The Key Laboratory of Mountain Environment Evolution and Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu, Sichuan 610041, China.
Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA.
Tree Physiol. 2021 Jun 7;41(6):992-1003. doi: 10.1093/treephys/tpaa169.
Rising atmospheric carbon dioxide (CO2) may enhance tree growth and mitigate drought impacts through CO2 fertilization. However, multiple studies globally have found that rising CO2 has not translated into greater tree growth despite increases in intrinsic water-use efficiency (iWUE). The underlying mechanism discriminating between these two general responses to CO2 fertilization remains unclear. We used two species with contrasting stomatal regulation, the relatively anisohydric Qilian juniper (Sabina przewalskii) and the relatively isohydric Qinghai spruce (Picea crassifolia), to investigate the long-term tree growth and iWUE responses to climate change and elevated CO2 using tree ring widths and the associated cellulose stable carbon isotope ratios (δ13C). We observed a contrasting growth trend of juniper and spruce with juniper growth increasing while the spruce growth declined. The iWUE of both species increased significantly and with similar amplitude throughout the trees' lifespan, though the relatively anisohydric juniper had higher iWUE than the relatively isohydric spruce throughout the period. Additionally, with rising CO2, the anisohydric juniper became less sensitive to drought, while the relatively isohydric spruce became more sensitive to drought. We hypothesized that rising CO2 benefits relatively anisohydric species more than relatively isohydric species due to greater opportunity to acquire carbon through photosynthesis despite warming and droughts. Our findings suggest the CO2 fertilization effect depends on the isohydric degree, which could be considered in future terrestrial ecosystem models.
大气中二氧化碳（CO2）浓度的升高可能通过 CO2 施肥来促进树木生长并减轻干旱的影响。然而，全球多项研究发现，尽管内在水分利用效率（iWUE）有所提高，但 CO2 浓度的升高并未转化为更大的树木生长。区分这两种对 CO2 施肥的一般反应的潜在机制仍不清楚。我们使用了两种具有不同气孔调节特性的物种，相对异水的祁连圆柏（Sabina przewalskii）和相对等水的青海云杉（Picea crassifolia），通过树木年轮宽度和相关的纤维素稳定碳同位素比值（δ13C）来研究气候变化和升高 CO2 对树木长期生长和 iWUE 的影响。我们观察到圆柏和云杉的生长趋势截然不同，圆柏的生长增加，而云杉的生长下降。两种物种的 iWUE 都显著增加，且在整个树木的生命周期内幅度相似，尽管相对异水的圆柏在整个时期的 iWUE 都高于相对等水的云杉。此外，随着 CO2 浓度的升高，异水的圆柏对干旱的敏感性降低，而相对等水的云杉对干旱的敏感性增加。我们假设，由于在变暖与干旱的情况下通过光合作用获得碳的机会更大，CO2 施肥对相对异水的物种的益处大于相对等水的物种。我们的研究结果表明，CO2 施肥效应取决于等水程度，这在未来的陆地生态系统模型中应予以考虑。
