Physiological responses of germinant Pinus palustris and P. taeda seedlings to water stress and the significance of the grass-stage.

• Rapid investment into needles leaves Pinus palustris vulnerable to drought. • P. palustris operates closer to hydraulic failure and inhibits gas exchange earlier. • Potential tradeoff between fire and drought tolerance early on in ontogeny. Loblolly pine (Pinus taeda L.) has gradually replaced longleaf pine (P. palustris Mill.) across much of the coastal southeastern United States. Given recent interest in the regeneration of longleaf pine ecosystems, understanding how these two species will respond to drier conditions associated with climate change will be critically important. We conducted a progressive water-stress experiment on germinants of P. palustris and P. taeda over the course of 9 weeks, measuring gas exchange, allometry, water relations, and tracheid anatomy. P. palustris developed more overall biomass, had wider tracheids, allocated more of its biomass into leaf tissue, and maintained a narrower safety margin against hydraulic failure than P. taeda. Both species reduced leaf gas exchange as water stress increased, though P. palustris experienced earlier and greater declines in photosynthetic rates than P. taeda. The greater aboveground growth and narrower hydraulic safety margin observed in P. palustris represents a trade-off between growth and hydraulic safety and is likely related to the species' defensive strategy against fire. We suggest that this trade-off may limit P. palustris germinant regeneration on sandier, xeric sites as climate change intensifies. [ABSTRACT FROM AUTHOR]