Sensitization rather than acclimation to chronic atmospheric aridity in invasive and native dryland C4 grasses

Despite extreme heat and limited water availability, invasive plant species are rapidly expanding in arid ecosystems, threatening native biodiversity. To understand the physiological mechanisms underlying invasive success in water-limited ecosystems, we analyzed photosynthetic, hydraulic and morphological traits of the invasive African grass Cenchrus ciliaris and three native North American dryland grasses (Bouteloua gracilis, Schizachyrium scoparium, and Sporobolus cryptandrus) in a full factorial growth chamber experiment combining two vapor pressure deficit (VPD) levels and three soil moisture levels. We hypothesized that C. ciliaris would exhibit greater physiological acclimation and morphological plasticity than native grasses, enabling superior tolerance to atmospheric and soil drought. Our results showed that rather than a simple invasive vs. native dichotomy, species grouped into functional strategies: a risk-prone, anisohydric group (C. ciliaris and S. scoparium) and a conservative, drought-avoidant group (B. gracilis and S. cryptandrus). The invasive C. ciliaris distinguished itself not through superior photosynthetic capacity, but through a unique ability to proactively allocate carbon to roots under high VPD combined with high soil moisture, differing from all native species. Contrary to our hypothesis, none of the species showed stomatal acclimation to sudden increases in VPD; instead, stomatal sensitivity to instantaneous VPD spikes operated as an intrinsic, species-level trait inflexible to prior environmental conditioning. Together, these findings suggest that the dominance of C. ciliaris in water-limited conditions is explained by allocation plasticity and anisohydric tolerance rather than superior carbon gain or stomatal acclimation.