High risk of plant invasion in the understory of eucalypt plantations in South China

Evidence illustrates that biodiversity contributes to ecosystem functioning1,2 and that a loss of biodiversity alters ecosystem processes that are essential to the productivity and sustainability of ecosystems3,4. Natural forests, which provide habitat to the richest terrestrial biodiversity in the world, have been shrinking in size. This environmental change is largely attributed to anthropogenic causes, the most significant of which is deforestation to meet wood supply5,6,7. Plantation forests generally maintain lower biodiversity levels than natural forests8,9,10; however, developing high-yield plantation forests in a sustainable manner may help preserve natural forests and biodiversity. Eucalypts comprise a group of approximately 800 woody species11 that belong to Eucalyptus, Corymbia or Angophora in Myrtaceae12, which are mostly native to Australia but not in China. Eucalypts are among the most fast-growing trees in the world; they adapt well in warm environments and provide materials for timber and pulp13. In South China, eucalypt plantations have expanded rapidly in the last decade, from 2.54 M ha in 2008, to 3.6 M ha in 201114 and to 4.40 M ha in 201315. According to the Chinese State Forest Administration, eucalypt plantations in China constituted 22% of the world’s eucalypt plantation area in 2013 and contributed 25% to Chinese wood production15. However, despite the remarkable growth of wood production in China, the self-sufficiency rate has declined in recent years (see Supplementary Fig. S1 online) mainly because of the rapid increase in wood consumption. The sustainable development of fast-growing eucalypt plantations is supported by the Chinese government, so as to enhance wood production and to preserve natural forests. The expansion of eucalypt plantations out of their natural distribution zones has generated concerns about their effects on biodiversity and ecosystem functions. Previous studies have found that the understory plant diversity of eucalypt plantations are important for reducing nutrient loss and soil erosion16 and maintaining soil microbial communities17; such diversity may have positive effects on the productivity of eucalypts18. Eucalypt plantations in South China have been found to maintain a lower diversity of understory plant species than natural forests9,19. However, other types of plantations, such as Pinus massoniana and Dimocarpus longan plantations, and secondary shrub grasslands are more common than natural forests in South China, and theses may serve as potential field sources of eucalypt plantations. Whether eucalypt plantations maintain less understory plant diversity than other common types of vegetation remains unclear. Furthermore, previous studies usually focus on diversity indices. However, community structure, such as the relative importance of species or family, typically contains important messages and provides links with the ecological function of community. In another aspect, according to the theory of fluctuating resource availability proposed by Davis et al.20, the disturbance of understory communities during plantation, fertilization and short rotations of eucalypt plantations may provide opportunities for invasive species to capture light, water and nutrients. Nevertheless, questions like whether eucalypt plantations have more invasive species understory than other common types of vegetation and which environmental factors may affect the invasion risk have been rarely investigated. We first compare the understory plant diversity, including diversity indices and community structure, in eucalypt plantations with those in other common types of vegetation in South China. Thereafter, we test if more invasive species exist in eucalypt plantations than in contrast vegetation. Finally, we detect the environmental factors that may affect plant invasion in both eucalypt plantations and contrast vegetation.