Climate change, comprising an increase in carbon dioxide levels coupled with elevated temperature, may favor invasive plants, as they possess traits that will facilitate adaptation to a new climate. In particular, alien plants of subtropical origin introduced to a colder region are expected to increase the number and size of their populations and spread farther with climate change. Seedlings of three such woody alien species in New Zealand (Archontophoenix cunninghamiana, Psidium guajava, and Schefflera actinophylla) were grown in environmental chambers under the combination of two temperature (23.7 and 26 C [74.7 and 78.8 F]) and two CO₂ (450 and 900 ppmv) regimes, simulating current conditions and conditions projected for the end of the century. Total biomass of S. actinophylla was 45% higher and total leaf area 35% larger under doubled CO₂ compared to current CO₂. Root ∶ shoot ratio was higher under doubled CO₂ across all species, and the number of branches was increased for P. guajava. The only significant interactive effect of elevated temperature and doubled CO₂ was for relative growth rate of the height of S. actinophylla seedlings. This study provides strong evidence of more vigorous growth of S. actinophylla under future conditions, particularly increased CO₂, whereas the other two species appear likely to maintain current growth rates. Better knowledge of the types of future conditions that may benefit such species, together with results of species distribution models and competition and eco-physiology studies will ensure robust weed risk assessments.

Nomenclature: Bangalow palm, Archontophoenix cunninghamiana (H. Wendl.) H. Wendl. & Drude, common guava, Psidium guajava L., Queensland umbrella tree, Schefflera actinophylla (Endl.) Harms.

Management Implications: Archontophoenix cunninghamiana, Psidium guajava, and Schefflera actinophylla are popular ornamental alien woody plants in New Zealand and elsewhere. Such plants from subtropical and tropical origin may benefit from climate change, naturalize, and become invasive in more temperate regions once climatic constraints in colder climates are removed. Responses to climate change, such as rising temperatures and CO₂ levels appear to vary, even within a single vegetation type such as woody plants. Of the three subtropical species in this study, S. actinophylla grew more vigorously under doubled CO₂ levels, resulting in increased biomass and leaf area. It also grew taller under the combination of doubled CO₂ and elevated temperature. Growth rates of the other two species did not differ under elevated temperature and doubled CO₂ compared to current levels. Consequently, climate change is likely to have a neutral or positive effect on growth of these three alien species. Although S. actinophylla is at a very early stage of invasion in New Zealand, early control of this species should be considered, due to its possible increase in performance under rising CO₂ levels. If co-occurring native species show negative growth responses to climate change, A. cunninghamiana and P. guajava may also have a competitive advantage, but this will have to be further assessed within a community context in another study. Woody alien species have been shown to be important invaders, and there are numerous subtropical plants introduced to temperate regions globally. This study highlights that such woody, bird-dispersed, subtropical species, which may benefit from climate change, should be screened as potential sleeper weeds.