Plant–soil feedback links negative distance dependence and light gradient partitioning during seedling establishment

Tree species coexistence has often been explained through either negative distance-dependent/density-dependent (NDD) mortality or resource-based niche partitioning. However, these two influential mechanisms for structuring community dynamics have rarely been investigated simultaneously. Negative conspecific effects, like those caused by plant–soil feedbacks (PSF), could link these two mechanisms by being restricted to low-light environments and determined by seedling shade intolerance. We conducted a 10 week long greenhouse experiment where we assessed survivorship of eight temperate tree species that varied in local adult abundance and seedling shade tolerance, to non-sterile vs. sterile soils collected under the canopy of conspecific vs. heterospecific adult trees grown at low and high light. We found that half of our species experienced strong negative conspecific feedbacks. For three out of four species, the PSF were biotic mediated. However, their effects on survival were often restricted to low-light conditions. Species sensitivity to negative PSF also increased with local adult rarity and with seedling shade intolerance. Our findings taken together indicate that biotic-mediated PSF may enhance the variety of recruitment niches among coexisting species via NDD processes as well as through intensifying light gradient partitioning among tree species.