Biotic Interactions and Plant Invasions in Subtropical Riparian Ecosystems: Experimental and Demographic Evidence from Hangzhou, China

Biotic interactions—encompassing herbivory, pathogen infection, mutualistic pollination, and belowground microbial associations—govern the establishment, colonization, and persistence of non-native plant species. While the Enemy Release Hypothesis (ERH) posits that invasive taxa proliferate due to the absence of co-evolved specialist antagonists, the Biotic Resistance Hypothesis (BRH) suggests that native assemblages suppress exotic spread. Few empirical studies, however, integrate multiple antagonist and mutualist guilds within a unified demographic framework to quantify their net influence on population trajectory (λ). This study investigates the multi-trophic biotic interactions regulating the invasive perennial Solidago canadensis L. in comparison with its co-occurring native congener Solidago decurrens Lour. across riparian and wetland fringes in Hangzhou, Zhejiang Province, China. Utilizing a multi-site, full-factorial field experiment (insect exclusion via systemic insecticide, soil-borne fungal pathogen suppression via fungicide drench, and pollinator exclusion) replicated across six riparian ecosystems in the Hangzhou urban-rural ecotone, we monitored vital rates over three consecutive growing seasons (2023–2025). We embedded these empirical demographic rates into stage-structured matrix population models to estimate asymptotic population growth rates (λ) and elasticity values. Native S. decurrens suffered pronounced demographic penalties under ambient conditions due to severe root-associated fungal pathogen loads, depressing its projected growth rate (λ=0.934±0.038). Conversely, S. canadensis demonstrated pronounced tolerance to generalist foliar insect herbivory and neutral plant-soil feedbacks under ambient microbiota, while simultaneously capitalizing on native generalist pollinators to secure substantial seed set. The projected population growth rate for S. canadensis remained well above unity across all treatments (λ_(ambient)=1.158±0.041). Elasticity analyses revealed that the demographic persistence of S. canadensis is heavily cushioned by clonal ramet survival and vegetative transitions, rendering it resilient against local antagonistic pressures. Effective ecological suppression of S. canadensis in subtropical China necessitates integrated management strategies targeting clonal vegetative expansion alongside native plant-soil community restoration, rather than relying exclusively on single-guild biological controls.