Effects of allelopathy and competition for water and nutrients on survival and growth of tree species in Eucalyptus urophylla plantations

Allelopathy and resource competition are considered to be two primary mechanisms responsible for loss of biodiversity in plantations of Eucalyptus species. However, these two processes are usually studied separately, and they have been rarely tested on native woody species. In this study, we used a bioassay to assess sensitivities of twenty broad-leaved tree species on roots, stem growth and seed germination to leaf aqueous extracts of Eucalyptus urophylla S.T. Blake and categorized them into two types: inhibited and uninhibited (including stimulated and unaffected). To compare the relative importance of allelopathy and resource competition, we planted these two species groups into the E. urophylla plantation separately and treated with three gradients of irrigation-nutrients. The results showed that the allelopathic effects of aqueous extract of E. urophylla were species-specific and could be inhibitory, neutral or stimulatory. Compared to the inhibited species, the uninhibited species grew faster and survived better after they were planted in an E. urophylla plantation for approximately 10 years, suggesting that allelopathy from E. urophylla is an important restraining factor on native woody communities. Individuals from both species groups grew faster following higher resource treatment at the early but not the late stage of growth. Saplings did not vary in their survival rates across resource gradients. This indicates that resource competition between E. urophylla and native woody species has only a limited role in reducing the diversity of native species in an E. urophylla plantation. We conclude that allelopathy is more important than resource competition in mediating the reduction in plant biodiversity in E. urophylla plantations. Our study suggests that mixing certain types of species (e.g. Helicia cochinchinensis, Pterospermum lanceaefolium, Cinnamomum burmanni, Machilus chinensis, Acmena acuminatissima and Castanopsis chinensis) in E. urophylla plantations can mitigate against plant diversity loss.