Use of the Bray-Curtis similarity measure in cluster analysis of foraminiferal data

Transformation of data effectively limits the distortion by outlying values on the Bray-Curtis similarity measure. It represents an effective method of using cluster analysis in distinguishing biotopes of benthic foraminifera.     

Vegetation and Community Changes of Elm （Ulmuspumila） Woodlands in Northeastern China in 1983-2011

Elm (Ulmus pumila), widely distributed in the north temperate zone, contributes to a special savanna-like woodland in typical grassland region in the northeastern China. This woodland performs a variety of ecological functions and environmental significance, such as decreasing soil erosion, stabilizing sand dunes, preserving species diversity. However, in the last approximate 30 years, the species composition, productivity and distribution area of elm woodland has decreased severely. A series of studies have been carried out to find out whether the climate changes or human disturbances caused the degradation of elm woodland and how these factors affected elm woodland. In this study, undisturbed, plowing and grazing elm woodland were investigated in 1983 and 2011 by using Point-Centered Quarter method. The relationship between vegetation changes and environmental factors was analyzed by Bray-Curtis ordination. The results show that in 2011, species diversity and understory productivity of undisturbed elm woodland decrease slightly compared to those of undisturbed elm woodland in 1983. However, nearly 60% of the species is lost in the plowing and grazing elm woodland relative to the species undisturbed elm woodland in 1983. Interestingly, plowing stimulates the growth of elm and certain understory species through furrowing soil and accelerating soil nutrient turnover rate. Grazing disturbance not only leads to species loss and productivity decrease, but also induces changes in elm growth (small, short and twisted). The mean age of the elm was 29 ± 2 yr in undisturbed and plowing elm woodland, while only 15 yr in the grazing elm woodland. The results of Bray-Curtis ordination analysis show that all sample stands clustered to three groups: Group I including the undisturbed sample stands of 83UE (undisturbed elm woodland in 1983) and 11UE (undisturbed elm woodland in 2011); Group II including sample stands of PE (elm woodland disturbed by plowing); Group III including samples stands of GE (elm woodland disturbed by grazing). The results indicate that the long time disturbance of the plowing and grazing have converted elm woodland to different community types. Climate change is not the primary reason causing the degradation of elm woodland, but plowing and grazing disturbance. Both plowing and grazing decrease the vegetation composition and species diversity. Grazing further decreases vegetation productivity and inhibits the growth of elm tree. Therefore, we suggest that reasonable plowing and exclusive grazing would be favorable for future regeneration of degraded elm woodland.     

生物集群相似性系数分析的适用性探讨

相似性系数的选择可能会影响生物集群间的β多样性分析。Bray-Curtis相似性系数是目前使用最广的系数之一,还有一些研究使用了欧氏距离等系数。通过非度量多维标度的结果直观对比了两个相似性系数在处理生物数据时的结果,以便了解两个系数的差异。分析结果表明,Bray-Curtis相似性系数比欧氏距离更适合处理包含很多零值的生物数据。生物数据中的零值与环境数据中的不同,不应该影响集群间相似性的计算。如果遇到完全没有生物的集群,可以选用零调整的Bray-Curtis系数,或者剔除此类集群后再计算Bray-Curtis相似性系数。一些研究忽略了相似性系数的差异,在比较和分析多元分析结果时应该慎重,以免在生态环境评价时被误导。