Isotope Bioscience Laboratory - ISOFYS, Department of Green Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University, Gent, Belgium.
Computational and Applied Vegetation Ecology - CAVElab, Department of Environment, Faculty of Bioscience Engineering, Ghent University, Gent, Belgium.
Glob Chang Biol. 2020 Aug;26(8):4449-4461. doi: 10.1111/gcb.15145. Epub 2020 May 29.
Forests exhibit leaf- and ecosystem-level responses to environmental changes. Specifically, rising carbon dioxide (CO ) levels over the past century are expected to have increased the intrinsic water-use efficiency (iWUE) of tropical trees while the ecosystem is gradually pushed into progressive nutrient limitation. Due to the long-term character of these changes, however, observational datasets to validate both paradigms are limited in space and time. In this study, we used a unique herbarium record to go back nearly a century and show that despite the rise in CO concentrations, iWUE has decreased in central African tropical trees in the Congo Basin. Although we find evidence that points to leaf-level adaptation to increasing CO -that is, increasing photosynthesis-related nutrients and decreasing maximum stomatal conductance, a decrease in leaf δ C clearly indicates a decreasing iWUE over time. Additionally, the stoichiometric carbon to nitrogen and nitrogen to phosphorus ratios in the leaves show no sign of progressive nutrient limitation as they have remained constant since 1938, which suggests that nutrients have not increasingly limited productivity in this biome. Altogether, the data suggest that other environmental factors, such as increasing temperature, might have negatively affected net photosynthesis and consequently downregulated the iWUE. Results from this study reveal that the second largest tropical forest on Earth has responded differently to recent environmental changes than expected, highlighting the need for further on-ground monitoring in the Congo Basin.
森林表现出对环境变化的叶片和生态系统水平的响应。具体来说，在过去一个世纪中，二氧化碳（CO ）水平的上升预计会提高热带树木的内在水分利用效率（iWUE），而生态系统则逐渐受到渐进的养分限制。然而，由于这些变化具有长期性，因此在空间和时间上都受到限制，难以获得验证这两种范式的观测数据集。在本研究中，我们利用独特的标本记录回溯近一个世纪，结果表明，尽管 CO 浓度上升，但在刚果盆地的中非热带树木中，iWUE 却在下降。尽管我们有证据表明，叶片水平上存在适应 CO 增加的现象，即增加与光合作用相关的养分和降低最大气孔导度，但叶片 δ C 的减少清楚地表明，iWUE 随时间呈下降趋势。此外，自 1938 年以来，叶片中的碳氮和氮磷化学计量比一直保持不变，这表明养分并没有逐渐限制生物群落的生产力，这表明没有出现渐进性的养分限制。总的来说，这些数据表明，其他环境因素，如温度升高，可能对净光合作用产生负面影响，从而降低 iWUE。本研究的结果表明，地球上第二大热带森林对近期环境变化的反应与预期不同，这凸显了在刚果盆地进一步进行实地监测的必要性。
