Genetic variation in first-year slash pine shoot components and their relationship to mature field performance

Seedlings of 64 open-pollinated slash pine (Pinus elliottii var. elliottii Engelm.) families were grown from seed to 24 weeks of age in raised outdoor boxes under two nitrogen treatments (5 and 50 ppm). Twenty-six shoot characteristics were measured, of which the most promising 12 were evaluated for use in a multitrait selection index to predict parental breeding values of 5- and 15-year field volume growth. Genetic parameters were estimated for each seedling trait and shoot components were analyzed for their contribution to total height increment. Number of stem units was the most important contributor to total height in both nitrogen treatments. Heritabilities were generally higher for all traits in the high-nitrogen regime (h(2) = 0.13-0.66). All traits displayed moderate to high genetic stability across both treatments (type B genetic correlations = 0.64-1.32). Total number of stem units (low-nitrogen treatment), free growth stem unit number (low-nitrogen treatment), and caliper (high-nitrogen treatment) exhibited the strongest genophenotypic correlations with 15-year volume (rjm = 0.35-0.39). All possible combinations of two- and three-trait indices were calculated to derive correlations between predicted genetic values and true genetic values (corr(g,g)). Cyclic growth length (high-nitrogen treatment), total height (low-nitrogen treatment), and free growth stem units (low-nitrogen treatment) combined to form the most precise three trait index for predicting 15-year volume growth (corr(g,g) = 0.56). Total number of stem units, total flushes, and total mean stem unit length in the low-nitrogen treatment along with cyclic number of stem units and caliper in the high-nitrogen treatment were also determined to be of potential use in a multitrait selection index based on their heritabilities, juvenile-mature correlations, and performance in two- and three-trait indices.