Timing of root dormancy and tolerance to root waterlogging in clonal Sitka spruce

Actively growing root tips of Picea sitchensis (Bong.) Carr. plants are highly susceptible to damage if waterlogged, but they are known to have some tolerance after they stop growing in the autumn. This paper describes the selection of clones on the basis of root dormancy timing and the corresponding responses of their roots to overwinter waterlogging. Sitka spruce transplants of Alaska, Queen Charlotte Islands (QCI), and Washington provenances were screened for early or late root dormancy over 2 successive years. Cuttings were propagated from the selected plants and after growing on for 2 years, they were planted in transparent acrylic tubes within outdoor 'root observation chambers'. Extension of main roots and the timing of onset of root dormancy was recorded on the clonal plants. The tubes were flooded in November and maintained with a water table 280 mm below the soil surface until March of the next year. Waterlogging caused most main root tips to die back, but within 2 months of draining regeneration occurred on the main roots below the waterlogging level. This regeneration was most commonly the growth of existing lateral tips or production of new lateral roots. Roots of early-dormant Washington plants died back on average 129 mm less than late-dormant Washington plants, and early-dormant Alaska plants had 173 mm less dieback than late-dormant Alaska plants. Differences between the clones of the QCI provenance were not significant. The 40% and 52% increases in survival depth of roots in early-dormant Washington and Alaska clones respectively indicates a potential for improving the rooting depth of Sitka spruce on seasonally waterlogged soils by planting clones selected on the basis of root dormancy.