Trees affect seedlings through many pathways, including via interactions with soil and surrounding vegetation, that can shape seedling recruitment. Plant-soil feedbacks (PSFs) occur when a plant modifies soil biota or abiotic factors in a way that influences the performance of subsequent or co-occurring plants. PSFs shape plant community dynamics by affecting species relative abundance. After a plant initiating PSFs dies, legacies of PSFs occurring as soil signatures that influence subsequent plants could persist for unknown duration. In this dissertation, I investigated PSFs and PSF legacies of two species, Prunus serotina and Acer rubrum, and the effectiveness of a Hawaiian tree for forest restoration. To examine PSF legacies, my approach was to plant seedlings in soils collected around live trees and stumps of varying ages in greenhouse experiments. PSFs were measured in live tree soils and PSF legacies as the difference between seedling performance in live tree and stump soils. For P. serotina, negative PSF legacies were short-lived, lasting up to 0.5 years after tree removal and occurred under 5% but not 30% full sun. Though restricted to low light, short-lived legacies of P. serotina PSFs could have lasting impacts on plant community dynamics during crucial post-disturbance regeneration by disfavoring P. serotina in small tree-fall gaps. To examine how long soil pathogens outlive tree hosts in gap soils, I studied the presence of oomycetes in soils near live Prunus serotina trees, and 0.5 and 1.5-year-old stumps. I isolated five species of oomycetes from soils, two of which were pathogenic (Pythium intermedium and Pythium irregulare) to P. serotina and present in soils of all stump ages. Continued presence of pathogens of P. serotina in gap soils demonstrates the potential for impacts on conspecific regeneration after tree death, though pairing this finding with improved seedling survival after the first growing season suggests that pathogen effects may weaken with time. For A. rubrum, I found positive PSFs and PSF legacies lasting > 8 years in conspecific soils. These results demonstrate that escaping soil enemies is not a mechanism contributing to the historical rise of A. rubrum abundance in many forests of eastern North America. Rather, positive PSFs and multi-year legacies will promote A. rubrum seedling recruitment near conspecific trees and reinforce the growing dominance of A. rubrum across many forests. In a restoration context, I examined mature tree effects on seedling regeneration through a study of mechanisms of grass suppression and facilitation by planted trees; grass suppression is a crucial condition needed to create opportunities for seedling recruitment. Using stands of a native N-fixer, Acacia koa (koa), on Hawaiʻi Island, I evaluated whether koa suppresses invasive grasses and if so, by which mechanisms. I found consistent effects of grass suppression by koa via shading and litter accumulation, but importantly, total grass suppression rarely occurred. Grass persistence under koa canopies may be driven by a shift in composition to more shade tolerant species. If complete grass suppression and/or more diverse forest are desired, then further management interventions, like diverse understory plantings, could amplify mechanisms of grass suppression and thereby create seedling recruitment opportunities. Taken together, these findings demonstrate the relevance of PSF legacies for forest community dynamics and how PSF legacies can vary in duration and direction depending upon the tree species involved. When planting trees to suppress invasive grasses, multiple mechanisms are involved with a net effect of suppressing grasses, but they may be insufficient to meet goals.
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