Can light-saturated photosynthesis in lowland tropical forests be estimated by one light level?

Leaf-level net photosynthesis (An) estimates and associated
photosynthetic parameters are crucial for accurately parameterizing
photosynthesis models. For tropical forests such data are poorly available
and collected at variable light conditions. To avoid over- or
underestimation of modelled photosynthesis, it is critical to know at which
photosynthetic photon flux density (PPFD) photosynthesis becomes light
saturated. We studied the dependence of An on PPFD in two tropical forests
in French Guiana. We estimated the light saturation range, including the
lowest PPFD level at which Asat (An at light saturation) is reached, as
well as the PPFD range at which Asat remained unaltered. The light
saturation range was derived from photosynthetic light-response curves, and
within-canopy and interspecific differences were studied. We observed wide
light saturation ranges of An. Light saturation ranges differed among
canopy heights, but a PPFD level of 1000 µmol/m²/s was common across all
heights, except for pioneer trees species that did not reach light
saturation below 2000 µmol/m²/s. A light intensity of 1000 µmol/m²/s
sufficed for measuring Asat of climax species at our study sites,
independent of the species or the canopy height. Because of the wide light
saturation ranges, results from studies measuring Asat at higher PPFD
levels (for upper canopy leaves up to 1600 µmol/m²/s) are comparable with
studies measuring at 1000 µmol/m²/s.
Methods
We refer to the methods section of the manuscript for more details.

叶片水平净光合速率（An）的估算值及其相关光合参数，对精准参数化光合模型而言至关重要。针对热带森林，此类数据不仅稀缺，且多在可变光环境下采集。为避免光合模型模拟结果出现高估或低估，明确光合达到光饱和时的光合有效辐射通量密度（PPFD）阈值至关重要。
本研究在法属圭亚那的两处热带森林中，探究了An随PPFD的变化规律。我们估算了光饱和范围，包括达到饱和光合速率（Asat，即光饱和状态下的An）的最低PPFD水平，以及Asat保持稳定的PPFD区间。光饱和范围通过光合光响应曲线推导得出，同时本研究还分析了冠层内部以及物种间的差异。
本研究发现An的光饱和范围跨度较大；不同冠层高度的光饱和范围存在差异，但除先锋树种外，所有冠层高度的物种均在1000 µmol/m²/s的PPFD水平达到光饱和，而先锋树种在2000 µmol/m²/s以下均未出现光饱和现象。在本研究样地中，1000 µmol/m²/s的光强足以测定顶极树种的Asat，且该值不受物种类型或冠层高度的影响。由于光饱和范围跨度较大，在更高PPFD水平（冠层上层叶片可达1600 µmol/m²/s）下测定Asat的研究结果，与在1000 µmol/m²/s下测定的结果具有可比性。
研究方法
详细实验方法参见论文的方法章节。