Post-fire resprouting of shortleaf pine is facilitated by a morphological trait but fire eliminates shortleaf × loblolly pine hybrid seedlings

Concurrent with fire exclusion, shortleaf pine × loblolly pine hybrids have increased throughout the southeastern USA and may threaten the genetic integrity of shortleaf pine. Historically, fire favored shortleaf pine over other southern pine species, especially loblolly pine, with which it shares a broad sympatric natural range. Shortleaf pine seedlings have a morphological adaptation (basal crook) that lowers dormant buds to the soil surface where they are presumably protected from fire to facilitate resprouting after topkill. To evaluate this presumption, we tested (1) the functional role of the basal crook in protecting dormant buds by measuring resprouting after exposing the lower stem of shortleaf pine to fire and protecting from fire the lower stem (and dormant buds) of loblolly pine (which lacks a basal crook) and (2) the occurrence and importance of the basal crook morphological adaptation in F1 shortleaf × loblolly pine seedlings by comparing morphology and post-fire resprouting of the hybrids to both parents. Fire exposure of shortleaf pine seedling dormant buds caused seedling mortality, while protecting loblolly pine dormant buds facilitated resprouting. Hybrid pines have basal crooks intermediate to the strong crook of shortleaf pine and loblolly pine's non-crooked stem. Fire top-killed loblolly pine and shortleaf × loblolly pine seedlings, and they did not resprout, while 57% of shortleaf pine resprouted after topkill during two dormant season and one growing season burns. This highly significant difference shows that the basal crook is important for shortleaf pine resprouting after topkill by fire. Fire is an important tool to reduce shortleaf × loblolly pine hybrids, and to provide a competitive advantage to shortleaf pine relative to loblolly pine, for ecosystem restoration and maintaining the genetic integrity of shortleaf pine.