Spacing of silver birch seedlings grown in containers of equal size affects their morphology and its variability.

Silver birch (Betula pendula Roth) seedlings were grown individually in containers arranged in rows radiating from a central point (Nelder plot) at densities spanning the range from 207 to 891 plants m(-2). Height of one set of seedlings was measured at weekly intervals and additional seedlings were harvested each week for dry mass and leaf area measurements. Height and shoot:root dry mass ratio increased with plant density. Seedling-to-seedling variability in dry mass, but not height, increased with increasing plant density. The red to far-red (R:FR) photon ratio in horizontally propagated radiation decreased with increasing density, even when plant densities and leaf area index values were low. In a separate experiment, elongating internodes of seedlings were irradiated locally by red and far-red light emitting diodes and stem elongation measured. Far-red light markedly increased stem elongation, suggesting that changes in light quality sensed by growing internodes are involved in the observed responses to growth density.