Herbivory and elevated levels of CO2 and nutrients separately, rather than synergistically, impacted biomass production and allocation in invasive and native plant species

Large parts of the Earth are experiencing environmental change caused by
alien plant invasions, rising atmospheric concentration of carbon dioxide
(CO2), and nutrient enrichments. Elevated CO2 and nutrient concentrations
can separately favour growth of invasive plants over that of natives but
how herbivory may modulate the magnitude and direction of net responses by
the two groups of plants to simultaneous CO2 and nutrient
enrichments remains unknown. In line with the enemy release hypothesis,
invasive plant species should reallocate metabolites from costly
anti-herbivore defences into greater growth following escape from intense
herbivory in the native range. Therefore, invasive plants should have
greater growth than natives under simultaneous CO2 and nutrient
enrichments in the absence of herbivory. To test this prediction, we grew
nine congeneric pairs of invasive and native plant species that naturally
co-occurred in grasslands in China under two levels each of nutrient
enrichment (low-nutrient vs. high-nutrient), herbivory (with herbivory vs.
without herbivory) and under ambient (412 ± 0.6 ppm) and elevated (790.1 ±
6.2 ppm) levels of CO2 concentrations in open-top chambers in a
common garden. Elevated CO2 and nutrient enrichment separately increased
total plant biomass, while herbivory reduced it regardless of the plant
invasive status. High-nutrient treatment caused the plants to allocate a
significantly lower proportion of total biomass to roots, while herbivory
induced an opposite pattern. Herbivory suppressed total biomass production
more strongly in native plants than invasive plants. The plants exhibited
significant interspecific and intergeneric variation in their responses to
the various treatment combinations. Overall, these results suggest that
elevated CO2 and nutrients and herbivory may separately, rather than
synergistically, impact productivity of the invasive and co-occurring
native plant species in our study system. Moreover, interspecific
variation in resource-use strategies was more important than invasive
status in determining plant responses to the various treatment
combinations.