Laffitte Benjamin, Seyler Barnabas C, Wang Wenzhi, Li Pengbo, Du Jie, Tang Ya
Department of Environment, College of Architecture and Environment, Sichuan University, No. 24, South Section One, First Ring Road, Chengdu, Sichuan 610065, China.
The Key Laboratory of Mountain Environment Evolution and Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu, Sichuan 610041, China.
Heliyon. 2022 Oct 23;8(10):e11219. doi: 10.1016/j.heliyon.2022.e11219. eCollection 2022 Oct.
Though rising atmospheric CO concentrations (C) harm the environment and society, they may also raise photosynthetic rates and enhance intrinsic water-use efficiency (iWUE). Numerous short-term studies have investigated tree growth under elevated CO (eCO) conditions, but no long-duration study has investigated eCO impacts on tree growth and iWUE under natural conditions. Utilizing a new dendrochronological experimental design in a heavily-touristed nature preserve in Southwest China (Jiuzhaigou National Nature Reserve), we compared tree growth (e.g., basal area increment) and iWUE in two biophysically and environmentally similar valleys with contrasting anthropogenic activities. Trees in the control valley with ambient CO benefited from increasing C, possibly due to the CO fertilization effect and optimal environmental conditions. However, trees in the treatment valley with intensive tourism experienced comparatively higher localized eCO and growth rate declines. While iWUE increased (1959-2017) in the control (25.3%) and treatment sites (47.8%), declining tree growth rates in the treatment site was likely because comparatively extreme CO exposure levels encouraged stomatal closures. As the first long-term study investigating eCO impacts on tree growth and iWUE under natural conditions, we demonstrate that increased forest iWUE is unlikely to overcome negative drought stress and rising temperature impacts. Thus, forest potential for mitigating eCO and global climate change is likely overestimated, particularly under dry temperate conditions.
尽管大气中二氧化碳浓度（C）的上升对环境和社会有害，但它们也可能提高光合速率并增强内在水分利用效率（iWUE）。许多短期研究调查了高浓度二氧化碳（eCO）条件下树木的生长情况，但尚无长期研究调查自然条件下eCO对树木生长和iWUE的影响。在中国西南部一个游客众多的自然保护区（九寨沟国家级自然保护区），我们采用了一种新的树木年代学实验设计，比较了两个生物物理和环境相似但人为活动不同的山谷中树木的生长情况（如断面积增量）和iWUE。对照山谷中处于环境二氧化碳浓度下的树木受益于不断增加的C，这可能是由于二氧化碳施肥效应和最佳环境条件。然而，处于高强度旅游区的处理山谷中的树木经历了相对较高的局部eCO，生长速率下降。虽然对照点（1959 - 2017年）和处理点的iWUE均有所增加（分别为25.3%和47.8%），但处理点树木生长速率下降可能是因为相对极端的二氧化碳暴露水平促使气孔关闭。作为第一项在自然条件下调查eCO对树木生长和iWUE影响的长期研究，我们证明森林iWUE的增加不太可能克服干旱胁迫和气温上升的负面影响。因此，森林缓解eCO和全球气候变化的潜力可能被高估了，特别是在干燥的温带条件下。
