Neighborhood size effects on mortality, growth and crown morphology of paper birch

Neighborhood removal treatments are commonly used to increase forest tree productivity, but it is unknown if they correspond with target zone of perception or if they differentially affect mortality, growth and crown morphology. In 12-13-year-old stands, naturally regenerated paper birch (Betula papyrifera Marsh.) was removed in neighborhoods 1 m, 2 m, 3 m, and 4 m in radius using three methods (single cutting, double cutting, or triclopyr). Zone of neighbor perception for target birch ranged between 0.9 and 4 m, depending on the response variable. The birch-free radius where neighbors started to reduce target performance was 3-4 m for diameter increment and 2-3 m for crown size. Crown diameter was reduced in 1 m smaller radii than where height-to-live crown was increased. Self-thinning occurred in 0.9-1.3 m spaced controls, but target mortality was not reduced with increased birch-free radius. Triclopyr killed most treated birch in all radii within 1 year, and also many target trees because of inter-tree chemical transfer through roots and soil. Most cut birch sprouted, but sprout mortality increased and height decreased with decreasing radius. A critical radius of 2-3 m resulted in increased target diameter growth while still restricting crown expansion and suppressing neighbor sprouting. These results highlight the need to recognize variation in zone of perception for different response variables when evaluating competitive effects or designing removal treatments.