Beyond the tropics: forest structure in a temperate forest mapped plot

Question: How do the diversity, size structure, and spatial pattern of woody species in a temperate (Mediterranean climate) forest compare to temperate and tropical forests? Location: Mixed evergreen coastal forest in the Santa Cruz Mountains, California, USA. Methods: We mapped, tagged, identified, and measured all woody stems (≥1 cm diameter) in a 6‐ha forest plot, following Center for Tropical Forest Science protocols. We compared patterns to those found in 14 tropical and 12 temperate forest plots. Results: The forest is dominated by Douglas‐fir (Pseudotsuga menziesii) and three species of Fagaceae (Quercus agrifolia, Q. parvula var. shrevei, and Lithocarpus densiflorus), and includes 31 woody species and 8180 individuals. Much of the diversity was in small‐diameter shrubs, treelets, and vines that have not been included in most other temperate forest plots because stems <5‐cm diameter had been excluded from study. The density of woody stems (1363 stems ha^?1) was lower than that in all but one tropical plot. The density of large trees (diameter ≥30 cm) and basal area were higher than in any tropical plot. Stem density and basal area were similar to most other temperate plots, but were less than in low‐diversity conifer forests. Rare species were strongly aggregated, with the degree of aggregation decreasing with abundance so that the most common species were significantly more regular than random. Conclusions: The patterns raise questions about differences in structure and dynamics between tropical and temperate forests; these need to be confirmed with additional temperate zone mapped plots that include small‐diameter individuals.     

Variations in resistance to canopy disturbances and their interactions with the spatial structure of major species in a cool‐temperate forest

Question: How does typhoon‐related disturbance (more specifically, disturbance in the understorey due to tree‐fall and branch‐fall) affect different species mortality rates in a vertically well‐structured forest community? Location: Cool‐temperate, old‐growth forest in the Daisen Forest Reserve, Japan. Methods: We investigated the canopy dynamics and mortality rate trends of trees ≥5 cm diameter at breast height in a 4‐ha study plot, and analysed the effects of tree diameter and spatial structure on the mortality risks for major tree species in the understorey. Results: Significant differences were found in the mortality rates and proportions of injured dead stems between census periods, which were more pronounced in the understorey than in the canopy. Acer micranthum, which showed increased mortality during typhoon disturbance periods, had a clumped distribution. In contrast, Acer japonicum and Viburnum furcatum, which showed similar mortality rates between census periods, had a loosely clumped spatial distribution and a negative association with canopy trees, respectively. In the understorey stems of Acanthopanax sciadophylloides and Fagus crenata, whose spatial distribution patterns depended on canopy gaps, significant increases in mortality rates were observed only during severe typhoon‐related disturbance periods. Conclusions: The sensitivity of trees to typhoon‐related canopy disturbance is more pronounced in the lower layers of vertically structured forest communities. Differences in mortality patterns generated through the combined effects of spatial variation in disturbance regime and species‐specific spatial distribution patterns (spatial aggregation, association with canopy trees, and canopy gap dependency) contribute to the co‐existence of understorey species in forest communities that are subject to typhoon‐related disturbance.     

Dominant network interactions are not correlated with resource availability: a case study using mistletoe host interactions

Network theory in ecology has been central to understanding species co‐occurrence patterns, specialization and community stability. However, network theory has traditionally focused on the ‘higher’ trophic level where exploitation of network ‘partners’ (i.e. individual interactions in response to resource availability) have remained underappreciated. In this study we tested how clumping and host availability influenced mistletoe–host interactions in a semi‐arid woodland, central Australia. We used a hierarchical approach that evaluated individual interactions by modifying the traditional randomization technique to simulate clumping and host exploitation. Using published literature we then compared our results with mistletoes from other genera. We found that mistletoes clump on fewer trees than predicted, even though interaction strength was no different from random expectations, and we found no evidence that common trees were heavily infected as predicted by the host availability hypothesis. The rate of host exploitation (measured as the proportion of trees infected) in semi‐arid Australia is similar to that for mistletoe genera in other parts of the world. We hypothesize that specific host trees act as a focal point for infection that facilitates the spread and overall population size of mistletoes. Overall our results indicate that resources, such as the number of trees in a mistletoe network, are less important than clumping of individual plants. We suggest that exploitation of available resources may play a similar role in other networks that extend beyond antagonistic relationships such as parasite or herbivore interactions.     

Liana Abundance,Diversity, and Distribution on Barro Colorado Island,Panama

Lianas are a key component of tropical forests; however, most surveys are too small to accurately quantify liana community composition, diversity, abundance, and spatial distribution – critical components for measuring the contribution of lianas to forest processes. In 2007, we tagged, mapped, measured the diameter, and identified all lianas ≥1 cm rooted in a 50-ha plot on Barro Colorado Island, Panama (BCI). We calculated liana density, basal area, and species richness for both independently rooted lianas and all rooted liana stems (genets plus clones). We compared spatial aggregation patterns of liana and tree species, and among liana species that varied in the amount of clonal reproduction. We also tested whether liana and tree densities have increased on BCI compared to surveys conducted 30-years earlier. This study represents the most comprehensive spatially contiguous sampling of lianas ever conducted and, over the 50 ha area, we found 67,447 rooted liana stems comprising 162 species. Rooted lianas composed nearly 25% of the woody stems (trees and lianas), 35% of woody species richness, and 3% of woody basal area. Lianas were spatially aggregated within the 50-ha plot and the liana species with the highest proportion of clonal stems more spatially aggregated than the least clonal species, possibly indicating clonal stem recruitment following canopy disturbance. Over the past 30 years, liana density increased by 75% for stems ≥1 cm diameter and nearly 140% for stems ≥5 cm diameter, while tree density on BCI decreased 11.5%; a finding consistent with other neotropical forests. Our data confirm that lianas contribute substantially to tropical forest stem density and diversity, they have highly clumped distributions that appear to be driven by clonal stem recruitment into treefall gaps, and they are increasing relative to trees, thus indicating that lianas will play a greater role in the future dynamics of BCI and other neotropical forests.     

Scales of aboveground and below-ground competition in a semi-arid woodland detected from spatial pattern

Abstract. Semi-arid woodlands are two-phase mosaics of canopy and inter-canopy patches. We hypothesized that both aboveground competition (within canopy patches), and below-ground competition (between canopy patches), would be important structuring processes in these communities. We investigated the spatial pattern of trees in a Pinus edulis-Juniperus monosperma woodland in New Mexico using Ripley's K-function. We found strong aggregation of trees at scales of 2 to 4 m, which indicates the scale of canopy patches. Canopy patches were composed of individuals of both species. Crown centers of both species were always less aggregated than stem centers at scales less than canopy patch size, indicating morphological plasticity of competing crowns. In the smallest size classes of both species, aggregation was most intense, and occurred over a larger range of scales; aggregation decreased with increasing size as is consistent with density-dependent mortality from intraspecific competition. Within canopy patches, younger trees were associated with older trees of the other species. At scales larger than canopy patches, younger trees showed repulsion from older conspecifics, indicating below-ground competition. Hence, intraspecific competition was stronger than interspecific competition, probably because the species differ in rooting depth. Woodland dynamics depend on the scale and composition of canopy patches, aggregated seed deposition and facilitation, above- and below-ground competition, and temporal changes in the spatial scale of interactions. This woodland is intermediate in a grassland-forest continuum (a gradient of increasing woody canopy cover) and hence we expected, and were able to detect, the effects of both above- and below-ground competition.     

Spatial patterns and dynamics of woody vegetation in an arid savanna

The spatial distribution of woody plants was studied in an arid savanna in Botswana. The study included stands of mixed species and sizes as well as monospecific even-sized stands of different size classes of the tree Acacia erioloba and the shrub Acacia mellifera. In the case of A. mellifera both dense stands on overgrazed land and more open stands were included. The analysis used all plant-to-plant distances, and individuals were represented with a realistic canopy extension. The mixed stands showed aggregated distribution of individuals, mainly caused by strong clumping of small shrubs. In A. erioloba saplings were aggregated, small trees were randomly or regularly distributed and large trees were randomly spaced. In open stands of A. mellifera aggregation increased with size of the shrubs, while in dense stands with overgrazing aggregation decreased with increasing size. The different patterns are discussed in relation to the relative importance of inter- and intraspecific competition for water and of disturbance by fire as regulatory mechanisms for total amount and spatial distribution of woody plants in this savanna.     

Hollow occurrence and abundance varies with tree characteristics and among species in temperate woodland Eucalyptus

Tree hollows are a critical but diminishing resource for a wide range of fauna around the world. Conservation of these fauna depends on sustainable management of tree species that produce the hollows on which they depend. This study addressed the need for empirical data about intraspecific and interspecific variation in hollow occurrence and abundance in woodland trees in Australia. We measured and performed hollow surveys on 1817 trees of seven species of woodland Eucalyptus in central‐western New South Wales, Australia. Trees were surveyed at 51 one‐hectare sites and about 30% of trees surveyed had multiple stems. Generalized linear mixed models that accounted for nestedness of stems within trees and trees within sites detected a significant amount of variation in hollow occurrence and abundance. Models for individual tree stems of live trees showed hollow probability and abundance increased with diameter at breast height (DBH) and with increasing senescence (form). Stems of Eucalyptus microcarpa Maiden had a higher probability of having hollows than similar DBH stems of Eucalyptus camaldulensis Dehnh., Eucalyptus melliodora A.Cunn. ex Schauer or Eucalyptus populnea ssp. bimbil L.A.S.Johnson & K.D.Hill. Dead stems in live trees were more likely to have hollows than live stems of similar DBH. Each stem in a multi‐stemmed tree had a lower probability of hollow occurrence and lower abundance of hollows than single‐stemmed trees of similar DBH. For stems of dead trees, hollow occurrence and abundance increased with DBH and differed depending on stage of senescence. A comparison of our data with other studies indicates regional variation of hollow abundances within tree species.     

Disturbance affects spatial patterning and stand structure of a tropical rainforest tree

The distribution and spatial patterns of plant populations in natural ecosystems have recently received much attention; yet the impacts of human‐induced disturbances on these patterns and underlying processes remain poorly understood. We used the sub‐canopy tree, Ryparosa kurrangii (Achariaceae), to explore the possible effects of such disturbances on stand structure and spatial patterning in an Australian tropical rainforest. We studied three populations that differed in their extent of habitat modification: anthropogenic disturbance (proximate settlement and roads) and internal damage by an invasive alien species, the feral pig (Sus scrofa). Populations were mapped, characterized, and three size cohorts (seedlings, saplings, trees) were analysed using a suite of spatial point pattern analyses (univariate: Diggle's G and F and Ripley's K; bivariate: Diggle's G and Ripley's K). Ryparosa kurrangii has a typical stand structure for a sub‐canopy tree species, but occurs at high densities locally (>400 stems ha^?1). At all sites, the tree cohort were randomly distributed and saplings were spatially aggregated at distances of up to 2–3 m. Between sites there were distinct differences in the size structure and spatial pattern of seedlings, the cohort most affected by recent habitat modification. That is, the least disturbed site had no aggregation among seedlings, the site with the greatest anthropogenic disturbance had many small, clustered seedlings that were spatially associated with trees, and the site with pig damage had clustered seedlings that had no spatial relationship with trees. We propose that habitat modification by anthropogenic and pig disturbance disrupts seed dispersal and establishment regimes, which leads to altered seedling spatial patterns. These disturbances could have long‐term implications for the population structure and health of R. Kurrangii.     

Composition,structure and diversity of cove forest stands in the Great Smoky Mountains: a patch dynamics perspective

Abstract. Cove forests of the Great Smoky Mountains are North American examples of old-growth temperate forest. Ecological attributes of seven stands were studied using one 0.6 - 1.0 ha plot per stand. Stand basal area (39 - 55 m²/ha) and biomass (326 - 471 Mg/ha) were high for temperate deciduous forest. Density ranged from 577 to 1075 stems/ha. All stands had a mixture of deciduous canopy species. Only rarely did a single species comprise more than half of the stand by density, basal area or biomass. Shade-intolerant species were present at low levels (1 - 5 % of total stand density). A wide range of stem diameters was characteristic of most species. However, some species lacked small stems, indicating discontinuous regeneration. Stands tended to have 10 - 20 tree species per ha and at least five species had biomass levels > 10 Mg/ha, indicating high evenness. Canopy gaps covered 10 % of the total area (2 - 21 % by stand). Gaps and conspecific patches of canopy trees > 0.05 ha in size were infrequent. Spatial analyses revealed a variety of patterns among species at inter-tree distances of 1 to 25 m. When all species were combined, juveniles showed aggregation, and adults were often hyperdispersed. Analyses for individual species confirmed that the mosaic of canopy species is influenced by non-random spatial processes. Adults of several species were aggregated at distances > 10 m. Juveniles of all major species exhibited aggregation. Several species exhibited regeneration near conspecific adults. This pattern suggested limited mobility for such species within the shifting mosaic. A diverse patchwork resulted despite the fact that many species did not exhibit segregation of adults and juveniles. Further understanding of patch dynamics and the potential for compositional steady state in cove forests requires long-term study with spatial data.     

Temperate tree expansion into adjacent boreal forest patches facilitated by warmer temperatures

Temperate and boreal forests are forecast to change in composition and shift spatially in response to climate change. Local‐scale expansions and contractions are most likely observable near species range limits, and as trees are long‐lived, initial shifts are likely to be detected in the understory regeneration layers. We examined understory relative abundance patterns of naturally regenerated temperate and boreal tree species in two size classes, seedlings and saplings, and across two spatial scales, local stand‐scale ecotones (tens of meters) and the regional temperate–boreal transition zone (?250 km) in central North America, to explore indications of climate‐mediated shifts in regeneration performance. We also tested for the presence of strong environmental gradients across local ecotones that might inhibit species expansion. Results showed that tree regeneration patterns across ecotones varied by species and size class, and varied across the regional summer temperature gradient. Temperate tree species regeneration has established across local ecotones into boreal forest patches and this process was facilitated by warmer temperatures. Conversely, boreal conifer regeneration exhibited negative responses to the regional temperature gradient and only displayed high abundance at the boreal end of local ecotones at cool northern sites. The filtering effects of temperature also increased with individual size for both boreal and temperate understory stems. Observed regeneration patterns and the minor environmental gradients measured across local ecotones failed to support the idea that there were strong barriers to potential temperate tree expansion into boreal forest patches. Detectable responses, consistently in the directions predicted for both temperate and boreal species, indicate that summer temperature is likely an important driver of natural tree regeneration in forests across the temperate–boreal transition zone. Regeneration patterns point toward temperate expansion and reduced but continued boreal presence in the near‐future, resulting in local and regional expansions of mixed temperate‐boreal forests.     

Variation in tree growth,mortality and recruitment among topographic positions in a warm temperate forest

Questions: Do the population dynamics of trees differ among topographic positions and, if so, how does topographic position affect the population dynamics of species that are distributed in a topography‐specific manner? Which is the most important life stage in determining vegetation patterns? Location: Primary and secondary warm temperate evergreen broad‐leaved forest (40 ‐ 280 m a.s.l.) on the western part of Yakushima Island, Japan. Methods: Mortality, recruitment, DBH growth and distribution of stems (= 5 cm DBH) in a 2.62‐ha plot were surveyed in 1992 and 2002 to determine the relationships between population parameters and (1) topography and (2) distribution patterns of 17 common tree species. Results: Common species (n = 17) were classified into three distribution pattern groups: group A, distributed mainly on convex slopes; group B, on concave slopes, and group C, not aggregated with respect to topographic position. Stem mortality, recruitment and DBH growth were greater in group A than in group B within each topographic class. The hierarchy of stem mortality among topographic classes for groups A and B was convex > planar > concave. Stem recruitment density was relatively high on the convex and concave slopes, respectively, for groups A and B. Conclusions The topographical positions of adult trees were not always most suited for adult survival and growth. For group A, the distribution pattern of adults was determined in the juvenile stage, while this was not the case for group B. Studies of juvenile stages are important for understanding the demographic basis of vegetation distribution patterns.     

沙棘木蠹蛾蛹的空间分布

沙棘木蠹蛾（Holcocerus hippophaecolus Hua, Chou , Fang et Chen）是近几年在内蒙古、辽宁、山西、宁夏和陕西等地大面积爆发的一种钻蛀性害虫,该虫约4a完成1代,主要以幼虫危害沙棘（Hippophae rhamnoidea）的根部和干部,老熟幼虫在土壤中化蛹.为了解种群的空间结构,从而有效控制其危害,应用生物学统计方法和地质统计学（Geostatistics）方法对沙棘木蠹蛾蛹的空间分布特性进行了分析研究.结果表明：约90%的蛹在6月初到7月末之间羽化,而7月份羽化的数量占总数的一半之多.在调查样地中,雌雄蛹的比例基本为1：1.每株沙棘树周围,蛹的数量为0～4个,有蛹株率仅为24.3%.蛹在距离根基部周围1.3m的范围内均有分布,不同分布区间内蛹的数量变化没有一定的规律性,但90%的蛹分布在距根基部1m的范围内.沙棘木蠹蛾蛹的种群呈现较明显的空间聚集状态,空间依赖范围大小为11.1m,局部空间连续性强度为90.7%,呈现较明显的斑块状分布,在整个区域内有很多聚集点.对不同样方大小的变异曲线图进行比较得知：样方边长分别为5、6、7m时,变程、空间局部连续性强度和基台值的变化幅度均很小,几乎相等,而样方边长为5m时的决定系数较大,此样方为最适样方大小.     

Diversity of dry forest epiphytes along a gradient of human disturbance in the tropical Andes

Question: Disturbance effects on dry forest epiphytes are poorly known. How are epiphytic assemblages affected by different degrees of human disturbance, and what are the driving forces? Location: An inter‐Andean dry forest landscape at 2300 m elevation in northern Ecuador. Methods: We sampled epiphytic bryophytes and vascular plants on 100 trees of Acacia macracantha in five habitats: closed‐canopy mixed and pure acacia forest (old secondary), forest edge, young semi‐closed secondary woodland, and isolated trees in grassland. Results: Total species richness in forest edge habitats and on isolated trees was significantly lower than in closed forest types. Species density of vascular epiphytes (species per tree) did not differ significantly between habitat types. Species density of bryophytes, in contrast, was significantly lower in edge habitat and on isolated trees than in closed forest. Forest edge showed greater impoverishment than semi‐closed woodland and similar floristic affinity to isolated trees and to closed forest types. Assemblages were significantly nested; habitat types with major disturbance held only subsets of the closed forest assemblages, indicating a gradual reduction in niche availability. Distance to forest had no effect on species density of epiphytes on isolated trees, but species density was closely correlated with crown closure, a measure of canopy integrity. Main conclusions: Microclimatic changes but not dispersal constraints were key determinants of epiphyte assemblages following disturbance. Epiphytic cryptogams are sensitive indicators of microclimate and human disturbance in montane dry forests. The substantial impoverishment of edge habitat underlines the need for fragmentation studies on epiphytes elsewhere in the Tropics.     

Characteristics of hollows and hollow‐bearing trees in semi‐arid river red gum woodland and potential limitations for hollow‐dependent wildlife

Up to 37 species of the birds and microbats inhabiting inland Australia are dependent on tree cavities for breeding or roosting. The river red gum (Eucalyptus camaldulensis), a well‐known hollow‐bearing tree species, occurs in linear semi‐arid woodland along thousands of kilometres of ephemeral river channels and is the only tree species that provides widespread, aggregated hollow resources across a landscape otherwise dominated by shrublands. Here we assess the type and quantity of hollows available along ephemeral rivers of the MacDonnell Ranges bioregion in central Australia and determine which characteristics of river red gums best predict the abundance and characteristics of different tree hollows, as first steps towards assessing the current availability of hollows in the region. Approximately a third of all river red gums sampled were hollow‐bearing, but individual trees with abundant hollows were rare. Further, 36% of hollows had an entrance ≤ 5 cm, and 37% had entrances which were 6–10 cm in diameter, whereas only 13% of hollows had an entrance diameter > 20 cm suitable for larger hollow‐using species. Large and high hollows only occurred on trees that did not display post‐disturbance resprouting. Trees with multiple and diverse hollows were rare and tended to be in advanced stages of senescence and had larger stems (82.3 ± 3.33 cm) and were taller (14.4 ± 0.53 m) compared to non‐hollow‐bearing trees (23.44 ± 1.68 cm, 8.0 ± 0.34 m). Further research is required to establish whether the current abundance of hollows and diversity of hollow types are limiting to cavity‐dependent wildlife, and to identify any threats to availability of hollows.     

Spatial distribution of Holcocerus hippophaecolus (Lepidopetera: Cossidae) pupae in a seabuckthorn (Hippophae rhamnoides) stand.

The seabuckthorn carpenter moth,Holcocerus hippophaecolus,which has a generation time of four years,is recently becoming one of the major pests of the seabuckthorn (Hippophae rhamnoides) in Inner Mongolia,Liaoning,Shanxi,Ningxia and Shaanxi of China (Hua et al.,1990).The larvae of the H.hippophaecolus mainly damage the stems and roots of the seabuckthorn,and the mature larvae pupate in the soil.The spatial distribution of the pupae was analyzed by using biostatistics and geostatistics in order to effectively control the insect and further study the spatial distribution of the population.Results show that most of the pupae (90%) had an eclosion time span from early June to the end of July.The sex ratio of the pupae was nearly 1:1 in the woodland samples.In addition,24.3% of the 971 trees investigated had pupae and it ranged from 0 to 4 per tree within a distance of 1.3 m from the base of the stem.90% of the pupae were aggregated within a distance of 1 m from the base of the stem.The pupae show intense spatial aggregation in the sampled woodland which had an 11.1 m spatial dependence and a 90.7% intensity in the local spatial continuity.Moreover,the population presented an intensive spotted distribution and many aggregated spots were found in the woodlands.As for the relationship between grid size and variogram of the pupae,the variations in the range,the intensity of local spatial continuity and the sill were all very low or non-existent when the grid size was 5 m,6 m or 7 m.Whereas,the value of the decisive coefficient was the biggest when the grid size was 5 m making it the ideal grid size.     

Pattern detection in null model analysis

Synthesis The identification of distinctive patterns in species x site presence‐absence matrices is important for understanding meta‐community organisation. We compared the performance of a suite of null models and metrics that have been proposed to measure patterns of segregation, aggregation, nestedness, coherence, and species turnover. We found that any matrix with segregated species pairs can be re‐ordered to highlight aggregated pairs, indicating that these seemingly opposite patterns are closely related. Recently proposed classification schemes failed to correctly classify realistic matrices that included multiple co‐occurrence structures. We propose using a combination of metrics and decomposing matrix‐wide patterns into those of individual pairs of species and sites to pinpoint sources of non‐randomness. Null model analysis has been a popular tool for detecting pattern in binary presence–absence matrices, and previous tests have identified algorithms and metrics that have good statistical properties. However, the behavior of different metrics is often correlated, making it difficult to distinguish different patterns. We compared the performance of a suite of null models and metrics that have been proposed to measure patterns of segregation, aggregation, nestedness, coherence, and species turnover. We found that any matrix with segregated species pairs can be re‐ordered to highlight aggregated pairs. As a consequence, the same null model can identify a single matrix as being simultaneously aggregated, segregated or nested. These results cast doubt on previous conclusions of matrix‐wide species segregation based on the C‐score and the fixed‐fixed algorithm. Similarly, we found that recently proposed classification schemes based on patterns of coherence, nestedness, and segregation and aggregation cannot be uniquely distinguished using proposed metrics and null model algorithms. It may be necessary to use a combination of different metrics and to decompose matrix‐wide patterns into those of individual pairs of species or pairs of sites to pinpoint the sources of non‐randomness.     

Plant metacommunity structure remains unchanged during biodiversity loss in English woodlands

The metacommunity concept provides important insights into large‐scale patterns and dynamics of distributions of interacting species. However, temporal change of metacommunity structure is little studied and has not been previously analysed in the context of biodiversity change. As metacommunity structure is determined by multiple species distributions, it is expected to change as a result of biodiversity loss. To examine this process, we analysed structural change of a southern English woodland metacommunity at two points in time, seven decades apart. During this interval, the metacommunity lost β‐diversity through taxonomic homogenization. We performed an ‘elements of metacommunity structure’ (EMS) analysis to examine metacommunity structure, based upon three structural elements: coherence (i.e. gaps in species range along a structuring gradient), spatial turnover (replacements), and species range boundary clumping. We predicted that metacommunity structure would decrease in spatial turnover and thus become more nested over time. We tested for change in individual structural elements with z‐scores and examined the role of spatial and environmental variables as potential structuring mechanisms through correlation with EMS ordination axes. Our results demonstrated that the metacommunity had a Clementsian structure that was maintained over time. Despite no change in broad structure, coherence and species range boundary clumping increased. Spatial turnover increased along the first structuring gradient but decreased on the second gradient. We hypothesise that this difference between gradients may reflect the presence of competing processes affecting spatial turnover. The mechanisms of biological structuring involved both environmental and spatial factors at the scale of the individual woodland. Therefore, our results suggest that broad metacommunity structure would not be a good landscape‐scale indicator for conservation status. Conversely, knowledge that metacommunity structure does not change over time could assist in long‐term conservation strategy because fundamental metacommunity structural processes are resistant to environmental change.     

Structure and pattern in temperate seasonal forests

Demography, spatial pattern, and diversity of canopy and subcanopy trees, shrubs, and lianas were compared in two cool and two warm temperate North American forests, paired at 30° and 40° north latitudes. All woody stems 1 cm dbh in 16 randomly located, non-contiguous plots totalling 1 ha at each of the four sites were measured, mapped, and identified. Basal area and overall density did not differ between latitudes. Demographic and spatial analyses revealed remarkable similarity in spatial dispersion, irrespective of density or species composition. At all sites, dispersion of canopy trees was random but all understory stems were uniformly distributed relative to all canopy trees. Species diversity and vertical structure differed between the warm and cool temperate sites, especially in species composition of individual strata. Associations of understory species relative to canopy species were more random at 30° than at 40° north, where a higher degree of association between canopy and understory species' patterns, coupled with their size class distributions, suggested more lengthy regeneration cycles and an alternation of species assemblages. The forests at 30°, those subject to periodic canopy disturbance by hurricanes, had more vertical mixing of species (i.e., canopy species represented in all size classes), more tree saplings, and significantly more shrub and liana species.     

Gap pattern and colonization opportunities in plant communities: effects of species richness, mortality, and spatial aggregation

We investigate how perturbations that induce mortality transform original spatial patterns in plant communities into binary spatial patterns of survivors and perished individuals. By means of computer simulation, we analyse effects of average mortality, interspecific variation of mortality around the mean, spatial distribution of the species (clumping degree), and species richness. Gap spatial pattern is quantified by four spatial indices or landscape metrics (gap area, density, shape and coherence). In single‐species communities, the emerging gap patterns are subject to critical phenomena: opportunities for colonizers to establish increase with mortality, but more rapidly at specific mortality thresholds. In multi‐species communities, neither species richness nor interspecific variation of mortality influences gap spatial pattern when community assembly is random. Colonization opportunities would therefore not be affected by local species extinction in such a system, nor by the presence of species with divergent sensitivities to perturbation. In a community that is highly spatially aggregated, increases in interspecific mortality variation shift the pattern towards fewer gaps that are larger and more isodiametric, which suggests increased establishment chances for colonizers. Similar changes are induced in communities characterized by large interspecific mortality differences if clumping degree is increased. Loss of species richness only modifies gap spatial pattern to a substantial extent if mortality variation is high: in this case, depauperate communities exhibit a wider variety of colonization opportunities (more gaps which are on average smaller, but the largest gap is larger) than species‐richer ones. These findings may explain the contrast between the negative diversity‐invasibility relationship often found in small‐scale experimental studies and the positive diversity‐invasibility relationship found in observational studies at larger scale. They also demonstrate that the pre‐disturbance spatial structure of a community significantly affects colonization opportunities for alien species, and is therefore a likely determinant of the trajectory of secondary succession following perturbation.     

Merged trees in second-growth, fire-origin forests in Patagonia, Chile: positive spatial association patterns and their ecological implications

?Premise of the Study: Negative density-dependent processes have been thought to be the primary cause of shifting spatial patterns of tree populations through time. The existence of adult tree clusters might challenge this classical prediction. Here, we document the prevalence of merged stems (clustering of mature trees leading to stem fusion) in second-growth forests of Nothofagus pumilio and hypothesize that it is nonrandom but predictable in space. ?Methods: We stem-mapped nine sites in second-growth edge and interior forests of fire origin and in mature forests of N. pumilio (>3500 trees) in central Patagonia, Chile. The spatial structure of stand-level and individual-level features was estimated with spatial analyses (pair-correlation function and nearest-neighbor distances). ?Key Results: Multistemmed trees were merged clusters of separate individuals. Merged trees were predominantly found at the edge of the second-growth forests. We found strong clustering (≤5 m) at forest edge sites and none at interior sites. Nearest-neighbor distance distributions were unimodal for unmerged trees and monotonically decreasing for merged trees; interstem distances were much smaller at the edge sites than at the interior sites. ?Conclusions: The occurrence of merged trees at the forest edge, and the resulting high spatial aggregation of stems, is consistent with the hypothesis that establishment was probably aggregated. The spatial pattern found at the forest edge changes the standard spatial pattern sequence through time in temperate forests, altering traditional forest-stand-dynamics models.