Nitrogen Addition Additively Boosts Climate-Driven Salsola tragus (Russian Thistle) Invasion Into Southwestern USA Grasslands

Abstract

Effective management of invasive, non-native plants requires understanding key drivers of invasion, including nitrogen enrichment from atmospheric deposition and agriculture. Long-term studies are essential for capturing complex invasion dynamics yet remain uncommon. We used 22 years of observations and 14 years of nitrogen experiments to examine invasion by Salsola tragus (Russian thistle) into three semi-arid grasslands of central New Mexico. Invasion patterns varied, with rapid expansion in Chihuahuan Desert grassland and the Desert-Plains ecotone but stability in Plains grassland. Under ambient nitrogen, precipitation most strongly correlated with Russian thistle biomass and high native grass biomass was associated with Russian thistle declines. Across four nitrogen experiments, biomass increased 208% with nitrogen addition, with effects varying by year and experiment. In two cases, nitrogen also accelerated invasion rates. Nitrogen effects were not mitigated by climate or community traits. These findings highlight the substantial role of nitrogen in promoting Russian thistle invasion, especially under future eutrophication scenarios