Department Biology, San Diego State University, San Diego, California, USA.
School of Biosciences, University of Sheffield, Sheffield, UK.
Glob Chang Biol. 2023 Mar;29(5):1267-1281. doi: 10.1111/gcb.16487. Epub 2022 Nov 10.
Long-term atmospheric CO concentration records have suggested a reduction in the positive effect of warming on high-latitude carbon uptake since the 1990s. A variety of mechanisms have been proposed to explain the reduced net carbon sink of northern ecosystems with increased air temperature, including water stress on vegetation and increased respiration over recent decades. However, the lack of consistent long-term carbon flux and in situ soil moisture data has severely limited our ability to identify the mechanisms responsible for the recent reduced carbon sink strength. In this study, we used a record of nearly 100 site-years of eddy covariance data from 11 continuous permafrost tundra sites distributed across the circumpolar Arctic to test the temperature (expressed as growing degree days, GDD) responses of gross primary production (GPP), net ecosystem exchange (NEE), and ecosystem respiration (ER) at different periods of the summer (early, peak, and late summer) including dominant tundra vegetation classes (graminoids and mosses, and shrubs). We further tested GPP, NEE, and ER relationships with soil moisture and vapor pressure deficit to identify potential moisture limitations on plant productivity and net carbon exchange. Our results show a decrease in GPP with rising GDD during the peak summer (July) for both vegetation classes, and a significant relationship between the peak summer GPP and soil moisture after statistically controlling for GDD in a partial correlation analysis. These results suggest that tundra ecosystems might not benefit from increased temperature as much as suggested by several terrestrial biosphere models, if decreased soil moisture limits the peak summer plant productivity, reducing the ability of these ecosystems to sequester carbon during the summer.
长期大气 CO 浓度记录表明，自 20 世纪 90 年代以来，变暖对高纬度碳吸收的积极影响有所减弱。为了解释近年来随着气温升高，北方生态系统净碳汇减少的现象，提出了多种机制，包括植被水分胁迫和近年来呼吸作用增强。然而，由于缺乏一致的长期碳通量和原位土壤湿度数据，严重限制了我们识别导致近期碳汇强度降低的机制的能力。在这项研究中，我们利用了分布在整个北极地区的 11 个连续多年冻土苔原站点近 100 个站点年的涡度协方差数据记录，测试了总初级生产力 (GPP)、净生态系统交换 (NEE)和生态系统呼吸 (ER)在夏季不同时期（早期、峰值和后期）的温度（表示为生长度日，GDD）响应，包括主要苔原植被类群（禾本科植物和苔藓，以及灌木）。我们进一步测试了 GPP、NEE 和 ER 与土壤湿度和水汽压亏缺的关系，以确定植物生产力和净碳交换的潜在水分限制。我们的研究结果表明，在峰值夏季（7 月），两种植被类群的 GPP 都随着 GDD 的升高而降低，在偏相关分析中，在统计控制 GDD 后，峰值夏季 GPP 与土壤湿度之间存在显著关系。这些结果表明，如果土壤湿度降低限制了夏季植物生产力，那么苔原生态系统可能不会像一些陆地生物圈模型所建议的那样从升温中受益，从而降低了这些生态系统在夏季固定碳的能力。
