Across large portions of the western United States, drought intervals are increasing, often accompanied by larger-magnitude rainfalls. Semi-arid ecosystems are expected to be especially responsive to such temporal repackaging of rainfall because of their high sensitivity to variation in soil moisture. We conducted a field manipulation experiment to evaluate the impacts of summer rainfall repackaging (small/many events to large/few events with a fixed total seasonal amount) on a semi-arid mixed annual/perennial bunchgrass ecosystem. We monitored the sensitivity of plant greenness and productivity to soil moisture under rainfall pulses by combining automated, high-frequency repeat digital images at both plot- and plant functional type- level, whole plot CO₂ uptake measurements, and continuous in-situ soil moisture data. We found that plot greenness was closely correlated with gross primary productivity across all rainfall repackaging treatments (R = 0.82). Plot greenness was weakly correlated to soil moisture at the beginning of the growing season but showed a significant positive correlation during peak and late growing season. Notably, we found a significant lag time of ∼5 days between changes in soil moisture and canopy greenness under large/few events, while there was no lag under small/many events. This time lag was found to be driven by perennial grass greenness and its sensitivity to the relatively deep soil water infiltration observed with large/few rainfall events. Predicting semi-arid grassland responses to soil moisture dynamics under ongoing rainfall repackaging, and subsequent impacts on the regional to global carbon budget, should consider plant communities’ phenological characteristics and functional type composition.

在美国西部的大部分地区，干旱间隔正在增加，通常伴随着更大规模的降雨。由于半干旱生态系统对土壤水分的变化高度敏感，因此预计它们对这种降雨的时间重新包装特别敏感。我们进行了一项田间操作实验，以评估夏季降雨重新包装（小/多事件到大/少数事件，具有固定的总季节量）对半干旱混合一年生/多年生束草生态系统的影响。我们通过结合小区和植物功能类型级别的自动化高频重复数字图像、整小区 CO 2 吸收测量_和连续就地土壤水分数据。我们发现，在所有降雨重新包装处理中，地块绿化度与总初级生产力密切相关 (R = 0.82)。在生长季节开始时，地块绿度与土壤水分呈弱相关，但在生长旺季和后期显示出显着的正相关。值得注意的是，我们发现在大/少事件下土壤水分和冠层绿度变化之间存在约 5 天的显着滞后时间，而在小/多事件下没有滞后。这一时间滞后被发现是由多年生草的绿色及其对在大/少降雨事件中观察到的相对深层土壤水渗透的敏感性所驱动的。预测半干旱草地在持续降雨重新包装下对土壤水分动态的反应，以及随后对区域到全球碳收支的影响，