Native tree promotes invasion when native grasses are absent

Despite the expected co‐occurrence of effects from resident species, the environment, and disturbance on non‐natives, these factors have largely been treated as acting separately. Co‐occurring factors may, however, act in combination to result in a powerful, yet little explored, mechanism of invasion. Here, we addressed three hypotheses to explain the localized invasion of the non‐native Chenopodium album ( Chenopodium ) under the canopy of the dominant native tree Neltuma caldenia ( Neltuma ) in central Argentina. First, we evaluated that Neltuma favors Chenopodium by modifying environmental conditions, and specifically assessed the importance of soil versus that of the microclimate created by Neltuma shade. Second, we tested that Neltuma benefits Chenopodium by promoting under its canopy (calden microsite) the dominance of a grass species, Nassella tenuissima ( Nassella ), that exerts weaker competitive effects on Chenopodium than those exerted by the dominant grass, Piptochaetium napostaense ( Piptochaetium ), in adjacent communities free of the tree (open microsite). Lastly, we explored that the invasion of Chenopodium is mediated by disturbance. Chenopodium displayed increased biomass and fecundity (performance) in soil from calden relative to that from open microsites, whereas it exhibited no response to shade treatments. Also, Chenopodium performance in pots with Nassella was similar to that in pots with Piptochaetium . Finally, Chenopodium performance was much greater in pots with no initial plants than in those with grasses. Importantly, in pots without plants, Chenopodium performed much better in calden than open microsites, but in pots with grasses, Chenopodium performance exhibited no differences between microsites. These findings suggest that disturbance mediates Chenopodium invasion in calden microsites. Specifically, disturbance may weaken strong competitive effects from native grasses on Chenopodium , allowing this non‐native herb to benefit from favorable soil conditions under Neltuma . The combined action of natives enhancing resource levels and disturbance providing access to them could be a general mechanism of invasion.