Data from: Elevated CO2 induces substantial and persistent declines in forage quality irrespective of warming in mixedgrass prairie

Increasing atmospheric [CO2] and temperature are expected to affect the productivity, species composition, biogeochemistry, and therefore the quantity and quality of forage available to herbivores in rangeland ecosystems. Both elevated CO2 (eCO2) and warming affect plant tissue chemistry through multiple direct and indirect pathways, such that the cumulative outcomes of these effects are difficult to predict. Here, we report on a 7-year study examining effects of CO2 enrichment (to 600 ppm) and infrared warming (+1.5°C day/3°C night) under realistic field conditions on forage quality and quantity in a semiarid, mixedgrass prairie. For the 3 dominant forage grasses, warming effects on in vitro dry matter digestibility (IVDMD) and tissue [N] were detected only in certain years, varied from negative to positive, and were relatively minor. In contrast, eCO2 substantially reduced IVDMD (2 most abundant grasses) and [N] (all 3 dominant grass species) in most years, except the two wettest years. Furthermore, eCO2 reduced IVDMD and [N] independent of warming effects. Reduced IVDMD with eCO2 was related both to reduced [N] and increased acid detergent fiber (ADF) content of grass tissues. For the 6 most abundant forage species (representing 96% of total forage production), combined warming+eCO2 increased forage production by 38% and reduced forage [N] by 13% relative to ambient climate. Although the absolute magnitude of the decline in IVDMD and [N] due to combined warming+eCO2 may seem small (e.g. from 63.3 to 61.1% IVDMD and 1.25 to 1.04% [N] for Pascopyrum smithii), such shifts could have substantial consequences for the rate at which ruminants gain weight during the primary growing season in the largest remaining rangeland ecosystem in North America. With forage production increases, declining forage quality could potentially be mitigated by adaptively increasing stocking rates, and through management such as prescribed burning, fertilization at low rates, and legume interseeding to enhance forage quality.

大气二氧化碳浓度（atmospheric [CO₂]）升高与气温上升，预计将影响牧场生态系统的生产力、物种组成与生物地球化学循环，进而改变草食动物可获取饲草的数量与质量。升高的二氧化碳浓度（eCO2, elevated CO2）与增温均会通过多种直接与间接途径调控植物组织的化学特征，因此二者效应的综合影响难以预测。本研究报道了一项为期7年的野外原位实验结果：在半干旱混合草草原的真实田间条件下，设置二氧化碳富集（至600 ppm）与红外增温（日间增温1.5℃/夜间增温3℃）处理，探究其对饲草数量与质量的影响。针对3种优势饲草禾草，仅在部分年份检测到增温对体外干物质消化率（IVDMD, in vitro dry matter digestibility）与组织氮浓度（[N]）的影响，其效应方向从负向转为正向，整体影响相对微弱。与之形成鲜明对比的是，在多数年份中，eCO2显著降低了两种优势禾草的IVDMD以及所有3种优势禾草的组织[N]，仅在降水最充沛的两个年份未出现该效应。此外，eCO2对IVDMD与[N]的降低作用独立于增温的影响。eCO2处理下IVDMD的降低，与禾草组织[N]的下降以及酸性洗涤纤维（ADF, acid detergent fiber）含量的升高均存在显著关联。针对占总饲草产量96%的6种优势饲草物种，相较于对照气候（ambient climate），增温与eCO2联合处理使饲草产量提升38%，同时使饲草[N]降低13%。尽管增温+eCO2联合处理导致的IVDMD与[N]绝对降幅看似较小（例如，Pascopyrum smithii的IVDMD从63.3%降至61.1%，[N]从1.25%降至1.04%），但这类变化可能对北美现存最大的牧场生态系统中反刍动物在主要生长季的增重速率产生显著影响。随着饲草产量提升，可通过适应性调整载畜率、采用计划火烧、低剂量施肥以及豆科植物混播以改善饲草品质等管理措施，潜在缓解饲草品质下降的问题。