[1] We developed a water-centric monthly scale simulation model (WaSSI-C) by integrating empirical water and carbon flux measurements from the FLUXNET network and an existing water supply and demand accounting model (WaSSI). The WaSSI-C model was evaluated with basin-scale evapotranspiration (ET), gross ecosystem productivity (GEP), and net ecosystem exchange (NEE) estimates by multiple independent methods across 2103 eight-digit Hydrologic Unit Code watersheds in the conterminous United States from 2001 to 2006. Our results indicate that WaSSI-C captured the spatial and temporal variability and the effects of large droughts on key ecosystem fluxes. Our modeled mean (±standard deviation in space) ET (556 ± 228 mm yr −1 ) compared well to Moderate Resolution Imaging Spectroradiometer (MODIS) based (527 ± 251 mm yr −1 ) and watershed water balance based ET (571 ± 242 mm yr −1 ). Our mean annual GEP estimates (1362 ± 688 g C m −2 yr −1 ) compared well (R 2 = 0.83) to estimates (1194 ± 649 g C m −2 yr −1 ) by eddy flux-based EC-MOD model, but both methods led significantly higher (25-30%) values than the standard MODIS product (904 ± 467 g C m −2 yr −1 ). Among the 18 water resource regions, the southeast ranked the highest in terms of its water yield and carbon sequestration capacity. When all ecosystems were considered, the mean NEE (−353 ± 298 g C m −2 yr −1 ) predicted by this study was 60% higher than EC-MOD's estimate (−220 ± 225 g C m −2 yr −1) in absolute magnitude, suggesting overall high uncertainty in quantifying NEE at a large scale. Our water-centric model offers a new tool for examining the trade-offs between regional water and carbon resources under a changing environment.

Summary• Hydraulic redistribution (HR) of water via roots from moist to drier portions of the soil occurs in many ecosystems, potentially influencing both water use and carbon assimilation.• By measuring soil water content, sap flow and eddy covariance, we investigated the temporal variability of HR in a loblolly pine (Pinus taeda) plantation during months of normal and below-normal precipitation, and examined its effects on tree transpiration, ecosystem water use and carbon exchange.• The occurrence of HR was explained by courses of reverse flow through roots. As the drought progressed, HR maintained soil moisture above 0.15 cm 3 cm )3 and increased transpiration by 30-50%. HR accounted for 15-25% of measured total site water depletion seasonally, peaking at 1.05 mm d )1. The understory species depended on water redistributed by the deep-rooted overstory pine trees for their early summer water supply. Modeling carbon flux showed that in the absence of HR, gross ecosystem productivity and net ecosystem exchange could be reduced by 750 and 400 g C m )2 yr )1 , respectively.• Hydraulic redistribution mitigated the effects of soil drying on understory and stand evapotranspiration and had important implications for net primary productivity by maintaining this whole ecosystem as a carbon sink.

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.

Increasing variability of rainfall patterns requires detailed understanding of the pathways of water loss from ecosystems to optimize carbon uptake and management choices. In the current study we characterized the usability of three alternative methods of different rigor for quantifying stand-level evapotranspiration (ET), partitioned ET into tree transpiration (T), understory transpiration, interception, and soil evaporation (E S ) and determined their sensitivity to drought, and evaluated the reliability of soil moisture measurements by taking into account deep soil moisture dynamic. The analyses were conducted in an early-and in a mid-rotation stand of loblolly pine, the predominant species of southern US forest plantations. The three alternative methods for estimating ET were the eddy covariance measurements of water vapor fluxes (ET EC ), the water table fluctuation (ET WT ), and the soil moisture fluctuation (ET SM ). On annual and monthly scales, the three methods agreed to within 10 -20%, whereas on a daily scale, the values of ET SM and ET EC differed by up to 50% and ET SM and ET WT differed by up to 100%. The differences between the methods were attributed to root water extraction below measurement depth and to the sampling at different spatial scales. Regardless of the method used, ET at the early-rotation site was 15-30% lower than that at the mid-rotation site. The dry years did not affect ET at the mid-rotation site but reduced significantly ET at the early-rotation site. Soil moisture trends revealed the importance of measuring water content at several depths throughout the rooting zone because less than 20% of the water is stored in the top 30 cm of soil. Annually, E S represented approximately 9 and 14% of ET EC at the mid-rotation site and the early-rotation site, respectively. At the mid-rotation site, T accounted for approximately 70% of ET EC . Canopy interception was estimated to be 5-10% of annual precipitation and 6 -13% of total ET EC . At the early-rotation site, T accounted for only 35% of ET EC . At this site, transpiration from subdominant trees and shrubs represented 40 -45% of ET EC , indicating that understory was a significant part of the water budget. We concluded that the eddy covariance method is best for estimating ET at the fine temporal scale (i.e., daily), but other soil moisture and water table-based methods were equally reliable and cost-effective for quantifying seasonal ET dynamics. FOR. SCI. 58(5):497-512.Keywords: eddy covariance, loblolly pine, Pinus taeda, sapflow, soil moisture probes, water table L OBLOLLY PINE (PINUS TAEDA L.) REPRESENTS ONE-HALF OF THE STANDING PINE VOLUME in the southern United States (11.7 million ha) and is by far the single most commercially important forest tree species for the region, with more than 1 billion seedlings planted annually (McKeand et al. 2003). More than 1 million ha of intensively managed loblolly pine plantations are found along the lower coastal plain in eastern North Carolina. Large areas of the North Carolina...

Deep root water uptake and hydraulic redistribution (HR) have been shown to play a major role in forest ecosystems during drought, but little is known about the impact of climate change, fertilization and soil characteristics on HR and its consequences on water and carbon fluxes. Using data from three mid-rotation loblolly pine plantations, and simulations with the process-based model MuSICA, this study indicated that HR can mitigate the effects of soil drying and had important implications for carbon uptake potential and net ecosystem exchange (NEE), especially when N fertilization is considered. At the coastal site (C), characterized by deep organic soil, HR increased dry season tree transpiration (T) by up to 40%, and such an increase affected NEE through major changes in gross primary productivity (GPP). Deep-rooted trees did not necessarily translate into a large volume of HR unless soil texture allowed large water potential gradients to occur, as was the case at the sandy site (S). At the Piedmont site (P) characterized by a shallow clay-loam soil, HR was low but not negligible, representing up to 10% of T. In the absence of HR, it was predicted that at the C, S and P sites, annual GPP would have been diminished by 19, 7 and 9%, respectively. Under future climate conditions HR was predicted to be reduced by up to 25% at the C site, reducing the resilience of trees to precipitation deficits. The effect of HR on T and GPP was predicted to diminish under future conditions by 12 and 6% at the C and P sites, respectively. Under future conditions, T was predicted to stay the same at the P site, but to be marginally reduced at the C site and slightly increased at the S site. Future conditions and N fertilization would decrease T by 25% at the C site, by 15% at the P site and by 8% at the S site. At the C and S sites, GPP was estimated to increase by 18% and by >70% under future conditions, respectively, with little effect of N fertilization. At the P site, future conditions would stimulate GPP by only 12%, but future conditions plus N fertilization would increase GPP by 24%. As a consequence, in all sites, water use efficiency was predicted to improve dramatically with future conditions. Modeling the effect of reduced annual precipitation indicated that limited water availability would decrease all carbon fluxes, including NEE and respiration. Our simulations highlight the interactive effects of nutrients and elevated CO(2), and showed that the effect of N fertilization would be greater under future climate conditions.