Longer-term responses of Amazonian vegetation to fire

热带森林比任何其他类型的陆地环境都拥有更多的动植物物种。它们还在树木和土壤中储存了大量的温室气体。巴西亚马逊地区占所有剩余森林的40%，因此对保护地球物种和气候具有巨大的全球重要性。潮湿的热带森林通常不会燃烧，但由于人为火灾的威胁，它们的未来正变得越来越不确定。伐木和森林碎片化等人类活动打开了森林栖息地，使其更容易燃烧，而毗邻剩余森林的地区的农业大大增加了火灾事件的数量。此外，现在有令人信服的证据表明，预测下个世纪的区域气候变化可能会使亚马逊地区变得更加干燥和炎热，从而增加极端干旱事件的频率和强度，并增加大片森林的可燃性。亚马逊的严重干旱发生在1997年和2005年，并导致大范围的森林大火摧毁了数千平方公里的森林。当森林第一次燃烧时，火焰高度很少会超过30-40厘米，而且火会穿过落叶层。尽管这些大火看起来相对无害，但它们实际上具有很强的破坏性，因为大多数热带雨林树木对热的耐受性很低，而且它们的传播速度很慢，这意味着火焰与树木接触的时间很长。因此，即使是低强度的火灾也会导致8%到40%的直径超过10厘米的树木死亡。它们也是正反馈循环的第一步，在正反馈循环中，树木死亡率的增加会导致森林更加开阔，森林地面上可用的燃料也会增加，使森林更容易燃烧，并增加随后任何火灾的严重性。燃烧不止一次的森林已经失去了大约80%的直径为10厘米的树木。我们才刚刚开始了解野火对热带森林植被的影响，许多重要的问题仍然存在。例如，我们还不能解释为什么森林地区的树木死亡率从8%到40%不等，我们几乎没有关于火灾对热带森林树木的长期影响的信息。我们需要更多信息，了解森林需要多长时间才能恢复(如果它们确实可以完全恢复)，反复发生火灾后树木物种的组成会发生什么，以及这些反应在亚马逊盆地可能会有什么不同。这项提案将通过研究巴西亚马逊地区八个不同地区的森林在最后一次燃烧11-12年后发生的情况来解决这些不确定的领域。这些区域包括四个区域，这些区域位于核心的成熟亚马逊森林，在过去的数千年里，这些区域受到火灾的影响最小；以及四个过渡性森林区域，这些区域靠近森林-大草原的边界，高度要小得多(仅占总木材蓄积量的一半左右)。所有这些地区都有大片曾被烧毁过一次的森林，在过去20年里被烧毁过一次以上的森林，以及没有近期火灾历史的未被烧毁的森林。我避免选择已经被广泛砍伐的森林。通过对这些地区三种烧毁处理(烧毁一次、烧毁两次、未烧毁)的树木和树苗进行采样，我将能够测试对亚马逊过渡和核心森林火灾后树木死亡和再生的预测。这些信息将提高我们对热带森林对大面积和长期火灾的反应的了解。此外，它将提供必要的信息，以评估亚马逊对人类压力的适应能力，以及土地利用和气候变化的潜在后果。

Tropical forests hold more species of plant and animal than any other kind of terrestrial environment. They also store vast amounts of greenhouse gases in their trees and soils. The Brazilian Amazon accounts for 40% of all these remaining forests, and is therefore of enormous global importance for protecting Earth's species and climate. Humid tropical forests do not normally burn, yet their future is becoming increasingly uncertain due to the threat of man-made fires. Human activities such as logging and forest fragmentation open up the forest habitat and make it more flammable, while agriculture in areas adjacent to remaining forest has greatly increased the number of fire incidents. Furthermore, there is now convincing evidence that regional climatic changes predicted for the next century could make the Amazon both dryer and hotter, increasing the frequency and intensity of extreme drought events and raising the flammability of enormous areas of forest. Severe Amazonian droughts occurred in 1997 and 2005, and led to widespread forest fires devastating thousands of square kilometers of forest. When a forest burns for the first time flame heights rarely exceed 30-40 cm, and the fire moves through the leaf litter. Although these fires appear relatively innocuous, they are actually highly destructive as most rainforest trees have a low tolerance of heat, and their slow rate of spread means that flames stay in contact with trees for long periods of time. As a result, even low-intensity fires kill between 8 and 40 % of trees larger than 10cm in diameter. They are also the first step in positive feedback cycle, where increased tree mortality produces a more open forest and increases the fuel available on the forest floor, making the forest more flammable, and increasing the severity of any subsequent fire. Forests that have burned more than once have lost around 80% of their trees >10cm in diameter. We are only beginning to understand the implications of wildfires for tropical forest vegetation, and many important questions remain. For example, we cannot yet explain why rates of tree mortality vary from 8 to 40% depending on the region of forest, and we have almost no information on the longer-term effects of fire on tropical forest trees. We need more information on how long forests take to recover (if indeed they can fully recover), what happens to the makeup of tree species following recurrent fires, and how these responses may vary across the Amazon basin. This proposal will address these areas of uncertainty by examining what happens to forests 11-12 years after they last burned, in eight different regions of the Brazilian Amazon. These include four regions in core, mature Amazonian forests which have experienced a minimal influence from fires over the last millennia, and four regions of transitional forests which are close to the forest-savannah boundaries and much smaller in stature (holding only about half of the total wood volume). All these areas contain extensive areas of forest that have burned once, forest that have burned more than once in the past two decades, and unburned forest that has no recent history of fire. I have avoided selecting forests that have been extensively logged. By sampling tress and saplings in the three burn treatments (burned once, burned twice, unburned) in each of these areas, I will be able to test predictions about tree mortality and regeneration after fire in 'transitional' and 'core' Amazonian forests. This information will improve our understanding of the response of tropical forests to fires over a large area and over long time periods. Furthermore, it will provide information essential for assessing the resilience of the Amazon to human pressure, and the potential consequences of land-use and climate change.