Mixed evidence for plant-soil feedbacks in forest invasions.

Plant-soil feedbacks (PSFs) are plant-mediated changes to soil properties that ultimately influence plant performance, and can, thus, determine plant diversity, succession, and invasion. We hypothesized that PSFs influence invasion processes and that PSF mechanisms are largely driven by changes in soil properties produced by specific plant species. To test these hypotheses, we studied the effects of different soils collected from under common plant species on the growth of the invasive plant Phytolacca americana. We found that PSFs may interfere with invasion resistance because P. americana seedlings showed reduced growth (lower biomass) in soils collected from underneath some native species compared with soils collected from underneath P. americana and two non-native plants. We then selected eight co-occurring native and non-native plant species, and examined PSF dynamics and mechanisms in a pairwise conditioned soil greenhouse experiment. Plant species-specific conditioning effects regarding soil nutrients and enzyme activities were observed. Phytolacca americana had a high ability to use soil N, which may be related to its high invasion ability. Soil P was significantly lower in Quercus acutissima-conditioned soil, indicating that low P availability in Q. acutissima forests may enhance resistance to plant invasion. However, surprisingly, some native plants did not produce PSF effects that decreased the relative performance of invasive plants, nor did the invasive plants produce PSF effects that increased their own performance. We speculate that these PSF findings from greenhouse experiments cannot be extrapolated to field conditions because the litter and allelochemicals of some plants may be important for invasion resistance.