Response of Mid-Rotation Loblolly Pine (Pinus taeda L.) Physiology and Productivity to Sustained, Moderate Drought on the Western Edge of the Range

Abstract

Abstract:The productivity of the approximately 11 million ha of loblolly pine plantations in the southeastern USA could be threatened by decreased water availability in a future climate. To determine the effects of sustained drought on leaf gas exchange, whole-tree water use, and individual tree growth, we examined the response of loblolly pine trees to 100% throughfall exclusion cumulatively spanning the sixth and seventh growing seasons of a plantation in southeastern Oklahoma. Throughfall exclusion reduced volumetric soil water content for 0-12 cm soil depth from 10.8% to 4.8% and for 12-45 cm soil depth from 24.2% to 15.6%. Compared to ambient throughfall trees, leaf water potential of the throughfall exclusion trees became more negative, −0.9 MPa vs. −1.3 MPa for predawn measurements and −1.5 MPa vs. −1.9 MPa for midday measurements. Throughfall exclusion did not significantly reduce leaf gas exchange or tree water use. However, throughfall exclusion significantly reduced leaf biomass by 21% and stem volume growth by 23%. These results indicate that sustained drought may cause downward shifts in leaf quantity to conserve water rather than reducing leaf-level water use.

Lagged mortality among tree species four years after an exceptional drought in east Texas

“…1). At a finer scale, loblolly pine employs morphological water conservation strategies under moderate drought by reducing leaf area and maintaining leaf water potential high enough to avoid mortality (Maggard et al 2016), representing an isohydric water conservation strategy. Thus, predictions are underestimates but still highlight general trends for this species.…”

“…Recent tree-ring studies of pine species across the U.S. highlight the effectiveness of thinning and reduced stand densities at mitigating water stress (D'Amato et al 2013, Bottero et al 2017, Gleason et al 2017) lending support to our observations in east Texas. At a finer scale, loblolly pine employs morphological water conservation strategies under moderate drought by reducing leaf area and maintaining leaf water potential high enough to avoid mortality (Maggard et al 2016), representing an isohydric water conservation strategy. These physiological strategies also may have allowed many pines to avoid immediate mortality under the severe 2011 drought conditions.…”

“…Recent evidence examining these mechanisms suggests that hydraulic failure is most important in driving drought mortality across a range of species with reductions in carbon playing a smaller role, more so in gymnosperms than angiosperms (Adams et al 2017). Gymnosperms, such as loblolly pine (Pinus taeda L.) which dominates throughout east Texas, are capable of employing physiological adaptations to avoid drought stress by shedding needles and maintaining high leaf water potentials (Maggard et al 2016), falling more under the isohydric strategy. Gymnosperms, such as loblolly pine (Pinus taeda L.) which dominates throughout east Texas, are capable of employing physiological adaptations to avoid drought stress by shedding needles and maintaining high leaf water potentials (Maggard et al 2016), falling more under the isohydric strategy.…”

“…A recent global meta-analysis highlights these trends in that gymnosperms and drought-tolerant species typically experience long-lasting growth declines following a drought event prior to mortality (i.e., carbon starvation) while angiosperms may not show long or notable growth declines prior to mortality (i.e., hydraulic failure; Cailleret et al 2017). Gymnosperms, such as loblolly pine (Pinus taeda L.) which dominates throughout east Texas, are capable of employing physiological adaptations to avoid drought stress by shedding needles and maintaining high leaf water potentials (Maggard et al 2016), falling more under the isohydric strategy. Angiosperms, such as oaks, are prevalent throughout Texas and the south-central United States and, oaks in particular, are generally drought tolerant having deeper roots, ring-porous xylem anatomy, and operating at higher hydraulic safety margins (Abrams 1990, Hoffmann et al 2011, Klein 2014.…”

The Impact of Drought and Vascular-Inhabiting Pathogen Invasion inPinus taedaHealth

“…Many pathologists have had a false dichotomy of drought vs. biotic attack [17]. Many studies have focused primarily 2 International Journal of Forestry Research on individual factors: (i) drought and its subsequent effect on plant physiology [18,19] or (ii) Biotic agents and its subsequent impact tree health [15,16]. However, the evidence for the mechanisms suggested by these individual factors is inconclusive and a more integrated approach focusing on relations between drought and biotic agents on tree growth and functioning is needed.…”