Invasive Acer platanoides inhibits native sapling growth in forest understorey communities

1. Over three growing seasons, we tested how an invasive tree species (Acer platanoides) affected native plant growth in understorey communities of a suburban forest in central New Jersey, USA. We planted similar aged and sized saplings (>= 0.25 m tall) into experimental plots identified with one of three treatments (0%, 25% and 50% of total stems are invasive species) and hypothesized native species would grow better in communities lacking invasive plants. 2. There was a plant survival rate of 90% for the duration of the experiment, but in treatments where natives competed with A. platanoides, growth of native species was significantly less than in the purely native stand. In 2006, the mean height of A. rubrum was 110 cm (+/- 4 SE) in communities with the highest proportion of A. platanoides, while it was 149 cm (+/- 7 SE) in the 0% invasive communities. Conversely, A. platanoides grew similarly in treatments where it comprised two different proportions and beneath both canopy types (i.e. invasive and native). Native saplings were 28% shorter beneath an invasive canopy (i.e. A. platanoides), compared with a native canopy. A striking interaction existed between community treatment and canopy type, as the invasive canopy had such a strong negative effect on native growth that the presence of invasive saplings was irrelevant. However, beneath a native canopy, the absence of invasive saplings significantly increased growth of native saplings. As the extent and rate of invasive proliferation often makes complete removal unrealistic, this study supports episodic removal (every 2-3 years) of this invasive sapling. Synthesis. This experiment showed that native sapling growth was inhibited (i) when growing beneath an invasive canopy and (ii) when competing with A. platanoides in forest understorey communities. It appears canopy type is more important, because the negative effects from an invasive canopy were strong enough that the co-occurrence of invasive saplings had no impact on native growth. The capability of A. platanoides to inhibit native saplings through understorey competition and overstorey canopy effects, while not affecting conspecifics, may contribute to its success as an invader of North American forests.