Effects of Elevated CO2 on a Mojave Desert Ecosystem

9524144当前大气二氧化碳浓度比工业化前水平高出25%以上。根据目前向大气中输入碳的速度所作的预测表明，在不到一个世纪的时间里，大气中的二氧化碳含量将翻一番。尽管对于这种增加是否会导致显著的全球变暖存在争议，但毫无疑问，这种增加将对植物生产产生潜在的巨大影响，并可能导致世界主要生态系统植物组成的变化。有必要了解由于大气中温室气体浓度上升，特别是二氧化碳浓度上升，将在世界主要生态系统类型上发生的一系列反应，至关重要的是，这些实验必须在完整的生态系统中进行，而不是在温室或生长室等人工条件下进行。沙漠占地球陆地面积的最大百分比，并通过全球荒漠化的过程，地表面积的增长是最快的。沙漠也是被预测对全球变化最敏感的生态系统类型，因为升高的二氧化碳缓解了植物体内水分胁迫的影响。二氧化碳的升高可能对沙漠生态系统的生产力和生物多样性产生重大影响，并导致这种普遍存在的生态系统类型的功能和土地利用模式发生重大变化。然而，目前还没有任何关于全球变化对沙漠生态系统影响的大规模研究。莫哈韦沙漠是北美最干旱的沙漠之一，与世界其他地区的真正沙漠最相似，因此有可能作为研究沙漠植被对二氧化碳升高的反应的模型系统。自由空气二氧化碳富集（FACE）系统目前正在内华达州南部莫哈韦沙漠内华达试验场建造。这项技术需要一系列实验性的“环”，其中不断向每个环内的完整生态系统提供升高的二氧化碳，并且不会通过人工围栏改变场地的气候。本研究将进行的具体实验包括分析植物生产、繁殖和生存以及景观水平衡对二氧化碳升高的响应。这些具体的实验将通过一系列基于生态系统的模型进行整合，这些模型将专门为莫哈韦沙漠地区开发。建模工作将特别强调两个整合的概念：(1)在极端干燥和贫瘠的沙漠土壤下，植物之间的激烈竞争控制着生态系统的功能，二氧化碳的升高可能会改变目前存在的竞争平衡；(2)荒漠植物的补充和生存是由偶发条件（干旱和异常湿循环）决定的，二氧化碳的升高可能会改变植物对这些偶发条件的反应。在二氧化碳浓度升高的情况下，沙漠生态系统的生产力可能会提高，用水效率也会提高，但从长期来看，复杂的生态系统反馈过程可能会限制这种反应。这样的实验将提供我们预测这些变化将如何影响干旱土地所需的数据。

9524144 Current atmospheric carbon dioxide levels are more than 25% higher than pre-industrial levels. Projections based upon the present rate of carbon input into the atmosphere indicates that the atmospheric carbon dioxide content will double in less than a century. Although there is debate as to whether this increase will result in significant global warming, there is little doubt that this increase will have potentially dramatic effects on plant production and will possibly result in changes in plant composition of major world ecosystems. There is a need to understand the spectrum of responses that will occur over the major ecosystem types of the world as a consequence of rising atmospheric greenhouse gas concentrations, most notably carbon dioxide, and it is critical that these experiments be conducted in intact ecosystems, not in artificial conditions such as greenhouses or growth chambers. Deserts account for the largest percentage of the earth's land surface area, and through the process of global desertification, are showing the most rapid increase in surface area. Deserts are also the ecosystem type that are predicted to be the most sensitive to global change because elevated carbon dioxide relaxes the effects of water stress in plants. Elevated carbon dioxide may significantly impact the productivity and biodiversity of desert ecosystems and result in a substantial change in the function and land-use patterns of this prevalent ecosystem type. However, there have yet to be any large-scale studies of the effects of global change on desert ecosystems. The Mojave Desert is one of the most arid deserts in North America and most resembles true deserts in other parts of the world, giving the potential to serve as a model system for the study of the response of desert vegetation to elevated carbon dioxide. A Free Air carbon dioxide Enrichment (FACE) system is currently being constructed on the Nevada Test Site within the Mojave Desert of southern Nevada. This technology entails a series of experimental "rings" in which elevated carbon dioxide is constantly supplied to an intact ecosystem within each ring, and does not otherwise alter the climate of the site with artificial enclosures. Specific experiments to be conducted in this study include analysis of the responses of plant production, reproduction and survival, and landscape water balance to elevated carbon dioxide. These specific experiments will be integrated through a series of ecosystembased models that will be developed specifically for the Mojave Desert site. The modeling effort will particularly emphasize two integrating concepts: (1) under the extremely dry and infertile soils of deserts, intense competition between plants controls the function of the ecosystem, and elevated carbon dioxide may change the competitive balance that currently exists; and (2) recruitment and survival of plants in deserts are determined by episodic conditions (droughts and abnormal wet cycles), and elevated carbon dioxide may change plant responses to these episodic conditions. Desert ecosystems may become more productive and will use water more efficiently under elevated carbon dioxide, but complex ecosystem feedback processes may limit this response over the long term. Experiments like this will provide the data we need to predict how these changes will affect arid lands.