Experimental evidence for a phylogenetic Janzen-Connell effect in a subtropical forest

Observational evidence increasingly suggests that the Janzen-Connell effect extends beyond the species boundary. However, this has not been confirmed experimentally. Herein, we present both observational and experimental evidence for a phylogenetic Janzen-Connell effect. In a subtropical forest in Guangdong province, China, we observed that co-occurring tree species are less phylogenetically related than expected. The inhibition effects of neighbouring trees on seedling survival decreased with increasing phylogenetic distance between them. In a shade-house experiment, we studied seedling survival of eight species on soil collected close to Castanopsis fissa relative to their survival on soil close to their own adult trees, and found that this relative survival rate increased with phylogenetic distance from C. fissa. This phylogenetic signal disappeared when seedlings were planted in fungicide-treated soil. Our results clearly support negative effects of phylogenetically similar neighbouring trees on seedling survival and suggest that these effects are caused by associated host-specific fungal pathogens.     

神农架米心水青冈-曼青冈群落的地形异质性及其生态影响

地貌单元和地形的局部变化严重制约着光照、温度、水分甚至土壤资源的再分配 ,从而对植被的分布产生影响。以永久性监测样地为基础 ,利用数字高程模型模拟 0 .96 hm2样地的地形表面 ,在地理信息系统的支持下分析了米心水青冈 -曼青冈群落的地形异质性状况 ,并结合群落调查的结果探讨了地形因素对群落组成的影响。结果表明研究范围内样方的平均坡度在 18.2～ 74 .6°之间。坡向可以分为南坡、西南坡及东南坡 3部分 ,坡度及坡向差异显著。对坡度、坡向等地形因素及群落的物种组成进行半方差分析 ,发现三者的空间异质性尺度分别为 2 85 .8m,2 80 .5 m及 6 .1m。地形因素的变化尺度远大于植被组成的变化 ,说明小尺度下地形因素不是影响群落内种群分布格局的唯一因素。坡度和坡向因子的块金值分别为 2 1.8及 4 98.0 ,说明两者在小尺度下的某些过程不能忽视。植物与地形因素的相关分析表明不同的种对地形的适应性不同 ,群落中的 2 4种主要植物可以分为 4个生态种组。因此 ,群落的物种组成变化受到多种因素的影响 ,这种影响可以在地形变化中得到反映。     

The Janzen-Connell effect on the population dynamics of a Fagus engleriana- Cyclobalanopsis oxyodon community in a subtropical zone of China

The Janzen-Connell (J-C) hypothesis provides a mechanism explaining the high species diversity in tropical rainforests. It postulates that predation could cause greater mortality on seeds and seedlings near their parental trees. In this study, we tested the hypothesis in a subtropical zone, a mixed evergreen-deciduous broad-leaved forest dominated by the Fagus engleriana and Cyclobalanopsis oxyodon. The study area was in the Shennongjia region, a key area of biodiversity conservation in both China and the world. The recruitment probability index was used to detect the J-C effect on nine species of the community, which were more than 50 individuals. Six large adults of each species were selected, and the numbers of saplings and adults were counted at the distance intervals of 0–5, 5–10, 10–15, 15–20, and 20–25 m from each focal tree. Two species in saplings stage and six in adult stage supported the J-C hypothesis, but their χ² was not significant. Three species, the F. engleriana, Rhododendron hypoglaucum, and Toona sinensis, showed a strong Hubbell pattern in the adult stage. Because of these results, we reject the J-C hypothesis and conclude that species could recruit near the conspecific trees in subtropical forest. The reasons why the J-C hypothesis fails to explain the species diversity in this community are the shortage of seed-consuming agents of subtropical forest and the influence of microsite topographic variation.     

The full path of Janzen-Connell effects: genetic tracking of seeds to adult plant recruitment

The Janzen-Connell (J-C) model (Janzen 1970; Connell 1971) has been a dominant yet controversial paradigm for forest community dynamics for four decades, especially in the tropics. With increasing distance from the parent plant, the density of dispersed seeds decreases and, because of a reduced impact of distance- and density-responsive seed and seedling enemies, propagule survival increases, resulting in peak recruitment at some distance from the parent and little recruitment near adult conspecifics. This spacing generates gaps near adult trees for the recruitment of heterospecifics, enhancing species coexistence and species richness. Field studies, primarily focused on seeds and young seedlings, have repeatedly demonstrated increasing survival with increasing distance from parents or decreasing density of propagules (e.g. Clark & Clark 1984; Gilbert et al. 1994; Swamy & Terborgh 2010). Yet a meta-analysis of distance-dependent propagule survival failed to support a general pattern of survival increasing with distance from adult conspecifics, suggesting that there is no need for further experimental tests of the J-C hypothesis in terms of diversity enhancement-results are species-specific, not general (Hyatt et al. 2003). However, a lack of consistent experimental results is not surprising. The outcome of tests of the hypothesis can vary as a function of many factors that can affect successive recruitment stages differently (Schupp 1992; Hyatt et al. 2003; Swamy & Terborgh 2010). This highlights a critical gap-a full test of the J-C model requires data demonstrating that effects carry over to recruitment of new reproductive adults, yet few studies have gone beyond early stages. There is strong inferential evidence that adult trees can show the imprint of J-C effects (e.g. Nathan et al. 2000; Howe & Miriti 2004), and focal individual modelling has clearly demonstrated that J-C effects can operate from sapling through adult stages in a significant number of species (Peters 2003). It is likely that such results are not unusual, but there have been few attempts to demonstrate J-C spacing at the adult stage. In this issue of Molecular Ecology, Steinitz et al. (2011) studied the Mediterranean pine Pinus halepensis (Aleppo pine) and combined a unique situation with an innovative approach to provide the most elegant demonstration yet that adult recruits are spaced further from parents than expected from the initial seed distribution, clear evidence of a J-C effect carrying over to reproductive adults. A major advancement of this study is that it incorporates estimates of the initial patterns of seed dispersal and parentage analysis of adult-offspring relationships, illustrating the value of combined field and genetic approaches.     

Soil nitrogen availability intensifies negative density-dependent effects in a subtropical forest

负密度制约在维持森林群落物种多样性方面起重要作用。然而,在不同的非生物条件下密度制约效应的强度和方向有可能发生改变。理论预测密度制约效应随土壤可利用氮浓度上升而增强,但仍有待实验结果的验证。本研究基于黑石顶常绿阔叶林群落长达10年的幼苗动态调查数据,以18个常见树种的新生幼苗为研究对象,分析邻体密度制约作用如何在不同环境梯度下变化。通过收集上述目标种中的5个树种的野外幼苗进行根系损伤观察实验,研究在不同可利用氮环境中,土壤病原菌驱动的负密度制约效应是否存在差异。幼苗动态分析结果表明,负密度制约作用的强度随土壤可利用氮浓度的上升而增强。同种幼苗密度在土壤可利用氮浓度低的生境中表现为中性或正密度依赖效应,在土壤可利用氮浓度高的生境中则表现为负密度制约效应。根系损伤观察实验中,在土壤可利用氮浓度高的情况下,5个目标物种中有3个物种表现出更强的负密度制约作用,与幼苗动态分析的结果一致。此外,物种丰富度在土壤可利用氮浓度高的环境中比在浓度低的环境中更高。上述结果表明,在土壤可利用氮浓度高的环境中,更强的土壤病害加剧了负密度制约作用。我们的研究结果强调了负密度制约研究中考虑背景环境的重要性。     

中国亚热带森林转换对土壤呼吸动态及通量的影响

通过用静态碱吸收法对中国亚热带福建三明格氏栲自然保护区内的格氏栲天然林和33年生的格氏栲人工林及杉木人工林的土壤呼吸进行为期2a的定位研究,结果表明,3种森林土壤呼吸速率季节变化均呈单峰曲线,最大值出现在5月至6月份,最小值出现在12月至翌年1月份。格氏栲天然林、格氏栲人工林和杉木人工林土壤呼吸速率一年中变化范围分别在403.47～1001.12mgCO2m-2h-1、193.89～697.86mgCO2m-2h-1和75.97～368.98mgCO2m-2h-1之间。2002年土壤呼吸速率主要受土壤温度影响,但在极端干旱的2003年则主要受土壤湿度的影响。双因素关系模型(R=aebTWc)拟合结果优于仅考虑土壤温度或土壤湿度的单因素关系模型,土壤温度和土壤湿度共同解释不同年份不同森林土壤呼吸速率季节变化的80%～96%。杉木林土壤呼吸对气候变化敏感性高于格氏栲天然林和人工林。格氏栲天然林、格氏栲人工林和杉木人工林土壤呼吸年通量分别为13.742、9.439和4.543tC·hm-2·a-1,前者分别约是后二者的1.5倍和3.0倍。森林转换对土壤呼吸通量的影响可能与枯落物数量和质量、根系呼吸、土壤有机质数量和质量的变化有关。     

南亚热带森林演替过程中土壤线虫群落结构变化

森林演替会通过改变植物群落组成和土壤环境影响土壤生物群落, 反过来, 土壤生物群落的变化也会对生态系统的演替产生反馈作用, 但迄今南亚热带森林演替过程中土壤生物群落的变化特征尚不清晰。本研究以广东省鼎湖山的南亚热带森林演替序列(马尾松(Pinus massoniana)林-针阔叶混交林-季风常绿阔叶林)为对象, 研究了森林演替过程中土壤线虫多样性和群落结构的动态变化及其影响因素。通过采集不同演替阶段的土壤样品, 分析和比对了不同演替阶段土壤线虫的多度、多样性、群落组成、土壤线虫生态指数以及土壤理化性质的差异。结果表明: (1)在南亚热带森林演替过程中, 针阔叶混交林和季风常绿阔叶林土壤线虫的α多样性显著高于马尾松林, 但土壤线虫总数和各营养类群多度及其相对丰度并无显著变化; (2)针阔叶混交林中土壤线虫富集指数显著高于马尾松林, 表明其土壤养分状况要好于马尾松林, 而季风常绿阔叶林土壤线虫结构指数较高, 表明其受干扰程度较低; (3)针阔叶混交林的土壤含水量和土壤理化性质(除土壤总磷含量)已达到季风常绿阔叶林的水平, 但两者的土壤pH值均显著低于马尾松林, 而土壤pH值和土壤含水量是影响土壤线虫群落动态变化的主要因素。综上所述, 南亚热带森林中土壤线虫多度、多样性和群落结构对森林演替的响应略有不同, 演替过程中土壤环境因素的趋同是导致针阔叶混交林和季风常绿阔叶林中土壤线虫多样性和群落特征相似的主要原因。     

Effect of clear-cutting silviculture on soil respiration in a subtropical forest of China

Aims Clear-cutting is a common forest management practice, especially in subtropical China. However, the potential ecological consequences of clear-cutting remain unclear. In particular, the effect of clear-cutting on soil processes, such as the carbon cycle, has not been quantified in subtropical forests. Here, we investigated the response of soil respiration (Rs) to clear-cutting during a 12-month period in a subtropical forest in eastern China.Methods We randomly selected four clear-cut (CC) plots and four corresponding undisturbed forest (UF) plots. Measurements of Rs were made at monthly time points and were combined with continuous climatic measurements in both CC and UF. Daily Rs was estimated by interpolating data with an exponential model dependent on soil temperature. Daily Rs was cumulated to annual Rs estimates.Important findings In the first year after clear-cutting, annual estimates of Rs in CC (508±23g C m ?2 yr^-1) showed no significant difference to UF plots (480±12g C m ?2 yr^-1). During the summer, soil temperatures were usually higher, whereas the soil volumetric water content was lower in CC than in UF plots. The long-term effects of clear-cutting on Rs are not significant, although there might be effects during the first several months after clear-cutting. Compared with previous work, this pattern was more pronounced in our subtropical forest than in the temperate and boreal forests that have been studied by others. With aboveground residuals off-site after clear-cutting, our results indicate that the stimulation of increasing root debris, as well as environmental changes, will not lead to a significant increase in Rs. In addition, long-term Rs will not show a significant decrease from the termination of root respiration, and this observation might be because of the influence of fast-growing vegetation after clear-cutting in situ .     

亚热带森林凋落叶分解过程土壤节肢动物群落的变化特征

土壤动物群落结构和多样性可能随凋落物分解进程和基质质量的变化不断改变。为了解亚热带森林凋落叶分解过程中土壤节肢动物群落变化特征,以四川盆地亚热带森林麻栎（Quercus acutissima）和柳杉（Cryptomeria fortunei）凋落叶为对象,于2011-2015年采用分解袋法研究了2种凋落叶分解过程中土壤节肢动物组成、结构和多样性动态变化。整个研究期间,柳杉和麻栎凋落叶分解袋中共捕获土壤节肢动物3855只,分属于16目51科,且均以等节跳科和棘跳科为优势类群;麻栎凋落叶中土壤节肢动物的个体密度随分解进程呈现增加趋势,在分解的1079天达最高值后降低,而柳杉凋落叶则在分解的156天急剧增加后快速降低,2种凋落叶中土壤节肢动物类群数量具有相似的动态变化过程;2种凋落叶中土壤节肢动物总体以菌食性数量比例最高,腐食性最低,且随凋落叶分解进程,植食性土壤节肢动物占比明显下降,菌食性则上升;非度量多维尺度（NMDS）分析显示,2种凋落叶中土壤节肢动物群落组成具有显著差异,聚类分析表明,2种凋落叶土壤节肢动物群落结构相似性随分解进程不断降低。亚热带森林凋落叶分解过程中土壤节肢动物群落组成、结构和多样性受凋落叶类型影响。     

中国东部亚热带青冈种群叶片的生态解剖

运用生态解剖学的方法对我国亚热带东部分布的１０个青冈种群叶片的解剖特征进行了比较分析,结果如下：①青冈叶片结构在种群间存在着广泛的差异,这种差异是青冈适应不同环境条件的结构基础。青冈叶片各部分结构特征在种群间的变化不晃同步的,变异系数（ＣＶ）在６．０％～２０．５％之间;变异幅度最大的是栅栏组织和上角质膜厚度,最小的是下表皮厚度。②相关分析表明,温度和降雨是影响青冈叶片地理变异的主导因子,其中温度、     

Variation of Soil Nutrition in a Fagus engleriana Seem.- Cyclobalanopsis oxyodon Oerst. Community Over a Small Scale in the Shennongjia Area, China

Soil nutrition is a key factor influencing species composition in a community, but it has clearly scaledependent heterogeneity. In the present study, geostatistics methods and canonical correspondence analysis （CCA） were used to detect： （i） the variation range of soil spatial heterogeneity; （ii） the influence of topographic factors on the distribution of soil nutrition; and （iii） the relationships between soil chemical properties and species in the community. In all, 23 soil variables were measured, including total N and organic C, AI, Ba, Ca, Cr, Cu, Fe, Ga, Li, Mg, Mn, Na, NH4-N, Ni, NO3-N, Pb, pH, P, Sr, Ti, V, and Zn. Semi-variograms of these variables were calculated and mapped. All indices showed autocorrelatlons, with ranges between 29 and 200 m. When the sample method was larger than these distances, spatial autocorrelations were avoided. The distribution patterns of Ca, Cr, Ga, K, Mg, organic C, P, Pb, and pH, and total N were related to the microtopography and the distribution of these compounds was clumped in water catchments area. The CCA method was used to investigate the relationship between plant species and soil properties in this community. Fagus engleriana Seem., Lindera obtusiloba BI. Mus., and Acer griseum （Franch.） Pax were correlated with organic C, available N, and P.     

亚热带红壤丘陵区四种人工林凋落物分解动态及养分释放

应用网袋分解法,连续2a对我国亚热带红壤丘陵区内有代表性的人工林类型马尾松（Pinus,massoniana）、湿地松（Pinus elliottii）、杉木（Cunninghamialanceolata）、木荷（Schimasuperba）＋马尾松（Pinus,massoniana）混交林的凋落物的分解速率,及其C、N元素释放动态进行了研究,凋落物样品分地上、地下两组处理方式。4种林分凋落物地上组的第1、2年分解速率（凋落物的年失重率）依次为马尾松林〉混交林〉湿地松林〉杉木林,马尾松林〉混交林〉杉木林〉湿地松林;地下组的第1、2年分解速率顺序分别为马尾松林〉混交林〉杉木林〉湿地松林,马尾松〉杉木林〉湿地松林〉混交林。各林分地上组凋落物分解速率明显快于地下部分,马尾松林凋落物的分解速率在不同时期均高于其它林分。4种林分凋落物的分解动态符合Olson指数衰减模型。根据拟合方程得出的凋落物分解95％时间为4～01a,介于暖温带常见树种凋落物95％被分解所需时间8～17a,地处南亚热带季风区的鼎湖山凋落物分解95％所需的时间2～8a。养分元素释放率的变化因不同林分和分解时期而异。C在各林分中始终表现为净释放,地上组凋落物的释放率大多数时间均高于地下组。N则于湿地松林、马尾松林和混交林中前期表现出富集现象,而后开始净释放,其中湿地松林凋落物的N富集现象最为显著,释放速率在两个试验年度均为各林分中最低,凋落物中初始的高C／N比是导致上述现象的原因。杉木林凋落物具有最低的初始C／N比,没有出现N富集现象,且在两个试验年度末期均维持了较高的N释放率.     

The effect of conspecific density,herbivory, and bamboo on seedling dynamics of a dominant oak in a Neotropical highland forest

Conspecific negative density dependence (CNDD) is one of the main mechanisms influencing diversity maintenance in tropical forests. Tropical highland forests, in contrast to most lowland forests, are commonly dominated by a few tree species, and testing the importance of density dependence effects on seedling establishment of dominant trees may provide insights on the mechanisms regulating population dynamics and forest composition of tropical highlands. We tested the effect of CNDD regulation on seedling survival and recruitment of Quercus costaricensis, a monodominant oak in the Talamanca highland forests of Costa Rica. We used Ripley's K and generalized linear mixed models to test the effects of conspecific density, distance to the nearest adult, density of Chusquea bamboo shoots, and herbivory on the annual survival probability of 3579 seedlings between 2014 and 2017. We did not find a significant effect of CNDD on seedling survival. However, bamboo density and herbivory both significantly decreased oak seedling survival. All seedlings had signs of herbivory and predator satiation may explain the lack of density dependent regulation in seedlings of this species. We argue that the lack of intraspecific density regulation at the seedling stage may contribute to explain the dominance of Q. costaricensis in the highland forests of Costa Rica. Local seedling dynamics of this endemic oak are instead regulated by herbivory and the density of Chusquea. Abstract in Spanish is available with online material.     

亚热带中山地区草芦无性系种群的密度制约

亚热带中山立地条件下草芦无性种群密度制约的定量分析结果表明,不同时期草芦无性系种的平均蘖重,平均蘖叶重和平均蘖茎重均随着密度的增加呈函数下降。     

The demography of a newly established Osprey Pandion haliaetus population in France

Using long‐term mark–resighting data acquired over 27 years in continental France, we estimated demographic parameters and modelled the dynamics of a newly established population of Ospreys Pandion haliaetus using a life‐history model. We then performed prospective and retrospective analyses to estimate the sensitivity of the population growth rate to demographic parameters, and to quantify their contribution to the observed variation in abundance. The observed population growth rate was estimated at 1.150 (from one to 38 pairs in the period 1985–2011), and the stochastic population growth rate was estimated at 1.156. The number of fledglings per nest made the largest contribution to the variance of the observed population growth rate. Breeding productivity was stable across years. In contrast, the prospective analysis indicated that the sensitivity of the population growth rate was greatest for immigration and adult survival. Our results suggest that the increase of a new and recently established breeding population of Ospreys was mainly driven by local dynamics (high productivity and high proportion of breeding individuals), with no sign of density‐dependence except for juvenile survival. This probably reflects highly favourable conditions for breeding. Our results show that productivity can be a major driver in recovering raptor populations, and conservation work should aim to protect occupied nest‐sites and their surrounding habitat and to maintain highly favourable foraging areas in the vicinity of breeding sites.     

古田山常绿阔叶林主要树种2002-2007年间更新动态

种群更新是森林发育和演替过程中的重要环节.为了解古田山常绿阔叶林主要树种的更新动态,本研究以古田山自然保护区5 ha常绿阔叶林动态样地为对象,在对胸径(DBH)≥1 cm的木本植物个体2002年和2007年两次每木调查的基础上,比较主要树种的补员(recruitment)和死亡状况,分析种群大小变化趋势,研究了群落中主要木本植物的更新特点.结果表明:(1)5年间样地内木本植物个体从13,338株增长到16,812株,增幅达26.05%.新增4,758株胸径≥1 cm的补员个体,同时死亡1,384株.(2)乔木、小乔木或灌木、小灌木3种生长型的补员个体数目均随径级的增大而下降,但死亡个体却表现出不同的径级分布特点.(3)43个主要树种的平均年死亡率和年补员率分别为2.26%和5.90%.其中,19种小乔木或灌木在径级Ⅰ(1 cm≤DBH<2.5 cm)的补员率均高于死亡率,但径级Ⅱ(2.5cm≤DBH<5 cm)、径级Ⅲ(DBH≥5 cm)的补员率均和死亡率差别不大:而24种乔木各径级死亡率和补员率的关系则相对复杂.(4)43个主要树种中有31个为增长型种群,其余12个呈小幅度负增长.10个种群大小变化率在5%以上,属于快速变动的种群,其中以柳叶蜡梅(Chimonanthus salicifolius)(45.13%)和矩形叶鼠刺(Itea oblonga)(16.35%)的增幅最大,橙木(Loropetalum chinense)(-3.10%)和短柄袍(Quercus serrata vat.brevipetiolata)(-1.86%)的降幅最大.研究发现,3种生长型中,不同物种及其各个径级的补员率和死亡率各异,种群更新动态也有所差别,我们推测与密度制约和牛境波动有关.     

Elevational variation in density dependence in a subtropical forest

Density‐dependent mortality has been recognized as an important mechanism that underpins tree species diversity, especially in tropical forests. However, few studies have attempted to explore how density dependence varies with spatial scale and even fewer have attempted to identify why there is scale‐dependent differentiation. In this study, we explore the elevational variation in density dependence. Three 1‐ha permanent plots were established at low and high elevations in the Heishiding subtropical forest, southern China. Using data from 1200 1 m² seedling quadrats, comprising of 200 1 m² quadrats located in each 1‐ha plot, we examined the variation in density dependence between elevations using a generalized linear mixed model with crossed random effects. A greenhouse experiment also investigated the potential effects of the soil biota on density‐dependent differentiation. Our results demonstrated that density‐dependent seedling mortality can vary between elevations in subtropical forests. Species found at a lower elevation suffered stronger negative density dependence than those found at a higher elevation. The greenhouse experiment indicated that two species that commonly occur at both elevations suffered more from soilborne pathogens during seed germination and seedling growth when they grew at the lower elevation, which implied that soil pathogens may play a crucial role in density‐dependent spatial variation.