Rising atmospheric CO₂ is expected to increase global temperatures, plant water-use efficiency, and carbon storage in the terrestrial biosphere. A CO₂ fertilization effect on terrestrial vegetation is predicted to cause global greening as the potential ecospace for forests expands. However, leaf-level fertilization effects, such as increased productivity and water-use efficiency, have not been documented from fossil leaves in periods of heightened atmospheric CO₂. Here, we use leaf gas-exchange modeling on a well-preserved fossil flora from early Miocene New Zealand, as well as two previously published tropical floras from the same time period, to reconstruct atmospheric CO₂, leaf-level productivity, and intrinsic water-use efficiency. Leaf gas-exchange rates reconstructed from early Miocene fossils, which grew at southern temperate and tropical latitudes when global average temperatures were 5–6 ^∘C higher than today, reveal that atmospheric CO₂ was ∼450–550 ppm. Early Miocene CO₂ was similar to projected values for 2040 CE and is consistent with an Earth system sensitivity of 3–7 ^∘C to a doubling of CO₂. The Southern Hemisphere temperate leaves had higher reconstructed productivity than modern analogs, likely due to a longer growing season. This higher productivity was presumably mirrored at northern temperate latitudes as well, where a greater availability of landmass would have led to increased carbon storage in forest biomass relative to today. Intrinsic water-use efficiency of both temperate and tropical forest trees was high, toward the upper limit of the range for modern trees, which likely expanded the habitable range in regions that could not support forests with high moisture demands under lower atmospheric CO₂. Overall, early Miocene elevated atmospheric CO₂ sustained globally higher temperatures, and our results provide the first empirical evidence of concomitant enhanced intrinsic water-use efficiency, indicating a forest fertilization effect.

中文翻译：

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在中新世早期，CO ₂升高，叶片水平生产力提高，水利用效率提高

预计大气中CO _2的增加将增加全球温度，植物用水效率以及陆地生物圈中的碳储存。甲CO ₂对陆地植物施肥效果被预测为导致全球绿化作为用于森林膨胀的潜在生态空间。然而，在大气CO ₂升高的时期，化石叶片中未见叶水平的施肥效应，例如提高的生产率和水分利用效率。在这里，我们对新西兰中新世早期保存完好的化石植物群以及同一时期的两个先前发表的热带植物群进行了叶片气体交换模型，以重建大气中的CO _2。，叶级生产力和内在的用水效率。从早期中新世化石中重建的叶片气体交换率在全球平均温度为5-6时在南部温带和热带纬度生长。^{一种？？？？}C比今天高，表明大气中的CO ₂为 –450 –550 –ppm。早期的中新世CO ₂与2040年CE的预测值相似，并且与地球系统的3 7 7敏感性一致。^{一种？？？？}C到CO ₂的两倍。南半球温带叶的重建生产力比现代类似植物更高，这可能是由于生长季节更长。据推测，这种较高的生产率也反映在北部温带地区，那里的土地数量更多，这将导致如今森林生物量的碳储存增加。温带和热带林木的内在用水效率都很高，接近现代林木的用水上限，这可能扩大了在大气CO ₂含量较低且不能满足高水分需求的森林的地区的可居住范围。总体而言，中新世早期大气CO ₂升高 全球持续高温，我们的研究结果提供了第一个经验性证据，表明内在的用水效率随之提高，表明了森林施肥的效果。