GENETIC-VARIATION IN 1ST-YEAR SLASH PINE SHOOT COMPONENTS AND THEIR RELATIONSHIP TO MATURE FIELD PERFORMANCE

Seedlings of 64 open-pollinated slash pine (Pinus elliottii var. ellio ttii Engelm.) families were grown from seed to 24 weeks of age in rais ed outdoor boxes under two nitrogen treatments (5 and 50 ppm). Twenty- six shoot characteristics were measured, of which the most promising 1 2 were evaluated for use in a multitrait selection index to predict pa rental 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. Numbe r of stein units was the most important contributor to total height in both nitrogen treatments. Heritabilities were generally higher for al l traits in the high-nitrogen regime (h2 = 0.13-0.66). All traits disp layed moderate to high genetic stability across both treatments (type B genetic correlations = 0.64-1.32). Total number of stem units (low-n itrogen treatment), free growth stem unit number (low-nitrogen treatme nt), and caliper (high-nitrogen treatment) exhibited the strongest gen ophenotypic correlations with 15-year volume (r(jm) = 0.35-0.39). All possible combinations of two- and three-trait indices were calculated to derive correlations between predicted genetic values and true genet ic values (corr(g,g)). Cyclic growth length (high-nitrogen treatment), total height (low-nitrogen treatment), and free growth stem units (lo w-nitrogen treatment) combined to form the most precise three trait in dex for predicting 15-year volume growth (corr(g,g) = 0.56). Total num ber of stem units, total flushes, and total mean stem unit length in t he low-nitrogen treatment along with cyclic number of stem units and c aliper in the high-nitrogen treatment were also determined to be of po tential use in a multitrait selection index based on their heritabilit ies, juvenile mature correlations, and performance in two- and three-t rait indices.