How thinning benefits growth for all trees

A new study on intensively managed loblolly pine plantations in the southern United States reveals how trees compete for light and nutrients, and how thinning reshapes that competition. The findings, published in Forest Ecosystems, provide new insight into how forest management affects both tree growth patterns and overall productivity.

Researchers from Zhejiang University and Stephen F. Austin State University tracked 48 plots of loblolly pine across Texas and Louisiana for seven years after mid-rotation thinning. They used two indicators to assess forest structure: the Gini index (GI), which measures differences in tree size, and growth dominance (GD), which identifies which trees contribute most to the stand’s total growth.

In unthinned stands, two contrasting competitive patterns emerged. Larger trees dominated diameter growth, expanding faster than their smaller neighbors. At the same time, smaller trees gained height more quickly relative to their size, an adaptive strategy to escape shade and compete for sunlight.

When foresters thinned the stands by removing every fifth row and taking out weaker trees, these dynamics shifted dramatically. Tree sizes became more uniform, and the big-tree advantage in diameter growth declined. Thinning levels the playing field. By reducing crowding, it weakens the dominance of large trees and gives smaller ones a better chance to catch up.

The researchers also observed that thinning strengthened the upward race for light. Shorter trees were stimulated to grow taller at a faster rate than before, enhancing structural balance within the stand. However, total stand volume growth declined slightly in the years after thinning.

Still, the study found a clear link between forest structure and productivity. In unthinned plots, stands with stronger diameter dominance grew more slowly overall, while stands where height growth was more evenly shared produced more wood.

The researchers emphasize that thinning doesn’t remove competition, but it adjusts it. The underlying relationships between dominance and growth remained the same after thinning, but the change slowed. These findings suggest that thinning can be used as a tool to fine-tune competition and maintain forest productivity over time.

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