Abstract The stable carbon isotopic composition (δ13C) of terrestrial leaf wax components, such as n-alkanes (δ13Calk), is used extensively to address questions about past changes in vegetation, climate and the carbon cycle. To interpret sedimentary δ13Calk values, characterization of the environmental and biological controls on carbon isotopic fractionation during plant metabolism is required, especially for fractionation that occurs during photosynthesis (ɛleaf-CO2 or Δleaf) and n-alkane biosynthesis (ealk-leaf). Although much is understood about controls on ɛleaf-CO2, little is known about seasonal or within canopy variation in n-alkane composition and ealk-leaf. To address the gap, we sampled 5 common tree species (Acer rubrum, Acer saccharum, Juniperus virginiana, Sassafras albidum and Ulmus americana) from buds to senescing leaves from within a single temperate forest in southwestern Ohio, USA. We measured n-alkane concentration, δ13Calk, leaf biomass δ13C (δ13Cleaf), δ13C of atmospheric CO2 within the canopy, luminous flux and specific leaf area (SLA). In angiosperms, δ13Calk values were low in the buds, increased substantially (4–6‰) to the highest values in young leaves and then decreased (1–3‰) in mature leaves. Seasonal variation in δ13Calk values generally tracked changes in δ13Cleaf, but the fractionation between leaf and n-alkanes (ealk-leaf) was variable, with lower values in buds (−6.7 ± 1.1‰) than in young leaves (−4.9 ± 2.3‰). Later in the growing season, ealk-leaf values stabilized and ranged between −2.5 and −6.5‰, with a mean of −4.3 ± 1.4‰ in angiosperms. Stabilization of δ13Calk values coincided with settling of SLA and increase in average chain length (ACL). For the conifer, temporal variation in δ13Calk values was smaller (ca. 2‰) compared with the angiosperms, and ealk-leaf values slightly increased when n-alkane production increased. Average ealk-leaf value in the conifer at the end of the growing season was −5.0 ± 0.7‰. We speculate that ealk-leaf variation in both angiosperms and the conifer reflects changes in the source of carbon for plant metabolism, as well as the timing of wax synthesis. We also tested canopy effects on n-alkane concentration and carbon isotopic fractionation at different heights and extent of canopy closure. n-Alkane concentrations were higher at the top of the canopy by 2–8 times than in the lower canopy. The eleaf-CO2 values were more negative at the lower two thirds of the canopy than the upper one third of the canopy by 2–4‰ in two species with closed canopies. The ealk-leaf values did not vary with height and extent of canopy closure. To further explore the link between canopy leaves and leaf litter, we compared δ13Calk values of mature canopy leaves with those in leaf litter collected during senescence, and the offsets were within ca. 1‰. The results confirm indications that leaf litter δ13Calk values reflect those of the upper canopy leaves due to not only high leaf biomass in the upper canopy but also to high n-alkane production.

中文翻译：

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温带森林正构烷烃及其碳同位素的季节性和冠层高度变化

摘要 陆地叶蜡成分（如正烷烃 (δ13Calk)）的稳定碳同位素组成 (δ13C) 被广泛用于解决过去植被、气候和碳循环变化的问题。为了解释沉积 δ13Calk 值，需要对植物代谢过程中碳同位素分馏的环境和生物控制进行表征，尤其是在光合作用（ɛleaf-CO2 或 Δleaf）和正烷烃生物合成（ealk-leaf）期间发生的分馏。尽管对 ɛleaf-CO2 的控制了解很多，但对正构烷烃成分和 ealk-leaf 的季节性或冠层变化知之甚少。为了弥补这一差距，我们对 5 种常见树种进行了采样（红色 Acer、Acer saccharum、Juniperus virginiana、檫 albidum 和 Ulmus americana）从美国俄亥俄州西南部的一个温带森林中的芽到衰老的叶子。我们测量了正构烷烃浓度、δ13Calk、叶生物量 δ13C (δ13Cleaf)、冠层内大气 CO2 的 δ13C、光通量和比叶面积 (SLA)。在被子植物中，芽中的δ13Calk值较低，在幼叶中显着增加（4-6‰）至最高值，然后在成熟叶中降低（1-3‰）。δ13​​Calk 值的季节性变化通常跟踪 δ13Cleaf 的变化，但叶和正烷烃（ealk-leaf）之间的分馏是可变的，芽中的值 (-6.7 ± 1.1‰) 低于幼叶 (-4.9 ± 2.3‰) ）。在生长季节后期，ealk-leaf 值趋于稳定，范围在 -2.5 到 -6.5‰ 之间，被子植物的平均值为 -4.3 ± 1.4‰。δ13​​Calk 值的稳定与 SLA 的稳定和平均链长 (ACL) 的增加相吻合。对于针叶树，与被子植物相比，δ13Calk 值的时间变化较小（约 2‰），当正烷烃产量增加时，ealk-leaf 值略有增加。生长季末针叶树的平均榆叶值为-5.0±0.7‰。我们推测被子植物和针叶树的鹰嘴豆叶变异反映了植物代谢碳源的变化，以及蜡合成的时间。我们还测试了冠层对不同高度和冠层关闭程度的正烷烃浓度和碳同位素分馏的影响。冠层顶部的正烷烃浓度比冠层下部高 2-8 倍。在两个具有封闭冠层的物种中，冠层下部三分之二处的叶二氧化碳值比冠层上部三分之一处更负 2–4‰。ealk-leaf 值不随树冠关闭的高度和程度而变化。为了进一步探索冠层叶片和凋落物之间的联系，我们比较了成熟冠层叶片的 δ13Calk 值与衰老期间收集的落叶中的值，偏移量在约 1‰。结果证实了叶凋落物 δ13Calk 值反映了上部冠层叶片的迹象，不仅因为上部冠层中的高叶片生物量，而且还因为高正烷烃产量。为了进一步探索冠层叶片和凋落物之间的联系，我们比较了成熟冠层叶片的 δ13Calk 值与衰老期间收集的落叶中的δ13Calk 值，偏移量在约 1‰。结果证实了叶凋落物 δ13Calk 值反映了上部冠层叶片的迹象，不仅因为上部冠层中的高叶片生物量，而且还因为高正烷烃产量。为了进一步探索冠层叶片和凋落物之间的联系，我们比较了成熟冠层叶片的 δ13Calk 值与衰老期间收集的落叶中的值，偏移量在约 1‰。结果证实了叶凋落物 δ13Calk 值反映了上部冠层叶片的迹象，不仅因为上部冠层中的高叶片生物量，而且还因为高正烷烃产量。