Abstract Georeferenced digital aerial photographs were used to assess changes in overstorey vegetation cover since 1948 in the Victoria River District, Northern Territory, Australia, across a range of lowland tropical savanna habitats and with explicit consideration of known and variable site‐specific grazing and fire management histories. Vegetation surveys at corresponding locations on the ground identified five distinct woody vegetation communities defined primarily by water drainage and secondarily by soil characteristics. Air‐photo analyses revealed that, contrary to popular perceptions and in contrast to results from other habitats, there has been no generalized net increase in overstorey woody vegetation cover across the full range of lowland savanna habitats. Rather, different habitats exhibited distinctly different vegetation change mechanisms: low‐lying seasonally inundated ‘wet’ habitats have experienced woody vegetation increase since 1948, whereas well‐drained ‘dry’ habitats have experienced overstorey vegetation stability or loss. In almost every instance woody vegetation increase could be attributed to the invasion or proliferation of a single species, Melaleuca minutifolia F.Muell. The extent of M. minutifolia increase was unrelated to historical grazing/fire regime. Demographic analyses for this species revealed that recruitment was often episodic and that synchronized recruitment events occurred uniformly across the full range of historical management treatments, most likely as a consequence of favourable climatic conditions in years with an extended wet season. Heavy grazing facilitated juvenile survival and/or recruitment, most likely by reducing grassy fuel loads and eliminating landscape fire. We conclude that while there has been no generalized net increase in overstorey woody vegetation cover in lowland environments, savanna dynamics are complex, and multiple change mechanisms have occurred simultaneously in different habitats, some of which have been significantly transformed since 1948. Where net woody vegetation increase has occurred it is primarily a natural consequence of episodic M. minutifolia establishment in climatically favourable years, but the extent and magnitude of this effect is likely mediated by fire/grazing regime.

中文翻译：

---

木本植被净增加仅限于北领地维多利亚河区澳大利亚热带稀树草原季节性淹没的低地


摘要 使用地理参考数字航空照片评估自 1948 年以来澳大利亚北领地维多利亚河区一系列低地热带稀树草原栖息地的植被覆盖变化，并明确考虑已知和可变的特定地点放牧和火灾管理历史。对地面相应位置的植被调查确定了五个不同的木本植被群落，主要由排水系统定义，其次由土壤特征定义。航空照片分析显示，与普遍的看法相反，也与其他栖息地的结果相反，整个低地稀树草原栖息地的上层木本植被覆盖范围并未出现普遍的净增加。相反，不同的栖息地表现出明显不同的植被变化机制：自 1948 年以来，低洼的季节性淹没的“潮湿”栖息地经历了木本植被的增加，而排水良好的“干燥”栖息地则经历了高层植被的稳定或丧失。几乎在所有情况下，木本植被的增加都可以归因于单一物种（Melaleuca minutifolia F.Muell）的入侵或增殖。 M. minutifolia 的增加程度与历史放牧/火灾状况无关。对这个物种的人口统计分析表明，招募通常是间歇性的，并且同步的招募事件在整个历史管理处理过程中统一发生，很可能是雨季延长年份有利气候条件的结果。大量放牧有利于幼崽的生存和/或补充，最有可能是通过减少草地燃料负荷和消除景观火灾。 我们的结论是，虽然低地环境中的上层木本植被覆盖没有出现普遍的净增加，但稀树草原动态是复杂的，并且不同的栖息地同时发生了多种变化机制，其中一些自 1948 年以来已经发生了显着的转变。增加的发生主要是气候有利年份中偶发的小叶山楂建立的自然结果，但这种影响的程度和程度可能是由火灾/放牧制度介导的。