Native tree promotes invasion when native grasses are absent

Abstract

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 sepa-rately. Co-occurring factors may, however, act in combination to result in a powerful,yet little explored, mechanism of invasion. Here, we addressed three hypotheses toexplain the localized invasion of the non-native Chenopodium album (Chenopodium)under the canopy of the dominant native tree Neltuma caldenia (Neltuma) in centralArgentina. First, we evaluated that Neltuma favors Chenopodium by modifying envi-ronmental conditions, and specifically assessed the importance of soil versus that ofthe microclimate created by Neltuma shade. Second, we tested that Neltuma benefitsChenopodium by promoting under its canopy (calden microsite) the dominance of agrass species, Nassella tenuissima (Nassella), that exerts weaker competitive effects onChenopodium than those exerted by the dominant grass, Piptochaetium napostaense(Piptochaetium), in adjacent communities free of the tree (open microsite). Lastly, weexplored that the invasion of Chenopodium is mediated by disturbance. Chenopodiumdisplayed increased biomass and fecundity (performance) in soil from calden rela-tive to that from open microsites, whereas it exhibited no response to shade treat-ments. Also, Chenopodium performance in pots with Nassella was similar to that inpots with Piptochaetium. Finally, Chenopodium performance was much greater in potswith 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 withgrasses, Chenopodium performance exhibited no differences between microsites. Thesefindings suggest that disturbance mediates Chenopodium invasion in calden microsites.Specifically, disturbance may weaken strong competitive effects from native grasses onChenopodium, allowing this non-native herb to benefit from favorable soil conditionsunder Neltuma. The combined action of natives enhancing resource levels and distur-bance providing access to them could be a general mechanism of invasion.