Growth, survival, and physiology of Douglas-fir seedlings following root wrenching and fertilization

Abstract

Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedlings were subjected to wrenching treatments during their 2nd year of growth in two experiments at different nurseries on Vancouver Island. Seedlings were wrenched with a fixed blade at 20–25 cm below the bed surface. In experiment 1 wrenching reduced water potential of trees on unirrigated loam soil by an average of 300 kPa during August and September. Wrenched trees lifted in October, which were stored at 2 °C until May, showed 25% higher survival than unwrenched trees 1 year after planting. Trees lifted in December had uniformly high survival (98%) and showed no effect of wrenching. Wrenched trees from irrigated plots had lower shoot length relative growth rates (RGR) than unwrenched trees during the year after planting. In experiment 2 wrenching treatments were applied to seedlings, growing in a loamy sand, for different periods between 15 May and 11 September as follows: (i) unwrenched, (ii) early summer, (iii) midsummer, (iv) late summer, (v) all summer. Three different levels of fertilization were applied to each wrenching treatment, and seedlings were lifted for storage at 2 °C in October and December. Stored trees and freshly lifted trees were planted at 700 m elevation on 3 March. Wrenching increased root dry weight, particularly when additional fertilizer was applied, but had no measurable effect on cold hardiness or root growth capacity. Nevertheless, late summer wrenching increased survival 5 and 7% above control 1 and 2 years after planting, when average survival was 47% after 1 year. Wrenching had little subsequent effect on new shoot growth of planted trees during 2 years after planting. However, late summer wrenched trees showed significantly more new shoot growth than all summer wrenched trees. Increasing fertilization reduced cold hardiness and survival of cold-stored trees, but increased root growth capacity. Cold hardiness, measured by electrical impedance, was correlated with survival of cold-stored trees after planting (r2 = 0.82). Root growth capacity, averaged over all fertilizer levels, was closely correlated with survival of stored and freshly lifted trees (r2 = 0.93). Foliar nutrient concentrations were reduced by wrenching, but fertilization increased nutrient reserves within the seedling.