Experimental field comparison of native and non-native maple seedlings: natural enemies, ecophysiology, growth and survival

1. Acer platanoides (Norway maple) is an important non-native invasive canopy tree in North American deciduous forests, where native species diversity and abundance are greatly reduced under its canopy. We conducted a field experiment in North American forests to compare planted seedlings of A. platanoides and Acer saccharum (sugar maple), a widespread, common native that, like A. platanoides, is shade tolerant. Over two growing seasons in three forests we compared multiple components of seedling success: damage from natural enemies, ecophysiology, growth and survival. We reasoned that equal or superior performance by A. platanoides relative to A. saccharum indicates seedling characteristics that support invasiveness, while inferior performance indicates potential barriers to invasion. 2. Acer platanoides seedlings produced more leaves and allocated more biomass to roots, A. saccharum had greater water use efficiency, and the two species exhibited similar photosynthesis and first-season mortality rates. Acer platanoides had greater winter survival and earlier spring leaf emergence, but second-season mortality rates were similar. 3. The success of A. platanoides seedlings was not due to escape from natural enemies, contrary to the enemy release hypothesis. Foliar insect herbivory and disease symptoms were similarly high for both native and non-native, and seedling biomass did not differ. Rather, A. platanoides compared well with A. saccharum because of its equivalent ability to photosynthesize in the low light herb layer, its higher leaf production and greater allocation to roots, and its lower winter mortality coupled with earlier spring emergence. Its only potential barrier to seedling establishment, relative to A. saccharum, was lower water use efficiency, which possibly could hinder its invasion into drier forests. 4. The spread of non-native canopy trees poses an especially serious problem for native forest communities, because canopy trees strongly influence species in all forest layers. Success at reaching the canopy depends on a tree's ecology in previous life-history stages, particularly as a vulnerable seedling, but little is known about seedling characteristics that promote non-native tree invasion. Experimental field comparison with ecologically successful native trees provides insight into why non-native trees succeed as seedlings, which is a necessary stage on their journey into the forest canopy.