Global warming may intensify the hydrological cycle and lead to increased drought severity and duration, which could alter plant community structure and subsequent ecosystem water and carbon dioxide cycling. We report on the net ecosystem exchange of carbon dioxide (NEE) of a semidesert grassland through a severe drought which drove succession from native bunchgrasses to forbs and to eventual dominance by an exotic bunchgrass. We monitored NEE and energy fluxes using eddy covariance coupled with meteorological and soil moisture variables for 6 years at a grassland site in southeastern Arizona, USA. Seasonal NEE typically showed a springtime carbon uptake after winter-spring periods of average rainfall followed by much stronger sink activity during the summer rainy season. The two severe drought years (2004 and 2005) resulted in a net release of carbon dioxide (25 g C m~(-2)) and widespread mortality of native perennial bunchgrasses. Above average summer rains in 2006 alleviated drought conditions, resulting in a large flush of broad-leaved forbs and negative total NEE (-55 g C m~(-2) year~(-1)). Starting in 2007 and continuing through 2009, the ecosystem became increasingly dominated by the exotic grass, Eragrostis lehmanniana, and was a net carbon sink (-47 to -98 g C m~(-2) year~(-1)) but with distinct annual patterns in NEE. Rainfall mediated by soils was the key driver to water and carbon fluxes. Seasonal respiration and photosynthesis were strongly dependent on precipitation, but photosynthesis was more sensitive to rainfall variation. Respiration normalized by evapotranspiration showed no interannual variation, while normalized gross ecosystem production (i.e., water use efficiency) was low during drought years and then increased as the rains returned and the E. lehmanniana invasion progressed. Thus, when dry summer conditions returned in 2009, the potential for ecosystem carbon accumulation was increased and the ecosystem remained a net sink unlike similar dry years when native grasses dominated ecosystem structure.
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机译：全球变暖可能会加剧水文循环并导致干旱加剧和持续时间延长，这可能会改变植物群落结构以及随后的生态系统水和二氧化碳循环。我们报告了半干旱草原通过严重干旱造成的净生态系统二氧化碳交换（NEE），这导致了从天然束草到Forbs的演替，并最终导致了异国束草的统治。我们在美国亚利桑那州东南部的一块草地上，使用涡度协方差以及气象和土壤湿度变量监测了6年的NEE和能量通量。季节性NEE通常在冬春季平均降雨量后显示春季碳吸收，然后在夏季雨季更强的汇活动。这两个严重干旱年份（2004年和2005年）导致二氧化碳的净释放（25 g C m〜（-2））和本地多年生束草的广泛死亡。 2006年夏季降雨高于平均水平，缓解了干旱条件，导致大量阔叶前哨草泛滥，总NEE为负（-55 g C m〜（-2）年〜（-1））。从2007年开始，一直持续到2009年，该生态系统越来越多地由外来草（Eragrostis lehmanniana）主导，是净碳汇（-47至-98 g C m〜（-2）年〜（-1）），但随着NEE中不同的年度模式。土壤介导的降雨是水和碳通量的主要驱动力。季节性呼吸和光合作用强烈依赖于降水，但是光合作用对降雨变化更为敏感。通过蒸散量归一化的呼吸没有年际变化，而在干旱年中归一化的生态系统总产量（即水利用效率）较低，然后随着雨水的回落和雷曼氏梭菌的入侵而增加。因此，当2009年夏季干旱恢复时，生态系统碳积累的潜力增加了，而生态系统仍然是一个净汇，这与类似的干旱年不同，当时当地草占主导地位。
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