Selective mortality leads to competitive release of a foundational perennial grass in an aridifying grassland

Vegetation dynamics in dryland ecosystems are defined by competition for water, a limiting and increasingly variable resource. As aridification creates warmer and drier conditions, with more frequent and intense droughts, the nature of competitive relationships may be altered, leading to changes in community composition. Reduced water availability is expected to decrease the cover and abundance of cool-season C₃ grasses on the Colorado Plateau, with unknown impacts on other plant functional types, such as drought-tolerant C₄ species. Here, we explore the effects of competitive release on Pleuraphis jamesii, a dominant C₄ grass with rooting and photosynthetic traits that may enable it to capitalize on the mortality of less drought tolerant C₃ species. To accomplish this, we used herbicide to create three removal treatments (C₃ grass and forb removal, C₄ grass removal, and control (no removal)) surrounding P. jamesii focal individuals and measured the subsequent impacts on water availability and plant water relations and growth. The experimentally induced mortality resulted in greater water availability for focal individuals in the C₃ removal treatment. Increased water availability in C₃ removal plots translated to less water stress and greater biomass production of focal P. jamesii plants, suggesting that competitive release can serve as the basis for shifts in community composition towards drought tolerant C₄ plants observed regionally. These responses were not observed in the C₄ removal treatment despite removing roughly equivalent amounts of total plant cover. Our results suggest that reductions in C₃ competitors may benefit drought-tolerant C₄ species through increased resource availability. These findings contribute to a mechanistic understanding of competitive interactions in increasingly water-limited systems.