Abstract
Urban forests are more vulnerable to exotic species invasions than natural forests and are often a pathway for exotic invasions into natural areas. Investigating the mechanisms responsible for species coexistence in urban ecosystems is important to prevent forest invasions and conserve native biodiversity. In this experiment, we studied seedling recruitment for two exotic invasive (Acer platanoides and Rhamnus cathartica) and two native tree species (Acer saccharum and Betula papyrifera) in two urban forests. We measured the effects of distance from a mature tree on the growth of conspecific seedlings and their belowground interactions (mutualisms and pathogens). We expected that native seedlings growing in close proximity to a mature conspecific tree would more likely be damaged by co-specific pathogens than those growing further away. In contrast, considering that exotic invaders have not coevolved with the local soil pathogens, distance from the adult conspecific tree would not affect their seedlings. We collected undisturbed soil cores at five incremental distances from each adult tree and grew conspecific seeds in these cores. After three months of growth, we measured plant biomass, mycorrhizal root colonization and root lesions. We found that biomass increased with distance from the mature conspecific tree only for A. platanoides and no distance dependent signal was detected for other response variables. Our results show that distance from a conspecific mature tree may not determine exotic species invasibility in an urban forest and that, instead, this may contribute to promote native and invasive species coexistence in urban forest systems.
Highlights
Exotic plant species invasion is a major threat to global biodiversity and natural ecosystems
In natural forest ecosystems, native species coexistence is maintained by native seedling recruitment limitation assuming that host specific parasites are more likely to damage conspecific seeds and seedlings the closer they germinate to the parent tree (distance-dependent coexistence mechanism explained by the Janzen and Connell Hypothesis (Connell 1971; Janzen 1970))
Biogeographic studies have supported the parent tree distance-dependent mechanism to explain species coexistence across latitudinal gradients from boreal to temperate systems and across different ecological guilds (Comita et al 2014). It is still unknown if distance-dependent mechanisms in regard to recruitment limitation apply to closed-canopy urban forest ecosystems invaded by exotic invasive plant species
Summary
Exotic plant species invasion is a major threat to global biodiversity and natural ecosystems. Biogeographic studies have supported the parent tree distance-dependent mechanism to explain species coexistence across latitudinal gradients from boreal to temperate systems and across different ecological guilds (Comita et al 2014) It is still unknown if distance-dependent mechanisms in regard to recruitment limitation apply to closed-canopy urban forest ecosystems invaded by exotic invasive plant species (hereafter invasive species). In this study we tested the conspecific tree distance effect on the seedling growth and the belowground interactions of two native and two invasive exotic species coexisting in mixed deciduous urban forests. We hypothesized that distance-dependent effects on conspecific seedlings are present for the native species in urban forests but not for invasive species since exotic invaders may have escaped from their co-evolved specific pathogens and may potentially have acquired even better belowground mutualisms than those in their native range. This does not apply to exotic invasive plant species; H2 – Root mycorrhizal colonization : lesions ratio increases with distance from native conspecific trees but remains unchanged for exotic invasive plant species
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