Morphological Variability Among Loblolly and Shortleaf Pines of East Texas with Reference to Natural Hybridization

Needle, bud and cone characteristics were measured on 164 trees sampled from natural loblolly-shortleaf pine stands in east Texas. Hybrid index, cluster analysis and principal components analysis were used to characterize the morphological variability of the sample population. Results indicated the existence of two relatively distinct populations (loblolly and shortleaf pine). It was concluded that introgression of loblolly and shortleaf pines is not widespread in the area. Introgression with shortleaf pine has been proposed by several workers as an explanation for the apparent rust resistance of east Texas loblolly pine. In view of our conclusions, alternative explanations include: (1) Ancestral hybridization of the two species has been followed by subsequent speciation and separation; (2) Natural selection of rust-resistant loblolly pine has occurred in east Texas; (3) Unfavorable climatic conditions result in low rust infestation in east Texas, while rust resistance of east Texas loblolly pine when grown in other areas results from incompatibility of this race with eastern races of Cronartium fusiforme.