategic light manipulation as a restoration strategy to reduce alien grasses and encourage native regeneration in Hawaiian mesic forests

Question: Is there a light level at which alien grass biomass is reduced while still supporting growth and survival of native woody species, allowing for native species regeneration in abandoned pastures? Location: Island of Hawaii, USA. Methods: In a two‐part study we examined the effect of light availability on common native woody and alien grass species found in secondary forests in Hawaii. A field survey was conducted to examine the relationship between light availability and canopy type (open pasture, planted canopy and secondary forest) on understory grass biomass and litter accumulation. We then experimentally manipulated light levels to determine the effect of light availability on growth and survival of six native woody species and three alien grasses. Low‐light (5%), medium‐light (10%) and high‐light (20‐30%) treatments were created using shade structures erected beneath the existing secondary koa canopy. Results: In the field survey, alien grass biomass was greatest under the open pasture and lowest in the secondary forest. There was a positive correlation between understory light availability and alien grass biomass. In the experimental study, large reductions in relative growth rates were documented for all of the grass species and four of the six woody species under the lowest light level. Although growth at 5% light is substantially reduced, survival is still high (84‐100%), indicating that these species may persist under closed canopy. Conclusion: Low‐light conditions result in the greatest reduction in alien grass biomass while creating an environment in which native woody species can grow and survive.     

连栽杉木林林下植被生物量动态格局

用空间一致时间连续的定位研究方法,在湖南会同杉木林生态系统国家野外科学观测研究站试验基地的第2集水区,对连栽杉木林林下植被生物量进行了12 a的监测,研究了林下植被种类的变化、生物量动态特征、生物量的组成与分布变化格局。结果表明:连栽杉木林在14a生长发育过程中,林下植物种类呈现波动性的减少趋势,其中木本植物物种数下降率为40.0%,草本植物物种数下降率为47.1%。林下植被生物量由杉木林3年生29.48 t/hm²下降至14年生的2.53 t/hm²,其中木本植物生物量由7.07 t/hm²,下降至1.25 t/hm²,下降了82.3%;草本植物由22.41 t/hm²,下降至1.28 t/hm²,下降了94.3%。在此期间,木本与草本植物生物量的高低均出现波动现象。3年生杉木林下木本植物以乔木树种生物量6068.97 kg/hm²最高,占总生物量85.88%,藤本植物生物量736.97 kg/hm²为次,占10.44%,灌木植物生物量259.87 kg/hm²最低,仅占3.68%。14年生杉木林下木本植物以灌木植物生物量881.87 kg/hm²为首,占总生物量70.73%,藤本植物生物量247.07 kg/hm²为次,占19.82%,乔木树种生物量117.87 kg/hm²最少,只占9.45%。3年生杉木林下草本植物以蕨类植物生物量8391.44 kg/hm²最高,占总生物量的37.44%,过路黄生物量36.77 kg/hm²最低,仅占0.16%。杉木14年生时,以芒生物量573.00 kg/hm²最大,占总生物量44.78%,金毛耳草生物量2.93 kg/hm²最小,仅占0.23%。研究结果,可为研究杉木林养分循环、碳平衡、维护和提高林地地力及可持续经营管理提供科学依据。     

A genetic evaluation of seed dispersal in the neotropical tree Jacaranda copaia (Bignoniaceae)

Seed dispersal is a critical but poorly understood life-history stage of plants. Here we use a genetic approach to describe seed dispersal patterns accurately in a natural population of the Neotropical tree species Jacaranda copaia (Bignoniaceae). We used microsatellite genotypes from maternally derived tissue on the diaspore to identify which individual of all possible adult trees in the population was the true source of a given seed collected after it dispersed. Wind-dispersed seeds were captured in two different years in a large array of seed traps in an 84-ha mapped area of tropical forest on Barro Colorado Island, Panama. We were particularly interested in the proportion of seeds that traveled long distances and whether there was evidence for direct dispersal into gaps, which are required for successful recruitment of this pioneer tree species. Maximum likelihood procedures were used to fit single- and multiple-component dispersal kernels to the distance data. Mixture models, with separate distributions near and far, best fit the observed dispersal distances, albeit with considerable uncertainty in the tail. We discuss the results in light of different mechanisms responsible for separate distributions near the adult source and in the tail of the curve.     

Establishment and Control of Hay-scented Fern: A Native Invasive Species

Hay-scented fern (Dennstaedtia punctilobula (Michx.) Moore) is a native forest understory species that behaves as an invasive plant under certain conditions. Previous work has shown that both increased understory light intensity following overstory thinning and removal of competing plants by herbivores can lead to accelerated growth of hay-scented fern, allowing it to develop dense, nearly monospecific understories that inhibit tree seedling regeneration. To study the relationship between these two factors, we sampled 28 forest stands thinned at different times and subjected to different levels of browsing by white-tailed deer (Odocoileus virginianus), and concluded that more than 15 years of intensive browsing following thinning was necessary for ferns to form closed understory canopies with densities of >90 fronds/m² and canopy heights of 60–80 cm; thinning alone or intensive browsing alone was not sufficient to cause this level of fern invasion. We applied three treatments to dense fern understories to determine the relative importance of the fern canopy and the dense mat of roots, rhizomes, and dead fronds in the inhibition of tree seedling establishment. Results after two years were: (1) complete removal of the organic mat produced a large germination response of woody and herbaceous species; (2) mixing the organic mat into the mineral soil produced an initial germination response but poor seedling survivorship, as the fern canopy regrew to near pretreatment density; (3) repeatedly clipping the ferns for two years without disturbing the organic mat resulted in a lower germination response than the removal treatment, but rapid growth of seedlings.     

Forest community structure and composition following containment treatments for the fungal pathogen oak wilt

In temperate oak forests in Ohio, USA, we examined variability in forest communities within containment treatment sites for oak wilt (Bretziella fagacearum), a fungal pathogen lethal to susceptible oak species. Containment treatments included quarantine lines in soil for limiting belowground fungal spread and sanitation cutting of 1–3 mature black oak (Quercus velutina) trees within oak wilt infection patches. At 28 sites, we compared tree structure and understory plant communities across a gradient of 1- to 6-year-old treatments and reference forest (untreated and without evidence of oak wilt). While oak seedlings were abundant, oak saplings (1–10 cm in diameter) were absent. In contrast, many native understory plant community measures were highest in oak wilt treatments. Plant species richness 100 m^?2 doubled in treatments, regardless of age, compared with reference forest. Plant cover increased with treatment age, with 6-year-old treatments exhibiting 5?×?more cover than reference forest. Non-native plants averaged only a small proportion (<?0.12) of cover across treatments and reference forest. Variability in understory communities was mostly predictable using treatment age, tree canopy cover, and geographic location, as 20 of 25 understory measures had at least 72% of their variance modeled. While oak wilt treatments did not facilitate oak regeneration nor many conservation-priority species of open savanna-woodland habitats, the treatments did diversify and increase cover of native understory communities with minimal invasion of non-native plants.

     

Fire rather than nitrogen addition affects understory plant communities in the short term in a coniferous‐broadleaf mixed forest

Increasing fire risk and atmospheric nitrogen (N) deposition have the potential to alter plant community structure and composition, with consequent impacts on biodiversity and ecosystem functioning. This study was conducted to examine short‐term responses of understory plant community to burning and N addition in a coniferous‐broadleaved mixed forest of the subtropical‐temperate transition zone in Central China. The experiment used a pair‐nested design, with four treatments (control, burning, N addition, and burning plus N addition) and five replicates. Species richness, cover, and density of woody and herbaceous plants were monitored for 3 years after a low‐severity fire in the spring of 2014. Burning, but not N addition, significantly stimulated the cover (+15.2%, absolute change) and density (+62.8%) of woody species as well as herb richness (+1.2 species/m², absolute change), cover (+25.5%, absolute change), and density (+602.4%) across the seven sampling dates from June 2014 to October 2016. Light availability, soil temperature, and prefire community composition could be primarily responsible for the understory community recovery after the low‐severity fire. The observations suggest that light availability and soil temperature are more important than nutrients in structuring understory plant community in the mixed forest of the subtropical‐temperate transition zone in Central China. Legacy woody and herb species dominated the understory vegetation over the 3 years after fire, indicating strong resistance and resilience of forest understory plant community and biodiversity to abrupt environmental perturbation.     

吉林东部天然林林下光质分布特征

天然林林下光质对乔木幼苗以及灌草的组成与更新具有重要的生态学意义。但目前对于林下光质的研究仍然有限。以吉林东部地区天然林为例，通过调查乔木数据和林下光质数据，基于移动窗口法分析不同空间尺度森林冠层结构与林下光质的关系。结果表明：不同林型下红光光子通量密度(R)与蓝光光子通量密度(B)存在差异。其中沙松-千金榆-花楷槭混交林林下蓝光光子通量密度最小，而沙松-紫椴-臭冷杉混交林和长白落叶松纯林林下最大。随着尺度的增大，天然林乔木胸高断面积与R/PFD(红光/光子通量密度比值)和B/PFD(蓝光/光子通量密度比值)的比值呈显著正相关(P<0.05)。并且随着尺度的增加，相关系数总体逐渐增大，在35m处达到峰值。在此基础上在南向、东向和西向各延伸10m时呈现显著正相关(P<0.05)。在该尺度下分析优势树种对林下R/PFD和B/PFD比值的影响时发现：R/PFD与B/PFD比值随着针叶林胸高断面积的增加而增加。相对于阔叶林来说，多数林型针叶林下的冠层结构与林下R/PFD和B/PFD比值之间显著正相关(P<0.05)。在不同树种下，乔木冠层结构对R/PFD和B/PFD比值的影响...     

Habitat fragmentation,tree diversity,and plant invasion interact to structure forest caterpillar communities

Habitat fragmentation and invasive species are two of the most prominent threats to terrestrial ecosystems. Few studies have examined how these factors interact to influence the diversity of natural communities, particularly primary consumers. Here, we examined the effects of forest fragmentation and invasion of exotic honeysuckle (Lonicera maackii, Caprifoliaceae) on the abundance and diversity of the dominant forest herbivores: woody plant-feeding Lepidoptera. We systematically surveyed understory caterpillars along transects in 19 forest fragments over multiple years in southwestern Ohio and evaluated how fragment area, isolation, tree diversity, invasion by honeysuckle and interactions among these factors influence species richness, diversity and abundance. We found strong seasonal variation in caterpillar communities, which responded differently to fragmentation and invasion. Abundance and richness increased with fragment area, but these effects were mitigated by high levels of honeysuckle, tree diversity, landscape forest cover, and large recent changes in area. Honeysuckle infestation was generally associated with decreased caterpillar abundance and diversity, but these effects were strongly dependent on other fragment traits. Effects of honeysuckle on abundance were moderated when fragment area, landscape forest cover and tree diversity were high. In contrast, negative effects of honeysuckle invasion on caterpillar diversity were most pronounced in fragments with high tree diversity and large recent increases in area. Our results illustrate the complex interdependencies of habitat fragmentation, plant diversity and plant invasion in their effects on primary consumers and emphasize the need to consider these processes in concert to understand the consequences of anthropogenic habitat change for biodiversity.     

Floristics of soil seed banks on agricultural and disturbed land cleared of tropical forests

Little is known about how soil seed banks vary in germination, composition, and density under different land uses after tropical forest conversion. Seed banks can potentially act as one source of regeneration for reforestation of old agricultural lands. Our study documents the composition and density of germinants in soil seed banks from four land uses types surrounding the Sinharaja forest in southwest Sri Lanka. These include: (1) kekilla fern lands; (2) pine plantations; and (3) tea. These were compared to the adjacent (4) mature rainforest. During the 6‐month period of monitoring, we recorded 1,674 germinants (0.036 germinants/cm³ soil), representing 46 species. Germinants of tree and shrub species were restricted to the pine and rainforest soils and all of them are considered pioneers. The soils of the rainforest had the lowest species richness, density, and diversity of germinants; tea lands comprised much higher richness, Shannon diversity, and density. However, almost all germinants in tea were grasses and herbs as compared with other land uses. A multivariate analysis of the germinants of soil seed banks revealed that the four land use types comprise very different compositions and abundances, some of which can be associated with differences in growth habit (trees, shrubs, vines, herbs, grasses). Our results suggest that pine plantations may facilitate some tree and shrub regeneration. However, the seed banks beneath tea and kekilla fern land do not comprise any woody plant species. This may explain why agricultural lands such as tea do not revert back to forest easily.