A quicker return energy-use strategy by populations of a subtropical invader in the non-native range: a potential mechanism for the evolution of increased competitive ability

1. Many studies testing the evolution of increased competitive ability hypothesis have focused on whether plants from invasive populations of a species show reduced allocation to defence and increased allocation to growth than plants from native populations. But few have attempted to understand ecophysiological mechanisms by which decreased allocation to defence may increase growth. 2. Previously, we found that invasive Ageratina adenophora plants increase nitrogen allocation to photosynthesis and reduce allocation to cell walls compared with native Ageratina plants, suggesting a shift from defence to growth in invasive populations. Here, carrying this work forward, we measured construction costs and benefits associated with photosynthesis at light saturation to leaves. We hypothesized that invasive Ageratina populations might employ a quicker return energy-use strategy by increasing light-saturated photosynthetic rates and photosynthetic energy-use efficiency (PEUE) and by decreasing leaf construction costs. 3. Faster-growing plants from invasive populations (China and India) had significantly higher leaf nitrogen concentrations and specific leaf areas than plants from native populations (Mexico). Inconsistent with our prediction, leaf construction costs were not significantly different between plants from invasive and native populations, but higher light-saturated photosynthetic rates and in turn higher PEUE resulted in a significantly shorter payback time of construction costs, which allowed plants from invasive populations to grow faster. 4. Synthesis. Our results indicate that Ageratina plants from populations in non-native ranges have a distinct quick return energy-use strategy, a high PEUE and a short payback time but not lower construction costs, which might provide a mechanistic explanation for the commonly observed increase in growth when plants are introduced to new parts of the world. To our knowledge, this is the first study to compare energy-use strategy for plants from invasive and native populations of a noxious invasive species. We cannot exclude some alternative hypotheses for these patterns, such as founder effects, but these ecophysiological differences might provide mechanistic insight for how the evolution of decreased allocation to defence may increase growth and competitive ability.