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Global change agents are creating novel climatic and edaphic conditions that may favor introduced species. We attempted to identify mechanisms and impacts of Bromus tectorum invasion in the Colorado Front Range mixed-grass prairie under changing conditions.

We conducted an in-situ experiment with three removal treatments (removal of B. tectorum, removal of Pascopyrum smithii, and no removal) and two nitrogen treatments (ambient and low N), and measured plant community response. We used isotopic analysis of delta O-18 and delta D in plants and soils to identify seasonal source water of B. tectorum and P. smithii.

We found that dominance of B. tectorum was greatest under high resource conditions (ambient N, wet winter) now common due to increased N deposition and climate change. However, its removal had little impact on native plant abundance or composition. Isotopic results show B. tectorum and P. smithii partitioning water use between shallow versus deeper soil layers during the dry summer season.

Our results suggest that changing environmental conditions favor the introduced grass Bromus tectorum over native species, but probably not due to altered competitive relationships. Instead, B. tectorum appears to be opportunistically responding to expansion of a phenological niche and increased nutrient availability.