Physiological and morphological responses of invasive Ambrosia artemisiifolia (common ragweed) to different irradiances

Invasion by the exotic herb Ambrosia artemisiifolia L. has become a serious agricultural and environmental problem and of increasing research interest as the species continues its southward spread into subtropical regions of China. To better understand the possible physiological and morphological adaptations of A. artemisiifolia in this region, we conducted a greenhouse experiment using seedlings of A. artemisiifolia and a comparative native species, Urena lobata L. (Caesar’s weed), which is an erect annual shrub in the Malvaceae that commonly co-occurs with A. artemisiifolia in open, disturbed habitats in southern China. Seedlings of both plants were grown under four irradiance regimes (10%, 30%, 55%, and 100% irradiance) from 14 May to 13 July 2010. Ambrosia artemisiifolia showed significantly higher total biomass, total leaf area, specific leaf area, relative growth rate, and net assimilation rate but lower leaf mass fraction than U. lobata at the 30%–100% irradiance levels. With decreasing irradiance, A. artemisiifolia significantly increased biomass allocation to stems and decreased allocation to roots. Meanwhile, A. artemisiifolia exhibited higher light-saturated photosynthetic rates and light saturation points with wide variances except at the 10% irradiance treatment. These findings suggest that A. artemisiifolia has generally higher irradiance plasticity for traits pertaining to biomass partitioning, growth, and plant structure than U. lobata. The ability of A. artemisiifolia to tolerate high shade conditions while maintaining high growth rates at elevated irradiance levels may afford it a competitive advantage and may help explain its recent colonization and spread in southern China.