Bivariate Point Pattern Analyses Research Articles

Salt marsh vegetation and secured woody debris facilitate the recolonization of Avicennia germinans

AbstractQuestionsDoes the presence of salt marsh vegetation affect the long‐term regeneration of the pioneer mangrove species Avicennia germinans in a degraded dwarf forest? Does immobilized coarse woody debris (CWD) affect regeneration similarly? Do larger trees suppress or facilitate intraspecific saplings?LocationDwarf mangrove forest in the high intertidal zone on the Bragança peninsula in northern Brazil.MethodsThe spatial patterns of A. germinans, the herbaceous halophyte Sesuvium portulacastrum, and CWD were mapped in three sample plots (each 400 m2) during two consecutive vegetation surveys, conducted in 2011 and 2014. Inhomogeneous Poisson and Thomas point process models were used to assess the distribution of A. germinans life‐history stages (seedlings, saplings, and adult dwarf trees), conditioned on the presence of S. portulacastrum and CWD. In addition, intraspecific interactions between trees and regeneration were assessed based on crown projection mapping. Bivariate point pattern analyses were used to assess the dependence of advance regeneration on dwarf A. germinans trees and S. portulacastrum.ResultsAvicennia germinans saplings and trees were positively associated with S. portulacastrum and CWD, whereas seedlings were located around tree crowns. The density of fruit‐bearing trees was positively associated with sapling density, indicating that regeneration relied on locally dispersed propagules.ConclusionsHerbaceous vegetation and CWD have an important ecological function in degraded mangroves by retaining tidally dispersed propagules. Here, we show that herbaceous vegetation does not suppress the growth of seedlings but facilitates mangrove recolonization. Due to limited tidal dispersal, regeneration relies on local propagule supply. In addition to hydrological restoration, the observed vegetation patterns suggest that, in the absence of propagule‐retaining vegetation, restoration of high‐intertidal mangroves can be facilitated by establishing nuclei of planted trees and installing secured logs.

Identifying Ant-Mirid Spatial Interactions to Improve Biological Control in Cacao-Based Agroforestry System.

The use of ants for biological control of insect pests was the first reported case of conservation biological control. Direct and indirect community interactions between ants and pests lead to differential spatial pattern. We investigated spatial interactions between mirids, the major cocoa pest in West Africa and numerically dominant ant species, using bivariate point pattern analysis to identify potential biological control agents. We assume that potential biological control agents should display negative spatial interactions with mirids considering their niche overlap. The mirid/ant data were collected in complex cacao-based agroforestry systems sampled in three agroecological areas over a forest-savannah gradient in Cameroon. Three species, Crematogaster striatula Emery (Hymenoptera: Formicidae), Crematogaster clariventris Mayr (Hymenoptera: Formicidae), and Oecophylla longinoda Latreille (Hymenoptera: Formicidae) with high predator and aggressive behaviors were identified as dominant and showed negative spatial relationships with mirids. The weaver ant, O. longinoda was identified as the only potential biological control agent, considering its ubiquity in the plots, the similarity in niche requirements, and the spatial segregation with mirids resulting probably from exclusion mechanisms. Combining bivariate point pattern analysis to good knowledge of insect ecology was an effective method to identify a potentially good biological control agent.

Sapling growth rates reveal conspecific negative density dependence in a temperate forest.

Local tree species diversity is maintained in part by conspecific negative density dependence (CNDD). This pervasive mechanism occurs in a variety of forms and ecosystems, but research to date has been heavily skewed toward tree seedling survival in tropical forests. To evaluate CNDD more broadly, we investigated how sapling growth rates were affected by conspecific adult neighbors in a fully mapped 25.6 ha temperate deciduous forest. We examined growth rates as a function of the local adult tree neighborhood (via spatial autoregressive modeling) and compared the spatial positioning of faster‐growing and slower‐growing saplings with respect to adult conspecific and heterospecific trees (via bivariate point pattern analysis). In addition, to determine whether CNDD‐driven variation in growth rates leaves a corresponding spatial signal, we extended our point pattern analysis to a static, growth‐independent comparison of saplings and the next larger size class. We found that negative conspecific effects on sapling growth were most prevalent. Five of the nine species that were sufficiently abundant for analysis exhibited CNDD, while only one species showed evidence of a positive conspecific effect, and one or two species, depending on the analysis, displayed heterospecific effects. There was general agreement between the autoregressive models and the point pattern analyses based on sapling growth rates, but point pattern analyses based on single‐point‐in‐time size classes yielded results that differed markedly from the other two approaches. Our work adds to the growing body of evidence that CNDD is an important force in temperate forests, and demonstrates that this process extends to sapling growth rates. Further, our findings indicate that point pattern analyses based solely on size classes may fail to detect the process of interest (e.g., neighborhood‐driven variation in growth rates), in part due to the confounding of tree size and age.

Conspecific negative density dependence in American beech

BackgroundOne of the most important drivers of forest biodiversity is conspecific negative density dependence (CNDD), a reduction in performance when conspecific densities are high. While the majority of CNDD research has focused on tropical forests, evidence is mounting that CNDD may also play an important role in temperate forests. To further explore the potential reach of this phenomenon, we investigated CNDD in American beech (Fagus grandifolia) in a mature mid-Atlantic forest.MethodsWe used bivariate point pattern analyses to examine spatial relationships between large beech trees and conspecific saplings, and we also contrasted these patterns with comparable patterns for heterospecifics. In addition, to address the possibility of dispersal limitation and the associated effects on spatial patterns, we analyzed seedling density as a function of adult conspecific abundance.ResultsWe found that beech saplings were more repelled from large conspecifics than large heterospecifics, despite the fact that beech seedling density was positively correlated with beech basal area. However, saplings of other canopy tree species were also repelled from adult beech trees, suggesting a general suppressive effect. Nonetheless, the discrepancy between beech seedling and sapling densities beneath adult conspecifics suggests that beech seedling survival rates were reduced in vicinity of conspecific adults.ConclusionsRegardless of the extent to which beech inhibits heterospecific trees, a negative effect on conspecific recruits may be critical for biodiversity maintenance. Without this conspecific suppression, a dense layer of shade-tolerant beech saplings could form beneath adult beech trees. If this were to occur, beech would have a substantial head-start following canopy disturbance, and this late-successional species could potentially dominate a stand in perpetuity, through repeated disturbance cycles.

Fine-scale spatial patterns in oak sprouting and mortality in a newly restored coppice

Sprouting is a common life history strategy in tree communities and is crucial for coppice management of woodlands. Recently, there has been increasing interest in coppice restoration throughout Europe and several studies have explored the sprouting ability of tree stumps in the restored stands. Though the spatial distribution of trees has been previously found to affect ecological processes in many ecosystems, no study has so far considered the importance of spatial interactions during the process of coppice restoration. The present study attempts to fill this gap by analyzing fine-scale spatial patterns in sprouting and mortality in a newly restored oak coppice with different densities of residual timber trees. Univariate and bivariate point pattern analyses were used to quantify the observed spatial patterns. While no significant spatial associations were found between residual trees and dead (post-harvest non-sprouting) stumps, our mortality analysis revealed clear non-random interactions between stumps themselves. Negative distance- and density-dependent effects in post-harvest stump mortality were observed at short scales; stumps with more neighboring stumps at a close distance were significantly more likely to die after harvest. Our study demonstrated that non-random effects in mortality are not limited only to generative reproduction of trees but play also an important role in vegetative regeneration of one of the main European broadleaved tree species.

Assessing the impact of fire on the spatial distribution of Larrea tridentata in the Sonoran Desert, USA

In southwestern American deserts, fire has been historically uncommon because of insufficient continuity of fuel for spreading. However, deserts have been invaded by exotic species that now connect the empty space between shrubs to carry fire. We hypothesized that fire would change the spatial distribution of surviving Larrea tridentata shrubs. We established two study plots, one each in a burned and unburned area, and recorded location and living status of all shrubs. We performed univariate and bivariate point pattern analyses to characterize the impact of fire on the overall distribution of shrubs. Additionally, we used a simple wildfire model to determine how close we could come to reconstructing the observed spatial pattern of living and dead shrubs. We found a hyper-dispersed pattern of shrubs at finer scales and a random pattern at broader scales for both the unburned plot and for the living and dead shrubs combined in the burned plot, the latter providing an approximation of the pre-burn distribution of shrubs. After fire, living shrubs showed a clustered pattern at scales >2.5 m, whereas dead shrubs were randomly distributed, indicating that fire caused a change in the spatial pattern of the surviving shrubs. The fire model was able to partially reconstruct the spatial pattern of Larrea, but created a more clustered distribution for both living and dead shrubs. Our study reinforces the key role of fire in altering landscapes that had not been habituated to fire, and suggests the existence of potential cascading effects across the entire plant community.

Beech bark disease: spatial patterns of thicket formation and disease spread in an aftermath forest in the northeastern United States

Beech bark disease (BBD) has affected the composition, structure, and function of forests containing a significant proportion of American beech (Fagus grandifolia Ehrh.) across North America. BBD spread has been investigated at landscape and regional scales, but few studies have examined spatial patterns of disease severity and spread within stands where forest management mitigation measures can be implemented. We analyzed changes in forest composition between 1985 and 2009 and fine-scale spatial patterns of BBD between 2000 and 2009 in a ∼2 ha northern hardwood stand in the Adirondack Mountains of New York using location and disease severity of beech trees. A bivariate point pattern analysis was implemented to examine spatial patterns of beech thicket formation and BBD spread to beech saplings. Abundance of beech saplings increased near highly diseased canopy beech trees and around dead beech and sugar maple (Acer saccharum Marsh.). Disease severity of beech saplings was highest in close proximity to highly cankered canopy beech trees. Thus, BBD leads to the formation of beech thickets, and thickets are often located where saplings are most likely to become infected, increasing the likelihood that secondary killing fronts will develop and lead to heavy BBD-induced mortality in aftermath northern hardwood forests of North America.

Overstory succession in a mixed Quercus petraea–Fagus sylvatica old growth forest revealed through the spatial pattern of competition and mortality

Old-growth forests are often looked at as a reference for naturalness. Analyzing their successional pathways can provide valuable information on the main drivers of succession in the absence of anthropogenic influence. This knowledge could then prove particularly useful for the maintenance of mixed species stands as a management option to counteract potential changes in forest functions under foreseen climate scenarios. The study was conducted in an old growth Quercus petraea–Fagus sylvatica forest in the Runcu Grosi Natural Reserve, western Romania. Main goals of the research were to assess the structure of the old growth forest and understand how the spatial pattern of the two species is shaped by their competitive relationship. We wanted to figure out if the spatial relationship between Q. petraea and F. sylvatica is different between trees belonging to different layers, and how a possible difference can influence their successional dynamics. To accomplish these tasks, living and dead (both standing and lying) trees were mapped in four 0.56ha plots. The relationship between trees, considering species, vegetative and social status, was analysed through univariate and bivariate point pattern analyses. Spatial structure based on tree size was investigated with Moran’s correlograms and a local indicator of spatial association. The spatial relationship between understory and canopy trees was also assessed using crown cover data to test for differences among the species.The upper layer is dominated by Q. petraea, whereas F. sylvatica was the only species in the suppressed one. The high amount of deadwood was mostly due to oak contribution. F. sylvatica living trees presented a clumped structure, while Q. petraea exhibited a random pattern. F. sylvatica dead trees showed instead repulsion to living ones. Suppressed trees were clumped at all distances, whereas trees belonging to dominant and intermediate strata were randomly distributed. Repulsion was found between the two species in the dominant layer. Beech recruitment occurred more frequently outside the canopy of dominant trees. Those beeches established under dominant tree showed a preference for oak cover.The succession from oak-dominated to beech-dominated stands has already reached an advanced stage in this old growth forest. The two species competitive interactions are mostly driven by their different shade tolerance and crown plasticity. Quercus–Fagus mixed stands like the Runcu-Grosi one should be preserved to maintain forest diversity and enhance forest resilience. Oak decline could be reduced by the occurrence of natural disturbances, together with active management that should be promoted to reduce beech competitive pressure.

Spatial distribution of Tephrocactus aoracanthus (Lem.) Lem. in relation to shrubs in the hyperarid regions of west‐central Argentina

AbstractThe spatial intraspecific and interspecific distribution of Tephrocactus aoracanthus (Lem.) Lem. was studied using univariate and bivariate point pattern analysis, at landscape scale in four different physiographic units and at microscale in terms of the shrubs found in those units. The influence of shrubs on photosynthetically active radiation and on soil temperature under their canopies was analysed. Results show a high environmental variability in light and temperature under shrubs that generally has no influence on the distribution of T. aoracanthus. Apparently, this cactus has great ability to adapt to different conditions of both photosynthetically active radiation and temperature. However, intraspecific distribution was highly clustered in all physiographic units, independently of bushes. Possibly, the mode of seed dispersal and the rooting of cladodes at small scale are the main processes that determine the strong intraspecific clustering of T. aoracanthus.

Spatial structure of a natural mixed topodeme of subalpine Sorbus taxa

Spatial distribution and genetic variation of a population of <em>Sorbus chamaemespilus</em> (L.) Crantz and putative hybrids between <em>S. chamaemespilus</em>, <em>S. aria</em> and <em>S. aucuparia</em> growing in the nature reserve Skalnä Alpa (central Slovakia) were studied. The analysis of spatial patterns using Ripley's K-function revealed a significant clustering of the adults of both <em>S. chamaemespilus</em> and hybrid taxa at distances up to ~15 m and a strong affinity between both taxonomical groups, indicating similar ecological requirements. Bivariate point-pattern analysis considering cardinal direction showed that juvenile individuals of <em>S. chamaemespilus</em> are clustered around the adults up to the distance of ~2 m, whereas in hybrid taxa with larger and more dense crowns, juveniles are clustered at distances more than ~3 m from the adults. The analysis of genetic variation in a subset of adult shrubs using 4 nuclear microsatellite loci revealed that unlike expected, there was no variation in S. chamaemespilus but several genotypes were found in the group of hybrid taxa. Implications for the reproduction system and conservation of the investigated taxa are discussed.

A spatially explicit census reveals population structure and recruitment patterns for a narrowly endemic pine, Pinus torreyana

We conducted a census of the rare pine, Pinus torreyana ssp. torreyana, to determine (a) its population size and whether it is stable, growing, or declining; (b) the spatial variation in population structure; (c) the spatial patterning of trees in different life stages; and (d) the environmental factors that are related to seedling recruitment. Trees were classified into four stage classes: adult (>160 cm tall with cones); sub-adult (>160 cm without cones); saplings (30–160 cm); and seedlings (<30 cm). Stem diameter was measured for adults and sub-adults, and height for saplings and seedlings. Stands were defined by spatial clustering of the tree map. Univariate and bivariate point pattern analyses were used to explore spatial patterns for adult and juvenile trees and identify potential stand development processes such as density dependence, dispersal limitations, and patchy recruitment. Logistic regression was used to analyze seedling establishment and survival in relation to environmental variables derived from digital maps. Earlier studies reported little or no recruitment. We mapped 5,394 individuals, and tree size had “reverse J-shaped” distribution suggestive of a recruiting population. However, population structure was variable among stands. The predominant spatial pattern detected for adult and juvenile trees was clustering at lag distances <10 m. Bivariate pattern analysis did not suggest repulsion between adult and juvenile size classes. Seedlings tended to be found close to adults and on certain soil types. This suggests that the clustered patterns resulting from patchy recruitment of juveniles persist over time.

Patrón espacial de la regeneración de &lt;i&gt;Aspidosperma polyneuron &lt;/i&gt;Müll. Arg. en un bosque sometido a aprovechamiento selectivo en el norte de la Provincia de Misiones.

Se analizó el patrón espacial de la regeneración de Aspidosperma polyneuron Müll.Arg. en un sector remanente de la selva Paranaense en la provincia de Misiones, Argentina. Se mapearon los individuos presentes en un área de 210 m x 100 m (2,1 ha). Se aplicaron técnicas de análisis de patrones de puntos univariados y bivariados mediante el índice "O" de Wiegand &amp; Moloney. La regeneración presentó agrupamientos a escalas de hasta 10 metros resultando en conglomerados de 30 metros de diámetro. No se verificaron asociaciones entre la regeneración y los individuos mayores de la especie ni con los árboles muertos, mientras que se observaron asociaciones a pequeña escala con individuos de otras especies. Los resultados permiten concluir que la regeneración presenta patrones agregados que tienden a la homogeneidad en los adultos. La presencia de renovales está condicionada a la presencia de adultos en las cercanías y la instalación de la regeneración está asociada a individuos de más de 10 cm de diámetro al pecho siendo la regeneración en sitios abierto una excepción.

Response of pine natural regeneration to small‐scale spatial variation in a managed Mediterranean mountain forest

AbstractQuestions: What influence do management practices and previous tree and shrub stand structure have on the occurrence and development of natural regeneration of Pinus sylvestris in Mediterranean mountain forests? How are the fine‐scale and environmental patterns of resources affected and what impact does this have on the distribution of the regeneration?Location: A Pinus sylvestris Mediterranean mountain forest in central Spain.Methods: Upperstory trees and regeneration (seedlings and saplings) were mapped in four 0.5‐ha plots located in two types of stand with different management intensities (even‐aged and uneven‐aged stands). Environmental variables were recorded at the nodes of a grid within the plots. The relationships between the upperstory and regeneration were evaluated by bivariate point pattern analysis; redundancy analysis ordination and variation partitioning were performed to characterize regeneration niches and the importance of the spatial component.Results: Seedlings and saplings presented a clumped structure under both types of management and their distribution was found to be related to the spatial distribution of favourable microsites. Regeneration was positively related to conditions of partial cover with high soil water content during the summer. More than half of the explained variance was spatially structured in both types of stand. This percentage was particularly high in the even‐aged stands where the pattern of regeneration was highly influenced by the gaps created by harvesting.Conclusions: The spatial distribution of the tree and shrub upperstory strongly influences regeneration patterns of P. sylvestris. Current management practices, promoting small gaps, partial canopy cover and moderate shade in even‐aged stands, or favouring tree and shrub cover in the case of uneven‐aged stands, appears to provide suitable conditions for the natural regeneration of P. sylvestris in a Mediterranean climate.

A spatially explicit analysis of seedling recruitment in the terrestrial orchid Orchis purpurea

Seed dispersal and the subsequent recruitment of new individuals into a population are important processes affecting the population dynamics, genetic diversity and spatial genetic structure of plant populations. Spatial patterns of seedling recruitment were investigated in two populations of the terrestrial orchid Orchis purpurea using both univariate and bivariate point pattern analysis, parentage analysis and seed germination experiments. Both adults and recruits showed a clustered spatial distribution with cluster radii of c. 4-5 m. The parentage analysis resulted in offspring-dispersal distances that were slightly larger than distances obtained from the point pattern analyses. The suitability of microsites for germination differed among sites, with strong constraints in one site and almost no constraints in the other. These results provide a clear and coherent picture of recruitment patterns in a tuberous, perennial orchid. Seed dispersal is limited to a few metres from the mother plant, whereas the availability of suitable germination conditions may vary strongly from one site to the next. Because of a time lag of 3-4 yr between seed dispersal and actual recruitment, and irregular flowering and fruiting patterns of adult plants, interpretation of recruitment patterns using point patterns analyses ideally should take into account the demographic properties of orchid populations.

Spatial patterns on the sagebrush steppe/Western juniper ecotone

Analysis of the spatial patterns of woody plants is important to better understand the ecological processes that govern the worldwide expansion of woody plants across semi-arid ecosystems. Second-order characteristics of a marked spatial point pattern of western juniper (Juniperus occidentalis subsp. occidentalis) were analyzed using Ripley’s K-functions and the pair-correlation function g. The marked point process of crown diameters was produced via two-dimensional wavelet analysis of a fine scale aerial photograph at the woodland-steppe ecotone in the Reynolds Creek watershed in the Owyhee Mountains, southwestern Idaho. Colonization of J. occidentalis stems from mature juniper trees growing in rocky, fire resistant areas. Although these areas introduce components of natural heterogeneity within the landscape, the selected study area is situated within a single soil type, and we modeled the expansion of juniper plants into previously juniper-free sagebrush steppe as a homogeneous point process with constant intensity. Through this research we have identified two statistically significant spatial scales characteristic of J. occidentalis on the woodland/steppe ecotone: (1) We observed inhibition between J. occidentalis plants at distances <15 m, resulting in a regular pattern, rather than clumped or random. This short-distance inhibition can be attributed to competition for water and other resources. Recruitment of young J. occidentalis occurs significantly more often in a direction away from older plants, maximizing the utilization of water and light resources, and perpetuating the spread of the species into previously juniper-free shrub-steppe. (2) J. occidentalis on the ecotone exhibits significant clustering within a 30–60 m radius. Bivariate point pattern analyses provide evidence that, within a distance of 50–70 m, there is a spatial dependence in tree size such that medium trees are more likely than small trees to be close to large trees. We attribute these phenomena to the fact that juniper seeds are commonly dispersed by berry-eating birds with small territories (0.3–1 ha). Beyond a distance of 50–70 m, juniper plants are randomly distributed, suggesting that additional long-range seed dispersal processes are at work. We further acknowledge the importance of including a reference to spatial scale when formulating hypotheses in statistical analysis of spatio-temporal point patterns.

Human impact on size, age, and spatial structure in a mixed European larch and Swiss stone pine forest in the Western Italian Alps

Spatiotemporal development and human impact on dynamic processes were investigated in the mixed European larch (Larix decidua Mill.) and Swiss stone pine (Pinus cembra L.) subalpine forest of Lago Perso (Piedmont, Italy). We mapped and measured all 295 trees (DBH ≥4 cm) and 914 saplings (&gt;10 cm height, &lt;4 cm DBH) in a permanent plot (1 ha). One core per tree was extracted upslope at 50 cm height, and dendrochronological techniques were applied to reconstruct age structure and growth patterns. All of the data collected were stored in a GIS, and tree and stem crown maps were generated and analysed to quantify spatial patterns. Ripley's K(t) univariate and bivariate point pattern analyses were employed to assess the degree of spatial autocorrelation. Documentary research was conducted to reconstruct human land use. The stand is uneven-aged, and there were no obvious age cohorts or other evidence of major disturbances in the past. Stone pine saplings and trees and larch saplings exhibited a clumped structure. The same clumping was not so evident in larch trees. The observed structural changes are mainly related to human land use and grazing regime. Although human influence is still manifest, in the recent decades natural dynamics have become the predominant influence on the forest's structure and processes.

Shrub spatial aggregation and consequences for reproductive success

To link spatial patterns and ecological processes, we analysed the distribution of two shrub species (one large and dominant, the other smaller) and estimated the reproductive consequences of their distribution for the smaller species. We tested the significance of the spatial distribution pattern of the two shrubs by second-order bivariate point pattern analysis (Ripley's K function). Performance of Asparagus albus, the smaller shrub, was measured as (1) survival of transplanted seedlings in two contrasting habitats: patches of the dominant shrub (Ziziphus lotus), and open interspaces; and (2) reproductive output of plants naturally occurring in both habitats. The two species were significantly aggregated. Transplanted Asparagus albus seedlings had higher survival rates in patches than in the open. Plants produced more flowers, fruits, and showed a higher mass of seeds when living in aggregates than when isolated. The mechanisms responsible for this facilitative effect seem to be related to soil enrichment in patches. These results suggest that the spatial aggregation of species can be indicative of a positive interaction among them, directly affecting fitness of at least one of the species. Facilitation, by inducing variations in the reproductive performance may play a major role in the demography and dynamics of plant populations.

Influence of leaf-cutting ants (Atta mexicana) on performance and dispersion patterns of perennial desert shrubs in an inter-tropical region of Central Mexico

The influence of defoliation by leaf-cutting ants (Atta mexicana) on the dispersion patterns of xerophytic shrubs in the semi-arid valley of Zapotitlán, Mexico was analysed. All shrubs and ant nest entrances were mapped in a 65×50 m stand, and the vigour of each individual shrub was estimated. A regression analysis between the distance to the nearest nest and the vigour of plants showed a significant relationship for Acacia constricta, Castela tortuosa,Echinopteryx eglandulosa and Mimosa luisana, revealing higher vigour as distance to the nest entrances increases. A point pattern analysis showed a clear clumped pattern for the latter three species, plus Prosopis laevigata; whereasAcacia constricta and Cercidium praecox, as well as the ant nest entrances, did not show any pattern significantly different from chance. A bivariate point pattern analysis did not show the expected repulsion pattern among shrubs and nest entrances. According to an auto-correlation analysis, individuals of Mimosa luisana forming dense clusters away from the ant nests entrances showed higher vigour values than those plants with higher inter-individual distances closer to the nests entrances. Although the unveiling of the ecological implications of ant–shrub and shrub–shrub interactions demands a detailed experimental approach, our results reveal the importance of herbivory in the structuring of natural communities.