Resolving the current and future carbon dynamics of the dry tropics

干旱热带的生态系统正在变化：稀树草原、林地和干燥森林覆盖的面积加起来比热带雨林还大，它们既是森林砍伐和森林退化造成的碳排放来源，也是树木生长加速所造成的汇。然而，就其规模和原因而言，人们对这两个过程都知之甚少，净碳平衡及其未来仍不明朗。之所以出现这种认识差距，是因为我们没有一个系统的观测干旱热带植被变化的网络，因此，到目前为止，还不能利用对情况如何变化的观测来了解所涉及的过程并检验关键理论。卫星遥感与地面测量相结合，是克服这些挑战的理想方法，因为它可以以相对较低的成本提供定期、一致的监测。然而，干旱热带的大多数生态系统，特别是热带稀树草原，都是草和树木的混合体，许多光学遥感方法(类似于数码相机上的增强版传感器)难以区分两者之间的变化。长波雷达遥感避免了这个问题，因为它对树叶或草的存在不敏感，也不受云、烟或太阳角度的影响，所有这些都使光学遥感复杂化。因此，雷达遥感是监测干燥热带地区树木生物量的理想方法。我们已经成功地证明，这些数据可以用来准确绘制南部非洲全部500万平方公里的木本植物生物量变化图。在SECO中，我们将创建一个由600多个田间地块组成的网络，以了解干旱热带的植被是如何变化的。并与雷达遥感相辅相成，量化过去15年来干燥热带地区的碳循环变化。这将提供对所有干燥热带地区关键碳通量的第一次估计，包括森林退化和森林砍伐释放的碳量，以及该地区完整的植被正在吸收多少碳。通过了解这些过程在哪里发生，我们将提高对所涉及过程的了解。W将使用这些新数据来改进我们模拟干燥热带碳循环的方式，并测试关键理论。这一改进的理解被正式化为模型，将用于研究干旱热带地区将如何应对气候变化、土地利用变化和大气二氧化碳增加的影响。然后，我们将能够了解干燥热带的植被是否会减缓或加剧气候变化，我们将了解我们需要做些什么来维持干燥热带的结构和保护其生物多样性。总体而言，SECO将使我们能够了解干旱热带的植被正在如何变化，以及这对全球碳循环、稀树草原和干旱森林的生态以及减少气候变化的努力的影响。我们创建的数据和我们进行的分析将对其他研究人员开发从卫星上监测植被的方法很有用，也对那些对不同生态系统对气候和其他变化的反应进行建模的人有用。森林管理者、生态学家和发展实践者可以利用这些数据来了解世界上哪些地区的稀树草原和干旱森林变化最大，以及如何管理这些变化以避免威胁生物多样性和该地区10亿农村人口生计的负面影响，这些农村人口大多是贫穷的。

The ecosystems of the dry tropics are in flux: the savannas, woodlands and dry forests that together cover a greater area of the globe than rainforests are both a source of carbon emissions due to deforestation and forest degradation, and also a sink due to the enhanced growth of trees. However, both of these processes are poorly understood, in terms of their magnitude and causes, and the net carbon balance and its future remain unclear. This gap in knowledge arises because we do not have a systematic network of observations of vegetation change in the dry tropics, and thus have not, until now, been able to use observations of how things are changing to understand the processes involved and to test key theories.Satellite remote sensing, combined with ground measurements, offers the ideal way to overcome these challenges, as it can provide regular, consistent monitoring at relatively low cost. However, most ecosystems in the dry tropics, especially savannas, comprise a mixture of grass and trees, and many optical remote sensing approaches (akin to enhanced versions of the sensors on digital cameras) struggle to distinguish changes between the two. Long wavelength radar remote sensing avoids this problem as it is insensitive to the presence of leaves or grass, and also is not affected by clouds, smoke or the angle of the sun, all of which complicate optical remote sensing. Radar remote sensing is therefore ideal to monitor tree biomass in the dry tropics. We have successfully demonstrated that such data can be used to accurately map woody biomass change for all 5 million sq km of southern Africa.In SECO we will create a network of over 600 field plots to understand how the vegetation of the dry tropics is changing. and complement this with radar remote sensing to quantify how the carbon cycle of the dry tropics has changed over the last 15 years. This will provide the first estimates of key carbon fluxes across all of the dry tropics, including the amount of carbon being released by forest degradation and deforestation and how much carbon is being taken up by the intact vegetation in the region. By understanding where these processes are happening, we will improve our knowledge of the processes involved. W will use these new data to improve the way we model the carbon cycle of the dry tropics, and test key theories. The improved understanding, formalised into a model, will be used to examine how the dry tropics will respond to climate change, land use change and the effects of increasing atmospheric CO2. We will then be able to understand whether the vegetation of the dry tropics will mitigate or exacerbate climate change, and we will learn what we need to do to maintain the structure of the dry tropics and preserve its biodiversity. Overall, SECO will allow us to understand how the vegetation of the dry tropics is changing, and the implications of this for the global carbon cycle, the ecology of savannas and dry forests, and efforts to reduce climate change. The data we create, and the analyses we conduct will be useful to other researchers developing methods to monitor vegetation from satellites, and also to those who model the response of different ecosystems to climate and other changes. Forest managers, ecologists and development practitioners can use the data to understand which parts of the world's savannas and dry forests are changing most, and how these changes might be managed to avoid negative impacts that threaten biodiversity and the livelihoods of the 1 billion, mostly poor, rural people who live in this region.