Realized genetic gains from slash pine tree improvement

Realized genetic gains of slash pine (Pinus elliottii ENGLEM. var. elliottii), were estimated for tree volume and rust resistance using data from 2051 controlled-pollinated (CP) families grown in 175 CP tests. All CP families were divided into 5 groups for volume (groups V1 to V5) and 5 groups for rust (groups R1 to R5). For each variable the 5 groups represent the expected performance range from best to worst of the CP families based on parental breeding values (BVs) predicted by best linear prediction (BLP) from an independent data set of 367 open-pollinated (OP) progeny tests. Using simple linear regression, realized genetic gains were estimated by comparison among these groups and also by comparison of improved materials to unimproved materials (called CHECK) included in most tests. On a region-Ride basis, the ranking of realized gain ratios (in percent above unimproved material) for volume and rust of the 5 groups corresponded exactly to the order predicted by a priori classification based on the BLP predicted BVs. For example, for volume the realized gain increased linearly for the 5 volume groups. Regression analyses were used to examine how age and site characteristics (e.g., site index, rust hazard) and their interactions with the 2 types of genetic groupings (V1 to V5, R1 to R5) affected genetic gains in the 2 traits, volume and rust. Age was almost never significant for volume or rust suggesting that the percentage gain above unimproved material is constant over the range of ages 5 to 11. The results also suggest that superior families for tree volume express increased gain in volume (on a % basis) compared to the unimproved material on better quality sites, and that genetic differences among genetic groups are greater on better sites (SIclass 3, 4 and 5) relative to poorer sites (SIclass 1 and 2). Also, genetically superior families for rust resistance have more tree volume on the high hazard sites, and on these sites the most resistant families also had the highest volume gains relative to the CHECK.