Top Grafting Loblolly Pine in the Western Gulf Region
Flowering data were collected from top grafts made in 1996 and 1997 at the Mississippi Forestry Commission's Craig Seed Orchard near Lumberton, MS. Scion material from twenty loblolly pine (Pinus taeda) second-generation selections was grafted onto five loblolly pine and five slash pine (P. elliottii) interstocks. All interstocks had a history of good cone production. Data collected included the number of female strobili per graft, scion growth, the number of branches above the graft union and the numbers of conelets per graft in the second year after grafting. First-year measurements of 1996 grafts suggested that grafts on slash pine interstocks had an advantage over grafts on loblolly pine interstocks. Average survival of grafts on slash pine interstocks was 58%, of which 94% flowered with an average of 14.4 strobili per graft. Grafts on loblolly pine interstocks had, an average survival of only 25%, of which 43% flowered with an average of 4.8 strobili per graft. Second-year measurements of the 1996 grafts showed much smaller differences between interstock species. Average graft survival declined slightly to 50% for slash pine interstocks and 21% for loblolly pine interstocks. Flowering increased to 84% on the surviving grafts with 12.7 strobili per graft on slash pine interstocks and 86% flowering with 10.5 strobili per graft on loblolly pine interstocks. The similar performance of interstock species in the second-year data suggested that the first-year differences were the result of a differential response to a hard spring freeze that occurred at the orchard in 1996. Grafts made in 1997 had an average first-year survival of 91% on slash pine interstocks and 84% on loblolly pine interstocks. Of the surviving grafts, 56% flowered on slash pine interstocks producing an average 4.9 female strobili and 58% flowered on loblolly pine interstocks with an average of 5.2 female strobili. No significant differences were detected between the two interstock species in graft survival, average numbers of strobili, scion diameter or number of branches above the graft. Average scion length after one year's growth did differ between the slash pine (742 mm) and loblolly pine (586 mm) interstocks. Conelet survival after two years was not significantly different between loblolly pine interstocks (2.6 conelets per graft) and slash pine interstocks (2.2 conelets per graft). No significant interaction effect was observed between interstock species and scion clone for either the 1996 or 1997 grafts in the first or second year after grafting. Top grafting has been shown to be an effective way to shorten the breeding cycle for loblolly pine improvement programs. The rapid production of strobili using young scion material can greatly accelerate breeding and shorten generation intervals. Members of the Western Gulf Forest Tree Improvement Program are already using top grafting as a part of their regular breeding programs. The results of this study suggest that flowering of loblolly pine scions can be promoted by top grafting on to either slash pine or loblolly pine interstocks.