Light thinning promotes large-diameter tree growth and stand volume in Eucalyptus–Castanopsis hystrix mixed forests

Short-rotation Eucalyptus plantations struggle to produce large-diameter timber. Mixed plantations of Eucalyptus with the native climax tree Castanopsis hystrix under a multi-cycle management regime offer a strategy to balance short-term wood production with long-term timber goals. In a long-term field experiment, a gradient of strip-thinning intensities (0%–100% Eucalyptus removal) was applied to a mixed stand in southern China. Five years later, neighborhood-based spatial indices showed that light thinning created a random tree distribution, moderate species mingling, and competitive dominance of Eucalyptus . This spatial configuration maximized total stand volume (+12.1% vs. unthinned control) and promoted the development of large-diameter Eucalyptus trees, whereas heavy thinning favored large-diameter C. hystrix . Large-diameter trees of both species consistently occupied microsites with random distribution and dominant status but differed in their mingling needs. These findings provide a mechanistic, neighborhood-based rationale for thinning prescriptions that reconcile high productivity with large-diameter timber cultivation in mixed-species plantations.