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高度动态性，且已有证据表明，在无火灾干扰的情况下，森林会向稀树草原区域扩展。
2008年至2013年间，我们针对塞拉多（Cerrado）与亚马逊（Amazon）生物群系过渡带中连续11年未受火灾干扰的稀树草原植被外貌类型，评估了其物种组成、多样性以及木本植被结构的变化。该类植被外貌类型呈现出连续的梯度变化，从稀树灌丛（典型塞拉多稀树草原（Typical Cerrado - TC））、矮灌丛（密塞拉多稀树草原（Dense Cerrado - DC））直至灌丛（当地称为塞拉多大灌丛（Cerradão - CO））。
我们提出如下假说：1）相较于封闭型植被外貌（CO），开放型植被外貌（TC与DC）具有更高的动态性；2）在无火灾干扰的条件下，植被倾向于向更具森林特征的方向演替。
研究结果显示：1）TC的动态性显著高于CO与DC，具体表现为物种丰富度、多样性、植株多度以及基面积的增幅更大；2）三种植被外貌类型的基面积与植株个体多度均有所提升，但仅部分关键物种推动了此类变化。
上述结果表明，开放型植被外貌的动态性高于封闭型植被外貌，且在无火灾干扰的情况下，过渡带的稀树草原植被外貌会逐渐向森林化方向发展并累积生物量。