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Ripley's K (ripley + k)
Selected AbstractsLong-term fire frequency variability in the eastern Canadian boreal forest: the influences of climate vs. local factorsGLOBAL CHANGE BIOLOGY, Issue 5 2009ADAM A. ALI Abstract The influence of climatic and local nonclimatic factors on the fire regime of the eastern Canadian boreal forest over the last 8000 years is investigated by examining charred particles preserved in four lacustrine deposits. Herein, we compare the distribution of fire-free intervals (FFIs) and the synchronicity of fire events among sites, using Ripley's K -function to determine the extent of the role of local-scale vs. large-scale processes with respect to fire control. Between 8000 and 5800 cal. bp (calibrated years before present) the climatic and ecological conditions were less conducive to fire events than after this date. After 5800 cal. bp, the number of fires per 1000 years (fire frequency) progressively increased, reaching a maximum ca. 3400 cal. bp. There was a sharp decrease in fire frequency during the last 800 years. Between 8000 and 4000 cal. bp, comparable FFIs and synchronous fire episodes were determined for the study sites. During this period, the fire frequency was predominantly controlled by climate. After 4000 cal. bp, two sites displayed independent fire histories (different FFI distributions or asynchronous fire events), underlining the important influence of local factors, including short-term fuel wetness, characteristics of the watershed and landscape connectivity, in determining fire occurrence. We conclude that climatic changes occurred during the last 4000 years that induced a rise in the water table; this may explain the high spatial heterogeneity in fire history. Current and projected global climatic changes may cause similar spatial variability in fire frequency. [source] Spatial patterns of tree recruitment in a relict population of Pinus uncinata: forest expansion through stratified diffusionJOURNAL OF BIOGEOGRAPHY, Issue 11 2005J. Julio Camarero Abstract Aim, To infer future changes in the distribution of isolated relict tree populations at the limit of a species' geographical range, a deep understanding of the regeneration niche and the spatial pattern of tree recruitment is needed. Location, A relict Pinus uncinata population located at the south-western limit of distribution of the species in the Iberian System of north-eastern Spain. Methods,Pinus uncinata individuals were mapped within a 50 × 40-m plot, and their size, age and reproductive status were estimated. Data on seed dispersal were obtained from a seed-release experiment. The regeneration niche of the species was assessed based on the associations of seedling density with substrate and understorey cover. The spatial pattern of seedlings was described using point-pattern (Ripley's K) and surface-pattern (correlograms, Moran's I) analyses. Statistical and inverse modelling were used to characterize seedling clustering. Results, Pine seedlings appeared aggregated in 6-m patches. Inverse modelling estimated a longer mean dispersal distance (27 m), which corresponded to the size of a large cluster along the north to north-eastward direction paralleled by an eastward trend of increasing seedling age. The two spatial scales of recruitment were related to two dispersal processes. The small-scale clustering of seedlings was due to local seed dispersal in open areas near the edge of Calluna vulgaris mats: the regeneration niche. The long-range expansion might be caused by less frequent medium-distance dispersal events due to the dominant north-westerly winds. Main conclusions, To understand future range shifts of marginal tree populations, data on seed dispersal, regeneration niche and spatial pattern of recruitment at local scales should be obtained. The monitoring of understorey communities should be a priority in order to predict correctly shifts in tree species range in response to global warming. [source] Spatial patterns and associations in a Quercus-Betula forest in northern ChinaJOURNAL OF VEGETATION SCIENCE, Issue 3 2004J.H. Hou Abstract: Question: Are species-specific regeneration strategies and competition the dominant processes facilitating species coexistence in a Quercus liaotungensis dominated temperate deciduous forest? Location: Dongling Mountains, North China, 1300 m a.s.l. Methods: Ripley's K -function was used to characterize the spatial patterns and spatial associations of two dominant tree species, Quercus liaotungensis and Betula dahurica, and a common subcanopy species, Acer mono, at different growth stages (adult, sapling, seedling). Results: Seedlings, saplings and adults of all three species exhibited clumped distributions at most spatial scales. Quercus seedlings and saplings were positively associated with conspecific adult trees and spatially independent of dead trees suggesting that seed dispersal and vegetative regeneration influenced the spatial patterning of Quercus trees. Betula seedlings and saplings were positively associated with both live and dead trees of conspecific adults at small scales (<5 m) but negatively associated with live and dead trees of other species indicating sprouting as an important mechanism of reproduction. Saplings of Acer had a strong spatial dependence on the distribution of conspecific adult trees indicating its limited seed dispersal range. Negative associations between adult trees of Betula and Quercus demonstrated interspecific competition at local scales (<5 m). Conclusions: Different regeneration strategies among the three species play an important role in regulating their spatial distribution patterns, while competition between individuals of Betula and Quercus at the adult stage also contributes to spatial patterning of these communities. The recruitment limitations of Betula and Quercus may affect the persistence of these species and the long-term dynamics of the forest. [source] Spatial pattern of downed logs and wood-decaying fungi in an old-growth Picea abies forestJOURNAL OF VEGETATION SCIENCE, Issue 5 2001Mattias Edman See section on Field methods Abstract. Since many wood-living forest species are influenced by the dynamics of coarse woody debris (CWD), information about the spatial pattern of CWD under natural conditions is essential to understand species distributions. In this study we examined the spatial pattern of downed logs and wood-decaying fungi in an old-growth boreal Picea abies forest in northwestern Sweden that is governed by gap-phase dynamics. The spatial pattern of wood-decaying fungi was studied to draw conclusions about species dispersal abilities. A total of 684 logs with a diameter > 10 cm were mapped and analysed with Ripley's K -function. The distribution of all logs taken together displayed a significant aggregated pattern up to 45 m. The different decay stages also deviated from random expectations. Fairly fresh logs and logs in the middle decay stage were clumped up to about 25 and 35 m respectively, and late decayed logs aggregated up to 95 m. Logs with diameters from 10,29 cm were aggregated up to 25 m, whereas logs ,30 cm diameter were randomly distributed. The result suggests that gap-dynamics do have an impact on the spatial pattern of the CWD, creating fine-scale clumping. The random distribution of large logs may result from the slightly regular spacing of large living trees. The spatial patterns of 16 species (n > 20) of wood-decaying fungi were analysed with Ripley's K -function. Three patterns were aggregated, for Gloeophyllum sepiarium, Coniophora olivacea and Vesiculomyces citrinus. These results indicate that the distribution of most species at the stand level is generally not influenced by dispersal limitations. [source] A grid-based method for sampling and analysing spatially ambiguous plantsJOURNAL OF VEGETATION SCIENCE, Issue 4 2001Jeffrey S. Fehmi Hickman (1993). Abstract. Spatial data can provide much information about the interrelations of plants and the relationship between individuals and the environment. Spatially ambiguous plants, i.e. plants without readily identifiable loci, and plants that are profusely abundant, present non-trivial impediments to the collection and analysis of vegetation data derived from standard spatial sampling techniques. Sampling with grids of presence/absence quadrats can ameliorate much of this difficulty. Our analysis of 10 fully-mapped grassland plots demonstrates the applicability of the grid-based approach which revealed spatial dependence at a much lower sampling effort than mapping each plant. Ripley's K -function, a test commonly used for point patterns, was effective for pattern analysis on the grids and the gridded quadrat technique was an effective tool for quantifying spatial patterns. The addition of spatial pattern measures should allow for better comparisons of vegetation structure between sites, instead of sole reliance on species composition data. [source] Can isotropy vs. anisotropy in the spatial association of plant species reveal physical vs. biotic facilitation?JOURNAL OF VEGETATION SCIENCE, Issue 1 2001Peter Haase In dryland ecosystems and other harsh environments, a large part of the vegetation is often clustered, appearing as ,islands'. If ,independent' species, usually colonizers, can be distinguished from species which are ,dependent' on the presence of the colonizing species for successful establishment and/or persistence, the type of spatial pattern of the association - isotropic (spatially symmetric) or anisotropic (spatially asymmetric) - can give information on the underlying environmental factors driving the process of association. Modified spatial pattern analysis based on Ripley's K -function can be applied to bivariate clustered patterns by cardinal direction in order to detect possible anisotropy in the pattern of association. The method was applied to mapped distribution patterns of two types of semi-arid shrubland in southeastern Spain. In shrubland of Retama sphaerocarpa, low shrubs of Artemisia barrelieri were significantly clustered under the canopy of the Retama shrubs in all four cardinal directions, suggesting an isotropic facilitation effect. In low shrubland dominated by Anthyllis cytisoides and Artemisia barrelieri, Anthyllis shrubs occurred more frequently than expected on the eastern side (and downslope) of an Artemisia shrub. The possible environmental factors driving the two association patterns are discussed and recommendations for further applications of the analytical method are given. [source] Disturbance affects spatial patterning and stand structure of a tropical rainforest treeAUSTRAL ECOLOGY, Issue 4 2010BRUCE L. WEBBER Abstract 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. [source] Spatial distribution of communal nests in a colonial breeding bird: benefits without costs?AUSTRAL ECOLOGY, Issue 5 2008URS CHRISTIAN GIEßELMANN Abstract The spatial organization of individuals, or groups of individuals, within a population can provide valuable information about social organization and population dynamics. We analysed the spatial distribution of nests of the sociable weaver (Philetairus socius) on two farms in the Kalahari. Sociable weavers build large communal nests on big savannah trees, forming a pattern of trees with and without nests. We used two spatial statistics, Ripley's K and the pair correlation function, to describe characteristics of the point patterns over a range of distances. (i) At distances of 200 and 300 m, communal nests were clustered. (ii) At distances greater than 1000 m, communal nests were regularly distributed. These findings are independent of the spatial distribution of trees. Furthermore, we used Moran's I to analyse spatial autocorrelation of nest sizes. We expected negative autocorrelation because of competition between nests. But on both farms there was no significant autocorrelation of nest sizes for any distance class. The regular distribution observed at larger distances may indicate competition and/or territoriality among different nests, but the lack of spatial autocorrelation between nest sizes suggests that these interactions may happen between nest clusters rather than between single nests. This was confirmed by significant clustering of nests on small scales. We thus suggest, that colonies of P. socius consist of several nests on adjacent trees forming a cluster of subcolonies. The question why sociable weavers establish subcolonies instead of adding more chambers to the natal nest, could not simply be answered by limitation of nesting space. We suggest a strategy to avoid costs due to increasing colony size. [source] | ||||||||||