Intact forests provide only weak resistance to a shade-tolerant invasive Norway maple (Acer platanoides L.)

1 Intact, closed canopied forests appear highly resistant to exotic plant invasions, but there are few experimental studies of this observation. To test this issue and explore the conditions that foster resistance, we experimentally added Norway maple (Acer platanoides L.) seeds to intact forests for 3 years and monitored emergence, survivorship and height growth for 5 years. 2 Seed additions (250 seeds(-1) m(2)) were replicated in situ in combinations of light (deep shade vs. small gaps), soil fertility (NO3 and pH), and variation in soil moisture as influenced by topography (hill slopes vs. flat areas at slope bases) in five eastern deciduous forests in central New York and southern Connecticut. We then parameterized a model with this data to project long-term rates of sapling recruitment. 3 Seedlings had high survival and low growth rates in the deep shade, suggesting that this species readily forms a seedling bank in intact forests. By age 5, annual survivorship reached 93% in deep shade and 98% in small gaps. Median seedling heights were <= 10 cm in all treatments after 5 years, though the largest seedlings were markedly taller in gaps. Mean year 5 densities of all experimental seedlings (ages 3-5 years) were 14 seedlings(-1) m(2) (+/- 2.4 SE) in the shade and 19 seedlings(-1) m(2) (+/- 3.5 SE) in small gaps. 4 While seedlings colonized in all conditions, resistance to invasion varied markedly; resistance was highest in deeply shaded, acidic conditions (pH < 4.5). Small gaps initially promoted invasion, but by age 4 shade survivorship rates were equivalent to gaps. Higher soil pH (especially > 6) also significantly increased survivorship, particularly in deep shade. Shading's main effect was to suppress height growth. Overall, the model predicted impressive sapling recruitment; even in deeply shaded, acidic conditions, approximately 1% of seeds emerge and survive to become saplings. 5 Intact forests only weakly resisted A. platanoides colonization, but strongly suppressed its rate of invasion. As such, the frequency of disturbance, though ultimately unnecessary for A. platanoides invasions, will strongly influence its near-term invasiveness. Dispersal limitation and slow stand dynamics appear to be the primary constraints on its current distribution.