Dendrochronology in the dry tropics: the Ethiopian case

Dendrochronology is developing outside temperate and boreal regions. Over the past decade substantial progress has been made in Mediterranean and wet tropical regions. However, research in dry tropical regions, notably those of sub-Saharan Africa, has remained fragmentary. Here, we try to identify the unique challenges and opportunities of dendrochronology in the dry tropics. First, we briefly review the status of dendrochronology outside temperate and boreal regions with an emphasis on sub-Saharan Africa. Subsequently, we focus upon one of those areas where dendrochronology in the dry tropics is at the forefront of scientific advance: Ethiopia. A detailed review of tree ring studies in the lowlands and highlands highlights the complexity of ring formation and made us identify four major types of growth ring expression: anatomically not distinct rings, multiple rings per year, annual rings and multiple missing rings. This complex tree growth behaviour is associated with large-scale variations in precipitation regime (unimodal to multimodal) and relatively small-scale variations in tree sensitivity to water availability. Literature results are used to develop a scheme that can be used to predict differences in growth ring formation along gradients in these two factors. Because of the exceptional growth sensitivity of and the importance of local site conditions (topography, biological factors, etc.) for most trees sampled, those growing at the limits of their ecological amplitude are prone to possess multiple rings per year or multiple missing rings. In such circumstances, site selection should not always take place at the limits of the ecological amplitude of a species, but may sometimes have to be diverted to more mesic environments. Successful studies are now appearing, such as those reporting correlations between tree ring chronologies and Blue Nile river flows.

树木年轮学正在温带和寒带地区之外发展。在过去十年中，地中海地区和潮湿的热带地区取得了重大进展。然而，在干燥的热带地区，尤其是撒哈拉以南非洲地区的研究仍然是零碎的。在此，我们试图确定干燥热带地区树木年轮学所面临的独特挑战和机遇。首先，我们简要回顾温带和寒带地区之外树木年轮学的状况，重点是撒哈拉以南非洲地区。随后，我们聚焦于干燥热带地区树木年轮学处于科学前沿的地区之一：埃塞俄比亚。对低地和高地树木年轮研究的详细回顾凸显了年轮形成的复杂性，并使我们确定了四种主要的年轮表现类型：解剖结构上不明显的年轮、一年多轮、年轮和多年轮缺失。这种复杂的树木生长行为与降水模式的大规模变化（从单峰到多峰）以及树木对水分可利用性的敏感度的相对小规模变化有关。利用文献结果制定了一个方案，可用于预测这两个因素梯度上年轮形成的差异。由于大多数采样树木对生长环境的敏感度极高以及当地立地条件（地形、生物因素等）的重要性，那些在其生态幅度极限生长的树木容易出现一年多轮或多年轮缺失的情况。在这种情况下，样地选择不应总是在一个物种生态幅度的极限处进行，有时可能必须转向更湿润的环境。现在已经出现了一些成功的研究，比如那些报道树木年轮年表与青尼罗河流量之间相关性的研究。