Ecosystem process rates typically increase after plant invasion, but the extent to which this is driven by (i) changes in productivity, (ii) exotic species’ traits, or (iii) novel (non-coevolved) biotic interactions has never been quantified. We created communities varying in exotic plant dominance, plant traits, soil biota, and invertebrate herbivores and measured indicators of carbon cycling. Interactions with soil biota and herbivores were the strongest drivers of exotic plant effects, particularly on measures of soil carbon turnover. Moreover, plant traits related to growth and nutrient acquisition explained differences in the ways that exotic plants interacted with novel biota compared with natives. We conclude that novel biological interactions with exotic species are a more important driver of ecosystem transformation than was previously recognized.

Pathogen accumulation on an invasive plant species can occur over time, through co-invasion, or adaptation of native pathogen species. While accumulated pathogens can reduce the success and spread of an invasive species, they can also spill-over into native plant communities or valuable non-native populations. Transmission of pathogens may be density-dependent, with dense invasive populations creating better opportunities for pathogen spread than scattered individuals. Some pine species (Pinus) and some other Pinaceae (including Pseudotsuga) are extremely invasive trees in New Zealand but trees in the Pinaceae are also used extensively within the forestry industry. Little is known about the foliar pathogens present on invasive populations and whether they pose a risk to industry. We cultured foliar fungi from needles of Pseudotsuga menziesii and Pinus contorta found at both low and high densities of invasion. DNA from fungal cultures was extracted and sequenced using Sanger sequencing. We cultured fungi from a greater proportion of P. menziesii than P. contorta needles and a greater proportion of trees from low versus high densities of invasion. The richness of foliar fungi decreased as a function of density and P. menziesii hosted a greater richness of fungi than P. contorta. We observed no change in the richness of pathogens between P. menziesii and P. contorta or between low and high density invasions. However, we did observe a greater proportion of fungi that were potentially pathogenic at high density than at low density. We identified one major widespread pathogen (Nothophaeocryptopus gaeumannii) and a number of opportunistic potential pathogens (i.e. Sydowia polyspora, Lophodermium pinastri and Alternaria alternata), indicating the possibility of spill-over into commercial plantations.