Norby Richard J, Sholtis Johnna D, Gunderson Carla A, Jawdy Sara S
Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6422, USA.
Oecologia. 2003 Aug;136(4):574-84. doi: 10.1007/s00442-003-1296-2. Epub 2003 Jun 13.
Leaf area index (LAI) and its seasonal dynamics are key determinants of terrestrial productivity and, therefore, of the response of ecosystems to a rising atmospheric CO(2) concentration. Despite the central importance of LAI, there is very little evidence from which to assess how forest LAI will respond to increasing [CO(2)]. We assessed LAI and related leaf indices of a closed-canopy deciduous forest for 4 years in 25-m-diameter plots that were exposed to ambient or elevated CO(2) (542 ppm) in a free-air CO(2) enrichment (FACE) experiment. LAI of this Liquidambar styraciflua (sweetgum) stand was about 6 and was relatively constant year-to-year, including the 2 years prior to the onset of CO(2) treatment. LAI throughout the 1999-2002 growing seasons was assessed through a combination of data on photosynthetically active radiation (PAR) transmittance, mass of litter collected in traps, and leaf mass per unit area (LMA). There was no effect of [CO(2)] on any expression of leaf area, including peak LAI, average LAI, or leaf area duration. Canopy mass and LMA, however, were significantly increased by CO(2) enrichment. The hypothesized connection between light compensation point (LCP) and LAI was rejected because LCP was reduced by [CO(2)] enrichment only in leaves under full sun, but not in shaded leaves. Data on PAR interception also permitted calculation of absorbed PAR (APAR) and light use efficiency (LUE), which are key parameters connecting satellite assessments of terrestrial productivity with ecosystem models of future productivity. There was no effect of [CO(2)] on APAR, and the observed increase in net primary productivity in elevated [CO(2)] was ascribed to an increase in LUE, which ranged from 1.4 to 2.4 g MJ(-1). The current evidence seems convincing that LAI of non-expanding forest stands will not be different in a future CO(2)-enriched atmosphere and that increases in LUE and productivity in elevated [CO(2)] are driven primarily by functional responses rather than by structural changes. Ecosystem or regional models that incorporate feedbacks on resource use through LAI should not assume that LAI will increase with CO(2) enrichment of the atmosphere.
叶面积指数（LAI）及其季节动态是陆地生产力的关键决定因素，因此也是生态系统对大气中二氧化碳浓度上升响应的关键决定因素。尽管叶面积指数至关重要，但几乎没有证据可用于评估森林叶面积指数将如何响应二氧化碳浓度的增加。我们在一个自由空气二氧化碳富集（FACE）实验中，对直径25米的样地内的一个郁闭落叶林的叶面积指数及相关叶指标进行了4年评估，这些样地暴露于环境二氧化碳浓度或升高的二氧化碳浓度（542 ppm）下。这片胶皮糖香树（胶皮糖枫）林分的叶面积指数约为6，且年际间相对稳定，包括在二氧化碳处理开始前的两年。通过光合有效辐射（PAR）透过率数据、陷阱收集的凋落物质量以及单位面积叶质量（LMA）的综合数据，对1999 - 2002年生长季的叶面积指数进行了评估。二氧化碳浓度对叶面积的任何表现形式均无影响，包括叶面积指数峰值、平均叶面积指数或叶面积持续时间。然而，二氧化碳富集显著增加了冠层质量和单位面积叶质量。关于光补偿点（LCP）与叶面积指数之间的假设联系被否定，因为仅在全日照下的叶片中，二氧化碳富集降低了光补偿点，而在遮荫叶片中并非如此。光合有效辐射截获数据还可用于计算吸收的光合有效辐射（APAR）和光能利用效率（LUE），这是将陆地生产力的卫星评估与未来生产力的生态系统模型相联系的关键参数。二氧化碳浓度对吸收的光合有效辐射没有影响，观察到的高二氧化碳浓度下净初级生产力的增加归因于光能利用效率的提高，其范围为1.4至2.4克兆焦耳⁻¹。目前的证据似乎令人信服，即在未来二氧化碳浓度升高的大气中，非扩张性林分的叶面积指数不会有差异，且高二氧化碳浓度下光能利用效率和生产力的增加主要由功能响应而非结构变化驱动。纳入通过叶面积指数对资源利用反馈的生态系统或区域模型不应假定叶面积指数会随大气二氧化碳富集而增加。
