Relationship between size inequality and stand productivity is modified by self-thinning, age, site and planting density in Sassafras tzumu plantations in central China

Greater size inequality has been associated with lower forest productivity. Density-dependent mortality can reduce size inequality and potentially modify relationships between size inequality and productivity. We examined the effect of density-dependent mortality, age, planting density and site quality on this relationship using two Sassafras tzumu plantations in central China with three planting density levels (4.0 m × 5.0 m, 3.0 m × 4.0 m, and 2.0 m × 3.0 m) replicated at two sites. The stands were measured in winter every other year for 20 years, from 1996 to 2016. Generalized linear mixed effect models were used to examine the relationship between stand age, planting density, site quality, and size inequality (assessed using the Gini coefficient). Gini coefficients, and hence size inequality, increased with stand age, planting density and site quality. The onset of self-thinning reduced the rate at which size inequality increased. Productivity was negatively correlated with size inequality, and self-thinning modified this relationship such that productivity declined at a greater rate (with increasing size inequality) after self-thinning had begun to occur. The productivity-size inequality relationship was also modified by planting density, age and site quality, such that with all other things being equal, the negative effect of size inequality is greatest in young stands, dense stands and on low quality sites. The manipulation of stand size inequality (e.g. by thinning or mortality) influences growth more than simply by changing stand density. An understanding of the mechanisms driving the effect of density-dependent mortality on the relationship between productivity and size inequality may facilitate the further development of silvicultural regimes.