Effect of soil temperature on root and shoot growth and on budburst timing in conifer seedling transplants

Root growth in seedling transplants of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), Pacific silver fir (Abies amabilis(Dougl.) Forbes), noble fir (Abies procera Rehd.), lodgepole pine (Pinus contorta Dougl. ex Loud.) and ponderosa pine (Pines ponderosa Doug. ex Laws.) began when soil temperature exceeded 5 °C. Root growth increased rapidly after 10 °C and attained maximum values at 20 °C. At 30 °C, no root growth occurred in the firs; in the pines, root growth was 30 to 39% of maximum. Maximum shoot growth also occurred at 20 °C. In ponderosa pine, height growth of seedlings from a high-elevation source was unaffected by cold soil, but in low-elevation seedlings it was reduced. Budburst in Douglas-fir and the pines was delayed up to 11 days by cold soil, whereas in silver fir and noble fir, it was only slightly delayed. Prior to new root growth in ponderosa pine, xylem pressure potentials and stomatal conductances during the afternoon indicated reduced stomatal opening at all soil temperatures, whereas 23 days later, stomata were open to a greater degree when temperatures exceeded 10 °C.