Ground- and satellite-based evidence of the biophysical mechanisms behind the greening Sahel
Ground- and satellite-based evidence of the biophysical mechanisms behind the greening Sahel
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DOI：
10.1111/gcb.12807
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发表时间：
2015-04-01
影响因子：
11.6
通讯作者：
Fensholt, Rasmus
中科院分区：
文献类型：
--
作者：
Brandt, Martin;Mbow, Cheikh;Fensholt, Rasmus
在1970年代和1980年代经历了长期干旱的干旱期之后，最近的科学成果表明，萨赫勒地区几十年来异常干旱的状况已经因降雨量的正异常而逆转。各种遥感研究观察到，过去几十年来，植被绿化出现了积极趋势，这被称为萨赫勒地区的重新绿化。然而，在包括长期地面数据收集以评价和更好地了解这些发现背后的生物物理机制方面，几乎没有进行投资。因此，对生物量可能增加的扣减仍然是推测性的。我们的目标是弥合这些差距，并具体说明萨赫勒地区重新绿化的生物物理背景因素。因此，对基于卫星的植被和降雨数据的长时间序列（1987-2013年）以及对塞内加尔萨赫勒地区不同生态系统木本物种的叶生物量、草本植物生物量和木本物种丰度的地面观测进行了趋势分析。我们发现，卫星植被时间序列中观察到的正趋势（+36%）是由原位测量生物量的增加（+34%）引起的，这在很大程度上受降水量的控制（+40%）。草本植物生物量的年际波动很大，而不是一个明确的趋势，木本植物的叶生物量在27年内翻了一番（+103%）。这种木本生物量的增加并没有反映出生物多样性，在此期间，16种木本物种中有11种的丰度下降。我们的结论是，在塞内加尔萨赫勒地区观察到的绿化主要与树木覆盖率增加，导致卫星驱动的植被指数增加与降雨逆转。
After a dry period with prolonged droughts in the 1970s and 1980s, recent scientific outcome suggests that the decades of abnormally dry conditions in the Sahel have been reversed by positive anomalies in rainfall. Various remote sensing studies observed a positive trend in vegetation greenness over the last decades which is known as the re-greening of the Sahel. However, little investment has been made in including long-term ground-based data collections to evaluate and better understand the biophysical mechanisms behind these findings. Thus, deductions on a possible increment in biomass remain speculative. Our aim is to bridge these gaps and give specifics on the biophysical background factors of the re-greening Sahel. Therefore, a trend analysis was applied on long time series (1987-2013) of satellite-based vegetation and rainfall data, as well as on ground-observations of leaf biomass of woody species, herb biomass, and woody species abundance in different ecosystems located in the Sahel zone of Senegal. We found that the positive trend observed in satellite vegetation time series (+36%) is caused by an increment of in situ measured biomass (+34%), which is highly controlled by precipitation (+40%). Whereas herb biomass shows large inter-annual fluctuations rather than a clear trend, leaf biomass of woody species has doubled within 27years (+103%). This increase in woody biomass did not reflect on biodiversity with 11 of 16 woody species declining in abundance over the period. We conclude that the observed greening in the Senegalese Sahel is primarily related to an increasing tree cover that caused satellite-driven vegetation indices to increase with rainfall reversal.