Neighbourhood models of the effects of the invasive Acer platanoides on tree seedling dynamics: linking impacts on communities and ecosystems

1. Effects of invasive species on ecosystem processes are often thought to underlie the effects of invaders on community dynamics. Specifically, positive feedbacks in which invasive species alter ecosystem function in ways that favour their own growth have been suggested as an important mechanism contributing to the success of invasion. 2. In this study, we analysed the impacts of the invasive exotic tree Acer platanoides on survival and growth of conspecific and native tree seedlings, and explored whether these impacts can be explained by the ecosystem effects of the invader. Seedlings of Acer platanoides, Acer saccharum, Fraxinus americana and Prunus serotina were monitored in quadrats in three forest stands in north-western Connecticut. Soil resources and light levels were quantified in the same quadrats. 3. Maximum-likelihood methods were used to predict seedling survival and growth as a function of the size and spatial configuration of A. platanoides trees in the immediate neighbourhood (0-25 m). 4. The abundance of A. platanoides in the neighbourhood had moderate negative effects on survival of first-year conspecific seedlings, but did not affect survival of older conspecific or native seedlings. These negative effects on conspecifics were not correlated with soil nutrients or light levels, but were presumably related to Janzen-Connell effects. In contrast, A. platanoides had strong positive effects on the growth of seedlings of all four species. These positive effects appear to be related to the positive impacts of the invader on soil fertility. 5. Our results support the importance of canopy-seedling feedbacks as a mechanism regulating the rate of invasion in forests. However, they also indicate that the net consequences of feedbacks on the process of invasion are probably determined by the balance of positive and negative feedbacks acting at the same time on different aspects of regeneration (i.e. survival vs. growth). 6. Synthesis. Because the species with the highest inherent growth rates were the most responsive to the 'fertilizing' effect of A. platanoides, we conclude that the invasion of north-eastern forests by this exotic tree may facilitate canopy dominance by fast-growing native and exotic species associated with fertile soils.