Comparison analysis of seedling traits for hybrids between Pinus tabuliformisⅹP. yunnanensis and its parental species in P. densata habitat site.

Pinus densata had been suggested by genetic analyses to be a homoploid hybrid pine originated from natural hybridization between P. tabuliformis and P. yunnanensis. The alpine habitat of P. densata is distributed in southeastern Tibet between 2 700 and 4 200 m above sea level, exceeding that of its two parental species. To understand the ecological differentiations among these pines and investigate the mechanisms involved in hybrid adaptation to diverged niche, lines of artificial hybrid were built up and field comparison incorporating the natural hybrid pine, parental pines and the artificial pines, was applied in a typical habitat of the natural hybrid pine (Linzhi, Tibet). Ten traits of seedling validity and six traits of seedling growth were examined. Our results suggested that:1) the artificial hybrids (P. tabuliformisⅹP. yunnanensis) and P. densata displayed similar characteristics on most traits for seedling validity; 2) the artificial lines of hybrids were divergent in the traits by their origin, and some of them exhibited higher rates of bud-set and survival; 3) seedlings of P. tabuliformis had the highest validity,seedlings displayed highest performance in growth traits, while P. tabuliformis displayed the lowest; 5) in contrast to the parents, the phenomenon of diauxic growth in the second year was found in both the seedlings of P. densata and the artificial hybrids in similar percentage; 6) a line of artificial hybrid resembled the natural hybrid in most of the traits examined. Both the seedlings of natural and artificial hybrids exhibited superiority in both validity and growth under the habitat of high elevation, implying more developmental potential than their parental species in the alpine habitat. This study provides evidences for ecological speciation of P. densata via hybridization of P. tabuliformis and P. yunnanensis, and highlights the important role of ecological selection in the colonization of the Tibetan Plateau for hybrid pine. P. yunnanensis had the lowest, and P. densata was intermediated to its parents; 4) natural hybrid