Determinants of establishment success of rare and common alien and native plants. What roles play above- and belowground enemies and their interaction with disturbance?
Understanding the factors that drive commonness and rarity of plant species is a key question in ecology, in particular regarding invasive species. An essential aspect for a species success is the establishment of early stages from seed or seedlings. We aimed to tease apart what role above- and belowground enemies and their interaction with disturbance play for establishment success. We used a two-year multi-site field experiment with 20 species from 5 families of common and rare alien and native plant species. We tested the establishment success of seeds and seedlings, separately, under the following three treatments in a full factorial design. Tilling of the upper soil layer was applied as a disturbance treatment (1) and application of fungicide was used as belowground enemy exclusion treatment (2). Furthermore, we used net-cages for herbivore exclusion (3) but only for the seedlings in the experiment. This allowed us to test the influence of above- and belowground enemies simultaneously and to assess potential synergistic effects/roles of disturbance and pathogenic soil biota.
In concordance with the biotic resistance hypothesis, native species showed greater establishment success than alien species across all treatments, regardless of their commonness. Moreover, establishment success of all species was positively affected by the disturbance treatment. Notably, rare aliens showed lower establishment success in undisturbed sites with fungicide application. Release from pathogen pressure of the undisturbed resident community by fungicide application may explain this lower establishment success of rare aliens. These findings applied consistently to establishment from both seeds and seedlings, although less significantly so for seedlings, highlighting the role of pathogens in very early stages of establishment (i.e. after germination). Herbivore exclusion did not play a role for establishment success when initial size differences of seedlings were taken in account. In conclusion, we found evidence that alien species seem to benefit from soil-borne enemy release, as shown by reduced establishment success when the resident community is released from pathogenic pressure. In general, however, the intact grassland communities exhibited strong biotic resistance to species establishment success, whether they were native or alien.