Recent observational evidence suggests that nighttime temperatures are increasing faster than daytime temperatures, while in some regions precipitation events are becoming less frequent and more intense. The combined ecological impacts of these climatic changes on crassulacean acid metabolism (CAM) plants and their interactions with other functional groups (i.e., grass communities) remain poorly understood. Here we developed a growth chamber experiment to investigate how two CAM-grass communities in desert ecosystems of the southwestern United States and northern Mexico respond to asymmetric warming and increasing rainfall variability. Grasses generally showed competitive advantages over CAM plants with increasing rainfall variability under ambient temperature conditions. In contrast, asymmetric warming caused mortality of both grass species (Bouteloua eriopoda and Bouteloua curtipendula) in both rainfall treatments due to enhanced drought stress. Grass mortality indirectly favored CAM plants even though the biomass of both CAM species Cylindropuntia imbricata and Opuntia phaeacantha significantly decreased. The stem's volume-to-surface ratio of C. imbricata was significantly higher in mixture than in monoculture under ambient temperature (both P < 0.0014); however, the difference became insignificant under asymmetric warming (both P > 0.1625), suggesting that warming weakens the negative effects of interspecific competition on CAM plant growth. Our findings suggest that while the increase in intra-annual rainfall variability enhances grass productivity, asymmetric warming may lead to grass mortality, thereby indirectly favoring the expansion of co-existing CAM plants. This study provides novel experimental evidence showing how the ongoing changes in global warming and rainfall variability affect CAM-grass growth and interactions in dryland ecosystems.

中文翻译：

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不对称的气候变暖和降雨变化的增加间接地增加了CAM植物的生长。

最新的观测证据表明，夜间温度的升高快于白天的温度，而在某些地区，降水事件变得越来越少且更加强烈。这些气候变化对景天素酸代谢（CAM）植物及其与其他功能组（即草群落）的相互作用的综合生态影响仍然知之甚少。在这里，我们开发了一个生长室实验，以研究美国西南部和墨西哥北部沙漠生态系统中的两个CAM草群落对非对称变暖和降雨变化性的响应。在环境温度条件下，随着降雨变化的增加，草通常表现出优于CAM植物的竞争优势。相反，由于干旱胁迫的加剧，在两次降雨处理中，非对称变暖导致两种草种（Bouteloua eriopoda和Bouteloua curtipendula）的死亡。尽管CAM物种Cylindropuntia imbricata和Opuntia phaeacantha的生物量均显着降低，但草类死亡率间接地有利于CAM植物。在环境温度下，混合物中棉铃虫的茎体积与表面之比明显高于单一培养物中的二者（均P <0.0014）。然而，在不对称变暖下（P> 0.1625），差异变得微不足道，这表明变暖减弱了种间竞争对CAM植物生长的负面影响。我们的研究结果表明，虽然年内降雨变化的增加会提高草的生产力，但不对称的变暖可能会导致草的死亡，从而间接地有利于并存的CAM工厂的扩张。这项研究提供了新颖的实验证据，显示了全球变暖和降雨变化的持续变化如何影响干旱地区生态系统中的CAM草生长和相互作用。