Introduced ungulate browsing facilitates non-native tree invasions through enhanced compensatory growth and inducible defenses

Abstract

Invasive species often interact in novel ways affecting native ecosystems, but the role of herbivore-mediated plant responses to damage in such interactions remains understudied. We conducted a common garden experiment in northern Patagonia to assess the tolerance and resistance of native and non-native tree saplings—broadleaved and coniferous—to simulated browsing by introduced ungulates. Eight dominant tree species were subjected to mechanical clipping and sheep saliva application to mimic herbivory. We evaluated performance metrics, biomass allocation, leaf traits, induced chemical defenses, and subsequent leaf damage by insects (only in broadleaved species). Overall, plant responses to damage varied depending on whether the trees were native or non-native. In general, non-native broadleaved species exhibited strong compensatory regrowth, while native broadleaved species showed limited height recovery. However, broadleaved species showed less insect damage after simulated browsing, especially native ones, suggesting the induction of some defensive traits not evaluated here. In turn, non-native conifers showed reduced aboveground biomass but increased branch production and induced chemical defenses, including elevated levels of resin and monoterpenes, particularly γ-carene. In comparison, native conifers showed a conservative growth response and limited plasticity. Saliva application triggered specific defensive responses only in non-native species, suggesting evolved recognition of herbivore cues. These findings highlight distinct functional strategies: non-natives combined architectural plasticity with inducible defenses, while natives relied more on constitutive traits. Our results show that introduced ungulates may indirectly facilitate non-native tree invasion by differentially affecting native species recovery and defense, with implications for forest dynamics under ongoing invasion and co-invasion scenarios.