The expansion of rainforest pioneer trees into long-unburnt open forests has become increasingly widespread across high rainfall regions of Australia. Increasing tree cover can limit resource availability for understorey plant communities and reduce understorey diversity. However, it remains unclear if sclerophyll and rainforest trees differ in their competitive exclusion of understory plant communities, which contain most of the floristic diversity of open forests. Here, we examine dry open forest across contrasting fire histories (burnt and unburnt) and levels of rainforest invasion (sclerophyll or rainforest midstorey) to hindcast changes in understorey plant density, richness and composition. The influence of these treatments and other site variables (midstorey structure, midstorey composition and soil parameters) on understorey plant communities were all examined. This study is the first to demonstrate significantly greater losses of understorey species richness, particularly of dry open-forest specialists, under an invading rainforest midstorey compared to a typical sclerophyll midstorey. Rainforest pioneers displaced over half of the understorey plant species, and reduced ground cover and density of dry forest specialists by ~90%. Significant understorey declines also occurred with increased sclerophyll midstorey cover following fire exclusion, although losses were typically less than half that of rainforest-invaded sites over the same period. Understorey declines were closely related to leaf area index and basal area of rainforest and wattle trees, suggesting competitive exclusion through shading and potentially belowground competition for water. Around 20% of displaced species lacked any capacity for population recovery, while transient seed banks or distance-limited dispersal may hinder recovery for a further 68%. We conclude that rainforest invasion leads to significant declines in understorey plant diversity and cover in open forests. To avoid elimination of local native plant populations in open forests, fires should occur with sufficient frequency to prevent overstorey cover from reaching a level where shade-intolerant species fail to thrive.

中文翻译：

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雨林扩张降低了澳大利亚东部开阔森林的下层植物多样性和密度

在澳大利亚的高降雨量地区，将雨林先锋树扩展到长期未燃烧的开阔森林已变得越来越普遍。增加树木覆盖会限制林下植物群落的资源可用性并减少林下植物多样性。然而，目前尚不清楚硬叶树和雨林树木在对林下植物群落的竞争排斥方面是否有所不同，林下植物群落包含开放森林的大部分植物区系多样性。在这里，我们通过对比鲜明的火灾历史（燃烧和未燃烧）和雨林入侵水平（硬叶植物或雨林中层）来检查干燥的开阔森林，以了解下层植物密度、丰富度和组成的后发变化。这些处理和其他场地变量（中层结构、对下层植物群落的中层成分和土壤参数）都进行了检查。这项研究首次证明，与典型的硬叶林中层相比，在入侵的雨林中层下，层下物种丰富度的损失显着更大，尤其是干燥的开放森林专家。雨林开拓者取代了超过一半的林下植物物种，并将干燥森林专家的地面覆盖和密度减少了约 90%。火灾排除后，随着硬叶植物中层覆盖率的增加，林下层也出现了显着下降，尽管损失通常不到同期热带雨林入侵地点的一半。林下面积下降与雨林和荆树的叶面积指数和基面积密切相关，建议通过遮荫和潜在的地下水竞争来排除竞争。大约 20% 的流离失所物种缺乏任何种群恢复能力，而临时种子库或距离限制的传播可能会阻碍另外 68% 的种群恢复。我们得出的结论是，雨林入侵导致开放森林的下层植物多样性和覆盖率显着下降。为了避免在开阔的森林中消灭当地的本地植物种群，火灾发生的频率应该足够高，以防止覆盖层达到不耐阴物种无法繁衍的程度。我们得出的结论是，雨林入侵导致开放森林的下层植物多样性和覆盖率显着下降。为了避免在开阔的森林中消灭当地的本地植物种群，火灾发生的频率应该足够高，以防止上层覆盖达到耐阴物种无法繁衍的程度。我们得出的结论是，雨林入侵导致开放森林的下层植物多样性和覆盖率显着下降。为了避免在开阔的森林中消灭当地的本地植物种群，火灾发生的频率应该足够高，以防止上层覆盖达到耐阴物种无法繁衍的程度。