Abstract: Cinchona pubescens Vahl (red quinine) is an evergreen tree ranging in height from 10 to 25 m with broad leaves and white or pink fragrant flowers arranged in clusters. Growing at altitudes between 130 and 3,300 m, it is one of 23 species in the genus Cinchona and has a natural distribution from Costa Rica to Bolivia. Cinchona pubescens has been cultivated in tropical regions (e.g., in South America, Africa, China, India, and Indonesia) for its quinine-containing bark and has become invasive in some regions. This is especially the case in the Pacific region, where C. pubescens has invaded humid highland areas of Galapagos, Hawai'i, and Tahiti. It shades out and reduces cover of native plant species and adversely affects endemic birds. In addition, it changes microclimate and nutrient cycling in the soil, especially phosphorus, in Galapagos. Characteristics that make it such a successful invader include production of numerous, windborne seeds and vigorous vegetative reproduction by resprouting from underground stems and fallen trees. In Galapagos, C. pubescens is currently being manually controlled by uprooting the trees and by applying herbicides to cuts in the bark. However, this method requires continuous hand pulling of seedlings to be successful. Disturbance by control actions appears to facilitate establishment and invasion by other nonnative plant species, especially blackberry (Rubus niveus). Quinine and other alkaloids extracted from Cinchona bark are still being used for medicinal purposes today and the wood is increasingly used as construction material in Galapagos. Ironically, C. pubescens is now considered rare and endangered in its native range in Ecuador.

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Cinchona L. is likely the most commercially important genus of the family Rubiaceae after the genus Coffea L. due to its quinine-containing bark. Cinchona pubescens Vahl (syn. C. succimbra Pav. ex Klotzsch, 1857) (red quinine tree) has been introduced worldwide, mainly from Bolivia, Colombia, Ecuador, and Peru (Andersson 1998) (Figure 1), and is considered among the 100 worst invaders globally (ISSG 2015). It is highly invasive on several Pacific island groups, especially Galapagos, Hawai'i, and the Society Islands (Weber 2003), where it forms dense stands in the humid highland vegetation, reducing plant species diversity and abundance. The invaded islands are Santa Cruz in Galapagos (Jager et al. 2009); the islands of Hawai'i, Maui, and 0'ahu in Hawai'i (Starr et al. 2003); and Tahiti in the Society Islands (Meyer 2000). The introduction of the novel "tree" life-form to a formerly treeless environment in Galapagos led to significant changes in vegetation stand structure and changes in light, water, and nutrient regimes (Jager et al. 2007, 2009, 2013) (Figures 2 and 3). Consequently, C. pubescens is now considered an ecosystem engineer in Galapagos (Jones et al. 1994, Jager et al. 2009). In Hawai'i, C. pubescens has invaded native understory and forest canopy vegetation, as well as nonnative forests and disturbed areas, where it reduces the cover of other plant species (Starr et al. 2003, Fischer et al. 2009) (Figure 4). In Tahiti, C. pubescens has invaded secondary and primary rain forests and undisturbed cloud...