Isotopic and morphologic proxies for reconstructing light environment from fossil leaves: a modern calibration in the Daintree Rainforest, Australia

Premise: Within closed canopy forests, vertical gradients of light and atmospheric CO 2 drive variations in leaf carbon isotope ratios, leaf mass per area (LMA), and the micromorphology of leaf epidermal cells. Variations in such traits observed in preserved or fossilized leaves could enable inferences of past forest canopy closure and the habitat of individual taxa. However, as yet no calibration study has examined how multiple traits in combination reflect position within a modern closed canopy forest or how these could be applied to the fossil record. Methods: Leaves were sampled from throughout the vertical profile of the tropical forest canopy using the 48.5 m crane at the Daintree Rainforest Observatory, Queensland, Australia. Carbon isotope ratios, LMA, petiole metric (i.e. petiole-width 2 / leaf-area) and leaf micromorphology (i.e. undulation index and cell area) were compared within species across a range of canopy positions, as quantified by leaf area index (LAI). Key Results: Individually, cell area, δ 13 C, and petiole metric all correlated with both LAI and LMA, but the use of a combined model provided significantly greater predictive power. Conclusions: Using the observed relationships with leaf carbon isotope ratio and morphology to estimate the range of LAI in fossil floras can provide a measure of canopy closure in ancient forests. Similarly, estimates LAI and LMA for individual taxa can provide comparative measures of light environment and growth strategy of fossil taxa from within a flora. Please be aware that if you ask to have your user record removed, we will retain your name in the records concerning manuscripts for which you were an author, reviewer, or editor. In compliance with data protection regulations, you may request that we remove your personal registration details at any time.

研究背景：在郁闭林冠森林中，光照与大气二氧化碳的垂直梯度驱动了叶片碳同位素比值、比叶重（leaf mass per area, LMA）以及叶片表皮细胞微观形态的变异。对保存下来或化石叶片中这类性状变异的观测，可用于推断古代森林的林冠闭合程度以及类群的栖息环境。然而截至目前，尚无校准研究探讨多性状组合如何反映现代郁闭林冠森林中的植株位置，以及这些性状如何应用于化石记录。
研究方法：本研究依托澳大利亚昆士兰州丹特里雨林观测站的48.5米高架起重机，对热带林冠垂直剖面各层的叶片进行采样。以叶面积指数（leaf area index, LAI）量化林冠位置梯度，随后在同一物种内对比不同林冠位置的叶片碳同位素比值、比叶重、叶柄指标（即叶柄宽度²/叶面积）以及叶片微观形态（即起伏指数与细胞面积）。
主要结果：单性状分析显示，细胞面积、δ¹³C与叶柄指标均与叶面积指数和比叶重存在相关性，但联合模型的预测能力显著更高。
研究结论：利用观测到的叶片碳同位素比值与形态性状和林冠参数的关联，估算古植物群的叶面积指数范围，可量化古代森林的林冠闭合程度。同理，通过估算单个类群的叶面积指数与比叶重，可对比古植物群内化石类群的光照环境与生长策略。
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