(Table 1) Climate characteristics of the four North American Tundra Experiment (ITEX) sites

Climate warming is expected to differentially affect CO2 exchange of the diverse ecosystems in the Arctic. Quantifying responses of CO2 exchange to warming in these ecosystems will require coordinated experimentation using standard temperature manipulations and measurements. Here, we used the International Tundra Experiment (ITEX) standard warming treatment to determine CO2 flux responses to growing-season warming for ecosystems spanning natural temperature and moisture ranges across the Arctic biome. We used the four North American Arctic ITEX sites (Toolik Lake, Atqasuk, and Barrow [USA] and Alexandra Fiord [Canada]) that span 10° of latitude. At each site, we investigated the CO2 responses to warming in both dry and wet or moist ecosystems. Net ecosystem CO2 exchange (NEE), ecosystem respiration (ER), and gross ecosystem photosynthesis (GEP) were assessed using chamber techniques conducted over 24-h periods sampled regularly throughout the summers of two years at all sites.
At Toolik Lake, warming increased net CO2 losses in both moist and dry ecosystems. In contrast, at Atqasuk and Barrow, warming increased net CO2 uptake in wet ecosystems but increased losses from dry ecosystems. At Alexandra Fiord, warming improved net carbon uptake in the moist ecosystem in both years, but in the wet and dry ecosystems uptake increased in one year and decreased the other. Warming generally increased ER, with the largest increases in dry ecosystems. In wet ecosystems, high soil moisture limited increases in respiration relative to increases in photosynthesis. Warming generally increased GEP, with the notable exception of the Toolik Lake moist ecosystem, where warming unexpectedly decreased GEP >25%. Overall, the respiration response determined the effect of warming on ecosystem CO2 balance. Our results provide the first multiple-site comparison of arctic tundra CO2 flux responses to standard warming treatments across a large climate gradient. These results indicate that (1) dry tundra may be initially the most responsive ecosystems to climate warming by virtue of strong increases in ER, (2) moist and wet tundra responses are dampened by higher water tables and soil water contents, and (3) both GEP and ER are responsive to climate warming, but the magnitudes and directions are ecosystem-dependent.

气候变暖预计将对北极地区多样生态系统的二氧化碳（CO₂）交换产生差异化影响。要量化这些生态系统中CO₂交换对变暖的响应，需采用标准化温度操控与测量方法开展协同试验。本研究采用国际苔原实验（International Tundra Experiment, ITEX）的标准化增温处理方案，针对北极生物群区内横跨自然温度与湿度梯度的各类生态系统，探究其生长季增温下的CO₂通量响应特征。我们选取了横跨10个纬度的4个北美北极ITEX研究站点：美国的图利克湖、阿特萨萨克、巴罗以及加拿大的亚历山德拉峡湾。在每个站点中，我们分别探究了旱地与湿生/湿润生态系统对增温的CO₂响应。采用通量箱法在两年的夏季定期开展24小时连续采样，以评估生态系统净CO₂交换（Net ecosystem CO2 exchange, NEE）、生态系统呼吸（ER, ecosystem respiration）与总生态系统光合（GEP, gross ecosystem photosynthesis）。
在图利克湖站点，增温提升了湿润与旱地生态系统的净CO₂流失量。与之相反，在阿特萨萨克与巴罗站点，增温提升了湿生生态系统的净CO₂吸收量，却加剧了旱地生态系统的CO₂释放。在亚历山德拉峡湾站点，增温在两年内均提升了湿润生态系统的净碳吸收，但在湿生与旱地生态系统中，碳吸收在其中一年出现提升，另一年则出现下降。增温总体上提升了生态系统呼吸，其中旱地生态系统的呼吸增幅最大。在湿生生态系统中，较高的土壤湿度抑制了呼吸速率相对于光合速率提升的增幅。增温总体上提升了总生态系统光合，唯有图利克湖的湿润生态系统是显著例外——该站点的增温意外地使总生态系统光合下降了25%以上。总体而言，生态系统呼吸的响应决定了增温对生态系统CO₂平衡的影响效应。
本研究首次在横跨较大气候梯度的多个站点间，对比了北极苔原生态系统CO₂通量对标准化增温处理的响应。研究结果表明：（1）鉴于生态系统呼吸的显著提升，旱地苔原可能是当前对气候变暖响应最为敏感的生态系统类型；（2）较高的地下水位与土壤含水量会削弱湿润与湿生苔原的响应强度；（3）总生态系统光合与生态系统呼吸均对气候变暖存在响应，但其响应幅度与方向取决于生态系统类型。