Data from: Floristic variation within kerangas (heath) forest: re-evaluation of data from Sarawak and Brunei

The variation in species composition of trees greater-than-or-equal-to 7.6 cm gbh in thirty-eight plots (mostly c. 0.2 ha in extent) from physiognomically-defined kerangas forest were re-analyzed by principal components analysis ordination (species centering and standardization by sample norm). Analyses were performed separately on basal area abundances, on the densities of trees in three size classes (greater-than-or-equal-to 7.6, greater-than-or-equal-to 30.5 and greater-than-or-equal-to 61.0 cm gbh) and on the density of small and large trees (7.6- < 30.5 and 30.5- < 61.0 cm gbh). A total of 636 taxa were reduced to 381 for analysis, removing those of very low density and plot frequency. Three groups of plots were identified: forest at low elevation, and generally coastal, on deep humus podzols; forest at intermediate elevation on mostly red-yellow podzols with affinities to dipterocarp forest; and forest at high elevation on mostly peaty podzols. The first group was divisible into five subgroups along a drainage gradient, while the more poorly drained plots showing affinities to peat-swamp forest. Forty to eighty of the taxa, depending on the criteria for selection, were sufficient to define a stable, reduced spatial structure of the data matrix. Two subgroups, both coastal on deep podzols, represent the extreme form of kerangas forest per se. A comparison of Agathis borneensis- and Shorea albida-dominated plots revealed few other associated and differentiating taxa. Patterns were clearest from analyses of basal area data and of densities of all and small trees. Ordinations and grouping of plots for small, but not large, tree densities were similar to those for basal area. Different species were differentiated on the basis of the abundance measure, leading to group (tabular) definition of associations in a dual manner. A new system of summarization is presented which combines basal area, density and frequency in a graded hierarchical approach. The association between vegetation and soil type was difficult to unravel because of the limited environmental space sampled. Soil type was confounded with elevation, rainfall and geographical location. A major factor is clay content probably affecting nutrient status and water holding properties. 'Modal analysis' of small tree densities showed clearest patterns in this respect. There were no patterns at the family or genus level, nor in leaf size spectra within kerangas. Problems in the treatment, analysis and summarization of tropical forest data sets are discussed. These problems centre on the scale and intensity of field sampling and the advantages of measuring small trees leading to a dual basal area and density approach. All published studies, including this one, within kerangas forest have used inadequate sampling for the purposes of revealing species changes with respect to soil type and composition.本研究对38个样地（多数样地面积约0.2公顷）内胸高围度（girth at breast height，简称gbh）≥7.6 cm的乔木物种组成变异进行了重新分析，样地均取自以外貌特征界定的克朗加斯林（kerangas forest）。分析采用主成分分析排序（principal components analysis ordination）方法，对物种数据按样本范数进行中心化与标准化处理。研究分别针对三类数据开展独立分析：一是胸高断面积（basal area）丰度；二是三个径级（≥7.6 cm、≥30.5 cm、≥61.0 cm gbh）的乔木密度；三是小型与大型乔木（7.6~<30.5 cm、30.5~<61.0 cm gbh）的密度。研究共纳入636个分类单元，剔除密度及样地出现频率极低的类群后，最终保留381个用于分析。 研究共识别出三类样地：一是低海拔、多分布于沿海区域、生长于深腐殖质灰化土的克朗加斯林；二是中海拔、多以红黄灰化土为基质、与龙脑香林（dipterocarp forest）具有类群亲缘性的克朗加斯林；三是高海拔、多以泥炭质灰化土（peaty podzols）为基质的克朗加斯林。第一类样地可沿排水梯度划分为5个亚组，其中排水条件较差的亚组与泥炭沼泽林（peat-swamp forest）具有类群亲缘性。根据选取标准的不同，40~80个分类单元即可稳定定义数据矩阵经简化后的空间结构。两个均位于沿海深灰化土区域的亚组，本身即代表了克朗加斯林的极端类型。对以婆罗洲贝壳杉（Agathis borneensis）和白娑罗双（Shorea albida）为优势种的样地进行比较后发现，二者仅伴随极少数其他关联及区分类群。 基于胸高断面积数据以及全部乔木和小型乔木的密度数据所得到的群落格局最为清晰。小型乔木密度的排序与样地分组结果与胸高断面积分析结果相似，但大型乔木密度的分析结果则不然。不同物种可通过丰度指标进行区分，进而以双重方式实现类群（表格）关联的定义。本研究提出了一种新的概括方法，该方法以分级层级化的方式整合了胸高断面积、密度及出现频率三类数据。 由于采样覆盖的环境空间有限，植被与土壤类型之间的关联难以厘清。土壤类型与海拔、降雨量及地理位置存在混杂效应。其中，黏土含量是影响养分状况与持水特性的关键因素。对小型乔木密度开展的模态分析（modal analysis）在此方面展现出最清晰的格局。在克朗加斯林的类群分析中，科或属水平以及叶大小谱（leaf size spectra）均未呈现出显著格局。 本研究还探讨了热带森林数据集在处理、分析与概括过程中面临的诸多问题。这些问题的核心在于野外采样的尺度与强度，以及测定小型乔木所带来的优势——即可同时采用胸高断面积与密度的双重分析方法。目前所有已发表的克朗加斯林相关研究（包括本研究在内），在揭示物种随土壤类型与组成变化的分析中，均存在采样不足的问题。