1. Climate change is intensifying the hydrologic cycle globally, increasing both the size and frequency of extreme precipitation events, or deluges. Arid and semi‐arid ecosystems are expected to be particularly responsive to this change because their ecological processes are largely driven by distinct soil moisture pulses. However, since soil moisture, air temperature and plant phenology vary throughout the growing season, deluges will likely have differing impacts on these systems depending on when they occur.
2. We conducted a field experiment to assess how the seasonal timing (early, middle or late growing season) of a single deluge (70‐mm precipitation event) altered key ecological processes in the semi‐arid shortgrass steppe of North America.
3. Regardless of timing, a single deluge stimulated most ecosystem processes, but a deluge at mid‐season caused the greatest increase in soil respiration, canopy greenness, above‐ground net primary production (ANPP) and growth and flowering of the dominant plant species Bouteloua gracilis. In contrast, below‐ground net primary production (BNPP) was insensitive to deluge timing, with a consistent BNPP increase in all the deluge treatments that was almost twice as large as the ANPP response. This BNPP response was largely driven by enhanced root production at the 10–20 cm, rather than 0–10 cm, soil depth.
4. Synthesis. In a semi‐arid ecosystem, a single deluge can have season‐long impacts on many ecosystem processes, but responses can be mediated by event timing. Therefore, predicting responses of semi‐arid ecosystems to more dynamic precipitation regimes, and subsequent impacts on the global carbon budget, will require knowledge of how deluge magnitude, frequency and timing are being altered by climate change.

中文翻译：

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半干旱草原极端降雨事件的重要性及其时机

1. 气候变化在全球范围内加剧了水文循环，增加了极端降水事件或洪水的数量和频率。干旱和半干旱生态系统预计将对这种变化做出特别的反应，因为它们的生态过程主要是由不同的土壤水分脉冲驱动的。但是，由于土壤湿度，空气温度和植物物候在整个生长季节中都在变化，因此洪水可能会根据何时发生而对这些系统产生不同的影响。
2. 我们进行了一项野外试验，以评估单个洪水（70毫米降水事件）的季节时机（生长早期，中期或晚期）如何改变了北美半干旱短草草原的关键生态过程。
3. 不论时间长短，一次洪水都会刺激大多数生态系统进程，但是在中期，一次洪水会导致土壤呼吸，冠层绿色，地上净初级生产力（ANPP）以及优势植物物种Bouteloua gracilis的生长和开花最大。相比之下，地下净初级生产（BNPP）对洪水时机不敏感，所有洪水处理中BNPP持续增加，几乎是ANPP响应的两倍。这种BNPP响应很大程度上是由10-20厘米（而不是0-10厘米）土壤深度的根系产量增加推动的。
4. 综合。在半干旱的生态系统中，单个洪水会对许多生态系统过程产生为期一整年的影响，但响应可以通过事件时机来调节。因此，要预测半干旱生态系统对更动态的降水机制的响应以及对全球碳预算的后续影响，将需要了解气候变化如何改变洪水的数量，频率和时间。