Evolutionary history and novel biotic interactions determine plant responses to elevated CO2 and nitrogen fertilization

A major frontier in global change research is predicting how multiple agents of global change will alter plant productivity, a critical component of the carbon cycle. Recent research has shown that plant responses to climate change are phylogenetically conserved such that species within some lineages are more productive than those within other lineages in changing environments. However, it remains unclear how phylogenetic patterns in plant responses to changing abiotic conditions may be altered by another agent of global change, the introduction of non-native species. Using a system of 28 native Tasmanian <em>Eucalyptus</em> species belonging to two subgenera, <em>Symphyomyrtus</em> and <em>Eucalyptus</em>, we hypothesized that productivity responses to abiotic agents of global change (elevated CO₂ and increased soil N) are unique to lineages, but that novel interactions with a non-native species mediate these responses. We tested this hypothesis by examining productivity of 1) native species monocultures and 2) mixtures of native species with an introduced hardwood plantation species, <em>Eucalyptus nitens</em>, to experimentally manipulated soil N and atmospheric CO₂. Consistent with past research, we found that N limits productivity overall, especially in elevated CO₂ conditions. However, monocultures of species within the <em>Symphyomyrtus</em> subgenus showed the strongest response to N (gained 127% more total biomass) in elevated CO₂ conditions, whereas those within the <em>Eucalyptus</em> subgenus did not respond to N. Root:shoot ratio (an indicator of resource use) was on average greater in species pairs containing <em>Symphyomyrtus</em> species, suggesting that functional traits important for resource uptake are phylogenetically conserved and explaining the phylogenetic pattern in plant response to changing environmental conditions. Yet, native species mixtures with <em>E. nitens</em> exhibited responses to CO₂ and N that differed from those of monocultures, supporting our hypothesis and highlighting that both plant evolutionary history and introduced species will shape community productivity in a changing world.