绿色长城:绿化萨赫勒的希望?

grande muraille verte afrique - great green wall africa

  “非洲绿色长城”(GGW)是一项旨在防治撒哈拉沙漠和萨赫勒地区荒漠化的泛非倡议。GGW的最初构想是一个从西到东横穿非洲大陆的大规模植树造林行动,现已发展成为一系列改善环境和人类福祉的修复项目。鉴于既定的环境和社会目标,所有学科的研究人员都需要发挥重要作用,汇集专业知识,指导管理项目机构的决策。虽然GGW不再是政治行动者最初设想的“树墙”,但植物和植被重建仍然是该项目的核心,因为萨赫勒人地区民比其他任何地方都更加依赖植物资源来满足日常需要。在经历社会和生态转型的萨赫勒地区,真正的挑战是在保护植物资源和可持续利用植物资源之间找到平衡。

1. 绿色长城:非洲应对全球社会环境挑战的对策

  萨赫勒地区被认为是地球上最脆弱的地区之一。这是由草原、稀树草原和乔木组成的生态系统,从西到东横穿非洲大陆,标志着从非洲北部撒哈拉沙漠到非洲南部苏丹地区之间的生态地理过渡。由于该地区的人为压力和水文气候不稳定,这些主要服务于畜牧业的生态系统非常脆弱。

环境百科全书-绿色长城-非绿色长城的原始路线
图1. 位于萨赫勒地区中心的泛非绿色长城的原始路线,跨越参与该项目的11个国家。[来源:©SotaLCB2416(抄送BY-SA 4.0),来自维基媒体共享资源]
(图1 Zone sahélienne 萨赫勒地区;Tracé indicatif du projet de reforestation 植树造林项目;Pays concerné par le projet 项目所涉及的国家;La grande muraille verte : 绿色长城;7 600 km de long 15 km de large:长7600公里,宽15公里;Gambie 冈比亚;Sénégal 塞内加尔;Mauritanie 毛里塔尼亚;Mali 马里;Niger 尼日尔;Tchad 乍得;Soudan 苏丹;Burkina Faso 布基纳法索;Nigeria 尼日利亚;Soudan du Sud 南苏丹;Ethiopie 埃塞俄比亚;Erythrée 厄立特里亚;Djibouti 吉布提;SOURCE : AGENCE PANAFRICAINE DE LA GRANDE MURAILLE VERTE 资料来源:绿色长城泛非机构)

  GGW在这种高度受限的环境下开展起来,是防治荒漠化的关键因素(图1)。它覆盖了年降雨量100至400毫米的地带。在这个被确定为世界气候变化“热点”之一的萨赫勒地区[1],预计到2100年气温将上升3-6℃[2],预期后果尤其令人担忧。年降雨量低是植树造林恢复生物多样性的主要障碍。该地区降雨时间短,主要以偶发强降雨形式出现(集中于7月到9月)。降雨还具有年际变化大的特点,这是导致干旱的原因:1970年至1993年期间,该地区经历了几次危机,造成严重后果[3]。例如,在塞内加尔,1940-1969年和1970-1994年相比,年降水量为400-500毫米的地区向南移动约200公里,这导致“绿色长城”纬度的降水量急剧减少(图2A)。此外,如萨赫勒地区的降雨指数所示(图2B),三十年的湿润期之后出现了二十三年的异常干旱期(1971-1993年)。自1994年以来,萨赫勒西部的降雨情况再次好转,但与20世纪上半叶的情况不同。气候变化可能导致更极端的气候、更严重的干旱和更强烈但频率更低的降水[4]

  降雨持续不足,加上暴雨增加,通常意味着更极端的气候,其特点是干旱期更严重,降雨时期降雨量更大。

环境百科全书-绿色长城-降雨演变
图2. 20世纪初以来塞内加尔北部降雨的演变:A.1940-1969年和1970-1994年间400-500毫米等雨量线的纬度变化;B.1900年至2016年萨赫勒地区降雨指数的演变。[来源:修改自Peiry和Voldoire,参考文献[3]](图2 Tracé de la GMV GMV路线;lsoyetes 400-500mm 400-500mm等雨量线;Saint-Louis 圣路易斯;DAKAR 达喀尔;Podor 波多尔;MAURITANIE 毛里塔尼亚;Linguère 林盖尔;Matam 马塔姆;Bakel 巴克尔;MALI 马里;HUMIDITE 湿度;SECHERESSE 干燥度)

  自1970年代以来,国际发展机构定期在萨赫勒发生危机时进行干预。他们采取的行动往往以失败告终,因为此类工作趋向于应对短期和紧急情况下的挑战,解决表面问题而不是根本原因。为应对这些一再发生的社会生态危机,2007年,一项雄心勃勃的泛非方案启动——“撒哈拉和萨赫勒绿色长城倡议”(GGW),即建立一条横跨非洲大陆长7000多公里、宽15公里的植树造林林带。该倡议得到了11个创始国的国家元首采纳(图2)。这一倡议的最初目标是对抗荒漠化的有害影响,阻止沙漠扩张。由于以下原因,萨赫勒地区的一些专家和科学界对GGW表示质疑:

  • 对荒漠化的原因了解不足;
  • 大规模的造林行动被认为不可行,且具有不良影响,等等……

  自启动以来,GGW已朝着更现实的愿景发展,以更合理的方式应对区域挑战。它现在采取综合景观管理的方法开展一系列恢复行动,这些行动在地理上相互关联,目的是改善萨赫勒人口的环境和社会经济福祉

  自启动以来,GGW已高度多样化,包括建立围栏地块供树木自然再生以及防止过度放牧,建立由妇女管理的社区花园,设立动物保护区项目,发展养蜂业……然而,“合理的重新造林”和植物资源管理仍然是萨赫勒地区绿化和保护环境的重要手段之一。大规模植树造林是当前应对全球性问题中的一部分。例如,“波恩挑战”的目标是到2030年恢复35亿公顷被砍伐的森林土地。作为全球森林管理的一项行动,重新造林也符合联合国成员国制定的可持续发展目标其第15项目标包括“通过确保陆地生态系统的可持续利用、可持续管理森林、防治荒漠化、制止和扭转土地退化进程以及制止生物多样性的丧失,保护和恢复陆地生态系统”[5]

  为了实现其目标,GGW必须广泛利用各科学领域知识,将其融入决策过程。例如,萨赫勒地貌的明显同质性可能导致干预措施简单化和定型化(解决方案数量减少)。然而,萨赫勒地区由多种地貌组成,其中的社会生态系统在空间上相互交织。为了管理自然资源,不仅要考虑生态系统的生物物理成分,还要考虑人类的组成部分,以及两者之间存在的复杂的相互作用。正是这些相互作用决定了社会生态系统的特性。了解社会生态系统及其功能需要跨学科研究,研究人员需要将研究成果传递给管理人员,为GGW发挥更全面成熟的建设作用[6]。虽然GGW的行动随着时间的推移已经变得多样化,植被研究仍然是解决方案的核心,高质量再造林的标准之一是其生物多样性程度。

2. 使GGW沿线的生物多样性最大化

  生物多样性丧失是一个重大的全球问题[7]。由于其空间目标,GGW在扭转整个非洲大陆生物多样性流失方面可以发挥作用。在地方层面,萨赫勒人高度依赖木本植物的生物多样性为其提供生态系统服务,满足食物、健康、薪材、建筑木材等方面日常需求)。然而,在萨赫勒的大部分地区,木本物种的生物多样性正在下降:从数量上讲,在六钻林牧保护区(塞内加尔北部)建立了一个CNRS人类环境观测站,以监测GGW的社会环境影响,其中只记录了属于13个科的20个物种[8]

环境百科全书-绿色长城-造林
图3. GGW选择的树木生长在靠近移植地的苗圃中,这些树木都是苏丹-萨赫勒的物种,丰富程度不一。[来源:照片©J-L Peiry,2014]

  GGW提出一个由100%的本土物种组成的重新造林(图3)计划。物种的选择除了上述第一个标准外,还要考虑当地人对这些物种的传统利以及物种的生态适应性。为此,研究人员对当地人口进行了民族植物学研究,确定了树种(图4)。除了建立树种清单外,这项研究还可以确定树木的用途以及人们与树种的日常关系。在两个试验研究区内,对最具价值树种的再造林可行性进行了原位研究。来自GGW苗圃的13种萨赫勒物种植物被栽种,幼树在之后若干年得到监测(包括其存活率、生长状况、生物量生产)。这种方法将民族植物学和实验相结合,在此基础上向负责GGW的部门提出建议[9]

  • 重新造林增加的生物多样性不会超过木本植物自然再生(即在一定时间内将动物和人类排除在外,让自然得到休息)达到的生物多样性;
  • 尽管GGW沿线的生态地理条件具有显著相似性,但支持物种生长的条件并不一致。
环境百科全书-绿色长城-研讨会
图4. 研究人员发起研讨会,与当地人共同讨论并确定与植物资源相关的生态系统服务、植物资源的不同用途以及获得植物资源的首选区域。[来源:照片©H.Mazzore,2017]

  虽然在条件最恶劣的地点很少有物种能顽强生存,但在其他地点,可能成功的木本物种数量要多得多,这将增加生物量和生物多样性。这项研究还提供了提高物种存活率的技术信息(植物移栽时的年龄、种子来源、移栽条件,例如通过覆盖和添加有机物保护幼苗)。这些实验地点构成了露天实验室,可以在其中监测生态系统的其他组成部分(植物、动物、昆虫等)和气候参数,以衡量重新造林对“改良生态系统”的总体影响。

3. 更好地评估最具适应能力的木本物种

环境百科全书-绿色长城-塞内加尔金合欢树
图5. 维杜廷戈利(Widou Thiengoly)附近GGW项目中种植的塞内加尔金合欢树(Acacia senegal)(-15.262953°;15.979199°)。在2008年启动重新造林,八年后这些树木正在成熟,即将迎来首次阿拉伯树胶的开采和收获。[来源:照片©J-L Peiry,2016

  在GGW沿线上增加物种多样性很重要,但更好地利用该地区自然繁衍物种也很重要。在列出的20种常见木本植物中,有4种属于金合欢属(Acacia)植物。金合欢属(Acacia)植物的环境附加值是能够通过根系中的特殊共生根瘤固定大气中的氮。这种固氮作用有助于恢复萨赫勒极贫瘠土壤的肥力(参见《靠空气生存的植物》)。金合欢(Acacia)的种植常见于农林复合经营,这是一种将耕种植物与树木植被相结合的互利农业经营方式。其中,塞内加尔金合欢树(Acacia senegal)因其阿拉伯树胶生产的丰富性和质量,有着很好的经济前景(图5)。

环境百科全书-绿色长城-埃及槲果群
图6. 埃及槲果(Balanites aegyptiaca)群。该物种能够很好地适应萨赫勒地区的恶劣条件,因此在萨赫勒森林景观中占据非常重要的地位。[来源:照片©J-L Peiry,2014]

  在被考虑的物种中,有一个树种几乎在GGW所有沿线国家被一致用作再造林物种——埃及槲果(Balanites aegyptiaca),即“沙漠椰枣”。该物种在干旱地区具有无与伦比的生命力,因此在塞内加尔萨赫勒地区的景观中占主导地位,在某些地区占木本种群的50%[10]。该树种旺盛的生命力归功于生殖和营养繁殖能力,即使在最恶劣的条件下也能维系。从形态学上看,“沙漠椰枣”的叶子表面积很小,表皮硬,有一层厚厚的蜡状表皮层,叶绿素的部位呈刺状,所有这些都是为了避免水分流失(图6)。

  “沙漠椰枣”的根系同样壮观。将幼苗移栽后即使马上遇到旱季,其发育也非常迅速,而地上部分发育要慢得多。“沙漠椰枣”的根系形态具有双重系统,结构效率高:侧根生长在土壤表面附近,即使是稀薄的降水也能吸收;而主根生长得很深,可以汲取深层水资源[11]

  埃及槲果不仅健壮,而且根据民族植物学研究,它也被当地人评为最有用的木本物种。当地人主要将其用作食品、药品、薪材和建筑[12]。该植物对人类的重要性并不是近期才发现:其用途可以追溯到大约4000年前的埃及第十二王朝时期 [13]。今天,如果此树的所有部分都得到充分认识和利用,那么最重要的是其果实中蕴含的社会经济潜力。与其他热带水果相比,其果肉具有很高的营养价值:富含钾、钙、镁、铁和锌[14];果仁含有高达50%的优质油脂,富含不饱和脂肪酸(亚油酸和油酸)。提取的油可食用,也可以用于某些化妆品,但没有摩洛哥坚果油和乳木果油的利用普遍[15]。埃及槲果的销售受到当地非正式贸易的制约,该物种远未发挥其经济潜力。

  虽然研究让人们更好地了解了决定该物种经济价值的环境和遗传因素(果实大小和营养价值、果仁的含油量),但要使埃及槲果(Balanites)为萨赫勒地区人口带来显著经济收入还需要更大的努力。

4. 科学为绿色长城服务

环境百科全书-绿色长城-GGW在2008年种植的地块的评估
图7. 使用GEOSUD Teamx提供的SPOT 6-7图像对GGW项目在2008年某地块(675公顷,15.968327°N;-15.246496°E)的种植结果进行评估。使用了两种对比情况:1)雨季拍摄的红外/绿色/蓝色(A)图像显示了与草甸发育相关的光合活性,通过适当的统计处理可确定土壤湿度的空间对比(B);2) 旱季拍摄的全色图像(C)可在幼树树冠表面积超过 4m2时看到幼树,可通过卫星测量重新造林作业的成功率(D)。[来源:©J-L Peiry,2018]

  萨赫勒地区几乎不可能变成伊甸园,但是该地区确实在渐渐恢复绿色[16]。科学家用可靠的科学数据进行评估,就可采取的行动进行充分讨论,支持了GGW行动,推动了该地区重现绿色[17]

  在物种层面上,我们能够在古老且经过验证的传统实践与现代科学(选择更能适应干旱的新品种,预接种植物菌根真菌[19],利用可以改善植物矿物质营养和水分的有益土壤微生物)之间找到正确的平衡(zai、demi-lune[18])。在景观层面上,可以利用超高分辨率的卫星数据:

  • 使用空间分析技术定量评估GGW再造林(成功率取决于土壤蓄水能力,GGW的碳固存附加值),
  • 前瞻性地利用这些结果来确定和选择新的区域,在这些区域中,未来的重新造林行动有极高的成功几率(图7)。
环境百科全书-绿色长城-研究人员研讨会
图8. 研究人员正在GGW沿线国家的一个村庄Ranérou与当地利益相关者一起举办研讨会。在反思过程中,就该地区未来的愿景以及相关制约因素达成共识,并进行了相关讨论。这些愿景将作为讨论未来GGW行动的起点。[来源:照片©D.Goffner,2017]

  最后,为了减少萨赫勒地区人口易受反复发生的危机影响,科学家们正在研究社会生态恢复力,并试图找出增强恢复力的方法。通过集体工作,研究人员和利益相关者(治理参与者、环境管理者、当地居民和研究人员)分享了他们的知识和观点。他们一起研究自然资源管理、社会动态、实践和当地生产系统。该方法的最终目标是共同确定一套公平和可持续发展的目标,将改善人民生活条件与保护环境结合起来。他们的工作考虑到社会生态系统的复杂性、当地居民的愿望、社会和环境制约因素,并提出了最适当的自然资源管理办法。这些集体活动产生的具体行动需要赋予社会生态系统更大的恢复力,即提高其抵御未来灾害的能力,积极(甚至主动)适应负面影响,从而实现可持续繁荣发展。目前,新一代“恢复力评估”协议正在落实。这些协议共享相同的原则:一方面是跨学科原则,因为必须考虑所有社会和环境动态;另一方面是相关行动者的积极参与,因为他们的知识和期望是推动未来相关发展的必要条件(图8)[20]

5. 总结

  • 萨赫勒地区受到严重的水文气候和人为因素的制约,生态环境非常脆弱(土壤侵蚀、荒漠化、生物多样性丧失、营养不良)。
  • 由于前所未有的人口增长和持续气候变化的消极影响,其脆弱性在未来必然会增加。
  • 只要采取适当行动,其恢复能力很强。最先进的方法和现代研究工具的使用证明了这一潜力,特别是在增加生物量和生物多样性方面。
  • 绿色长城及其各类行动是改善萨赫勒地区状况的有效方法,但前提条件是将所有利益相关者(包括研究人员、管理人员和当地受益人群)密切联系起来。

  新的开发协议目前正在进行测试,利用了跨学科的集体合作方针,其中包括研究人员、受影响人群和GGW从业者。这一工作旨在确定新的行动方案,提高人们适应变化的能力和创造新资源的能力,与此同时以平衡和可持续的方式管理环境。

 


参考资料及说明

封面图片:自2008年塞内加尔相思树首次移植9年后,塞内加尔“绿色长城”第一块再造林地鸟瞰图。[来源:照片©J-L Peiry,2017]

[1] DIFFENBAUGH NS, GIORGI F (2012). Climate change hotspots in the CMIP5 global climate model together. Climatic Change. 114(3-4):813-822. doi:10.1007/s10584-012-012-0570-x.

[2] NIANG I, RUPPEL OC, ABDRABO MA, et al (2014). Africa, in: BARROS VR, FIELD BC, DOKKEN DJ et al (Eds.), Climate Change 2014: Impacts, Adaption, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Reprot of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom, pp. 1199-1265.

[3] PEIRY J-L and VOLDOIRE O. (2019) Climate framework and water resources in the Senegalese Great Green Wall area In: BOETSCH G, DUBOZ P., GUISSE A., SARR P. and. (Ed.). The Great Green Wall, an African response to climate change. CNRS Editions, Paris.

[4] LEBEL T, PANTHOU, G. & VISCHEL T. (2018)在萨赫勒地区,大旱灾后没有恢复正常。.See on The Conversation, URL: https://theconversation.com/au-sahel-pas-de-retour-a-la-normale-apres-la-grande-secheresse-106548 (in french)

[5] UN General Assembly, Transforming our world: the 2030 Agenda for Sustainable Development, 21 October 2015, A/RES/70/1, available at: http://www.refworld.org/docid/57b6e3e44.html[accessed 21 August 2018]

[6] BOETSCH G, BOCCANFUSO P (2016)科学与绿色长城(电影)

[7] ROCKSTRÖM J, STEFFEN W, NOONE K et al (2009) A safe operation space for humanity. Nature 461: 472-475.

[8] NIANG K, SAGNA MB, NDIAYE et al (2014) Revisiting tree species availability and usage in the Ferlo region of Senegal: a rationale for indigenous tree planting strategies in the context of the Great Green Wall of the Sahara and Sahel Initiative. Journal of Experimental Biology and Agricultural Sciences 2:529-537.

[9] WADE TI, NDIAYE O, MAUCLAIRE M et al (2018) Biodiversity field trials to inform reforestation and natural resource management strategies along the African Great Green Wall in Senegal. New Forests 49: 341-362. http://dx.doi.org/10.1007/s11056-017-9623-3

[10] BOËTSCH G and SPÄNI A. (2013) The Great Green Wall: trees against the desert. Editions Privat, France

[11] BREMAN H, KESSLER JJ (1995) Woody plants in agro-ecosystems of semi-arid regions with an emphasis on Sahelian countries. Springer-Verlag, Berlin, Germany

[12] SAGNA MB, NIANG K, GUISSE A, GOFFNER D (2014) Balanites aegyptiaca (L.) Delile: geographical distribution and ethnobotanical knowledge by local populations in the Ferlo (north Senegal). Biotechnol. Agron. Soc. Approximately. 18: 503-511

[13] NATIONAL RESEARCH COUNCIL (2008) Lost Crops of Africa: Volume III: Fruits. The National Academies Press, Washington DC.

[14] SAGNA MB, NDIAYE O, DIALLO A, GOFFNER D, GUISSE A (2014) Biochemical composition and nutritional value of Balanites aegyptiaca fruit pulp from the Ferlo region in northern Senegal. Afr J Biotechnol 13: 336-342.

[15] https://www.klorane.com/ch-fr/cheveux/dattier-du-desert

[16] DARDEL C. KERGOAT L, HIERNAUX P et al (2014) Re-greening Sahel: 30 years of remote sensing data and field observations (Mali, Niger) 140: 350-364. https://doi.org/10.1016/j.rse.2013.09.011

[17] http://future-sahel.blogspot.com/

[18] https://www.ilesdepaix.org/wp-content/uploads/2014/09/Le-zaï-et-la-demi-lune.pdf

[19] http://books.openedition.org/irdeditions/3304

[20] https://rethink.earth/wayfinder/


环境百科全书由环境和能源百科全书协会出版 (www.a3e.fr),该协会与格勒诺布尔阿尔卑斯大学和格勒诺布尔INP有合同关系,并由法国科学院赞助。

引用这篇文章: GOFFNER Deborah, PEIRY Jean-Luc (2024年3月10日), 绿色长城:绿化萨赫勒的希望?, 环境百科全书,咨询于 2024年11月21日 [在线ISSN 2555-0950]网址: https://www.encyclopedie-environnement.org/zh/vivant-zh/green-wall-hope-greening-sahel/.

环境百科全书中的文章是根据知识共享BY-NC-SA许可条款提供的,该许可授权复制的条件是:引用来源,不作商业使用,共享相同的初始条件,并且在每次重复使用或分发时复制知识共享BY-NC-SA许可声明。

The Great Green Wall: a hope for greening the Sahel?

grande muraille verte afrique - great green wall africa

The Great Green Wall of Africa (GGW) is a pan-African initiative to combat desertification in the Sahara Desert and the Sahel. Originally conceived as a massive tree plantation crossing the African continent from west to east, the GGW has evolved into a set of restoration projects aimed at environmental and human well-being. Given the stated environmental and social objectives, researchers from all disciplines have an important role to play in pooling their expertise to guide the decisions of the institutions managing the project. While the GGW is no longer a simple wall of trees as originally envisaged by political actors, plants and re-vegetation remain at the heart of the project because, more than anywhere else, the people of the Sahel depend heavily on plant resources to meet their daily needs. The challenge is to find a balance between the protection and sustainable use of plant resources in a Sahel undergoing social and ecological transformation.

1. The Great Green Wall: an African response to global socio-environmental challenges

The Sahel is considered one of the most vulnerable areas on Earth. It is a set of ecosystems made up of steppes and shrubby savannah with trees crossing the African continent from west to east and marking the eco-geographical transition between the Sahara desert in the north and the Sudanese zone in the south. These ecosystems, which are largely dedicated to pastoralism, are highly vulnerable due to anthropogenic pressure and hydro-climatic instability in the area.

pan-africa great green wall sahel
Figure 1. Original route of the Pan-African Great Green Wall in the heart of the Sahel region and crossing eleven states involved in the project. [Source: © SotaLCB2416 (CC BY-SA 4.0), from Wikimedia Commons]
It is in this highly restrictive context that the Great Green Wall operates, a key element in the fight against desertification (Figure 1). It covers a band where rainfall is 100 to 400 mm/year. In this Sahelian zone identified as one of the world’s “hotspots” of climate change [1], the expected consequences are particularly alarming with predictions of a temperature increase of 3-6°C by 2100 [2]. Low annual rainfall is the main obstacle to the reintroduction of biodiversity through reforestation. Rainfall occurs for a short period of time (mainly from July to September) in the form of intense and sporadic events. Rainfall is also characterized by high inter-annual variability leading to droughts: between 1970 and 1993, the area experienced several crisis situations with dramatic consequences [3]. For example, in Senegal, the area receiving 400-500 mm of precipitation annually moved about 200 km southward when comparing the periods 1940-1969 and 1970-1994, which resulted in a sharp decrease in rainfall at the latitude of the GGW (Figure 2A). In addition, as shown by the rainfall index in the Sahel (Figure 2B), three wet decades followed twenty-three abnormally dry years (1971-1993). Since 1994, the rainfall situation in the Western Sahel has become more favourable again, but not similar to that of the first half of the 20th century. Climate change could lead to a more extreme climate with more severe droughts and more intense but less frequent precipitation [4].

This persistence of a deficit of rainfall events, combined with an increase in heavy rainfall, typically corresponds to a more extreme climate characterized by both more severe dry periods and heavier rainfall when it rains.

grande muraille verte afrique - grande muraille verte panafricaine - evolution pluviometrie nord senegal - evolution of rainfall in northern senegal - great green wall
Figure 2. Evolution of rainfall in northern Senegal since the beginning of the 20th century: A. Change in latitude of isohyets 400-500 mm between the periods 1940-1969 and 1970-1994; B. Evolution of the rainfall index in the Sahel between 1900 and 2016. [Source: Modified from Peiry and Voldoire, ref. [3]]
Since the 1970s, international development institutions have regularly intervened during the recurrent crises in the Sahel. The actions they have taken have often ended in failure because the tendency has been to manage challenges in the short term and in the emergency by addressing symptoms rather than the root causes of problems. To combat these repeated socio-ecological crises, a particularly ambitious pan-African programme was launched in 2007: the “Great Green Wall Initiative for the Sahara and the Sahel“. Adopted by the heads of state of eleven founding countries (Figure 2), the GGW was designed as a reforestation belt more than 7,000 km long and 15 km wide across the African continent. Originally, the objective was to counter the harmful effects of desertification and to block the advance of the desert. The GGW has been received with great scepticism by some Sahel experts and the scientific community for various reasons:

  • causes of desertification poorly understood;
  • massive reforestation actions considered as infeasible and having undesirable effects, etc…

Since its adoption, the GGW has evolved towards a more realistic vision and a more appropriate response to regional challenges. It now takes the form of a series of restoration actions carried out on a landscape scale and which are geographically linked in order to respond to the environmental and socio-economic well-being of Sahelian populations.

Since its inception, GGW has been highly diversified: fencing plots to allow natural regeneration of trees and protection against overgrazing, installation of community gardens managed by women, animal reserve projects, development of beekeeping… However, “reasoned reforestation” and plant resource management remain one of the essential levers for greening and preserving the environment in the Sahel. They are part of a context where large-scale reforestation is clearly a global issue. For example, “The Bonn Challenge” aims to restore 350,000 million hectares of deforested land by 2030. As an action of the GGW, reforestation also meets the sustainable development objectives set by UN Member States. Its objective n°15 consists in particular in “preserving and restoring terrestrial ecosystems, by ensuring their sustainable use, sustainably managing forests, combating desertification, halting and reversing the process of land degradation and halting the loss of biodiversity” [5].

To meet its ambitions, the GGW must integrate knowledge from a wide range of scientific disciplines into the decision-making process. For example, the apparent homogeneity of Sahelian landscapes can lead to simplistic and stereotypical interventions (reduced number of solutions). However, in reality, the Sahelian zone is rather a mosaic of landscapes composed of socio-ecosystems that are intertwined in space. To manage natural resources, we must therefore consider not only the biophysical components of ecosystems but also the human components, as well as the complex interactions that exist between the two. It is all these interactions that determine the identity of the socio-ecosystem. Knowing socio-ecosystems and their functioning requires interdisciplinary study, with researchers then becoming full-fledged actors in the construction of GGW through the transfer of their research results to managers [6]. Even if GGW’s actions have diversified over time, vegetation research is still at the heart of the solutions to be proposed, one of the criteria for quality reforestation being primarily the degree of biodiversity of the plantations.

2. Maximize biodiversity on the GGW route

Biodiversity loss is a major global concern [7]. Because of its spatial ambition, the GGW has a role to play in reversing the trend on the scale of the African continent. At the local level, Sahelian populations are highly dependent on woody biodiversity to provide them with ecosystem services to meet their daily needs for food, health, fuelwood, construction wood, etc.). However, in a large part of the Sahel, the biodiversity of woody species is in decline: to give an order of magnitude, in the Six Drilling Sylvo-Pastoral Reserve (northern Senegal) where a CNRS Human-Media Observatory has been set up to monitor the socio-environmental impacts of GGW, only 20 species belonging to 13 families have been recorded [8].

grande muraille verte afrique arbres - great green wall africa trees
Figure 3. Trees selected by the GGW, grown in nurseries close to the plots where they will be transplanted, are all Sudano-Sahelian species present in variable abundance in the area [Source : Photo © J-L Peiry, 2014].
The GGW proposes a reforestation consisting 100% of indigenous species (Figure 3). In addition to this first criterion for selecting species, there are their traditional use by local populations and the ecological adaptability of the species selected for reforestation. Ethnobotanical research was conducted with populations to identify the trees used (Figure 4). In addition to the inventory of trees used, this research made it possible to identify their uses and the daily relationship that populations have with them. The feasibility of reforestation of the most highly valued species is studied in situ in two experimental research plots. Plants from GGW nurseries of 13 Sahelian species are planted and young trees are monitored (survival rate, growth, biomass production) over several years. This approach, combining ethnobotany and experimentation, has already made it possible to make recommendations to the services in charge of GGW [9]:

  • reforestation does not increase biodiversity beyond what natural regeneration of ligneous trees (by allowing nature to rest by excluding animals and humans for a given time) can achieve;
  • the conditions for the establishment of species are far from uniform despite the apparent similarity of eco-geographical conditions along the route.

grande muraille verte afrique - great green wall africa sahel
Figure 4. Workshop initiated by researchers with local populations to identify ecosystem services related to plant resources, their different uses and the preferred areas where they obtain their supplies [Source: Photo © H. Mazzero, 2017].
While on the most difficult sites very few species resist, on others the woody species likely to succeed are much more numerous, which would increase the biomass produced and biodiversity. This research also provides technical information to improve species survival rates (age of plants at transplanting, seed origin, transplanting conditions, for example by protecting the young plant by mulching and adding organic matter). These experimental sites thus constitute open-air laboratories in which the other components of the ecosystem (plants, animals, insects, etc.) and climate parameters can be monitored to measure the overall impacts on the “modified ecosystem” resulting from reforestation.

3. Better valorize the potential of the most adapted woody species

acacia senegal - grande muraille verte afrique - GMV plantation acacia great green wall africa
Figure 5. GGW plantation of Acacia senegal near Widou Thiengoly (-15.262953°; 15.979199°). Eight years after the reforestation operations that took place in 2008, the trees are maturing for the first bleeds and harvests of gum arabic [Source: Photo © J-L Peiry, 2016].
While it is important to diversify the species along the GVM route, it is also essential to better valorise the species that thrive naturally in the area. Of the twenty commonly listed woody species, four are of the genus Acacia. The environmental added value of acacias results in their ability to fix atmospheric nitrogen by specialized and symbiotic nodules in the root system. This nitrogen fixation contributes to restoring the fertility of the very poor soils of the Sahel (see Plants that live on air). Acacias are often used in agroforestry practices, an agricultural practice that combines cultivated plants with tree vegetation for mutual benefit. Among them, Acacia senegal, has strong economic hope for the abundance and quality of its gum arabic production (Figure 5).

Balanites aegyptiaca - grande muraille verte afrique - balanites aegyptiaca great green wall africa
Figure 6. Grouping of Balanites aegyptiaca. The species is so well adapted to the harsh Sahelian conditions that it often dominates the Sahelian wooded landscape very strongly [Source: Photo © J-L Peiry, 2014].
Among the species counted, another tree is almost unanimously used as a reforestation species in all the countries crossed by the GGW: it is the Balanites aegyptiaca, the “desert date”. The species is of unparalleled robustness in arid areas, and as a result dominates the landscapes of the Senegalese Sahel, representing up to 50% of the woody population in some areas [10]. It owes its success to its capacity for reproductive and vegetative regeneration, even under the most hostile conditions. Morphologically, the desert date palm has very small, tough leaves with a thick, waxy epidermal layer and chlorophyllous assimilating shoots reduced to thorns, all to avoid excessive perspiration (Figure 6).

The characteristics of the root system are equally spectacular. It develops very quickly, even during the first dry season after transplanting the young plant into the ground, while at the same time its aerial parts develop much more slowly. Root morphology has a dual system that is very structurally efficient: lateral roots grow near the soil surface, catching even the slightest surface rain; while a taproot grows deep, allowing access to deep water reserves [11].

Balanites aegyptiaca is not only robust, but according to ethnobotanical studies, it is also the woody species considered most useful by local populations. They use it mainly for food, medicine, firewood and construction [12]. Its importance for mankind is not recent: its uses are mentioned since the time of the 12th dynasty of Egypt, about 4000 years ago [13]. Today, if all parts of the tree are appreciated, it is above all the fruit that brings socio-economic hope. The pulp is of great nutritional value compared to other tropical fruits: it is rich in K, Ca, Mg, Fe, and Zn [14]. The almond contains up to 50% good quality oil, rich in unsaturated fatty acids (linoleic and oleic acids). If the oil is edible, it is also found in some cosmetic products, but much more discreetly than argan and shea butter [15]. Today, the sale of Balanites aegyptiaca fruits is subject to informal local trade, but the species is far from reaching its economic potential.

While research has led to a better understanding of the environmental and genetic factors dictating the economic interest of the species (fruit size and nutritional value, oil content of the almond), significant efforts still need to be made to enable Balanites to contribute significantly to the incomes of Sahelian populations.

4. Science at the service of the Great Green Wall

grande muraille verte afrique resultats arbres - GMV plantation great green wall africa
Figure 7. Evaluation of the results on a plot planted by the GGW in 2008 (675 ha, 15.968327° N ;-15.246496° E) using SPOT 6-7 images provided by the GEOSUD Teamx. Two contrasting situations were used: 1) an Infrared/Green/Blue (A) image taken in the wet season shows the photosynthetic activity related to the development of the herbaceous mat and, through appropriate statistical treatment, identifies spatial contrasts in soil moisture (B); 2) the panchromatic image taken in the dry season (C) allows young trees to be seen as soon as the surface area of their crown exceeds 4m2, thus making it possible to measure in space the success of reforestation operations (D) [Source : © J-L Peiry, 2018].
There is little chance that the Sahel will turn into a Garden of Eden, but regreening is progressing slowly in the area [16]. The support of scientists for GGW actions contributes to strengthening the phenomenon, by allowing their evaluation with solid scientific data and by contributing to the debate on the actions to be undertaken [17].

At the species level, we are able to identify the right balance between the practice of old and proven techniques (zai, half-moon [18]), and the contributions of modern science (new varieties better adapted to drought; pre-inoculation of plants with mycorrhizal fungi [19], beneficial soil microorganisms that improve mineral nutrition and water status of plants). At the landscape level, very high resolution satellite data now allow:

  • quantitatively evaluate GGW reforestation using spatial analysis techniques (success rate as a function of soil water retention capacity, added value of GGW for carbon sequestration),
  • use these results prospectively to identify and select new areas where future reforestation actions can be carried out with a good probability of success (Figure 7).

grande muraille verte afrique atelier populations locales - collective reflection great green wall
Figure 8. The researchers are conducting a workshop with a group of local stakeholders in Ranérou, a village on the GGW route. During a collective reflection, a consensus on the aspirations for the future of the area, as well as the associated constraints, is identified and discussed collectively. These aspirations will serve as a starting point for discussions on future GGW actions [Source: Photo © D. Goffner, 2017]
Finally, in an attempt to reduce the vulnerability of populations to recurrent crises in the Sahel, scientists are studying socio-ecological resilience and attempting to identify ways to improve it. Through collective work, researchers and stakeholders (governance actors, environmental managers, local populations and researchers) are encouraged to share their knowledge and points of view. Together, they examine natural resource management, social dynamics, practices and local production systems. The ultimate goal of the approach is to collectively define a set of objectives for a fair and sustainable development that combines the improvement of people’s living conditions and the preservation of the environment. Their work takes into account the complexity of socio-ecological systems, the aspirations of local populations, social and environmental constraints, and proposes the most appropriate natural resource management. The concrete actions resulting from these collective dynamics must give socio-ecological systems greater resilience, i.e. improve their ability to resist future disruptions, to adapt actively (or even proactively) to them and thus to prosper in a sustainable way. Currently, a new generation of “resilience assessments” protocols is being implemented. These protocols share the same principles: on the one hand, interdisciplinarity because all social and environmental dynamics must be considered; on the other hand, active engagement of the actors concerned because taking into account their knowledge and expectations is necessary for relevant and appropriate development (Figure 8) [20].

5. Messages to remember

  • The Sahel area is subject to severe hydroclimatic and anthropogenic constraints that make it highly vulnerable (soil erosion, desertification, biodiversity loss, malnutrition).
  • Subjected to unprecedented population growth and the negative consequences of ongoing climate change, its vulnerability is bound to increase in the future.
  • At the same time, its capacity for resilience is strong, provided that appropriate actions are taken. State-of-the-art methodologies and modern research tools demonstrate this potential, particularly in terms of increasing biomass and biodiversity.
  • The Great Green Wall and its various actions are an effective lever to improve the situation in the Sahel, provided, however, that all stakeholders, including researchers, managers and local beneficiary populations, are closely involved.

New development protocols are currently being scientifically tested using an interdisciplinary and collective approach involving researchers, affected populations and GGW practitioners. They aim to identify new sets of actions to increase people’s ability to adapt to ongoing changes and to generate new resources, while managing their environment in a balanced and sustainable way.

 


Notes and references

Cover image. Aerial view of the first reforestation plot of the Senegalese Great Green Wall, 9 years after the first transplants of Acacia senegal carried out in 2008 [Source: Photo © J-L Peiry, 2017].

[1] DIFFENBAUGH NS, GIORGI F (2012). Climate change hotspots in the CMIP5 global climate model together. Climatic Change. 114(3-4):813-822. doi:10.1007/s10584-012-012-0570-x.

[2] NIANG I, RUPPEL OC, ABDRABO MA, et al (2014). Africa, in: BARROS VR, FIELD BC, DOKKEN DJ et al (Eds.), Climate Change 2014: Impacts, Adaption, and Vulnerability. Part B: Regional Aspects. Contribution of Working Group II to the Fifth Assessment Reprot of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom, pp. 1199-1265.

[3] PEIRY J-L and VOLDOIRE O. (2019) Climate framework and water resources in the Senegalese Great Green Wall area In: BOETSCH G, DUBOZ P., GUISSE A., SARR P. and. (Ed.). The Great Green Wall, an African response to climate change. CNRS Editions, Paris.

[4] LEBEL T, PANTHOU, G. & VISCHEL T. (2018) Au Sahel, pas de retour à la normale après la « grande sécheresse. See on The Conversation, URL: https://theconversation.com/au-sahel-pas-de-retour-a-la-normale-apres-la-grande-secheresse-106548 (in french)

[5] UN General Assembly, Transforming our world: the 2030 Agenda for Sustainable Development, 21 October 2015, A/RES/70/1, available at:

http://www.refworld.org/docid/57b6e3e44.html[accessed 21 August 2018]

[6] BOETSCH G, BOCCANFUSO P (2016) Science and the Great Green Wall (film)

[7] ROCKSTRÖM J, STEFFEN W, NOONE K et al (2009) A safe operation space for humanity. Nature 461: 472-475.

[8] NIANG K, SAGNA MB, NDIAYE et al (2014) Revisiting tree species availability and usage in the Ferlo region of Senegal: a rationale for indigenous tree planting strategies in the context of the Great Green Wall of the Sahara and Sahel Initiative. Journal of Experimental Biology and Agricultural Sciences 2:529-537.

[9] WADE TI, NDIAYE O, MAUCLAIRE M et al (2018) Biodiversity field trials to inform reforestation and natural resource management strategies along the African Great Green Wall in Senegal. New Forests 49: 341-362. http://dx.doi.org/10.1007/s11056-017-9623-3

[10] BOËTSCH G and SPÄNI A. (2013) The Great Green Wall: trees against the desert. Editions Privat, France

[11] BREMAN H, KESSLER JJ (1995) Woody plants in agro-ecosystems of semi-arid regions with an emphasis on Sahelian countries. Springer-Verlag, Berlin, Germany

[12] SAGNA MB, NIANG K, GUISSE A, GOFFNER D (2014) Balanites aegyptiaca (L.) Delile: geographical distribution and ethnobotanical knowledge by local populations in the Ferlo (north Senegal). Biotechnol. Agron. Soc. Approximately. 18: 503-511

[13] NATIONAL RESEARCH COUNCIL (2008) Lost Crops of Africa: Volume III: Fruits. The National Academies Press, Washington DC.

[14] SAGNA MB, NDIAYE O, DIALLO A, GOFFNER D, GUISSE A (2014) Biochemical composition and nutritional value of Balanites aegyptiaca fruit pulp from the Ferlo region in northern Senegal. Afr J Biotechnol 13: 336-342.

[15] https://www.klorane.com/ch-fr/cheveux/dattier-du-desert

[16] DARDEL C. KERGOAT L, HIERNAUX P et al (2014) Re-greening Sahel: 30 years of remote sensing data and field observations (Mali, Niger) 140: 350-364. https://doi.org/10.1016/j.rse.2013.09.011

[17] http://future-sahel.blogspot.com/

[18] https://www.ilesdepaix.org/wp-content/uploads/2014/09/Le-zaï-et-la-demi-lune.pdf

[19] http://books.openedition.org/irdeditions/3304

[20] https://rethink.earth/wayfinder/


环境百科全书由环境和能源百科全书协会出版 (www.a3e.fr),该协会与格勒诺布尔阿尔卑斯大学和格勒诺布尔INP有合同关系,并由法国科学院赞助。

引用这篇文章: GOFFNER Deborah, PEIRY Jean-Luc (2019年8月16日), The Great Green Wall: a hope for greening the Sahel?, 环境百科全书,咨询于 2024年11月21日 [在线ISSN 2555-0950]网址: https://www.encyclopedie-environnement.org/en/life/green-wall-hope-greening-sahel/.

环境百科全书中的文章是根据知识共享BY-NC-SA许可条款提供的,该许可授权复制的条件是:引用来源,不作商业使用,共享相同的初始条件,并且在每次重复使用或分发时复制知识共享BY-NC-SA许可声明。