Papers by Jean-Christophe Domec
The New phytologist, Jan 5, 2017
Xylem vulnerability to embolism is emerging as a major factor in drought-induced tree mortality e... more Xylem vulnerability to embolism is emerging as a major factor in drought-induced tree mortality events across the globe. However, we lack understanding of how and to what extent climate has shaped vascular properties or functions. We investigated the evolution of xylem hydraulic function and diversification patterns in Australia's most successful gymnosperm clade, Callitris, the world's most drought-resistant conifers. For all 23 species in this group, we measured embolism resistance (P50 ), xylem specific hydraulic conductivity (Ks ), wood density, and tracheary element size from natural populations. We investigated whether hydraulic traits variation linked with climate and the diversification of this clade using a time-calibrated phylogeny. Embolism resistance varied widely across the Callitris clade (P50 : -3.8 to -18.8 MPa), and was significantly related to water scarcity, as was tracheid diameter. We found no evidence of a safety-efficiency tradeoff; Ks and wood density...
Plant, cell & environment, Jan 14, 2016
Here, we summarize studies on the effects of elevated [CO2 ] (CO2(e) ) on the structure and funct... more Here, we summarize studies on the effects of elevated [CO2 ] (CO2(e) ) on the structure and function of plant hydraulic architecture and explore the implications of those changes using a model. Changes in conduit diameter and hydraulic conductance due to CO2(e) vary among species. Ring-porous species tend towards an increase in conduit size and consequently conductivity. The effect in diffuse-porous species is much more limited. In conifers, the results are mixed, some species show minor changes in xylem structure, while other studies found increases in tracheid density and diameter. Non-woody plants generally exhibited the reverse pattern with narrower conduits and lower hydraulic conductivity under CO2(e) . Further, changes in drought-resistance traits suggest that non-woody plants were the most affected by CO2(e) , which may permit them to better resist drought-induced embolism under future conditions. Due to their complexity, acclimation in hydraulic traits in response to CO2(e)...
The New phytologist, Jan 21, 2016
In addition to buffering plants from water stress during severe droughts, plant water storage (PW... more In addition to buffering plants from water stress during severe droughts, plant water storage (PWS) alters many features of the spatio-temporal dynamics of water movement in the soil-plant system. How PWS impacts water dynamics and drought resilience is explored using a multi-layer porous media model. The model numerically resolves soil-plant hydrodynamics by coupling them to leaf-level gas exchange and soil-root interfacial layers. Novel features of the model are the considerations of a coordinated relationship between stomatal aperture variation and whole-system hydraulics and of the effects of PWS and nocturnal transpiration (Fe,night) on hydraulic redistribution (HR) in the soil. The model results suggest that daytime PWS usage and Fe,night generate a residual water potential gradient (Δψp,night) along the plant vascular system overnight. This Δψp,night represents a non-negligible competing sink strength that diminishes the significance of HR. Considering the co-occurrence of PW...
Ecological Applications, 2016
Canopy transpiration (EC ) is a large fraction of evapotranspiration, integrating physical and bi... more Canopy transpiration (EC ) is a large fraction of evapotranspiration, integrating physical and biological processes within the energy, water, and carbon cycles of forests. Quantifying EC is of both scientific and practical importance, providing information relevant to questions ranging from energy partitioning to ecosystem services, such as primary productivity and water yield. We estimated EC of four pine stands differing in age and growing on sandy soils. The stands consisted of two wide-ranging conifer species: Pinus taeda and Pinus sylvestris, in temperate and boreal zones, respectively. Combining results from these and published studies on all soil types, we derived an approach to estimate daily EC of pine forests, representing a wide range of conditions from 35° S to 64° N latitude. During the growing season and under moist soils, maximum daily EC (ECm ) at day-length normalized vapor pressure deficit of 1 kPa (ECm-ref ) increased by 0.55 ± 0.02 (mean ± SE) mm/d for each unit increase of leaf area index (L) up to L = ~5, showing no sign of saturation within this range of quickly rising mutual shading. The initial rise of ECm with atmospheric demand was linearly related to ECm-ref . Both relations were unaffected by soil type. Consistent with theoretical prediction, daily EC was sensitive to decreasing soil moisture at an earlier point of relative extractable water in loamy than sandy soils. Our finding facilitates the estimation of daily EC of wide-ranging pine forests using remotely sensed L and meteorological data. We advocate an assembly of worldwide sap flux database for further evaluation of this approach.
Tree physiology, Jan 26, 2016
Stable isotope ratios (δ(13)C and δ(18)O) of tree-ring α-cellulose are important tools in paleocl... more Stable isotope ratios (δ(13)C and δ(18)O) of tree-ring α-cellulose are important tools in paleoclimatology, ecology, plant physiology and genetics. The Multiple Sample Isolation System for Solids (MSISS) was a major advance in the tree-ring α-cellulose extraction methods, offering greater throughput and reduced labor input compared to traditional alternatives. However, the usability of the method for resinous conifer species may be limited by the need to remove extractives from some conifer species in a separate pretreatment step. Here we test the necessity of pretreatment for α-cellulose extraction in loblolly pine (Pinus taeda L.), and the efficiency of a modified acetone-based ambient-temperature step for the removal of extractives (i) in loblolly pine from five geographic locations representing its natural range in the southeastern USA, and (ii) on five other common coniferous species (black spruce (Picea mariana Mill.), Fraser fir (Abies fraseri (Pursh) Poir.), Douglas fir (Pse...
Plant physiology, Jan 9, 2016
The vascular system of grapevine has been reported as being highly vulnerable, even though grapev... more The vascular system of grapevine has been reported as being highly vulnerable, even though grapevine regularly experiences seasonal drought. Stomata would consequently remain open below water potentials that would generate a high loss of stem hydraulic conductivity via xylem embolism. This situation would necessitate daily cycles of embolism repair to restore hydraulic function.. However, a more parsimonious explanation is that some hydraulic techniques are prone to artifacts in species with long vessels, leading to overestimation of vulnerability. The aim of this study was to provide an unbiased assessment of (i) the vulnerability to drought-induced embolism in perennial and annual organs, and (ii) the ability to refill embolized vessels in two Vitis species. X-ray micro-CT observations on intact plants indicated that both V. vinifera and V. riparia were relatively vulnerable, with the pressure inducing 50% loss of stem hydraulic conductivity (Ψ50Stem) = -1.7 and -1.3MPa, respectiv...
Tree physiology, Jan 4, 2016
Water transport from soils to the atmosphere is critical for plant growth and survival. However, ... more Water transport from soils to the atmosphere is critical for plant growth and survival. However, we have a limited understanding about many portions of the whole-tree hydraulic pathway, because the vast majority of published information is on terminal branches. Our understanding of mature tree trunk hydraulic physiology, in particular, is limited. The hydraulic vulnerability segmentation hypothesis (HVSH) stipulates that distal portions of the plant (leaves, branches and roots) should be more vulnerable to embolism than trunks, which are nonredundant organs that require a massive carbon investment. In the current study, we compared vulnerability to loss of hydraulic function, leaf and xylem water potentials and the resulting hydraulic safety margins (in relation to the water potential causing 50% loss of hydraulic conductivity) in leaves, branches, trunks and roots of four angiosperms and four conifer tree species. Across all species, our results supported strongly the HVSH as leave...
Global Change Biology, 2016
Loblolly pine trees (Pinus taeda L.) occupy more than 20% of the forested area in the Southern Un... more Loblolly pine trees (Pinus taeda L.) occupy more than 20% of the forested area in the Southern United States, represent more than 50% of the standing pine volume in this region, and remove from the atmosphere about 500 g C m(-2) per year through net ecosystem exchange. Hence, their significance as a major regional carbon sink can hardly be disputed. What is disputed is whether the proliferation of young plantations replacing old forest in the Southern United States will alter key aspects of the hydrological cycle, including convective rainfall, which is the focus of the present work. Ecosystem fluxes of sensible (Hs) and latent heat (LE) and large-scale, slowlyevolving free atmospheric temperature and water vapor content are known to be first order controls on the formation of convective clouds in the atmospheric boundary layer. These controlling processes are here described by a zero-order analytical model aimed at assessing how plantations of different ages may regulate the persistence and transition of the atmospheric system between cloudy and cloudless conditions. Using the analytical model together with field observations, the roles of ecosystem Hs and LE on convective cloud formation are explored relative to the entrainment of heat and moisture from the free atmosphere. Our results demonstrate that cloudycloudless regimes at the land surface are regulated by a non-linear relation between the Bowen ratio Bo = Hs/LE and root-zone soil water content, suggesting that young/mature pines ecosystems have the ability to recirculate available water (through rainfall predisposition mechanisms). Such non-linearity was not detected in a much older pine stand, suggesting a higher tolerance to drought but a limited control on boundary layer dynamics. These results enable the generation of hypotheses about the impacts on convective cloud formation driven by afforestation/deforestation and groundwater depletion projected to increase following increased human population in the Southeastern United States. This article is protected by copyright. All rights reserved.
Global change biology, Jan 22, 2015
Rising atmospheric [CO2 ], ca , is expected to affect stomatal regulation of leaf gas-exchange of... more Rising atmospheric [CO2 ], ca , is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO2 ], ci , a constant drawdown in CO2 (ca - ci ), and a constant ci /ca . These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying ca . The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to ca . To assess leaf gas-exchange regulation strategies, we analyzed patterns in ci inferred from studies reporting C stable isotope ratios (δ(13) ...
The New phytologist, Jan 17, 2015
The evolution of lignified xylem allowed for the efficient transport of water under tension, but ... more The evolution of lignified xylem allowed for the efficient transport of water under tension, but also exposed the vascular network to the risk of gas emboli and the spread of gas between xylem conduits, thus impeding sap transport to the leaves. A well-known hypothesis proposes that the safety of xylem (its ability to resist embolism formation and spread) should trade off against xylem efficiency (its capacity to transport water). We tested this safety-efficiency hypothesis in branch xylem across 335 angiosperm and 89 gymnosperm species. Safety was considered at three levels: the xylem water potentials where 12%, 50% and 88% of maximal conductivity are lost. Although correlations between safety and efficiency were weak (r(2) < 0.086), no species had high efficiency and high safety, supporting the idea for a safety-efficiency tradeoff. However, many species had low efficiency and low safety. Species with low efficiency and low safety were weakly associated (r(2) < 0.02 in mos...
Forest Ecology and Management, 2015
With an increasing fraction of the world's forests being intensively managed for meeting humanity... more With an increasing fraction of the world's forests being intensively managed for meeting humanity's need for wood, fiber and ecosystem services, quantitative understanding of the functional changes in these ecosystems in comparison with natural forests is needed. In particular, the role of managed forests as long-term carbon (C) sinks and for mitigating climate change require a detailed assessment of their carbon cycle on different temporal scales. In the current review we assess available data on the structure and function of the world's forests, explore the main differences in the C exchange between managed and unmanaged stands, and explore potential physiological mechanisms behind both observed and expected changes. Two global databases that include classification for management indicate that managed forests are about 50 years younger, include 25% more coniferous stands, and have about 50% lower C stocks than unmanaged forests. The gross primary productivity (GPP) and total net primary productivity (NPP) are the similar, but relatively more of the assimilated carbon is allocated to aboveground pools in managed than in unmanaged forests, whereas allocation to fine roots and rhizosymbionts is lower. This shift in allocation patterns is promoted by increasing plant size, and by increased nutrient availability. Long-term carbon sequestration potential in soils is assessed through the ratio of heterotrophic respiration to total detritus production, which indicates that (i) the forest soils may be losing more carbon on an annual basis than they regain in detritus, and (ii) the deficit appears to be greater in managed forests. While climate change and management factors (esp. fertilization) both contribute to greater carbon accumulation potential in the soil, the harvest-related increase in decomposition affects the C budget over the entire harvest cycle. Although the findings do not preclude the use of forests for climate mitigation, maximizing merchantable productivity may have significant carbon costs for the soil pool. We conclude that optimal management strategies for maximizing multiple benefits from ecosystem services require better understanding of the dynamics of belowground allocation, carbohydrate availability, heterotrophic respiration, and carbon stabilization in the soil.
Background/Question/Methods Plant mortality events following droughts are occurring throughout th... more Background/Question/Methods Plant mortality events following droughts are occurring throughout the globe and rising mortality rates have been documented in major biomes. The American southwest and central plains regions have experienced extreme droughts both in the past few centuries that have resulted in dramatic changes in vegetation. There is currently considerable debate on the direct causes of mortality, particularly whether carbon starvation, hydraulic failure, or an interaction drives mortality. Although interest in simulating and forecasting vegetation mortality has grown substantially, no model has been developed specifically to simulate mortality because no one knows what are the empirical thresholds beyond which mortality is unavoidable? Our main goals were to investigate the advantages and disadvantages of different model assumptions for examining tree mortality during drought, and to determine the physiological mechanisms resulting in mortality, emphasizing hydraulic fa...
Background/Question/Methods Distributions of tree species under climate change may be attributed ... more Background/Question/Methods Distributions of tree species under climate change may be attributed to their hydraulic parameters, particularly as they pertain to drought-tolerance. The Edwards Plateau of central Texas experienced a severe drought during 2011-2013 which resulted in the death of hundreds of millions of trees. To determine if certain hydraulic parameters were related to greater mortality in different species we measured gas exchange, water potentials, leaf, root and branch hydraulic conductance and vulnerability to embolism and sapwood hydraulic capacitance in four co-occurring tree species (Quercus fusiformis, Prosopis glandulosa, Diospyros texana, and Juniperus asheii). Based on the summer aridity of this plant community (approximately 140 mm July-September rainfall and August mean maximum temperatures of 37.5˚C), we hypothesized that these species would be highly resistant to embolism. We also hypothesized that these species would have stomata that were highly sensiti...
Background/Question/Methods Over the past decade, much has been learned about climate change and ... more Background/Question/Methods Over the past decade, much has been learned about climate change and its potential impacts on vegetation. Many natural tree population distributions are predicted to change over the next century. However, most predictions about how these distributions will change are based primarily on parameters that have been measured on mature trees. The woody plant life stage with the greatest mortality, by far, is the germinant seedling stage and young seedling performance may be the most important for determination of species distributions. However, very little work has focused on the ecophysiology of seedlings (less than one year old). To have a better understanding of the ecophysiology of seedlings, we decided to work on three local species (Pinus taeda, Liriodendron tulipifera and Liquidambar styraciflua). Young seedlings (8 weeks old) were transplanted in experimental field in Duke forest in April 2013 and we measured seedling hydraulic properties, seedling wate...
Graduation date: 2002 Every wood anatomist knows that the wood near the center of a tree (juvenil... more Graduation date: 2002 Every wood anatomist knows that the wood near the center of a tree (juvenile wood) differs from the wood laid down at some distance from the pith (mature wood), and that the wood produced during the spring (earlywood) differs from the wood produced during the summer (latewood). There is a progressive increase in the dimensions of the cells from inner to outer growth rings. These differences affect the structure and function of the wood for water transport and mechanics. However, why do trees produce different wood quality as a function of cambial age? Is it as an adaptation to hydraulics or mechanical demands? No research has been undertaken in this area, because of the historic lack of communication between wood scientist and ecophysiologists, and because the hydraulic architecture field had few insights to offer until its recent wave of increasing sophistication. The chapters presented here are founded on these questions and provide new concepts and understan...
Every wood anatomist knows that the wood near the center of a tree (juvenile wood) differs from t... more Every wood anatomist knows that the wood near the center of a tree (juvenile wood) differs from the wood laid down at some distance from the pith (mature wood), and that the wood produced during the spring (earlywood) differs from the wood produced during the summer (latewood). There is a progressive increase in the dimensions of the cells from inner to outer growth rings. These differences affect the structure and function of the wood for water transport and mechanics. However, why do trees produce different wood quality as a function of cambial age? Is it as an adaptation to hydraulics or mechanical demands? No research has been undertaken in this area, because of the historic lack of communication between wood scientist and ecophysiologists, and because the hydraulic architecture field had few insights to offer until its recent wave of increasing sophistication. The chapters presented here are founded on these questions and provide new concepts and understandings of the trade-off...
Every wood anatomist knows that the wood near the center of a tree (juvenile wood) differs from t... more Every wood anatomist knows that the wood near the center of a tree (juvenile wood) differs from the wood laid down at some distance from the pith (mature wood), and that the wood produced during the spring (earlywood) differs from the wood produced during the summer (latewood). There is a progressive increase in the dimensions of the cells from inner to outer growth rings. These differences affect the structure and function of the wood for water transport and mechanics. However, why do trees produce different wood quality as a function of cambial age? Is it as an adaptation to hydraulics or mechanical demands? No research has been undertaken in this area, because of the historic lack of communication between wood scientist and ecophysiologists, and because the hydraulic architecture field had few insights to offer until its recent wave of increasing sophistication. The chapters presented here are founded on these questions and provide new concepts and understandings of the trade-off...
Trees grown in a short rotation coppice represent a potential source of bioenergy. Most of the re... more Trees grown in a short rotation coppice represent a potential source of bioenergy. Most of the research has been dedicated to genera Populus (poplars) and Salix (willows). However, poplars and willows require relatively high maintenance during the first years to eliminate the competition with weeds. We hypothesized that other autochtonous species not widely used for bioenergy purposes may provide comparable biomass production and may be better adapted to weed and other environmental pressures. To test this hypothesis we established a small-scale plot experiment with allochtonous hybrid poplar (Populus nigra x P. maximowiczii) and two autochtonous tree species American sycamore (Platanus occidentalis) and yellow poplar (Liriodendron tulipifera) in spring 2012. The trees were grown in a high density (10,000 trees per ha) in a randomized three-block design with four treatments: herbicide + insecticide (HI), herbicide + no insecticide (HC), no herbicide + insecticide (CI) and no herbici...
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Papers by Jean-Christophe Domec