Abstract

Woody encroachment—the spread of woody vegetation in open ecosystems—is a common threat to grasslands worldwide. Reversing encroachment can be exceedingly difficult once shrubs become established, particularly clonal species that resprout following disturbance. Single stressors are unlikely to reverse woody encroachment, but using multiple stressors in tandem could be successful in slowing or reversing encroachment. We explored whether increasing fire frequency in conjunction with multi-year drought could reduce growth and survival of encroaching shrubs in a tallgrass prairie in northeastern Kansas, USA. Passive rainout shelters (~ 50% rainfall reduction) were constructed over mature clonal shrubs (Cornus drummondii) and co-existing C₄ grasses in two fire treatments (1-year and 4-year burn frequency). Leaf- and whole-plant level physiological responses to drought and fire frequency were monitored in shrubs and grasses from 2019 to 2022. Shrub biomass and stem density following fire were unaffected by five years of consecutive drought treatment, regardless of fire frequency. The drought treatment had more negative effects on grass leaf water potential and photosynthetic rates compared to shrubs. Shrub photosynthetic rates were remarkably stable across each growing season. Overall, we found that five consecutive years of moderate drought in combination with fire was not sufficient to reduce biomass production or stem density in an encroaching clonal shrub (C. drummondii). These results suggest that moderate but chronic press-drought events do not sufficiently stress encroaching clonal shrubs to negatively impact their resilience following fire events, even when fire frequency is high.

中文翻译：

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火灾和干旱的综合影响不足以减缓高草草原的灌木侵蚀

摘要

木本侵占——木本植被在开放生态系统中的蔓延——是全世界草原面临的共同威胁。一旦灌木丛建立起来，扭转侵蚀就会变得极其困难，特别是在干扰后重新发芽的克隆物种。单一压力源不太可能扭转木质侵蚀，但串联使用多个压力源可能会成功减缓或扭转侵蚀。我们探讨了在美国堪萨斯州东北部的高草草原上，火灾频率的增加与多年干旱是否会减少入侵灌木的生长和生存。在两次火灾处理（1 年和 4 年燃烧频率）中，在成熟的克隆灌木（山茱萸）和共存的 C ₄草上建造了被动防雨棚（约减少 50% 的降雨量） 。2019年至2022年，监测了灌木和草丛的叶片和整株植物对干旱和火灾频率的生理反应。无论火灾频率如何，火灾后灌木生物量和茎密度均不受连续五年干旱处理的影响。与灌木相比，干旱处理对草叶水势和光合速率有更大的负面影响。灌木的光合速率在每个生长季节都非常稳定。总体而言，我们发现连续五年的中度干旱与火灾相结合不足以减少入侵的克隆灌木（C.rummondii）的生物量产量或茎密度。这些结果表明，中等但长期的干旱事件不会对侵入的克隆灌木产生足够的压力，从而对其在火灾事件后的恢复能力产生负面影响，即使火灾频率很高。