Physiological and morphological attributes of shortleaf × loblolly pine F1 hybrid seedlings: is there an advantage to being a hybrid?

Fire suppression and other anthropogenic activities have the potential to eliminate ecological barriers that previously kept related, sympatric species from interbreeding. We compared artificial F1 shortleaf pine (Pinus echinata Mill.) x loblolly pine (Pinus taeda L.) hybrids with their parents to determine potential traits that have allowed hybrid seedlings to dramatically increase in abundance since 1950s. Six open-pollinated families each of loblolly and shortleaf pine as well as 12 controlled crosses were compared over 2 years in Oklahoma, USA. Loblolly and hybrid seedlings had higher establishment and growth rates than shortleaf pine. The hybrid seedlings had similar numbers of sprouts following top-clipping as shortleaf, which were higher than those of loblolly pine. Formation of a strong basal crook, an adaptation to protect dormant buds from fire, was greatest in shortleaf and lower in hybrid and loblolly pine. Instantaneous water use efficiency indicated that hybrid pine had similar high water use efficiency as shortleaf pine compared with loblolly pine. In the absence of fire, the hybrid seedlings perform at least as well as the parent species, which points out the importance of fire to eliminate hybrids and maintain the genetic integrity of shortleaf pine native to fire-prone, xeric sites. [PUBLICATION ABSTRACT]