California Institute of Technology, Jet Propulsion Laboratory, Pasadena, California.
Institute of Meteorology and Climate Research (IMK-IFU), Karlsruhe Institute of Technology, Garmisch-Pertenkirchen, Germany.
Glob Chang Biol. 2018 Aug;24(8):3416-3435. doi: 10.1111/gcb.14283. Epub 2018 May 28.
The springtime transition to regional-scale onset of photosynthesis and net ecosystem carbon uptake in boreal and tundra ecosystems are linked to the soil freeze-thaw state. We present evidence from diagnostic and inversion models constrained by satellite fluorescence and airborne CO from 2012 to 2014 indicating the timing and magnitude of spring carbon uptake in Alaska correlates with landscape thaw and ecoregion. Landscape thaw in boreal forests typically occurs in late April (DOY 111 ± 7) with a 29 ± 6 day lag until photosynthetic onset. North Slope tundra thaws 3 weeks later (DOY 133 ± 5) but experiences only a 20 ± 5 day lag until photosynthetic onset. These time lag differences reflect efficient cold season adaptation in tundra shrub and the longer dehardening period for boreal evergreens. Despite the short transition from thaw to photosynthetic onset in tundra, synchrony of tundra respiration with snow melt and landscape thaw delays the transition from net carbon loss (at photosynthetic onset) to net uptake by 13 ± 7 days, thus reducing the tundra net carbon uptake period. Two global CO inversions using a CASA-GFED model prior estimate earlier northern high latitude net carbon uptake compared to our regional inversion, which we attribute to (i) early photosynthetic-onset model prior bias, (ii) inverse method (scaling factor + optimization window), and (iii) sparsity of available Alaskan CO observations. Another global inversion with zero prior estimates the same timing for net carbon uptake as the regional model but smaller seasonal amplitude. The analysis of Alaskan eddy covariance observations confirms regional scale findings for tundra, but indicates that photosynthesis and net carbon uptake occur up to 1 month earlier in evergreens than captured by models or CO inversions, with better correlation to above-freezing air temperature than date of primary thaw. Further collection and analysis of boreal evergreen species over multiple years and at additional subarctic flux towers are critically needed.
春季向北方森林和苔原生态系统的区域范围光合作用和净生态系统碳吸收的转变与土壤冻融状态有关。我们根据 2012 年至 2014 年卫星荧光和机载 CO 的诊断和反演模型提供了证据，表明阿拉斯加春季碳吸收的时间和幅度与景观融解和生态区相关。北方森林的景观融解通常发生在 4 月下旬（DOY111±7），直到光合作用开始有 29±6 天的滞后。北坡苔原的融解晚 3 周（DOY133±5），但直到光合作用开始只有 20±5 天的滞后。这些时间滞后的差异反映了苔原灌木在寒冷季节的高效适应，以及北方常绿植物更长的抗寒期。尽管苔原从融解到光合作用开始的过渡时间很短，但苔原呼吸与雪融和景观融解的同步使净碳损失（在光合作用开始时）到净吸收的过渡延迟了 13±7 天，从而减少了苔原的净碳吸收期。两个使用 CASA-GFED 模型的全球 CO 反演结果比我们的区域反演更早地预测了北部高纬度地区的净碳吸收，我们将其归因于：（i）光合作用开始的模型先验偏差，（ii）反演方法（缩放因子+优化窗口），以及（iii）阿拉斯加 CO 观测的稀疏性。另一个零先验估计的全球反演结果与区域模型的净碳吸收时间相同，但季节性幅度较小。对阿拉斯加涡度协方差观测的分析证实了苔原的区域尺度发现，但表明光合作用和净碳吸收比模型或 CO 反演更早发生，与超过冰点的空气温度的相关性比主要融解日期更好。进一步多年多物种收集和分析北方常绿植物，并在更多的亚北极通量塔进行分析，这是至关重要的。
