Temporal changes in species composition, diversity, and woody vegetation structure of savannas in the Cerrado-Amazon transition zone

ABSTRACT Vegetation in the transition between tropical forest and savanna is hyperdynamic and there is evidence that in the absence of fire, forest advances over savanna. Between 2008 and 2013 we evaluated changes in species composition and diversity and in the structure of the woody vegetation of savanna physiognomies in the transition between the Cerrado and Amazon biomes that were fire free for 11 years. The physiognomies form a gradient from savanna woodland (Typical Cerrado - TC), to low woodland (Dense Cerrado - DC), to woodland (locally called Cerradão - CO). We hypothesise that: 1) the more open physiognomies (TC and DC) are more dynamic compared to the closed physiognomy (CO); and 2) in the absence of fire vegetation tends to become more forested. We found that: 1) TC was more dynamic (e.g. greater increases in richness, diversity, and abundance of plants and basal area) than CO and DC; 2) The three physiognomies experienced an increase in basal area and abundance of individuals, but only certain key species contributed to these increases. These results indicate that the open physiognomies were more dynamic than the closed physiognomies, and in the absence of fire the savanna physiognomies became more forested and accumulated biomass.

摘要
热带森林与稀树草原过渡带的植被具有超动态变化特征，现有研究证据表明，在无火干扰的情况下，森林会向稀树草原区域扩张蔓延。2008年至2013年，我们针对塞拉多（Cerrado）与亚马逊（Amazon）生物群系过渡带内11年未受火干扰的稀树草原植被型（savanna physiognomies），开展了其物种组成、多样性及木本植被结构变化的评估。该区域的植被型沿环境梯度分布，依次为稀树疏林（典型塞拉多（Typical Cerrado，TC））、低郁闭灌丛（密灌塞拉多（Dense Cerrado，DC））与乔木林（当地俗称塞拉董（Cerradão，CO））。我们提出两项研究假说：其一，相较于封闭型植被型（CO），开放型植被型（TC与DC）具备更高的动态性；其二，在无火干扰的条件下，植被会愈发趋向森林化。本研究得到如下结果：1）典型塞拉多（TC）的动态性显著高于密灌塞拉多（DC）与塞拉董（CO），具体表现为物种丰富度、多样性、植株多度及基径断面积（basal area）的增幅更为显著；2）三种植被型的基径断面积与植株多度均有所提升，但仅部分关键物种推动了此类变化。上述结果表明，开放型植被型的动态性高于封闭型植被型，且在无火干扰的情况下，稀树草原植被型会愈发趋向森林化并积累生物量。