Intensive management and declines in soil nutrients lead to serious exotic plant invasion in Eucalyptus plantations under successive short-rotation regimes

The ongoing expansion of Eucalyptus plantations is changing the development of the world's forests. But, it also threatens ecological security regionally and worldwide. The expansion effect on biodiversity and soil nutrients is a universal problem, which has always been heatedly debated. In order to evaluate the long-term influence of intensive management of Eucalyptus plantations under successive short-rotation regime on understory plant diversity, soil nutrients and Eucalyptus tree growth, we chose a chronosequence representing the first to the sixth rotation of Eucalyptus plantations arrayed in the major Eucalyptus plantation areas in southern China and analyzed plant diversity indices, composition of plant functional groups (PFGs), soil nutrient concentrations and Eucalyptus tree growth at the stand level. Our results reveal that intensive management exerts a long-lasting negative effect on understory plant species diversity, soil nutrients and Eucalyptus tree growth over successive rotations. Changes in soil nutrient content related with C and P cycling and declines in soil total nitrogen and available phosphorus were related with understory PFGs degradation. Furthermore, the combination of declines in understory woody plants, grasses and soil N and P cycling indices led to serious exotic plant invasion in the understory. Our study suggests that serious alien plant invasions could threaten the biosecurity, soil and timber security in Eucalyptus plantations. We suggest that reducing management intensity, converting mono-cultured Eucalyptus plantations into mixed plantations with indigenous tree species, and changing from successive short-rotation to short-, medium- and long-term cyclic rotations are required to sustainably manage these plantations through improving plant diversity, restoring degraded soils and resisting plant invasions.