Toward understanding woody plant invasiveness in a changing environment: Phylogenetically independent contrasts of seedling performance under varying drought and nitrogen levels

Invasive species are often thought to opportunistically use available resources and/or to exhibit more uniform performance across different environments. In many parts of the world, atmospheric nitrogen deposition plays an important role in adding a resource to the environment. With climate change, many regions also expect increasing drought. Are there differences in how invasive and non-invasive species react to an added resource (nitrogen) and/or drought stress? Using commonly planted exotic horticultural woody angiosperms, we tested the responses of invasive species and their related much less-invasive counterparts (19 species forming 8 phylogenetically independent contrasts) to two nitrogen levels and three moisture levels. Plants were grown from seed at two nitrogen levels (8 ppm and 180 ppm) under well-watered conditions for two months. The drought treatments (none, medium, and high) then began, lasting one month. We examined the changes in survival, final biomass, relative growth rate and related variables, and a measure of plant physiological stress (total carotenoids: total chlorophyll) across the treatments. We found that for most traits, invasive species had different responses –behaving opportunistically in some contrasts, while maintaining trait levels across drought levels in others, both, or in some cases, neither strategy. A few clear patterns emerged: although plants were consistently smaller at the low nitrogen level, both invasive and much less-invasive species were much more tolerant of drought stress than at the high nitrogen level, and less invasive species exhibited significantly less physiological stress than their invasive counterparts in the high nitrogen-high drought treatment.