Decoupling the influence of leaf and root hydraulic conductances on stomatal conductance and its sensitivity to vapour pressure deficit as soil dries in a drained loblolly pine plantation

Abstract

The study examined the relationships between whole tree hydraulic conductance (Ktree) and the conductance in roots (Kroot) and leaves (Kleaf) in loblolly pine trees. In addition, the role of seasonal variations in Kroot and Kleaf in mediating stomatal control of transpiration and its response to vapour pressure deficit (D) as soil-dried was studied. Compared to trunk and branches, roots and leaves had the highest loss of conductivity and contributed to more than 75% of the total tree hydraulic resistance. Drought altered the partitioning of the resistance between roots and leaves. As soil moisture dropped below 50%, relative extractable water (REW), Kroot declined faster than Kleaf. Although Ktree depended on soil moisture, its dynamics was tempered by the elongation of current-year needles that significantly increased Kleaf when REW was below 50%. After accounting for the effect of D on gs, the seasonal decline in Ktree caused a 35% decrease in gs and in its sensitivity to D, responses that were mainly driven by Kleaf under high REW and by Kroot under low REW. We conclude that not only water stress but also leaf phenology affects the coordination between Ktree and gs and the acclimation of trees to changing environmental conditions.

A Comparison of Three Methods to Estimate Evapotranspiration in Two Contrasting Loblolly Pine Plantations: Age-Related Changes in Water Use and Drought Sensitivity of Evapotranspiration Components

“…More details on LAI measurements at this site are given in Domec et al (2009) and Noormets et al (2010). The optical method was not appropriate because of the open canopy at the early-rotation site, so minimum LAI (winter LAI) was measured destructively on 6 -10 trees harvested in late winter every year.…”

“…The optical method was not appropriate because of the open canopy at the early-rotation site, so minimum LAI (winter LAI) was measured destructively on 6 -10 trees harvested in late winter every year. The seasonal change in tree LAI at this site was calculated using the seasonal pattern in current-year needle elongation and previous-year needle loss (Domec et al 2009). The variation curve of total LAI (trees and understory vegetation combined) was determined based on the MODIS-LAI product (Knyazikhin et al 1998, Oak Ridge National Laboratory Distributed Active Archive Center 2010), which is an 8-day interval time series, and had a total of 43 scenes per year at our sites.…”

“…Preliminary results showed that there was no significant difference in azimuthal J s within trees (P ϭ 0.27, F-test), probably owing to the homogeneity of pine trunks. Therefore, we inserted all probes on the north-northwest side of the trees (Domec et al 2009). Thirty-minute averages of temperature difference data were computed and stored in data loggers (CR10; Campbell Scientific).…”

“…For tree species, a shift in ecologic and environmental growing conditions may result in a decline in tree performance and in productivity due to climate change-induced drought stress (Hanson and Weltzin, 2000, Irvine et al 2004, McDowell et al 2008). This consideration is especially true for loblolly pine trees that are responsive to prolonged low soil water content (Oren et al 1998, Ford et al 2005, Domec et al 2009), which decreases tree transpiration, gross primary productivity, and net carbon exchange . Soil water content is the essential state variable in hydrologic studies of Acknowledgments: This study was supported by the USDA Forest Service Raleigh Eastern Forest Environmental Threat Assessment Center cooperative agreements 03-CA-11330147-073 and 04-CA-11330147-238, and the DOE-BER Terrestrial Ecosystem Sciences program, grant 11-DE-SC-0006700.…”

Forest Management Challenges for Sustaining Water Resources in the Anthropocene

“…Consequently, predicting the future impacts of climate warming and hydrologic drought on watershed hydrology is difficult. While there is growing information about how drought interacts with hydraulic architecture and stomatal responses [60][61][62], we know very little (and hence, can't model) about how drought impacts root structure and function. Furthermore, in mixed species stands drought does not affect trees equally.…”