Previous analyses of historical aerial photography and satellite imagery have shown thickening of woody cover in Australian tropical savannas, despite increasing fire frequency. The thickening has been attributed to increasing precipitation and atmospheric CO2 enrichment. These analyses involved labour‐intensive, manual classification of vegetation, and hence were limited in the extent of the areas and the number of measurement times used. Object‐based, semi‐automated classification of historical sequences of aerial photography and satellite imagery has enabled the spatio‐temporal analysis of woody cover over entire landscapes, thus facilitating measurement, monitoring and attribution of drivers of change. Using this approach, we investigated woody cover change in 4000 ha of intact mesic savanna in the Ranger uranium lease and surrounding Kakadu National Park, using imagery acquired on 10 occasions between 1950 and 2016. Unlike previous studies, we detected no overall trend in woody cover through time. Some variation in cover was related to rainfall in the previous 12 months, and there were weak effects of fire in the year of image acquisition and the antecedent 4 years. Our local‐scale study showed a mesic eucalypt savanna in northern Australia has been resilient to short‐term variation in rainfall and fire activity; however, changes in canopy cover could have occurred in other settings. When applying this semi‐automated approach to similar studies of savanna dynamics, we recommend maximising the time depth and number of measurement years, standardising the time of year for image acquisition and using many plots of 1 ha in area, rather than fewer, larger plots.

中文翻译：

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澳大利亚季风热带中热带桉树稀树草原木质覆盖的多年代际稳定性

先前对历史航拍和卫星图像的分析表明，尽管火灾频率增加，但澳大利亚热带稀树草原的木质覆盖层增厚。增稠归因于降水增加和大气二氧化碳富集。这些分析涉及劳动密集型的人工植被分类，因此在区域范围和使用的测量次数方面受到限制。对航空摄影和卫星图像的历史序列进行基于对象的半自动分类，可以对整个景观的木质覆盖进行时空分析，从而促进对变化驱动因素的测量、监测和归因。使用这种方法，我们使用 1950 年至 2016 年间 10 次获取的图像，调查了 Ranger 铀租约和周围卡卡杜国家公园 4000 公顷完整中热带稀树草原的木质覆盖变化。 与之前的研究不同，我们没有发现木质覆盖随时间变化的总体趋势。覆盖的一些变化与前12个月的降雨有关，图像采集年份和前4年的火灾影响较弱。我们的本地规模研究表明，澳大利亚北部的中层桉树稀树草原对降雨和火灾活动的短期变化具有弹性；然而，树冠盖的变化可能发生在其他环境中。在将这种半自动方法应用于热带草原动力学的类似研究时，我们建议最大化时间深度和测量年数，