Scaling xylem sap flux and soil water balance and calculating variance: a method for partitioning water flux in forests

Oren, Ram; Phillips, Nathan; Katul, Gabriel G.; Ewers, B. E.; Pataki, Diane E.

doi:10.1051/forest:19980112

Cited by 215 publications

(193 citation statements)

References 35 publications

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“…Spruce 2 had two large branches, just above the Granier measuring point, which may have also affected the flow pattern as compared to Spruce 1, where there were no large branches near the measuring point. The radial variation in sapflow density was in the same range as those found in several studies on pine and spruce [6,25,34,37]. A sensor that covered the whole sapwood depth, and integrated the variation in sapflow, would probably best account for the radial variations in sapflow density.…”

Section: Radial Variation Of Granier Measurementssupporting

confidence: 62%

Evaluation of heat balance and heat dissipation methods for sapflow measurements in pine and spruce

2001

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-The tissue heat-balance method (Cÿ ermák) and the heat-dissipation method (Granier) were compared in three Scots pines and two Norway spruces in a forest in central Sweden. The Granier system measured up to 50% lower sapflow than the Cÿ ermák system at high flow rates. New coefficients for the Granier system were estimated, based on sapflow density from the Cÿ ermák measurements. Without compensation, natural temperature gradients may cause large errors in measurements made by the Granier system. By using a horizontal reference sensor, no compensation was necessary. It was also shown that radial flow patterns must be considered when calculating total tree sapflow. Transpiration of two adjacent stands, one measured by the Granier method and the other by the Cÿ ermák method, showed good agreement internally and with total evaporation measured by the eddy-correlation method. THB / HD / sap-flow / transpiration / Picea abies / Pinus sylvestrisRésumé -Évaluation des méthodes des équilibres thermiques et de dissipation thermique concernant la mesure de flux de sève pour le pin et l'épicéa. La méthode des équilibres thermiques (Cÿ ermák), et la méthode de dissipation thermique (Granier) ont été comparées pour trois Pins sylvestres et deux épicéas dans une forêt du centre de la Suède. Le système Granier a donné des valeurs de flux de sève jusqu'à 50 % plus faible que le système Cÿ ermák pour les flux élevés. De nouveaux coefficients basés sur les mesures de densité de flux obtenues avec le système Cÿ ermák ont été estimés pour le système Granier. Le système Granier sans compensation pour les gradients naturels de température dans le tronc peut présenter de larges erreurs, mais si un capteur de référence horizontal est utilisé, les compensations ne sont pas nécessaires. Il a aussi été démontré que les variations radiales de flux doivent être prises en compte lors des calculs de flux de sève pour l'arbre entier. Les transpirations de deux couverts adjacents, l'un mesuré avec la méthode Granier, et l'autre avec la méthode Cÿ ermák, sont en accord entre elles et avec l'évaporation totale mesurée par la méthode des corrélations turbulentes. THB / HD / flux de sève / transpiration / Picea abies / Pinus sylvestris

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Section: Radial Variation Of Granier Measurementssupporting

confidence: 62%

Evaluation of heat balance and heat dissipation methods for sapflow measurements in pine and spruce

2001

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“…5). Such summer droughts are common in the region (Oren et al 1998a, Ellsworth 1999. Again, the instantaneous scatter in modelled C i /C a was sufficiently large and indicated that processes affecting the dynamics of C i /C a were not resolved by all closure models, at least with their static parameters.…”

Section: Model Assessment With Independent Parametersmentioning

confidence: 82%

Modelling assimilation and intercellular CO₂ from measured conductance: a synthesis of approaches

Katul

Ellsworth

Lai

2000

Plant Cell & Environment

145

129

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A spectrum of models that estimate assimilation rate A from intercellular carbon dioxide concentration (Ci) and measured stomatal conductance to CO2 (gc) were investigated using leaf‐level gas exchange measurements. The gas exchange measurements were performed in a uniform loblolly pine stand (Pinus taeda L.) using the Free Air CO2 Enrichment (FACE) facility under ambient and elevated atmospheric CO2 for 3 years. These measurements were also used to test a newly proposed framework that combines basic properties of the A–Ci curve with a Fickian diffusion transport model to predict the relationship between Ci/Ca and gc, where Ca is atmospheric carbon dioxide concentration. The widely used Ball–Berry model and five other models as well as the biochemical model proposed by Farquhar et al. (1980) were also reformulated to express variations in Ci/Ca as a function of their corresponding driving mechanisms. To assess the predictive capabilities of these approaches, their respective parameters were estimated from independent measurements of long‐term stable carbon isotope determinations (δ13C), meteorological variables, and ensemble A–Ci curves. All eight approaches reproduced the measured A reasonably well, in an ensemble sense, from measured water vapour conductance and modeled Ci/Ca. However, the scatter in the instantaneous A estimates was sufficiently large for both ambient and elevated Ca to suggest that other transient processes were not explicitly resolved by all eight parameterizations. An important finding from our analysis is that added physiological complexity in modeling Ci/Ca (when gc is known) need not always translate to increased accuracy in predicting A. Finally, the broader utility of these approaches to estimate assimilation and net ecosystem exchange is discussed in relation to elevated atmospheric CO2.

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“…, equals E L only if J S represents the spatially weighted mean flux (Oren et al 1998a). Sap flux density was measured with a Granier-type sensor composed of a heated (constant heat flow) and unheated thermocouple pair connected in opposition to measure temperature difference, as described by Granier (1987).…”

Section: Methodsmentioning

confidence: 99%

“…Measured sap flux (J S , kg H 2 O m -2 sapwood s -1 ) in the xylem of trees is increasingly used to estimate G S (Köstner et al 1992;Granier and Loustau 1994;Phillips and Oren 1998;Oren et al 1998aOren et al , 1998b, based on the assumption that J S scaled by A S :A L is equal to transpiration rate per unit of leaf area (E L ). Thus, G S for conifers or other species with small leaves (Landsberg 1986;Phillips and Oren 1998;Ewers and Oren 2000) can be calculated using the function (modified from Monteith and Unsworth 1990): (2) where G S is the canopy stomatal conductance for water vapour (m s -1 ), γ is the psychrometric constant (kPa·K -1 ), λ is the latent heat of vaporization (J·kg -1 ), ρ is the density of air (kg·m -3 ), c p is the specific heat of air at constant pressure (J·kg -1 K -1 ), and D is the vapor pressure deficit (kPa).…”

Section: Methodsmentioning

confidence: 99%

Sensitivity of mean canopy stomatal conductance to vapor pressure deficit in a flooded Taxodium distichum L. forest: hydraulic and non-hydraulic effects

et al. 2001

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We measured the xylem sap flux in 64-yearold Taxodium distichum (L.) Richard trees growing in a flooded forest using Granier-type sensors to estimate mean canopy stomatal conductance of the stand (G S ). Temporal variations in G S were investigated in relation to variation in vapor pressure deficit (D), photosynthetic photon flux density (Q o ), and the transpiration rate per unit of leaf area (E L ), the latter variable serving as a proxy for plant water potential. We found that G S was only weakly related to Q o below 500 µmol m -2 s -1 (r 2 =0.29), but unrelated to Q o above this value. Above Q o =500 µmol m -2 s -1 and D=0.6 kPa, G S decreased linearly with increasing E L with a poor fit (r 2 =0.31), and linearly with lnD with a much better fit (r 2 =0.81). The decrease of G S with lnD was at a rate predicted based on a simple hydraulic model in which stomata regulate the minimum leaf water potential. Based on the hydraulic model, stomatal sensitivity to D is proportional to stomatal conductance at low D. A hurricane caused an ~41% reduction in leaf area. This resulted in a 28% increase in G S at D=1 kPa (G Sref ), indicating only partial compensation. As predicted, the increase in G Sref after the hurricane was accompanied by a similar increase in stomatal sensitivity to D (29%). At night, G Sref was 20% of the daytime value under non-limiting light (Q o >500 µmol m -2 s -1 ). However, stomatal sensitivity to D decreased only to ~46% (both reductions referenced to prehurricane daytime values), thus having more than twice the sensitivity expected based on hydraulic considerations alone. Therefore, non-hydraulic processes must cause heightened nighttime stomatal sensitivity to D.

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Scaling xylem sap flux and soil water balance and calculating variance: a method for partitioning water flux in forests

Cited by 215 publications

References 35 publications

Evaluation of heat balance and heat dissipation methods for sapflow measurements in pine and spruce

Evaluation of heat balance and heat dissipation methods for sapflow measurements in pine and spruce

Modelling assimilation and intercellular CO₂ from measured conductance: a synthesis of approaches

Sensitivity of mean canopy stomatal conductance to vapor pressure deficit in a flooded Taxodium distichum L. forest: hydraulic and non-hydraulic effects

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Scaling xylem sap flux and soil water balance and calculating variance: a method for partitioning water flux in forests

Cited by 215 publications

References 35 publications

Evaluation of heat balance and heat dissipation methods for sapflow measurements in pine and spruce

Evaluation of heat balance and heat dissipation methods for sapflow measurements in pine and spruce

Modelling assimilation and intercellular CO2 from measured conductance: a synthesis of approaches

Sensitivity of mean canopy stomatal conductance to vapor pressure deficit in a flooded Taxodium distichum L. forest: hydraulic and non-hydraulic effects

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Modelling assimilation and intercellular CO₂ from measured conductance: a synthesis of approaches