Plant-soil feedbacks in temperate and tropical forests

The Janzen-Connell (J-C) Model proposes that host-specific natural enemies maintain high tropical tree diversity by reducing seed and/or seedling survivorship near conspecific adults and/or at high conspecific densities. Such non-competitive distance or density-dependent (NCDD) mortality would favor establishment of heterospecific individuals, thus promoting species coexistence. Negative plant-soil feedback, whereby individual plants "culture" the soil community in which they grow to the detriment of themselves and other conspecific individuals, may be an important mechanism that could create NCDD mortality and/or reduced growth. I used a wet-sieving method to filter out biotic and water extractable chemical elements from soil that had been cultured by conspecific and heterospecific adults and seedlings. These soil extracts were used in greenhouse experiments with temperate and tropical tree species to examine (1) advantages to heterospecific and disadvantages to conspecific recruitment, (2) soil mechanisms underlying NCDD, (3) differences between common and rare species in sensitivity to J-C processes, (4) the strength of J-C processes in tropical versus temperate forests, (5) and the interactions of J-C processes with light availability. I found that susceptibility to microbial extract cultured by conspecific individuals was negatively correlated with seedling shade tolerance not a species' local abundance, thereby exaggerating apparent shade tolerance differences among species and likely contributing to species coexistence through heightening niche differentiation. When comparing effects of con- vs. hetero-specific cultured soils, I found that species-specific feedbacks between adult trees (not seedlings) and soil influenced seedling performance for all temperate and tropical species. Con- and hetero-specific effects had similar prevalence and magnitude of influence for temperate species whereas three of the six tropical species had decreased performance when grown with extract cultured by con- vs. all hetero-specific adults and an additional two species had decreased performance in con- vs. two or more heterospecific cultured extracts. In addition, in temperate forests, soils cultured by a particular species do not necessarily improve heterospecific seedling performance relative to conspecific seedlings which may impede the ability of these plant-soil feedbacks to enhance species coexistence. However, in tropical forests, heterospecific seedlings are favored relative to conspecific seedlings in soils cultured by a given species. Thus, J-C processes appear stronger in tropical vs. temperate forests, at least those mediated by plant-soil feedbacks. Surprisingly, chemical factors in the soil not micro-organisms seem to be primarily responsible for these feedbacks. Thus, my dissertation identifies a novel mechanism (feedback between adult trees and soil abiotic factors) that creates NCDD seedling mortality and/or reduced growth and moves the J-C Model beyond solely focusing on natural enemies.