Influence of nutrient supply and water vapour pressure on root architecture of Douglas-fir and western hemlock seedlings.

Root growth responses of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] and western hemlock (Tsuga heterophylla Raf. Sarg.) seedlings to three nutrient concentrations and two shoot vapour pressure deficits were measured. Both species gained dry mass at high and medium nutrient treatments throughout the experiment, but not at low nutrition. Low nutrition gave highest ratios of projected leaf surface area to total root length in both species. Douglas-fir geometry differed from that of hemlock, with longer interior link lengths, particularly at the lowest nutrition. Douglas-fir showed greater numbers of exterior-interior links than hemlock. More links were observed at medium and high nutrition than at low nutrition for both species. Exterior-interior links increased over time for the two highest nutrient treatments. Significant topological differences were observed between species, the lowest and two highest nutrient treatments, and high and low vapour pressure deficits. Both species showed herring-bone root architecture at the lowest nutrition. This architectural configuration became more pronounced in hemlock seedlings grown under higher vapour pressure deficits. Faster-growing Douglas-fir had a dichotomous architecture at medium and high nutrition that was not influenced by increased vapour pressure deficits. Douglas-fir topology appears to be adapted to exploit soil nutrient patches while hemlock appears to rely on efficient exploitation of soil volume.