LAUSR

Climate change may reduce the benefits of tree improvement. We investigated the response to limited water availability of seedlings from nine improved full-sibling families of Sitka spruce (Picea sitchensis (Bong.) Carr.) plus an unimproved seed lot (‘Washington control’). Seedlings were 1-year old at the start of the imposition of two water availability treatments, which were maintained for 2.5 years: full irrigation, in which all water transpired was replaced, and water limitation, in which irrigation was provided to match only 30% of the transpiration of the Washington control under full irrigation. Both family and water availability affected leader extension rate, height, root collar diameter and total dry mass at the end of the following 2 years. Water limitation reduced leaf area and fresh mass per unit stem length and delayed bud flush. Specific leaf area was affected by water limitation earlier but not later in the experiment; the reverse was true of root to shoot ratio and numbers of primordia. Under full irrigation, five improved families maintained superiority over the Washington control in terms of height, root collar diameter and total biomass at the end of the experiment. Under water limitation, however, by the end of the experiment, seedlings from the improved families had no greater total biomass than those of the Washington control, reflecting the homogenizing impact of water limitation on production of leader biomass. Thus, for Sitka spruce, planting the most vigorous material will maximize gains under optimal conditions, but have little benefit in stress conditions.