Root system development in naturally regenerated Douglas-fir saplings as influenced by canopy closure and crowding

Documented information on the growth dynamics and related silvicultural manipulation of naturally regenerat­ed Douglas-fir beneath a canopy shelter is scarce. We hypothesized that long regeneration phases creating dense cohorts of Douglas-fir advanced regeneration beneath a closed canopy of mature trees have a negative impact on the formation of a stable root system. We therefore studied the influence of shading and competition on root system characteristics of 28 naturally regenerated, ca 3 m tall Douglas-fir saplings growing either without immediate competitors or in dense neighbourhoods, and these situations were located either beneath a closed canopy or in large canopy openings. Both canopy shelter and neighbourhood crowding limited sapling growth and root system development. Saplings from groups located beneath the canopy, although significantly older than the saplings growing in the open, exhibited lower root-to-shoot ratios. When growing in the open, maximum width, total length, and total biomass of root systems was much lower for saplings with than without competitors. In contrast, reductions in root development owing to competition within the sapling cohort were less pronounced under closed canopy conditions. Our results suggest that irrespective of the level of canopy closure, high sapling densities in naturally regenerated young Douglas-fir stands should be reduced early to improve root system development and hence physical stability.