Relative Growth Rate and Functional Traits of a Hybrid Species Reflect Adaptation to a Low-Fertility Habitat

Hybrid species provide a unique opportunity to examine trait evolution. We previously demonstrated that the hybrid speciesHelianthus anomalusis more fit than its ancestral parents,Helianthus annuusandHelianthus petiolaris, in its habitat and hypothesized that nutrient limitations have been a major selective agent in that adaptive evolution. In this glasshouse study, we asked whetherH. anomalushas more conservative growth and resource use traits than its ancestral parents, as expected for low-nutrient-adapted species.Helianthus anomalushad a lower maximum relative growth rate, leaf area ratio, and specific leaf area as well as less of a decrease in relative growth rate with nitrogen limitation than its ancestral parents.Helianthus anomalusalso had higher nitrogen use efficiency, largely because of a longer mean residence time, as well as greater leaf toughness and longer leaf life span. However,H. anomalusdid not have a lower photosynthetic rate, higher ratio of fine root length to leaf area, or higher nitrogen resorption before leaf senescence. Thus,H. anomalushas acquired several conservative growth and resource use traits consistent with low nutrients as a selective agent during speciation and adaptation, but this leaves open the question of which growth and resource use components are the actual targets of selection.