Both light and soil fungi are important determinants of early regeneration success in tree species, but few studies have examined their role simultaneously. Soil fungal pathogens are key mediators of negative density-dependent mortality (NDD) in seeds and seedlings, and the ability to withstand pathogens in the shaded conditions of closed-canopy forests has been hypothesized to reinforce partitioning of the light-gradient by tree species. For four species of tropical trees with varying life history strategies—two shade-tolerant and two shade-intolerant—we conducted an in situ field experiment to examine the interactive effects of fungal pathogens, light, and seed density on germination and early seedling establishment in a fragmented forest in Western Ghats, India. For each species, seeds were sown into 1x1 m plots containing soil collected from underneath parent trees, with each plot assigned to high (100/m2) or low (10/m2) density (additional category of 1000 seeds/m2 for Macaranga peltata), high light or shaded understory, and fungicide or no fungicide application, in a fully factorial design. We analyzed probabilities of cumulative seedling emergence and final seedling establishment using generalized linear mixed-models with a logit link.
The two shade-intolerant species were strongly constrained by light; their seedlings eventually survived only in high light. Fungicide application improved seedling emergence, but not final seedling establishment, for shade-intolerant Toona ciliata. Gains from fungicide application for Toona were significantly higher in shade. Light was the only factor affecting seedling emergence of shade-intolerant Macaranga peltata, but its seedlings finally survived only in high light plots with highest seed density (1000/ m2) and treated with fungicide. Conversely, shade-tolerant Olea dioica and Heritiera papilio recruited equally well in light and shade. Recruitment of both shade-tolerant species clearly improved with fungicide application, suggesting a benefit from dispersal to locations where pathogens are low, e.g. away from conspecific adults. Notably, fungicide application improved Olea recruitment to a greater degree in high light, indicating how this species could colonize high-light locations. Our results suggest that the interaction of light and pathogens during early regeneration can be idiosyncratic among species and can vary even at different phases of early recruitment.
Edge-effects due to habitat modification or fragmentation alter recruitment conditions in human-impacted forests. Although changes to both light availability and plant-soil feedback can potentially affect species' regeneration success, the combined role of light and pathogens is yet to be examined in the context of plant community dynamics in human-impacted forests.