Spatial pattern and process in forest stands within the Virginia piedmont

Abstract. Question: Underlying ecological processes have often been inferred from the analysis of spatial patterns in ecosystems. Using an individual‐based model, we evaluate whether basic assumptions of species’life‐history, drought‐susceptibility, and shade tolerance generate dynamics that replicate patterns between and within forest stands. Location: Virginia piedmont, USA. Method: Model verification examines the transition in forest composition and stand structure between mesic, intermediate and xeric sites. At each site, tree location, diameter, and status were recorded in square plots ranging from 0.25 to 1.0 ha. Model validation examines the simulated spatial pattern of individual trees at scales of 1–25 m within each forest site using a univariate Ripley's K function. Results: 7512 live and dead trees were surveyed across all sites. All sites exhibit a consistent, significant shift in pattern for live trees by size, progressing from a clumped understorey (trees ± 0.1 m in diameter) to a uniform overstorey (trees > 0.25 m). Simulation results reflect not only the general shift in pattern of trees at appropriate scales within sites, but also the general transition in species composition and stand structure between sites. Conclusions: This shift has been observed in other forest ecosystems and interpreted as a result of competition; however, this hypothesis has seldom been evaluated using simulation models. These results support the hypothesis that forest pattern in the Virginia piedmont results from competition involving species’life‐history attributes driven by soil moisture availability between sites and light availability within sites.     

侧柏树干边材液流空间变化规律

利用热扩散探针配合自动气象站,于2005年在北京林业大学妙峰山试验林场对侧柏树干边材液流指标空间变化规律进行了研究.结果表明:侧柏树干不同高度边材液流速率随树干高度的升高而增加,而且高层液流峰值的出现时间要比低层早,高层液流曲线尖窄,曲线斜率大,低层液流曲线变化缓慢,曲线斜率小.不同高度的平均液流速率峰值:6.6m处为0.015cm·s-1,4.6m处为0.009cm·s-1,2.6m处为0.006cm·s-1,0.6m处为0.003cm·s-1;无论是液流速率还是连日耗水量,不同直径的单株均表现出沿直径的增加而增加,但液流速率随直径的变化并不是线性的;土壤的含水量限制了树木的耗水能力,日平均液流通量与土壤含水量呈现良好的S型关系,其中与土层20～40cm层关系更为密切;不同气象因子对树干的液流影响方式不同,太阳辐射强度、大气温度和风速与液流指标呈正相关,并且属于第一主分量,对液流的影响较为直接,空气相对湿度和土壤温度与液流指标呈负相关,属于第二主分量,对液流的影响较为缓慢.通过多元线性回归,得出各气象因子和液流相关性均比较高,说明通过气象因子对树干液流进行预测是切实可行的.     

古尔班通古特沙漠土壤水分与化学性质的空间分布

区域尺度上,气象和水文状况是影响植被分布格局和土壤性状的主要因子,当然,局部的地貌特征及植被分布对土壤性质的影响也是不能忽视的。以古尔班通古特沙漠为研究对象,对其南缘至腹地约25 km的土壤水分与化学性质的空间分布及其相互关系进行研究,结果显示:水平方向上,从沙漠南缘至腹地,土壤水分在丘底呈减小趋势,尤其是距边缘10 km以外,且在距边缘6—7 km出现1个峰值。同时土壤pH值和电导率的大小以及有机碳,全氮,有效氮,全磷和有效磷含量与土壤水分的变化状况相似,在丘底呈减小趋势,而在丘坡和丘顶上均呈波动分布。这说明丘底土壤性质的空间格局受到气象和水文状况的显著影响,而丘坡和丘顶土壤性质并没有受到气象和水文状况的显著影响,这种波动分布主要由采样点的设置及植物的分布状况所引起。丘底土壤水分及化学性质均显著高于丘坡和丘顶,这说明地貌特征对土壤性质的空间分布有显著影响。垂直方向上,不同深度土层土壤性质也存在显著差异,土壤水分,pH值和电导率随着土层深度增加而增加,而土壤养分随土层深度增加而减小。相关分析结果表明:仅4、5月份的土壤含水量及年平均土壤含水量与丘底土壤化学性质显著正相关,这可能由于冬季积雪的融化导致了土壤水分在4、5月份的聚集比较明显,使土壤水分对土壤化学性质发挥作用。总之,土壤空间异质性是由气象和水文以及地貌特征共同影响的,而土壤空间异质性的变异进而影响到植被的分布,植被的分布反过来又影响土壤的空间分布,因此土壤的空间分布和植被的空间分布是相互影响,共同作用的。     

Spatial pattern of pulmonary blood flow distribution is stable over days

Glenny, Robb W., Steven McKinney, and H. Thomas Robertson.Spatial pattern of pulmonary blood flow distribution is stableover days. J. Appl. Physiol. 82(3):902-907, 1997.Despite the heterogeneous distribution of regionalpulmonary perfusion over space, local perfusion remains stable overshort time periods (20-100 min). The purpose ofthis study was to determine whether the spatial distribution ofpulmonary perfusion remains stable over longer time periods (1-5days). Regional blood flow was measured each day for 5 days in five awake standing dogs. Fluorescent microspheres of differentcolors were injected into a limb vein over 30 s on each day. After thelast microsphere injection, the dogs were killed, and lungs wereflushed free of blood, excised, dried at total lung capacity, and dicedinto ~2-cm³ pieces(n = 1,296-1,487 per dog).Relative blood flow to each piece on each day was determined byextracting the fluorescent dyes and determining the concentrations ofeach color. We established that blood flow is spatiallyheterogeneous with a coefficient of variation of 29.5 ± 2%. Blood flow to each piece is highly correlated with flow to thesame piece on all days (r = 0.930 ± 0.006). The temporal heterogeneity of regional perfusion as measured by the coefficient of variation is 6.9 ± 0.7% over the 5 days and is nonrandom. The magnitude of spatial and temporal variationis significantly less than previously reported in a study in whichanesthetized and mechanically ventilated dogs were used. We concludethat spatial distribution of pulmonary blood flowremains stable over days and we speculate that in the normal awake dogregional perfusion is determined primarily by a fixed structure such asthe geometry of the pulmonary vascular tree rather than by localvasoactive regulators. Anesthesia and/or mechanical ventilationmay increase the temporal variability in regionalperfusion.

     

石羊河流域土地覆被空间演化及驱动机制

基于石羊河流域1998年、2002年、2006年、2010年NDVI、夜间灯光数据和TM影像提取的土壤信息,以城市地表人工覆被系统为基础,计算流域土地覆被指数(Land-Cover Index,LCI),利用小波分析、空间变差函数和间隙度指数等方法构建测度模型,分析了石羊河流域地区十多年来的土地人工覆被空间演化过程和格局,然后利用回归拟合方程找出影响地区土地覆被变化的驱动机制。结果表明:LCI大范围分布受地形限制,高值区(城镇用地区域)值越来越高,而低值区(植被覆盖区域)值在2002年形成了峰值后下降,即地区城镇化水平显著提高的同时,林草覆盖率缓慢变好,并且逐渐形成区域簇群,同时地区城镇化水平受人为因素的影响较大,植被覆盖度受自然因素的影响较大。     

应用空间点模式方法研究大兴安岭雷击火空间分布格局

林火空间分布格局的研究对于林火发生预测以及理解林火在森林景观变化过程中的作用具有重要的意义.运用空间点模式方法对大兴安岭地区1988～2005年间雷击火发生情况进行了统计分析进而确定雷击火空间分布情况,研究结果表明大兴安岭地区雷击火成聚集分布,并存在雷击火热点地区,其核心区地理坐标分别为123°06′E,52°20′N;123°41′E,51°34′N;124°08′E,50°48′N.     

Spatial ecology of a root parasite – from pattern to process

Abstract Occurrence patterns of parasitic plants are constrained by the distribution of suitable hosts and movement patterns of seed vectors and, accordingly, represent a simplified system to study many aspects of spatial ecology and determinants of distribution. Previous work has focused on the aerially hemiparasitic mistletoes, and it is unclear whether root parasites are affected by similar factors. Here, we evaluate spatial patterns in the root parasitic Santalum lanceolatum in an arid shrubland in north‐western New South Wales, central Australia. In this region, the principal host is a long‐lived nitrogen fixing shrub Acacia tetragonophylla closely associated with ephemeral creek‐lines. The location of 765 individuals of both species was mapped along a 250‐m section of creek‐line using a total survey station, and occurrence patterns of the root parasite related to host distribution and landscape context. We used Ripley's K‐function and the O‐ring statistic to determine whether the distribution of S. lanceolatum was random, aggregated or regular; the spatial scales at which these patterns occurred; and to quantify any spatial associations between the parasite and its host, A. tetragonophylla. While acacias were closely associated with the creek‐line, S. lanceolatum plants were more tightly clustered, displaying significant clustering at two spatial scales (1.2 m and 8.8 m). We suggest that host quality may act as an important constraint, with only those acacias growing in or near the creek‐line being physiologically capable of supporting a parasite to maturity. Insights gained from spatial analysis are used to guide ongoing research in this system, and highlight the utility of the O‐ring statistic for understanding patterns of distribution affected by multiple processes operating at critical scales.     

Mathematical simulation of a biofilm process

A mathematical model has been developed for a fixed-film biological process (film flow over a flat plate) that describes bulk liquid transport, diffusional transport of oxygen and organics across a stagnant film, diffusional transport of oxygen and organics into the biofilm, biochemical reactions by the individual cells within the biofilm, biofilm growth, and cell density changes within the biofilm due to cellular decay. Simulation studies are presented to show how contact time and diffusion layer thickness affect process performance.     

Spatial scaling of species abundance distributions

Species abundance distributions are an essential tool in describing the biodiversity of ecological communities. We now know that their shape changes as a function of the size of area sampled. Here we analyze the scaling properties of species abundance distributions by using the moments of the logarithmically transformed number of individuals. We find that the moments as a function of area size are well fitted by power laws and we use this pattern to estimate the species abundance distribution for areas larger than those sampled. To reconstruct the species abundance distribution from its moments, we use discrete Tchebichef polynomials. We exemplify the method with data on tree and shrub species from a 50 ha plot of tropical rain forest on Barro Colorado Island, Panama. We test the method within the 50 ha plot, and then we extrapolate the species abundance distribution for areas up to 5 km². Our results project that for areas above 50 ha the species abundance distributions have a bimodal shape with a local maximum occurring for the singleton classes and that this maximum increases with sampled area size.     

Spatial patterns in the distributions of polygenic characters

The spatial patterns in the mean and variance of a quantitative character that result from the interaction of spatially varying, optimizing selection and gene flow are considered. The model analyzed is an extension of those of Kimura (1965) and Lande (1976) for the distribution of a quantitative character maintained in a population by independent mutations. For weak selection, it is shown that there is only a small effect of gene flow on the variance of the character and that the mean value changes on a length scale that is large compared to the average dispersal distance. As in models of clines in allele frequencies, it is possible to define a “characteristic length” in terms of the average dispersal distance and strength of selection. The characteristic length is the smallest length scale environmental change to which the mean value of the character can significantly respond. It is also shown that, for weak selection, an asymmetry in dispersal can result in a significant shift in location of a cline. By considering an infinite linear cline in optimal values, it is shown that gene flow can increase the variance only when there is sufficient mixing in each generation of individuals from locations with different means. A model of selection in different niches is also considered. There is an increase in variance due to the effective weakening of the intensity of selection because of the differences in optimal values in different niches.The implications of the different models for maintenance of genetic polymorphism are discussed. Under some conditions gene flow can produce a significant increase in heterozygosity. It is also argued that spatial variation in selection on a polygenic character can be much more effective in increasing heterozygosity than temporal variation because of the potentially greater increase in phenotypic variance. The difference between some of the results for polygenic characters from those of similar models of one and two locus systems is accounted for by the fact that for normally distributed polygenic characters, changes in the variance are effectively decoupled from changes in the mean.     

Multimodal pattern formation in phenotype distributions of sexual populations

During bouts of evolutionary diversification, such as adaptive radiations, the emerging species cluster around different locations in phenotype space. How such multimodal patterns in phenotype space can emerge from a single ancestral species is a fundamental question in biology. Frequency-dependent competition is one potential mechanism for such pattern formation, as has previously been shown in models based on the theory of adaptive dynamics. Here, we demonstrate that also in models similar to those used in quantitative genetics, phenotype distributions can split into multiple modes under the force of frequency-dependent competition. In sexual populations, this requires assortative mating, and we show that the multimodal splitting of initially unimodal distributions occurs over a range of assortment parameters. In addition, assortative mating can be favoured evolutionarily even if it incurs costs, because it provides a means of alleviating the effects of frequency dependence. Our results reveal that models at both ends of the spectrum between essentially monomorphic (adaptive dynamics) and fully polymorphic (quantitative genetics) yield similar results. This underscores that frequency-dependent selection is a strong agent of pattern formation in phenotype distributions, potentially resulting in adaptive speciation.     

Spatial interactions in rapid pattern discrimination

We measured reaction times (RTs) for identification of a target among distracters under stabilized image conditions in which the positions of the target and the distracters were constant within a single experimental session. Under these conditions, the observer need not search for the target because its position is known. We nevertheless found that the presence of even a single distracter could elevate RTs. The magnitude of this effect depended on the distance of the distracter from the target and, for some observers, the distance of the distracter from the fovea. When we added not one but six background elements in a ring around the target, RT increased even more. If, apart from these neighboring distracters, the target was surrounded by more distracters located beyond the nearest neighbors, RT was, in general, not increased further. These findings suggest that adding background elements in a search task can elevate RTs in ways that are not dependent on the positional uncertainty of the target.