Browsing by Author "Domec, Jean-Christophe"
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Item Open Access A broad survey of hydraulic and mechanical safety in the xylem of conifers.(Journal of experimental botany, 2014-08) Bouche, Pauline S; Larter, Maximilien; Domec, Jean-Christophe; Burlett, Régis; Gasson, Peter; Jansen, Steven; Delzon, SylvainDrought-induced forest dieback has been widely reported over the last decades, and the evidence for a direct causal link between survival and hydraulic failure (xylem cavitation) is now well known. Because vulnerability to cavitation is intimately linked to the anatomy of the xylem, the main objective of this study was to better understand the xylem anatomical properties associated with cavitation resistance. An extensive data set of cavitation resistance traits and xylem anatomical properties was developed for 115 conifer species, with special attention given to the micro-morphology of bordered pits. The ratio of torus to pit aperture diameter, so-called torus overlap, increased with increasing cavitation resistance, while the flexibility of the margo does not seem to play a role, suggesting that air-seeding is located at the seal between the aspirated torus and pit aperture. Moreover, punctured tori were reported in various Pinaceae species. Species resistant to cavitation had thicker tracheid walls, while their lumen diameter (conduit size) was only slightly reduced, minimizing the impact on hydraulic conductance. The results also demonstrated (i) the existence of an indirect trade-off between hydraulic safety and mechanical strength; and (ii) a consistency between species distribution and xylem anatomy: species with a wide torus overlap and high valve effects are found in arid environments such as the Mediterranean region.Item Open Access 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(Forest Science, 2012-10-02) Domec, Jean-Christophe; Sun, Ge; Noormets, Asko; Gavazzi, Michael J; Treasure, Emrys A; Cohen, Erika; Swenson, Jennifer J; McNulty, Steve G; King, John SIncreasing 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.© 2012 by the Society of American Foresters.Item Open Access Co-occurring woody species have diverse hydraulic strategies and mortality rates during an extreme drought.(Plant Cell Environ, 2018-03) Johnson, Daniel M; Domec, Jean-Christophe; Carter Berry, Z; Schwantes, Amanda M; McCulloh, Katherine A; Woodruff, David R; Wayne Polley, H; Wortemann, Remí; Swenson, Jennifer J; Scott Mackay, D; McDowell, Nate G; Jackson, Robert BFrom 2011 to 2013, Texas experienced its worst drought in recorded history. This event provided a unique natural experiment to assess species-specific responses to extreme drought and mortality of four co-occurring woody species: Quercus fusiformis, Diospyros texana, Prosopis glandulosa, and Juniperus ashei. We examined hypothesized mechanisms that could promote these species' diverse mortality patterns using postdrought measurements on surviving trees coupled to retrospective process modelling. The species exhibited a wide range of gas exchange responses, hydraulic strategies, and mortality rates. Multiple proposed indices of mortality mechanisms were inconsistent with the observed mortality patterns across species, including measures of the degree of iso/anisohydry, photosynthesis, carbohydrate depletion, and hydraulic safety margins. Large losses of spring and summer whole-tree conductance (driven by belowground losses of conductance) and shallower rooting depths were associated with species that exhibited greater mortality. Based on this retrospective analysis, we suggest that species more vulnerable to drought were more likely to have succumbed to hydraulic failure belowground.Item Metadata only Conversion of natural forests to managed forest plantations decreases tree resistance to prolonged droughts(Forest Ecology and Management, 2015-01-01) Domec, Jean-Christophe; King, John S; Ward, Eric; Christopher Oishi, A; Palmroth, Sari; Radecki, Andrew; Bell, Dave M; Miao, Guofang; Gavazzi, Michael; Johnson, Daniel M; McNulty, Steve G; Sun, Ge; Noormets, Asko© 2015 Published by Elsevier B.V.Throughout the southern US, past forest management practices have replaced large areas of native forests with loblolly pine plantations and have resulted in changes in forest response to extreme weather conditions. However, uncertainty remains about the response of planted versus natural species to drought across the geographical range of these forests. Taking advantage of a cluster of unmanaged stands (85-130year-old hardwoods) and managed plantations (17-20year-old loblolly pine) in coastal and Piedmont areas of North Carolina, tree water use, cavitation resistance, whole-tree hydraulic (Ktree) and stomatal (Gs) conductances were measured in four sites covering representative forests growing in the region. We also used a hydraulic model to predict the resilience of those sites to extreme soil drying. Our objectives were to determine: (1) if Ktree and stomatal regulation in response to atmospheric and soil droughts differ between species and sites; (2) how ecosystem type, through tree water use, resistance to cavitation and rooting profiles, affects the water uptake limit that can be reached under drought; and (3) the influence of stand species composition on critical transpiration that sets a functional water uptake limit under drought conditions. The results show that across sites, water stress affected the coordination between Ktree and Gs. As soil water content dropped below 20% relative extractable water, Ktree declined faster and thus explained the decrease in Gs and in its sensitivity to vapor pressure deficit. Compared to branches, the capability of roots to resist high xylem tension has a great impact on tree-level water use and ultimately had important implications for pine plantations resistance to future summer droughts. Model simulations revealed that the decline in Ktree due to xylem cavitation aggravated the effects of soil drying on tree transpiration. The critical transpiration rate (Ecrit), which corresponds to the maximum rate at which transpiration begins to level off to prevent irreversible hydraulic failure, was higher in managed forest plantations than in their unmanaged counterparts. However, even with this higher Ecrit, the pine plantations operated very close to their critical leaf water potentials (i.e. to their permissible water potentials without total hydraulic failure), suggesting that intensively managed plantations are more drought-sensitive and can withstand less severe drought than natural forests.Item Open Access Drought and thinning have limited impacts on evapotranspiration in a managed pine plantation on the southeastern United States coastal plain(Agricultural and Forest Meteorology, 2018-11) Liu, Xiaodong; Sun, Ge; Mitra, Bhaskar; Noormets, Asko; Gavazzi, Michael J; Domec, Jean-Christophe; Hallema, Dennis W; Li, Jiyue; Fang, Yuan; King, John S; McNulty, Steven G© 2018 Managed and natural coastal plain forests in the humid southeastern United States exchange large amounts of water and energy with the atmosphere through the evapotranspiration (ET) process. ET plays an important role in controlling regional hydrology, climate, and ecosystem productivity. However, long-term studies on the impacts of forest management and climatic variability on forest ET are rare, and our understanding of both external and internal drivers on seasonal and interannual ET variability is incomplete. Using techniques centered on an eddy covariance method, the present study measured year-round ET flux and associated hydrometeorological variables in a drained loblolly pine (Pinus taeda L.) plantation on the lower coastal plain of North Carolina, U.S. We found that annual ET was relatively stable (1076 ± 104 mm) in comparison to precipitation (P) (1168 ± 216 mm) during the 10-year study period when the site experienced extreme climate (2007–2008) and forest thinning (2009). At the seasonal time scale, mean ET/P varied between 0.41 and 1.51, with a mean value of 1.12 ± 0.23 and 0.72 ± 0.16 for the growing and dormant seasons, respectively. The extreme drought during 2007–2008 (mean annual P, 854 mm) only resulted in a slight decrease (∼8%) in annual ET owing to the shallow groundwater common to the study area. Although changes in leaf area index and canopy structure were large after the stand was 50% thinned in the fall of 2009, mean annual ET was similar and averaged 1055 mm and 1104 mm before (2005, 2006 and 2009) and after (2010–2015) thinning, respectively. Data suggested that annual ET recovered within two years of the thinning as a result of rapid canopy closure and growth of understory. Further analysis indicated that available energy was the key driver of ET: approximately 69% and 61% of the monthly variations in ET were explained by net radiation during the dormant and growing seasons, respectively. Overall, we concluded that drought and forest thinning had limited impacts on seasonal and annual ET in this energy limited forest ecosystem with shallow groundwater. The results from this study help to better understand regional ecohydrological processes and projecting potential effects of forest management and extreme climate on water and carbon cycles.Item Open Access Drought will not leave your glass empty: Low risk of hydraulic failure revealed by long-term drought observations in world's top wine regions(Science Advances, 2018-01-01) Charrier, Guillaume; Delzon, Sylvain; Domec, Jean-Christophe; Zhang, Li; Delmas, Chloe EL; Merlin, Isabelle; Corso, Deborah; King, Andrew; Ojeda, Hernan; Ollat, Nathalie; Prieto, Jorge A; Scholach, Thibaut; Skinner, Paul; van Leeuwen, Cornelis; Gambetta, Gregory ACopyright © 2018 The Authors, some rights reserved;. Grapevines are crops of global economic importance that will face increasing drought stress because many varieties are described as highly sensitive to hydraulic failure as frequency and intensity of summer drought increase. We developed and used novel approaches to define water stress thresholds for preventing hydraulic failure, which were compared to the drought stress experienced over a decade in two of the world's top wine regions, Napa and Bordeaux. We identified the physiological thresholds for drought-induced mortality in stems and leaves and found small intervarietal differences. Long-term observations in Napa and Bordeaux revealed that grapevines never reach their lethal water-potential thresholds under seasonal droughts, owing to a vulnerability segmentation promoting petiole embolism and leaf mortality. Our findings will aid farmers in reducing water use without risking grapevine hydraulic integrity.Item Open Access Ecophysiological impacts of Esca, a devastating grapevine trunk disease, on Vitis vinifera L.(PloS one, 2019-01) Ouadi, Loris; Bruez, Emilie; Bastien, Sylvie; Vallance, Jessica; Lecomte, Pascal; Domec, Jean-Christophe; Rey, PatriceEsca is a Grapevine Trunk Disease (GTD) caused by a broad range of taxonomically unrelated fungal pathogens. These attack grapevine wood tissues inducing necroses even in the conductive vascular tissues, thus affecting the vine physiology and potentially leading to plant death. However, the influence of Esca on leaf and whole-plant water transport disruption remains poorly understood. In this paper, a detailed analysis of xylem-related physiological parameters in grapevines that expressed Esca-foliar symptoms was carried out. The experiments were conducted in a vineyard in the Bordeaux region (France) on cv. Cabernet-Sauvignon (Vitis vinifera L.) grapevines, which were monitored for Esca-foliar symptoms over a two-year period. Heat dissipation sap-flow sensors were installed during the summer on grapevines having expressed or not Esca-foliar symptoms. Leaf water potential, stomatal conductance and leaf transpiration were also measured. Physiological monitoring showed that sap flow density and whole-plant transpiration of Esca-infected grapevines decreased significantly a week before the first foliar symptoms appeared. When atmospheric water demand (Vapour Pressure Deficit, VPD) was the highest, both parameters tended to be about twice as low in symptomatic grapevines as in asymptomatic ones. Sap flow density data at the maximum transpiration-time, was systematically 29-30% lower in Esca-infected grapevines compared to control plants before or after the appearance of Esca-foliar symptoms. This trend was observed whatever the temperatures and VPD values measured. In Esca-diseased plants, larger amounts of necrotic wood, mainly white rot, were found in the trunk and cordon of symptomatic grapevines compared to healthy ones, suggesting necroses have an influence in reducing the whole-plant hydraulic capacity. This study reveals that the use of physiological monitoring methods, together with the visual monitoring of foliar symptoms, could prove useful in providing accurate measurements of Esca disease severity.Item Restricted Gold mining in the Peruvian Amazon: global prices, deforestation, and mercury imports.(PLoS One, 2011-04-19) Swenson, Jennifer J; Carter, Catherine E; Domec, Jean-Christophe; Delgado, Cesar IMany factors such as poverty, ineffective institutions and environmental regulations may prevent developing countries from managing how natural resources are extracted to meet a strong market demand. Extraction for some resources has reached such proportions that evidence is measurable from space. We present recent evidence of the global demand for a single commodity and the ecosystem destruction resulting from commodity extraction, recorded by satellites for one of the most biodiverse areas of the world. We find that since 2003, recent mining deforestation in Madre de Dios, Peru is increasing nonlinearly alongside a constant annual rate of increase in international gold price (∼18%/yr). We detect that the new pattern of mining deforestation (1915 ha/year, 2006-2009) is outpacing that of nearby settlement deforestation. We show that gold price is linked with exponential increases in Peruvian national mercury imports over time (R(2) = 0.93, p = 0.04, 2003-2009). Given the past rates of increase we predict that mercury imports may more than double for 2011 (∼500 t/year). Virtually all of Peru's mercury imports are used in artisanal gold mining. Much of the mining increase is unregulated/artisanal in nature, lacking environmental impact analysis or miner education. As a result, large quantities of mercury are being released into the atmosphere, sediments and waterways. Other developing countries endowed with gold deposits are likely experiencing similar environmental destruction in response to recent record high gold prices. The increasing availability of satellite imagery ought to evoke further studies linking economic variables with land use and cover changes on the ground.Item Open Access Growth and physiological responses of isohydric and anisohydric poplars to drought.(J Exp Bot, 2015-07) Attia, Ziv; Domec, Jean-Christophe; Oren, Ram; Way, Danielle A; Moshelion, MenachemUnderstanding how different plants prioritize carbon gain and drought vulnerability under a variable water supply is important for predicting which trees will maximize woody biomass production under different environmental conditions. Here, Populus balsamifera (BS, isohydric genotype), P. simonii (SI, previously uncharacterized stomatal behaviour), and their cross, P. balsamifera x simonii (BSxSI, anisohydric genotype) were studied to assess the physiological basis for biomass accumulation and water-use efficiency across a range of water availabilities. Under ample water, whole plant stomatal conductance (gs), transpiration (E), and growth rates were higher in anisohydric genotypes (SI and BSxSI) than in isohydric poplars (BS). Under drought, all genotypes regulated the leaf to stem water potential gradient via changes in gs, synchronizing leaf hydraulic conductance (Kleaf) and E: isohydric plants reduced Kleaf, gs, and E, whereas anisohydric genotypes maintained high Kleaf and E, which reduced both leaf and stem water potentials. Nevertheless, SI poplars reduced their plant hydraulic conductance (Kplant) during water stress and, unlike, BSxSI plants, recovered rapidly from drought. Low gs of the isohydric BS under drought reduced CO2 assimilation rates and biomass potential under moderate water stress. While anisohydric genotypes had the fastest growth under ample water and higher photosynthetic rates under increasing water stress, isohydric poplars had higher water-use efficiency. Overall, the results indicate three strategies for how closely related biomass species deal with water stress: survival-isohydric (BS), sensitive-anisohydric (BSxSI), and resilience-anisohydric (SI). Implications for woody biomass growth, water-use efficiency, and survival under variable environmental conditions are discussed.Item Open Access Hydraulic redistribution of soil water by roots affects whole-stand evapotranspiration and net ecosystem carbon exchange.(New Phytol, 2010-07) Domec, Jean-Christophe; King, John S; Noormets, Asko; Treasure, Emrys; Gavazzi, Michael J; Sun, Ge; McNulty, Steven G*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.Item Open Access Leaf phenology paradox: Why warming matters most where it is already warm(Remote Sensing of Environment, 2018-05-01) Seyednasrollah, Bijan; Swenson, Jennifer J; Domec, Jean-Christophe; Clark, James S© 2018 Elsevier Inc. Interactions between climate and ecosystem properties that control phenological responses to climate warming and drought are poorly understood. To determine contributions from these interactions, we used space-borne remotely sensed vegetation indices to monitor leaf development across climate gradients and ecoregions in the southeastern United States. We quantified how air temperature, drought severity, and canopy thermal stress contribute to changes in leaf flushing from mountainous to coastal plain regions by developing a hierarchical state-space Bayesian model. We synthesized daily field climate data with daily vegetation indices and canopy surface temperature during spring green-up season at 59 sites in the southeastern United States between 2001 and 2012. Our results demonstrated strong interaction effects between ecosystem properties and climate variables across ecoregions. We found spring green-up is faster in the mountains, while coastal forests express a larger sensitivity to inter-annual temperature anomalies. Despite our detection of a decreasing trend in sensitivity to warming with temperature in all regions, we identified an ecosystem interaction: Deciduous dominated forests are less sensitive to warming than are those with fewer deciduous trees, likely due to the continuous presence of leaves in evergreen species throughout the season. Mountainous forest green-up is more susceptible to intensifying drought and moisture deficit, while coastal areas are relatively resilient. We found that with increasing canopy thermal stress, defined as canopy-air temperature difference, leaf development slows following dry years, and accelerates following wet years.Item Open Access Remote Sensing Tree Physiology(2021-08-17) Frear, JoshuaTranspiration, or plant water loss, is a critical component of water balance and flux for terrestrial systems worldwide, yet uncertainty in large-scale estimates create significant challenges for water resource forecasting (Jasechko 2013, Coenders-Gerrits 2014). Effectively measuring changes in transpiration over time can provide insight into water availability issues facing a tree, a stand, or a landscape. As temperature and precipitation patterns change in the next century and cause shifts in water availability, forest health around the world is likely to change, too, and should be monitored. Currently, transpiration is often monitored at research sites with specialized eddy covariance flux towers, or sap-flux sensors installed in trees that measure the transpiration of individual trees or stands. To measure transpiration frequently across the Earth, a different approach is required. One possible tool for this is ECOSTRESS (Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station). ECOSTRESS is an experiment run by NASA’s Jet Propulsion Laboratory that measures elements of plant transpiration worldwide (Fischer 2020). Attached to the International Space Station, the ECOSTRESS radiometer measures latent heat flux and estimates plant water loss occurring around the world every day. The objective of this project is to investigate and compare ECOSTRESS to more traditional methods of measuring transpiration at three temperate forest research sites in the US and France. Comparing transpiration and evapotranspiration data at each site since the inception of ECOSTRESS in 2018 reveal significant but weak correlations between the two measurements (all r2 < 0.4). ECOSTRESS generally measured much higher transpiration for the days of overpass compared to sap-flux measurements. Time of day of overpass had a significant relationship with the difference between the two measurements, but sub-setting the data to exclude times of day with low transpiration or winter months generally did not improve the correlations, except for excluding morning observations. Based on these observations, ECOSTRESS should be used with caution for non-spatial time-series-type studies. Additionally, there remains a significant spatial difference between the two approaches, as one ECOSTRESS pixel covers about half a hectare.Item Open Access Role of aquaporin activity in regulating deep and shallow root hydraulic conductance during extreme drought(Trees, 2014-05-30) Johnson, Daniel M; Sherrard, Mark E; Domec, Jean-Christophe; Jackson, Robert BKey message Deep root hydraulic conductance is upregulated during severe drought and is associated with upregulation in aquaporin activity. In 2011, Texas experienced the worst single-year drought in its recorded history and, based on tree-ring data, likely its worst in the past millennium. In the Edwards Plateau of Texas, rainfall was 58 % lower and the mean daily maximum temperatures were >5 °C higher than long-term means in June through September, resulting in extensive tree mortality. To better understand the balance of deep and shallow root functioning for water supply, we measured root hydraulic conductance (KR) in deep (~20 m) and shallow (5-10 cm) roots of Quercus fusiformis at four time points in the field in 2011. Deep roots of Q. fusiformis obtained water from a perennial underground (18-20 m) stream that was present even during the drought. As the drought progressed, deep root KR increased 2.6-fold from early season values and shallow root KR decreased by 50 % between April and September. Inhibitor studies revealed that aquaporin contribution to KR increased in deep roots and decreased in shallow roots as the drought progressed. Deep root aquaporin activity was upregulated during peak drought, likely driven by increased summer evaporative demand and the need to compensate for declining shallow root KR. A whole-tree hydraulic transport model predicted that trees with greater proportions of deep roots would have as much as five times greater transpiration during drought periods and could sustain transpiration during droughts without experiencing total hydraulic failure. Our results suggest that trees shift their dependence on deep roots versus shallow roots during drought periods, and that upregulation of aquaporin activity accounts for at least part of this increase. © 2014 Springer-Verlag Berlin Heidelberg.Item Open Access Variability of sun-induced chlorophyll fluorescence according to stand age-related processes in a managed loblolly pine forest(Global Change Biology, 2018-07) Colombo, Roberto; Celesti, Marco; Bianchi, Remo; Campbell, Petya KE; Cogliati, Sergio; Cook, Bruce D; Corp, Lawrence A; Damm, Alexander; Domec, Jean-Christophe; Guanter, Luis; Julitta, Tommaso; Middleton, Elizabeth M; Noormets, Asko; Panigada, Cinzia; Pinto, Francisco; Rascher, Uwe; Rossini, Micol; Schickling, Anke© 2018 John Wiley & Sons Ltd Leaf fluorescence can be used to track plant development and stress, and is considered the most direct measurement of photosynthetic activity available from remote sensing techniques. Red and far-red sun-induced chlorophyll fluorescence (SIF) maps were generated from high spatial resolution images collected with the HyPlant airborne spectrometer over even-aged loblolly pine plantations in North Carolina (United States). Canopy fluorescence yield (i.e., the fluorescence flux normalized by the light absorbed) in the red and far-red peaks was computed. This quantifies the fluorescence emission efficiencies that are more directly linked to canopy function compared to SIF radiances. Fluorescence fluxes and yields were investigated in relation to tree age to infer new insights on the potential of those measurements in better describing ecosystem processes. The results showed that red fluorescence yield varies with stand age. Young stands exhibited a nearly twofold higher red fluorescence yield than mature forest plantations, while the far-red fluorescence yield remained constant. We interpreted this finding in a context of photosynthetic stomatal limitation in aging loblolly pine stands. Current and future satellite missions provide global datasets of SIF at coarse spatial resolution, resulting in intrapixel mixture effects, which could be a confounding factor for fluorescence signal interpretation. To mitigate this effect, we propose a surrogate of the fluorescence yield, namely the Canopy Cover Fluorescence Index (CCFI) that accounts for the spatial variability in canopy structure by exploiting the vegetation fractional cover. It was found that spatial aggregation tended to mask the effective relationships, while the CCFI was still able to maintain this link. This study is a first attempt in interpreting the fluorescence variability in aging forest stands and it may open new perspectives in understanding long-term forest dynamics in response to future climatic conditions from remote sensing of SIF.Item Open Access Vulnerability to cavitation and response of canopy conductance to root cavitation in five southeastern US pine species(2021-12-08) Zhang, HaoyuAlthough the Southeastern United States generally receives considerable precipitation and total precipitation has slightly increased, the region has experienced recent recurrent droughts and is anticipated to encounter more severe droughts as a result of increasing temperature. Knowledge of plant water transport and plant organ hydraulics facilitates our understanding of the capabilities of forests to withstand drought under a changing climate. In this project, we studied five pine species that are ecologically or economically important in the Southeastern US, namely Pinus virginiana (Virginia pine), P. echinata (shortleaf pine), P. taeda (loblolly pine), P. elliottii (slash pine), and P. palustris (longleaf pine). The overarching goal is to understand and compare their hydraulic performance under droughts, and we addressed the goal from two perspectives. First, we compared hydraulic properties associated with vulnerability to embolism curves for terminal branches and lateral roots in the topsoil. Second, we investigated the stomatal regulation strategies adopted by these species. For this purpose, we studied the response of canopy conductance index (GcI) to the estimated in situ loss of conductivity in roots (PLCroot) during the growing season of 2019. Two methods of estimating soil water potential, from which PLCroot was computed, were used by either assuming random noise in measurements on soil texture or assuming representation of microsite variability by the measurements. Overall, the results suggest that the two short-needle species, P. virginiana and P. echinata, are likely to be more resistant to drought by the combined benefit of higher resistance to cavitation in roots and moderate to conservative regulation of GcI. The study also calls attention to the assumption made for microsite variability in soil texture, when using common garden to study relationships among functions of different organs that entail soil water conditions.