Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83 Umeå, Sweden.
Department of Biological and Environmental Sciences, University of Gothenburg, SE-405 30 Gothenburg, Sweden.
Tree Physiol. 2018 Apr 1;38(4):558-569. doi: 10.1093/treephys/tpx130.
Trees are able to reduce their carbon (C) losses by refixing some of the CO2 diffusing out of their stems through corticular photosynthesis. Previous studies have shown that under ideal conditions the outflowing CO2 can be completely assimilated in metabolically active, young stem and branch tissues. Fewer studies have, however, been carried out on the older stem sections of large trees and, accordingly, the importance of refixation is still unclear under natural environmental conditions. We investigated the spatial and temporal variation in refixation in ~90-year-old boreal Scots pine (Pinus sylvestris L.) trees by utilizing month-long continuous measurements of stem CO2 efflux (Ec) made at four heights along the bole. Refixation rates were found to vary considerably along the bole, leading to a 28% reduction in long-term Ec in the upper stem compared with a negligible reduction at breast height. This vertical pattern correlated with variation in light availability, bark chlorophyll content and bark type. Analysis of the vertical and diurnal patterns in Ec further suggested that the influence of sap flow on the observed daytime reduction in Ec was small. The areal rates of corticular photosynthesis were much lower than previous estimates of photosynthetic rates per unit leaf area from the same trees, implying that the impact of refixation on tree-scale C uptake was small. However, upscaling of refixation indicated that 23-27% of the potential Ec was refixed by the bole and the branches, thereby significantly reducing the woody tissue C losses. Thus, our results suggest that refixation needs to be considered when evaluating the aboveground C cycling of mature P. sylvestris stands and that breast-height estimates should not be extrapolated to the whole tree.
树木通过皮层光合作用重新固定从茎中扩散出来的部分二氧化碳，从而减少碳（C）损失。先前的研究表明，在理想条件下，流出的 CO2 可以完全被代谢活跃的年轻茎和树枝组织同化。然而，对大树较老茎段的研究较少，因此，在自然环境条件下，重新固定的重要性仍然不清楚。我们通过在树干四个高度上进行长达一个月的连续茎 CO2 逸出（Ec）测量，研究了~90 年生北欧赤松（Pinus sylvestris L.）树木中重新固定的空间和时间变化。结果发现，重新固定率在树干上变化很大，导致与树干中部相比，上部树干的长期 Ec 降低了 28%，而树干中部的 Ec 降低可忽略不计。这种垂直模式与光照可用性、树皮叶绿素含量和树皮类型的变化相关。对 Ec 的垂直和昼夜模式的分析进一步表明，液流对观察到的白天 Ec 降低的影响很小。皮层光合作用的面积速率远低于从同一树木获得的单位叶面积光合作用速率的先前估计值，这意味着重新固定对树木尺度上的 C 吸收的影响很小。然而，重新固定的放大表明，树干和树枝重新固定了潜在 Ec 的 23-27%，从而显著减少了木质组织的 C 损失。因此，我们的结果表明，在评估成熟 P. sylvestris 林地上方的 C 循环时，需要考虑重新固定，并且不应将树干中部的估计值外推到整棵树。
