Extended leaf phenology has limited benefits for invasive species growing at northern latitudes

Many understory woody invasive plants in North America leaf out earlier or retain leaves later than their native associates. This extended leaf phenology is thought to grant invasive species an advantage over native species because spring and fall are crucial times for light access and carbon acquisition in understory habitats. However, it is unclear whether this advantage persists at northern latitudes where freezing temperatures constrain growing season length and low light levels reduce carbon gain. To investigate the costs and benefits of extended leaf phenology at northern latitudes, we observed leaf phenology, estimated total carbon gain, measured growth, and tested susceptibility to freezing temperatures for four native and four invasive woody shrubs in a disturbed forest in northern Minnesota, USA. We found that the invaders leafed out simultaneously with the natives in the spring but retained their leaves later in the autumn than native species. This extended fall phenology did not enable greater total carbon gain for the invaders because they assimilated less carbon earlier in the year than the natives. There was also no significant difference between native and invasive species in their susceptibility to freezing temperatures. Instead freezing tolerance related more to native range then leaf phenology. Our results suggest that freezing temperatures do not limit invasive species’ northern expansion and instead indicate that at the northern edge of their ranges, these species may lose any competitive advantage granted by extended leaf phenology over their native associates. This study demonstrates the importance of considering latitude and forest structure when investigating phenology and growth.