Quantifying the 2022-2023 altitudinal range-wide tree mass-mortality event and probability of forest loss in Central Chile

智利中部的地中海型生态系统具有高度的生物多样性和丰富的物种，这是其他地方没有的，而且它们代表了南美洲的整个地中海生物群。然而，由于农业、放牧和城市化、栖息地碎片化和森林火灾，栖息地丧失和退化，智利的地中海型生态系统受到威胁。气候危机不断升级的影响现在对他们的生存构成了严重威胁。2022年至2023年的夏天，智利中部出现了突然、严重和广泛的森林死亡。该地区正在经历一段被称为“特大干旱”(MD)的持续干旱时期，自2010年以来，该地区的降雨量至少比常年低25%。2019年和2021年出现了两个异常干旱的年份，降水量比平均水平低约80%，随后在2022年减少了50%。到2023年夏末，大面积的树木死亡和森林生态系统崩溃是显而易见的。我们对一个物种的地理分布中应该首先感受到干旱影响的理解表明，我们应该看到这些影响集中在物种范围内本已更热和更干燥的部分。然而，在智利中部的地中海气候地区，我们现在看到的是自然分布在山区的森林正在死亡-即使是那些生长在山区更凉爽的地方的森林也死于干旱造成的死亡。因此，以圣地亚哥树(圣地亚哥橡树)为代表的高海拔森林以及以Peumo树(智利橡子树)为代表的低海拔森林都出现了森林死亡，而Peumo树通常更耐干旱。这两种主要的森林形成树都没有长寿的种子。因此，随着成虫大规模死亡，树木再生的潜力将非常小，这是一个严重的风险。风险在于，森林将迅速被灌木生态系统取代，灌木生态系统的身高更小，储存的碳要少得多，高度易燃，相关的生物多样性也非常不同。智利这次异常广泛的森林大规模死亡事件为我们提供了一个前所未有的机会，帮助我们了解森林生态系统崩溃的模式、过程和影响。这样的机会非常难得，对帮助我们更好地了解全球范围内我们的森林所面临的风险是极其宝贵的。在这个项目中，我们将对枯树的大小和分布进行详细的调查，我们从种子和新梢中发现的任何树木再生，以及从灌木中获得的类似数据。我们还将调查种子库，以发现哪些物种最有可能从种子中再生。我们将在整个森林中使用温度和湿度传感器来了解树木正在经历的气候的小范围变化，并将其与再生和我们发现树木存活的情况联系起来。除了地块级别的数据外，我们还将访问详细的数字景观模型，使用无人机安装的相机对树木树冠进行调查，并将这些图像与现场调查数据结合起来，以便我们可以从上到下了解林分和树冠结构。这些基于无人机的数据还将使我们能够扩大到来自卫星的遥感数据，以便我们能够在更大的空间尺度上了解死亡程度和影响。结合起来，这些数据将使我们能够了解死亡对森林本身的影响程度和影响，森林更新的潜力，以及整个景观中乔木和灌木生存与更新之间的平衡。它将帮助我们了解森林死亡的地点和原因--以及树木死亡后将保留哪些植被，使我们能够更好地规划气候变化的影响，并量化森林丧失将对当地和全球树木碳吸收和储存模型产生的后果。

The Mediterranean-type ecosystems of central Chile are highly biodiverse and rich in species that occur nowhere else, furthermore, they represent the entire Mediterranean biome of South America. However, Chile's Mediterranean-type ecosystems are threatened due to habitat loss and degradation due to conversion for agriculture, grazing and urbanisation, habitat fragmentation and forest fires. The escalating impacts of the climate crisis now represent a critical threat to their survival. The Chilean summer of 2022-2023 has seen sudden, severe and extensive forest mortality in central Chile. The region is experiencing a period of prolonged drought dubbed the 'Mega Drought' (MD), where precipitation has been at least 25% lower than usual since 2010. Two exceptionally dry years occurred in 2019 and 2021, with precipitation some 80% lower than average, followed by a 50% reduction in 2022. By the end of summer 2023 widespread tree death and forest ecosystem collapse was apparent. Our understanding of where drought impacts should be felt first across the geographic distribution of a species suggests that we should see these impacts concentrated in the already hotter and drier parts of species ranges. However, in the Mediterranean climate region of Central Chile we are now seeing forests dying right across their natural distribution in mountain regions - with even those occurring in cooler locations higher in the mountains succumbing to drought-driven death. Consequently, forest mortality is witnessed across the higher elevation forests typified by the tree 'Roble de Santiago" (Santiago Oak) as well as those lower forests typified by the peumo tree (the Chilean acorn) which is usually much more drought resistant. Neither of these dominant forest- forming trees has long-lived seeds. Consequently, there is a serious risk that as the adults die on such large scale, there will be very little potential for trees to regenerate. The risk is that forest will rapidly be replaced by shrubland ecosystems which are smaller in stature, store much less carbon, are highly flammable and with very different associated biodiversity. This exceptionally widespread forest mass mortality event in Chile presents an unprecedented opportunity to help us understand the pattern, process and implications of forest ecosystem collapse. Such an opportunity is highly rare and exceptionally valuable to help us better understand the risks to our forests at the global scale. In this project, we will conduct a detailed survey of the size and distribution of dead trees, any tree regeneration that we find from seeds and shoots and similar data from shrubs. We'll also survey the seed bank to discover which species are most likely to regenerate from seed. We'll use temperature and moisture sensors throughout the forest to understand small-scale variation in the climate that the trees are experiencing and link this to regeneration and the occasions where we find tree survival. As well as the plot-level data, we will access a detailed digital landscape model and survey the tree canopy using a drone mounted camera and unite these images with the field survey data so we can understand stand and canopy structure from above and below. This drone-based data will also enable us to scale up to remote sensing data from satellites so that we can understand the mortality extent and impacts at much larger spatial scales. In combination, the data will enable us to understand the extent and impacts of mortality on the forest itself, the potential for forest regeneration and the balance between tree and shrub survival and regeneration across the landscape. It will help us to understand where and why forests are dying - and what vegetation will remain after the trees die, enabling us to better plan for the impacts of climate change and to quantify what consequences forest loss will have for local and global models of carbon uptake and storage by trees.