Data from: Invasive plant species support each other’s growth in low-nutrient conditions but compete when nutrients are abundant

Globally, many ecosystems are now co-invaded by multiple exotic plant species that can engage in competitive, facilitative, or neutral interactions with each other and with native plant species, both within and between species. The invaded ecosystems often exhibit spatial variation in soil moisture and nutrients. However, it is still unclear how the co-variation of soil moisture and nutrient levels can affect the competitive and facilitative interactions among invasive and native plant species. The stress-gradient hypothesis suggests that facilitative interactions between species become more common in stressful environments, while competitive interactions dominate in more favourable conditions. Additionally, the invasional meltdown hypothesis proposes that invasive species can facilitate the establishment and spread of other invasive species. Both hypotheses provide insights into how environmental stresses and interspecies interactions can affect invasion success and ecological impacts. However, these hypotheses have rarely been tested in the context of co-limitation of soil moisture and nutrients. Furthermore, studies of plant invasions across environmental gradients often overlook the relative importance of intraspecific vs. interspecific competition and facilitation. To address these knowledge gaps, we conducted a greenhouse experiment where we grew individuals (i.e., focal plants) of five congeneric pairs of invasive and native plant species under three levels of plant-plant interactions (intraspecific, interspecific involving a native and an invasive plant, and interspecific involving two native or invasive individuals within a pot) and fully crossed with two levels of water (drought vs. well-watered) and nutrient (low vs. high) treatments. Consistent with the stress-gradient and invasional meltdown hypotheses, our findings show that under low-nutrient conditions, biomass production of invasive focal plants was facilitated by interspecific neighbouring invasive species. Conversely, under high-nutrient conditions, biomass production of invasive focal plants was suppressed by interspecific neighbouring invasive species. Additionally, under low-nutrient conditions, native plant species similarly enhanced the biomass production of both invasive and native focal plants. Across the different water and nutrient treatment combinations, neither invasive nor native plants showed significant intraspecific competitive and facilitative interactions. Taken together, these results suggest that co-occurring invasive exotic plant species may facilitate each other in low-nutrient habitats but compete in high-nutrient habitats.