Density and rectangularity of planting influence 20-year growth and development of red alder

Red alder (Alnus rubra Bong.) seedlings were planted in northwestern Oregon, U.S.A., at five initial spacings: 0.6 × 1.2 m, 1.2 × 1.2 m, 1.2 × 1.8 m, 1.8 × 1.8 m, and 2.5 × 2.5 m. Up to about age 10, tree and stand characteristics were correlated primarily with initial planting density in the expected manner; through age 20, however, tree growth and stand development in plots planted at rectangular spacings were substantially more rapid than in the two closest square spacings. Mean stand diameter ranged from 19.2 cm in the widest spacing to 14.0 cm in the closest square (1.2 × 1.2 m) spacing; mean tree height decreased from nearly 24 m in the widest (2.5 × 2.5 m) spacing to about 18 m in the closest square spacing. Diameter–density relationships in the widest spacing were consistent with existing density management guidelines, but very dense spacings and rectangular plantings began to experience substantial mortality at smaller diameters than assumed in the guidelines. We suggest that rectangular planting of red alder at dense spacing enhanced stand differentiation, accelerated competition-related mortality, and thus led to improved growth of surviving trees.