Duursma Remko A, Gimeno Teresa E, Boer Matthias M, Crous Kristine Y, Tjoelker Mark G, Ellsworth David S
Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW, Australia.
UMR 1391 ISPA, INRA, Villenave d'Ornon, 33140, France.
Glob Chang Biol. 2016 Apr;22(4):1666-76. doi: 10.1111/gcb.13151. Epub 2016 Feb 9.
Canopy leaf area, quantified by the leaf area index (L), is a crucial driver of forest productivity, water use and energy balance. Because L responds to environmental drivers, it can represent an important feedback to climate change, but its responses to rising atmospheric [CO2] and water availability of forests have been poorly quantified. We studied canopy leaf area dynamics for 28 months in a native evergreen Eucalyptus woodland exposed to free-air CO2 enrichment (the EucFACE experiment), in a subtropical climate where water limitation is common. We hypothesized that, because of expected stimulation of productivity and water-use efficiency, L should increase with elevated [CO2]. We estimated L from diffuse canopy transmittance, and measured monthly leaf litter production. Contrary to expectation, L did not respond to elevated [CO2]. We found that L varied between 1.10 and 2.20 across the study period. The dynamics of L showed a quick increase after heavy rainfall and a steady decrease during periods of low rainfall. Leaf litter production was correlated to changes in L, both during periods of decreasing L (when no leaf growth occurred) and during periods of increasing L (active shedding of old foliage when new leaf growth occurred). Leaf lifespan, estimated from mean L and total annual litter production, was up to 2 months longer under elevated [CO2] (1.18 vs. 1.01 years; P = 0.05). Our main finding that L was not responsive to elevated CO2 is consistent with other forest FACE studies, but contrasts with the positive response of L commonly predicted by many ecosystem models.
通过叶面积指数（L）量化的冠层叶面积是森林生产力、水分利用和能量平衡的关键驱动因素。由于L对环境驱动因素有响应，它可以代表对气候变化的重要反馈，但其对森林大气[CO₂]升高和水分可利用性的响应尚未得到很好的量化。我们在亚热带气候中常见水分限制的原生常绿桉树林地中，对暴露于自由空气CO₂富集环境下的冠层叶面积动态进行了28个月的研究（EucFACE实验）。我们假设，由于预计生产力和水分利用效率会受到刺激，L应随[CO₂]升高而增加。我们从冠层漫射透过率估算L，并测量每月的落叶产量。与预期相反，L对[CO₂]升高没有响应。我们发现，在整个研究期间，L在1.10至2.20之间变化。L的动态变化显示，暴雨后迅速增加，低降雨期稳步下降。落叶产量与L的变化相关，无论是在L下降期间（无新叶生长）还是在L增加期间（新叶生长时老叶的主动脱落）。根据平均L和年总落叶产量估算的叶寿命，在[CO₂]升高的情况下长达2个月（1.18年对1.01年；P = 0.05）。我们的主要发现，即L对升高的CO₂没有响应，与其他森林FACE研究一致，但与许多生态系统模型通常预测的L的正响应形成对比。
