Department of Environmental Systems Science, ETH Zurich, Zurich, Switzerland.
Department of Biology, University of Toronto Mississauga, Mississauga, L5L1C6, Canada.
Oecologia. 2020 Feb;192(2):323-339. doi: 10.1007/s00442-019-04583-x. Epub 2020 Jan 4.
While foliar photosynthetic relationships with light, nitrogen, and water availability have been well described, environmental factors driving vertical gradients of foliar traits within forest canopies are still not well understood. We, therefore, examined how light availability and vapour pressure deficit (VPD) co-determine vertical gradients (between 12 and 42 m and in the understorey) of foliar photosynthetic capacity (A), C fractionation (∆), specific leaf area (SLA), chlorophyll (Chl), and nitrogen (N) concentrations in canopies of Fagus sylvatica and Abies alba growing in a mixed forest in Switzerland in spring and summer 2017. Both species showed lower Chl/N and lower SLA with higher light availability and VPD at the top canopy. Despite these biochemical and morphological acclimations, A during summer remained relatively constant and the photosynthetic N-use efficiency (PNUE) decreased with higher light availability for both species, suggesting suboptimal N allocation within the canopy. ∆ of both species were lower at the canopy top compared to the bottom, indicating high water-use efficiency (WUE). VPD gradients strongly co-determined the vertical distribution of Chl, N, and PNUE in F. sylvatica, suggesting stomatal limitation of photosynthesis in the top canopy, whereas these traits were only related to light availability in A. alba. Lower PNUE in F. sylvatica with higher WUE clearly indicated a trade-off in water vs. N use, limiting foliar acclimation to high light and VPD at the top canopy. Species-specific trade-offs in foliar acclimation to environmental canopy gradients may thus be considered for scaling photosynthesis from leaf to canopy to landscape levels.
虽然已经很好地描述了叶片与光、氮和水分可利用性的光合作用关系，但仍不清楚驱动森林冠层内叶片特征垂直梯度的环境因素。因此，我们研究了在 2017 年春季和夏季，光照可用性和蒸气压亏缺 (VPD) 如何共同决定瑞士混交林生长的欧洲山毛榉和白云杉冠层内叶片光合能力 (A)、C 分馏 (∆)、比叶面积 (SLA)、叶绿素 (Chl) 和氮 (N) 浓度的垂直梯度 (在 12 到 42 米之间和林下)。这两个物种在冠层顶部都表现出较低的 Chl/N 和较低的 SLA，而具有较高的光照可用性和 VPD。尽管有这些生化和形态适应，夏季的 A 仍然相对稳定，对于两个物种，随着光照可用性的增加，光合 N 使用效率 (PNUE) 降低，这表明冠层内的 N 分配不理想。与底部相比，两个物种的 ∆ 在冠层顶部较低，表明具有较高的水分利用效率 (WUE)。VPD 梯度强烈共同决定了 F. sylvatica 中 Chl、N 和 PNUE 的垂直分布，表明在冠层顶部光合作用受到气孔限制，而在 A. alba 中这些特征仅与光照可用性相关。F. sylvatica 中具有较高 WUE 的较低 PNUE 清楚地表明了在水与 N 使用之间的权衡，限制了叶片对冠层顶部高光和 VPD 的适应。因此，可能需要考虑叶片对环境冠层梯度的适应的种间权衡，以便将光合作用从叶片扩展到冠层再到景观水平。
