Soil and litter legacy effects of pine invasion on subsequent reinvasion and secondary invasion

Abstract

Invasive alien trees can alter the biotic and abiotic properties of invaded ecosystems. Even after intensive control programmes, areas can be prone to reinvasion by the same species or secondary invasion by other species. One key factor that might determine reinvasion or secondary invasion is soil and litter legacy effects of the primary invader. Alien conifers are a prominent case of invaders with the potential to create lasting soil and litter legacy effects. In New Zealand, lodgepole pine (Pinus contorta) is considered the most problematic invasive conifer species posing a threat to biodiversity, farming productivity and landscape values. We assessed if soil and litter microbiome legacies of controlled P. contorta invasions can facilitate the reinvasion by conspecifics and potential secondary invasion by the two most abundant commercially planted alien conifer species (Pinus radiata, Pseudotsuga menziesii). We grew plants in a sterile background soil inoculated with live or sterile soil and litter samples sourced from uninvaded grassland and stands of P. contorta that had been controlled by herbicide 2 years previously. Our experimental design aimed to disentangle the total observed legacy effect into its components of soil and litter microbiome‐mediated effects and their potential interaction on seedling establishment and early growth of the primary invader and the potential secondary invaders. We found an overall negative legacy effect of P. contorta invasion on its own performance, whereas P. menziesii and P. radiata were neutrally or even positively affected, respectively. This was mainly driven by the individual effect of the soil microbiome. In contrast, seedling establishment was positively affected by the physical litter component but negatively affected by the litter microbiome. Thereby, the soil and litter microbiome‐mediated components can interact creating non‐additive effects. Synthesis. Our results suggest an increased risk of secondary invasion by other alien conifers after control of P. contorta. The complex interactions of soil and litter effects on plant performance highlight the importance of simultaneously considering multiple dimensions of legacy effects and their possible interactions. Paying attention to such interactions may help to better understand reinvasion and secondary invasion processes and inform post‐invasion management practices.