基于零模型的宁夏荒漠草原优势种群点格局分析

植物种群空间分布格局是多种生态过程综合作用的结果。明确植物优势种群个体的空间分布格局与形成机制有助于认识种群生态适应对策与群落多样性维持机制。以宁夏荒漠草原优势种群蒙古冰草、短花针茅、牛枝子和牛心朴子为研究对象,采用完全空间随机零模型分析其种群空间分布格局特征,并通过异质泊松零模型与泊松聚块零模型探讨生境异质性、扩散限制等因子在其空间分布格局形成过程中的作用。结果显示:(1)完全空间随机零模型下,4个物种在4 m尺度范围内表现为聚集分布,随尺度增大,逐渐过渡到随机分布和均匀分布。(2)在排除生境异质性的异质泊松零模型下,蒙古冰草种群在整个研究尺度上表现为随机分布;牛枝子、短花针茅和牛心朴子种群仅分别在0—0.2、0.1—0.4 m与0—0.2 m尺度范围内发生偏离,表现为均匀分布与聚集分布,其他尺度均为随机分布。(3)在排除扩散限制的泊松聚块零模型下,所研究种群均表现为随机分布。综上,荒漠草原优势种群在小尺度范围内主要表现为聚集分布;生境异质性与扩散限制均是驱动其空间分布格局形成的重要因子,相对而言,小尺度空间范围内扩散限制的作用更为显著。     

辽东山区次生林木本植物空间分布

森林木本植物的空间格局有助于揭示群落结构的形成机制与潜在的生态学过程,且对林分经营具有一定指导意义。在0—50 m尺度范围内综合分析了辽东山区4 hm2温带次生林样地多度10的树种空间格局。研究发现:(1)在完全随机零模型下,大部分树种呈现聚集格局,聚集格局树种的比例随尺度增加而降低;在32 m的较大尺度下,随尺度增加,随机和规则格局成为树种分布的主要形式;(2)在异质性泊松过程零模型下,55.9%的树种呈现随机格局,其余大部分树种在10 m的尺度下呈现聚集格局,且随尺度增加,规则格局成为主要形式;(3)在完全随机零模型下,树种属性(林层、径级和多度)显著地影响种群聚集度,而在异质性泊松过程零模型下,树种属性对种群聚集度不存在显著影响。综上,生境异质性、扩散限制和树种属性部分解释了辽东山区次生林木本植物空间分布格局,相对而言,生境异质性的效应更为突出。研究结果有助于揭示次生林群落生物多样性的维持机制。     

辽东山区次生林乔木幼苗分布格局与种间空间关联性

在辽东山区次生林建立4 hm^2样地(200 m×200 m),研究0~50 m尺度范围内乔木幼苗分布格局及种间空间关联性.结果表明:在完全随机零模型下,0~20 m尺度上,95%的树种呈现聚集分布格局;0~16 m尺度上,19个树种呈现聚集分布;随着尺度的增加,聚集分布树种的比例逐渐减少,50 m尺度上,随机分布成为树种分布的主要形式;在异质性泊松过程零模型下,0~24 m尺度上,5%的树种呈现聚集分布,26~50 m尺度上,42%和58%的树种呈现随机和均匀分布.在完全随机零模型下,正相关树种对比例最高,且在50 m尺度下呈现正相关、负相关、无相关3种相关性的树种对比例相同;在异质性泊松过程零模型下,树种对主要呈现负相关,且随尺度增大,负相关的树种对比例逐渐升高.种子扩散限制和生境异质性在某种程度上解释了乔木幼苗的聚集分布格局,乔木幼苗强烈的聚集分布又促使种间空间关联性密切,更新群落稳定性较差.     

广西弄岗北热带喀斯特季节性雨林监测样地种群空间点格局分析

种群的空间分布格局是由多种机制的交互作用而形成,是探究生物多样性维持机制的基础。北热带喀斯特季节性雨林是我国北热带石灰岩山地具有地带性特征的植被,其生境的典型特征在于土层浅薄、岩石裸露率高、贮水能力低和周期性水淹,以及富钙强碱性环境等。本研究基于广西弄岗北热带喀斯特季节性雨林15 ha动态监测样地的第一次调查资料,采用双关联g(r)函数点格局方法,分析了雨林中出现个体数≥15株的160个种的种群空间分布格局以及不同类群间种群分布的差异。研究表明:160种木本植物中有146种在0–10 m的尺度上呈聚集分布,且随着空间尺度的增大,聚集度呈下降趋势;物种的种群聚集度与其物种多度、平均胸径和最大胸径成负相关;常绿物种的种群聚集度与落叶物种间差异不显著;不同生活型间的种群聚集度差异显著,总体表现为亚乔木层物种高于乔木层,灌木层物种显著高于亚乔木层;剔除生境异质性后大部分物种表现为随机分布,仅少部分物种的种群表现为聚集分布。这表明物种的功能属性如物种多度、生活型等可较好地预测物种的分布格局。此外,物种种群的分布格局还受生境异质性的影响,且不同物种受生境异质性影响的程度不同。     

长白山次生杨桦林种群空间点格局及密度制约效应

以长白山5.2 hm²次生杨桦林样地为研究对象,利用空间点格局分析的双相关函数g（r）以及随机标签零模型和案例-对照设计法,探讨了树木种群空间格局及其密度制约效应。研究结果表明：生境异质性对不同生活型以及不同树种的成熟个体在大尺度范围上具有强烈影响。剔除生境异质性带来的影响后,在14个常见树种中有12个树种在小尺度上呈显著的空间聚集分布格局;随着空间尺度的增加聚集性分布树种数量急剧下降,在18 m尺度上聚集率下降到0。全部14个常见树种均呈现出显著的密度制约效应,表明密度制约是调节温带森林树木种群空间结构的主要作用机制。此外,密度制约与物种多度呈负相关,与亚林层和灌木层树种的同种聚集强度呈正相关。并且随着空间尺度增大受密度制约影响的树种百分比逐渐减小,14个常见树种中有11个在0-1 m处达到最大的密度制约强度。     

浙江省典型天然次生林主要树种空间分布格局及其关联性

松阔混交林和常绿阔叶林是亚热带地区天然次生林代表性的森林类型,通过研究主要树种分布变动趋势,可以有效地指导森林经营措施。在该地区4个1hm~2典型样地,在0—30m尺度范围内综合分析了主要树种的空间分布格局及种间关联性,并对同一树种分发育阶段在不同森林群落中的种群空间分布格局进行了比较,以探讨亚热带地区天然次生林群落空间格局形成和种群维持机制。样地1和2为松阔混交林,其中样地1的马尾松密度较低,样地3和4为常绿阔叶林。研究结果表明:(1)以完全随机模型为零假设时,样地1的主要种群在小尺度(10 m)呈聚集分布,随尺度增加呈随机分布;样地2—4的主要种群在所有尺度呈聚集分布,随尺度增加聚集强度逐渐减弱;以异质泊松模型为零假设时,4个样地的主要种群在大部分尺度呈随机分布;(2)青冈和苦槠的小树(5.0 cm≤DBH10.0 cm)在4样地的所有尺度以聚集分布为主,大树(DBH≥10.0 cm)在松阔混交林样地呈随机分布趋势,但在常绿阔叶林样地青冈在0—20 m尺度、苦槠在所有尺度呈聚集分布;(3)松阔混交林中建群种马尾松和其他树种的种间关联性,在样地1的小尺度为负相关,随尺度增加为不相关,在样地2的所有尺度为负相关;常绿阔叶林中建群种青冈和其他树种的种间关联性在在样地3的小尺度为负相关,随尺度增加为不相关,在样地4的所有尺度为负相关;所有伴生树种的种间关联性以负相关为主。结果说明,种群空间分布格局及其关联性随群落结构和空间尺度的不同而出现变化,在松阔混交林和常绿阔叶林群落格局形成中除了扩散限制和生境异质性以外,密度制约机制在松阔混交林中发挥了重要作用,而在常绿阔叶林中其作用随着树木生活史阶段的提高而减弱。     

水杉原生种群结构及空间分布格局

水杉(Metasequoia glyptostroboides)是我国特有濒危极小种群物种, 其种群的状态一直被国内外学者广泛关注。分析现存水杉原生种群结构和空间分布格局及其空间关联性, 可以从空间格局角度深入认识水杉原生种群结构和分布格局及可能的形成机理。本文基于湖北利川境内水杉原生种群的野外调查数据, 分析其径级和高度级结构, 同时运用点格局分析中的成对相关函数g(r)以及3个零模型(完全空间随机模型、异质泊松模型、先决条件模型)分析水杉原生种群空间分布格局、各龄级空间分布格局及空间关联性。结果表明: (1)分布于研究区域内的水杉原生种群个体数共5,663株, 已死亡33株, 现存活5,630株, 其中40株濒临死亡, 465株处于衰弱状态, 部分个体呈现不同的形态特征和生长状况, 断梢和蚁害最为常见, 雷击是最致命的危害。(2)水杉原生种群结构分析显示, 其径级结构和高度级结构均呈纺锤型, 自然更新不良。(3)基于完全空间随机模型, 水杉原生种群在各尺度下均呈现聚集分布, 中龄树在较小尺度(r < 3,300 m)上呈现聚集分布, 成年树和老龄树在较大尺度(r < 4,700 m)上呈现聚集分布; 排除生境异质性影响后, 聚集尺度均减小, 所以种群及不同龄级的聚集尺度分别为0-3,000 m、0-2,100 m、0-2,900 m和0-2,500 m, 随后呈现为微弱的随机分布和均匀分布。(4)基于完全空间随机模型, 3个龄级之间在所有尺度均为正关联; 在排除生境异质性影响后, 不同龄级的正关联尺度减小, 均在0-2,800 m呈正关联。综上, 水杉原生种群个体数量正呈现逐步减少的趋势, 种群主要呈聚集分布, 各个龄级间具有正向的关系, 从现有的空间格局来看, 生境异质性、扩散限制和种内竞争是导致该格局的主要原因。     

北京暖温带次生林种群分布格局与种间空间关联性

种群分布格局和种间空间关联性研究有助于深入理解物种共存机制.本研究在北京地区5个1 ha典型暖温带森林样地,在0-50 m尺度范围内综合分析了常见种的种群分布格局及成年树种问的空间关联性.研究发现:(1)所有检验的物种都表现了聚集格局,主要发生在较小(0-15 m)的尺度范围内,并且同种聚集强度峰值普遍出现在目标个体周围1 m的距离内;在>15 m的较大尺度上,随着尺度增加,随机和规则格局成为物种分布的主要形式;(2)种间不相关联的比例高(～50%),即使种间存在显著的关联性,也是以隔离和部分重叠为主要的关联形式;很少的物种对(～4%)呈混合分布.种子扩散限制和牛境异质性在某种程度上解释了种群普遍聚集的格局,种群聚集分布又促使种间分布不相关联,或者种间呈现隔离和部分重叠格局,反映了物种分布与生境存在紧密的关联性.另外,种间隔离的格局会阻止种间个体相互竞争.然而,由于同种个体聚集分布,密度制约成为调节种群分布的主要形式.本结果将有助于揭示森林群落物种共存的潜在维持机制.     

南亚热带森林不同林型功能性状分布格局及其驱动机制

植物功能性状能反映植物对环境变化的响应,研究植物功能性状的分布格局有助于揭示群落的构建过程及其内在作用机制。该研究以鼎湖山南亚热带山地常绿阔叶林和沟谷雨林为研究对象,采集并测量了样地中木本植物的12种不同的功能性状,分别以5 m×5 m、10 m×10 m、20 m×20 m的样方为尺度单元,通过计算平均成对性状距离指数来探讨群落中功能性状的分布格局及其驱动机制。结果表明,两个林型的群落中12个功能性状均存在不同程度变异,但功能性状在群落间的差异不显著(P>0.05)。两个林型的群落中功能性状空间分布格局均具有尺度依赖性,但不同尺度的驱动机制有差异,随着空间尺度的增大,山地常绿阔叶林的功能性状空间分布格局主要驱动机制由环境过滤转为扩散限制;沟谷雨林的由环境过滤和相似性限制转为扩散限制,两个林型在20m×20m空间尺度上都是扩散限制。生态位分化和扩散限制综合作用于鼎湖山南亚热带山地常绿阔叶林和沟谷雨林的群落功能性状分布格局的产生及其群落构建过程,二者的贡献作用会随空间尺度发生变化。坡度是影响山地常绿阔叶林功能性状分布格局的最关键地形因子,海拔是影响沟谷雨林的最关键地形因子。     

密度制约对宝天曼落叶阔叶林锐齿栎死亡前后分布格局的影响

为了在温带和亚热带的过渡带中验证森林树木死亡是否受密度制约的影响, 我们在宝天曼国家级自然保护区选择1个100 m×100 m的暖温带落叶阔叶林样地, 用双变量函数g(r)(the pair-correlation function)研究了锐齿栎(Quercus aliena var. acuteserrata)死亡前(活树和枯木统称为死亡前树木)和死亡后(活树为死亡后树木)的分布格局。把样地中的锐齿栎分为幼树(1 cm ≤ DBH<10 cm)、小树(10 cm ≤ DBH<20 cm)和成年树(DBH≥20 cm)3个不同的生长阶段来研究密度制约对空间分布格局的影响机制。结果表明: (1)死亡前锐齿栎在r>5 m尺度呈聚集分布, 死亡后幼树(1 cm≤DBH<10 cm)和成年树(DBH≥20 cm)在1-25 m尺度呈现随机分布, 死亡后小树(10 cm≤DBH< 20 cm)在r<1.5 m和2.5-4.5 m的尺度为随机分布, 在r>5 m的尺度呈聚集分布; (2)采用随机标签零模型和案例-对照设计的方法, 排除生境异质性影响后, 将幼树和小树的分布格局作为案例, 将成年树的分布格局作为对照, 并代表生境异质性的作用, 通过小径级树木与成年树分布格局的对比发现, 密度制约效应对死亡前后的锐齿栎分布格局均具有影响; (3)幼树和小树在成年树周围的分布死亡前为显著聚集分布格局, 死亡后剩余树木的聚集强度下降; 随着与成年树之间距离的增加, 死亡后的幼树(仅包括现存的活树)逐渐向随机分布格局演替。本研究初步表明锐齿栎空间格局受生境异质性的影响并呈现出显著的聚集效应, 排除生境异质性影响后, 锐齿栎死亡前后的空间格局受到密度制约的影响, 这一结果为Janzen-Connell假说提供了支持。     

Point patterns of tree distribution determined by habitat heterogeneity and dispersal limitation

Understanding processes underlying spatial distribution of tree species is fundamental to studying species coexistence and diversity. This study modeled point patterns of tree distribution, expressed by Cartesian coordinates of individual trees within a mapped forest stand, for the purpose of identifying processes that may generate spatial patterns of tree communities. We used four primary point pattern processes (homogeneous Poisson process, inhomogeneous Poisson process, homogeneous Thomas process, and inhomogeneous Thomas process) to model tree distribution in two stem-mapped forests in Taiwan, Republic of China. These four models simulate spatial processes of habitat association and seed dispersal, allowing us to evaluate the potential contribution of habitat heterogeneity and dispersal limitation to the formation of spatial patterns of tree species. The results showed that the inhomogeneous Thomas process was the best fit model and described most of the species studied, suggesting that spatial patterns of tree species might be formed by the joint effects of habitat associations and dispersal limitation. The homogeneous Thomas process that models the effect of dispersal limitation was the second best model. We also found that the best fit models could be predicted by species attributes, including species abundance and dispersal mode. The significant traits, however, differed between the two study plots and demonstrated site-specific patterns. This study indicated that the interactive operation of niche-based (habitat heterogeneity) and neutral-based (dispersal limitation) may be important in generating spatial patterns of tree species in forest communities.     

Point process models,the dimensions of biodiversity and the importance of small-scale biotic interactions

Aims Recent mechanistic explanations for community assembly focus on the debates surrounding niche-based deterministic and dispersal-based stochastic models. This body of work has emphasized the importance of both habitat filtering and dispersal limitation, and many of these works have utilized the assumption of species spatial independence to simplify the complexity of the spatial modeling in natural communities when given dispersal limitation and/or habitat filtering. One potential drawback of this simplification is that it does not consider species interactions and how they may influence the spatial distribution of species, phylogenetic and functional diversity. Here, we assess the validity of the assumption of species spatial independence using data from a subtropical forest plot in southeastern China.Methods We use the four most commonly employed spatial statistical models—the homogeneous Poisson process representing pure random effect, the heterogeneous Poisson process for the effect of habitat heterogeneity, the homogenous Thomas process for sole dispersal limitation and the heterogeneous Thomas process for joint effect of habitat heterogeneity and dispersal limitation—to investigate the contribution of different mechanisms in shaping the species, phylogenetic and functional structures of communities.Important findings Our evidence from species, phylogenetic and functional diversity demonstrates that the habitat filtering and/or dispersal-based models perform well and the assumption of species spatial independence is relatively valid at larger scales (50×50 m). Conversely, at local scales (10×10 and 20×20 m), the models often fail to predict the species, phylogenetic and functional diversity, suggesting that the assumption of species spatial independence is invalid and that biotic interactions are increasingly important at these spatial scales.     

Strong plant-soil associations in a heterogeneous subtropical broad-leaved forest

The relative importance of deterministic and neutral processes on community assembly is currently a topic of much debate among ecologists. Analyzing species-environment associations is an effective way to assess the importance of deterministic process such as niche differentiation, but both habitat association and dispersal limitation can produce similar patterns of spatial aggregation in species. Therefore, it is crucial to control for the impact of dispersal limitation on species distributions when analyzing species-environment associations. We sampled soil with high resolutions in a 24 ha stem-mapped subtropical forest and tested plant-soil associations. We controlled for the influence of dispersal limitation by employing the homogeneous Thomas process to simulate the effect of dispersal limitation on the aggregation of tree species. After controlling for the effect of dispersal limitation, we found that the spatial heterogeneity of soil properties was associated with distributions of 88.2% (90 of 102 species) of tree species in this subtropical forest. Furthermore, not only did soil properties influence the distribution of tree species, but also tree species tended to affect properties of the soil around them. The soil factors most strongly influencing species distributions were TC, TN, TP, K, Mg, Si, soil moisture, and bulk density. We found the spatial heterogeneity of soil properties to be strongly associated with tree species distributions. Niche partitioning of soil gradients contributed substantially to species coexistence in this subtropical forest.     

Phylogenetic and functional diversity area relationships in two temperate forests

Phylogenetic diversity (PD, the diversity of lineages) and functional diversity (FD, the diversity of functional traits or groups in a biological community) reflect important yet poorly understood attributes of species assemblages. Until recently, few studies have examined the spatial variation of PD and FD in natural communities. Yet the relationships between PD and FD and area (termed PDAR and FDAR), like the analogous species–area relationship (SAR), have received less attention and may provide insights into the mechanisms that shape the composition and dynamics of ecological communities. In this study, we used four spatial point process models to evaluate the likely roles of the random placement of species, habitat filtering, dispersal limitation, and the combined effects of habitat filtering and dispersal limitation in producing observed PDARs and FDARs in two large, fully mapped temperate forest research plots in northeast China and in north‐central USA. We found that the dispersal limitation hypothesis provided a good approximation of the accumulation of PD and FD with increasing area, as it did for the species area curves. PDAR and FDAR patterns were highly correlated with the SAR. We interpret this as evidence that species interactions, which are often influenced by phylogenetic and functional similarity, may be relatively unimportant in structuring temperate forest tree assemblages at this scale. However, the scale‐dependent departures of the PDAR and FDAR that emerged for the dispersal limitation hypothesis agree with operation of competitive exclusion at small scales and habitat filtering at larger scales. Our analysis illustrates how emergent community patterns in fully mapped temperate forest plots can be influenced by multiple underlying processes at different spatial scales.     

Does spatial heterogeneity blur the signature of dispersal syndromes on spatial patterns of woody species? A test in a tropical dry forest

Spatial patterns of adult plants are a consequence of several ecological processes related to seed dispersal and recruitment. Dispersal limitation, mediated by dispersal syndrome, is considered a key factor in the formation of adult plant spatial patterns. Although this initial pattern determined by dispersal has been thoroughly studied, the subsequently modification by the effect of additional ecological factors, such as habitat heterogeneity is less understood. We explored the relative importance of dispersal syndrome and spatial heterogeneity on the realization of spatial patterns of adult trees in an Ecuadorian tropical dry forest. The spatial distribution of 28 species was modeled with four different spatial point processes each: homogeneous Poisson (HPP), inhomogeneous Poisson (IPP), homogeneous Poisson cluster (HPCP), and inhomogeneous Poisson cluster process (IPCP). These models allowed us to discern between effects of random processes, habitat heterogeneity, limited dispersal, and joint effects of habitat heterogeneity and limited dispersal. We employed Akaike's information criterion (AIC) to select the model which best fit the spatial pattern of each species. The best model of each species was used to analyze differences in cluster size and degree of aggregation, between dispersal syndromes. Seventy‐five percent of the species showed inhomogeneous patterns. IPCP yielded the best fit for the spatial distribution of 50% of species in the studied forest and was the prevalent model for the three dispersal syndromes. Thus, the effect of spatial heterogeneity was prevalent in the distribution of most species in this dry tropical forest. Only 21% of species had spatial patterns compatible with random mechanisms associated to limited dispersal around parent sources. Clearly, ignoring habitat heterogeneity could bias the analysis of relationships between dispersal syndrome and species patterns.     

Spatial associations among major tree species in a cool-temperate forest community under heterogeneous topography and canopy conditions

Analysis of the spatial pattern of plants may provide insight into the processes and mechanisms that promote species coexistence and community organization. Using torus-translation tests and point-pattern analyses for a heterogeneous Poisson process, we investigated habitat association and intra- and inter-specific spatial relationships of six major tree species in a cool-temperate forest community. All stems ≥5 cm in diameter at breast height were mapped on a 1.4-ha (100 × 140 m) plot and the topographic conditions (convexity and slope degree) and canopy state were assessed. Our results showed that all six species exhibited habitat associations with topographic and/or canopy conditions except for Magnolia salicifolia. Intra-specific aggregation was found for Acer japonicum, M. salicifolia, and Hamamelis japonica var. obtusata. Community-wide analysis of the inter-specific spatial patterns showed mainly mixed or partially overlapped patterns at a scale of up to 30 m, whereas individual pairwise analyses of inter-specific patterns revealed that Fagus crenata was positively associated with two Acer species and M. salicifolia at a spatial scale of up to 5 m. These results highlight that scale-dependant ecologically important processes, such as species-specific habitat preference, regeneration mode, seed dispersal, facilitation and niche complementarity, may operate simultaneously to shape tree distributional patterns, although their presence/absence as well as relative importance vary among species. Given the complexity of the process and mechanisms promoting species coexistence and community organization, more attention should be given to the effect of spatial scale in analyzing the spatial patterns of tree species in forest communities.     

Separating the effect of mechanisms shaping species‐abundance distributions at multiple scales in a subtropical forest

Species abundance distributions (SADs) play an important role in the current dispute over mechanisms shaping community assembly. Niche theory assumes differential occurrence of species in different habitats while neutral theory emphasizes stochastic events and dispersal. The previous tests of niche and neutral models shaping SADs lead to the claim that SADs are not informative for inferring underlying processes. Using spatial statistical models in a fully mapped 24‐ha subtropical forest in China, we first demonstrate that one can not distinguish between the effect of habitat heterogeneity and dispersal limitation on SADs by inspecting whether the observed SADs fall within 95% confidence intervals of the simulated SADs. Subsequently, we demonstrate that SADs can be used to detect mechanisms shaping SADS by comparing alternative process‐based models using model selection techniques. We found that dispersal limitation explain SADs at smaller spatial scales, while the combination of niche and dispersal limitation explain SADs at larger scales. These processes are linked with the degree of conspecific aggregation, informing further attempts to refine and parameterize the statistical theory of sampling SADs.