Drivers of floristic variation in biogeographic transitions: insights from the ecotone between the largest biogeographic domains of South America

ABSTRACT Ecological transitions usually represent vulnerable ecosystems and high biodiversity. Investigating their drivers is important from both biogeographic and conservationist perspectives. One of these transitions is situated between the two largest biogeographic domains of South America - the Amazon and the Cerrado. We evaluated variation in tree floristic composition throughout this transition and assessed which factors explain this variation. We used 17,240 records of occurrences of 2,530 tree species from the NeotropTree database. We investigated variation in floristic composition using UPGMA cluster analysis with bootstrap resampling and NMDS ordination, and modeled the distribution of species in relation to environmental and spatial predictors by means of transformation-based Redundancy Canonical Analysis (tb-RDA) and variance partitioning. We found four general floristic groups: 1) flooded forests; 2) white sand dwarf-forests; 3) savannic and dwarf forests; and 4) non-flooded forest types. Floristic variation along the transition was influenced by the environment, especially variables related to fire and soil moisture, and by space, especially variables acting on broader scales. Correspondence between the environmental heterogeneity found throughout the transition and our results demonstrates once again the importance of conserving biogeographical transition zones in general, and the most comprehensive of South America in particular.

摘要：生态过渡带往往兼具生态系统脆弱性与高生物多样性的特征。从生物地理学与保护生物学视角出发，探究其驱动机制均具有重要学术价值。其中一处过渡带坐落于南美洲两大核心生物地理域——亚马逊（Amazon）与塞拉多（Cerrado）之间。本研究针对该过渡带内的木本植物区系组成变化开展评估，并解析驱动该变化的关键影响因子。研究使用了来自新热带树木数据库（NeotropTree）的2530种木本植物的17240条分布记录。本研究采用带自举重采样的非加权配对算术平均法（UPGMA）聚类分析与非度量多维标度（NMDS）排序，探究区系组成的变化规律；并通过基于转换的典范冗余分析（tb-RDA）与方差分解方法，构建物种分布与环境、空间预测变量间的关联模型。研究共识别出四类典型植物区系类群：1）水淹林；2）白沙矮林；3）稀树草原与矮林；4）非水淹林型。该过渡带内的区系组成变化同时受环境因子（尤其是与火、土壤湿度相关的变量）与空间因子（尤其是大尺度作用变量）的共同调控。本研究结果与过渡带内的环境异质性特征高度契合，再次印证了保护生物地理过渡带的普遍重要性，尤其凸显了南美洲这一代表性过渡带的保护价值。