Processes controlling understorey evapotranspiration

1989 ◽  
Vol 324 (1223) ◽  
pp. 207-231 ◽  
Keyword(s):  
Stomatal Conductance ◽  
Forest Floor ◽  
Root Zone ◽  
Significant Proportion ◽  
Vapour Pressure Deficit ◽  
Water Vapour Pressure ◽  
Removal Experiments ◽  
Radiation Regime ◽  
Water Vapour Pressure Deficit

The understorey often accounts for a significant proportion of forest evapo- transpiration. In this paper we discuss the role of the understorey radiation regime, and the aerodynamic and stomatal conductance characteristics of the understorey in understorey evapotranspiration. Values of the McNaughton—Jarvis parameter Ω for the understorey in two mid-rotation Douglas-fir stands indicate considerable coupling between the understorey and the atmosphere above the overstorey. However, the stronger coupling between the overstorey and the atmosphere accounts for the observation that the fraction of stand evapotranspiration originating at the understorey increases as the water vapour pressure deficit increases and the soil dries. We also discuss the approaches to describing the process of evaporation from the forest floor and the results of understorey removal experiments. These show small decreases in stand evapotranspiration and root-zone soil water content, but significant increases in the transpiration and growth of the trees.

Stomatal conductance, transpiration, and resistance to water uptake in a Pinussylvestris spacing experiment

1984 ◽  
Vol 14 (5) ◽  
pp. 692-700 ◽  
Author(s):  
D. Whitehead ◽  
P. G. Jarvis ◽  
R. H. Waring
Keyword(s):  
Stomatal Conductance ◽  
Hydraulic Resistance ◽  
Vapour Pressure Deficit ◽  
Water Vapour Pressure ◽  
Canopy Conductance ◽  
Cross Sectional ◽  
Summer Maximum ◽  
The Mean ◽  
Monteith Equation ◽  
Water Vapour Pressure Deficit

Stomatal conductance was measured with porometers in two plots of Pinussylvestris L. with markedly different tree spacings (plot 1, 608 stems ha−1; plot 2, 3281 stems ha−1), and hourly rates of transpiration were calculated using the Penman–Monteith equation at intervals throughout one growing season. Stomatal conductance varied little in relation to height or age of foliage. There was a linear decrease in canopy conductance with increasing water vapour pressure deficit of the air. Transpiration rates on both plots increased during the summer (maximum 0.3 mm h−1); rates on plot 1 were always lower (ca. 0.7 times) than on plot 2. Needle water potentials were similar throughout the season and only slightly lower on plot 1 than on plot 2. The mean hydraulic resistance of the trees on plot 1 was 2.4 times that on plot 2. The results support a hypothesis that considers the changes in transpiration rate, conducting cross-sectional area, canopy leaf area, water potential, and hydraulic resistance following thinning as a set of homeostatic relationships.

Seasonal Variation in Stomatal Responses of Two Cultivars of Lychee (Litchi chinenis Sonn)

1992 ◽  
Vol 19 (3) ◽  
pp. 317 ◽  
Author(s):  
D Batten ◽  
J Lloyd ◽  
C Mcconchie
Keyword(s):  
Stomatal Conductance ◽  
Temperature Sensitivity ◽  
Vapour Pressure Deficit ◽  
Sensitivity Coefficient ◽  
Cultivar Differences ◽  
Water Vapour Pressure ◽  
Data Sets ◽  
Diurnal Changes ◽  
Whole Plant ◽  
Significant Difference

The effect of cultivar and environment on the stomatal conductance and plant water relations of lychee (Litchi chinensis Sonn.) was investigated. Diurnal changes in stomatal conductance (gs) and leaf water potentials (Ψ1) were determined for leaves of irrigated trees of cv. 'Bengal' and cv. 'Kwai May Pink' between July and December 1990. Leaves of Bengal always had much higher gs than Kwai May Pink in winter, but as summer approached, this difference became less. Ψ1 values at midday for Bengal were always much lower than for Kwai May Pink. The linear model, E = G(Ψ1-Ψsoil), where E is transpiration rate and G is whole plant conductance, was found to be valid for nearly all the data sets collected. The values of G for Kwai May Pink were higher than for Bengal, especially in summer, and the average values of G (Bengal 4.1 and Kwai May Pink 6.3 mmol H2O m-2 s-1 Mpa-1) indicate that the lychee has a relatively efficient water transport system compared with other fruit tree species. Laboratory measurements of the responses of these cultivars confirmed observed differences in gs in the field. The responses of each cultivar to irradiance (I), leaf temperature (Tl) and leaf-air water vapour pressure deficit (D) were obtained and used to model the orchard data. The equation (see pdf) where Topt is the temperature for maximum gs, gdark is the basal gs in the dark at given T1 and D, and kI, kT and kD are constants fitted by non-linear least squares, provided an acceptable fit for both cultivars (R2 = 0.68 for Bengal and 0.55 for Kwai May Pink). The fit was not improved by including Ψ1 in the model. There was a significant difference between cultivars in kT, the temperature sensitivity coefficient. Possible implications of inter-cultivar differences in temperature sensitivity are discussed.

scholarly journals Can birds do it too? Evidence for convergence in evaporative water loss regulation for birds and mammals

2017 ◽  
Vol 284 (1867) ◽  
pp. 20171478 ◽  
Author(s):  
E. C. Eto ◽  
P. C. Withers ◽  
C. E. Cooper
Keyword(s):  
Water Vapour ◽  
Water Loss ◽  
Vapour Pressure ◽  
Vapour Pressure Deficit ◽  
Ambient Air ◽  
Evaporative Water Loss ◽  
Water Vapour Pressure ◽  
Evaporative Heat Loss ◽  
Evaporative Water ◽  
Water Vapour Pressure Deficit

Birds have many physiological characteristics that are convergent with mammals. In the light of recent evidence that mammals can maintain a constant insensible evaporative water loss (EWL) over a range of perturbing environmental conditions, we hypothesized that birds might also regulate insensible EWL, reflecting this convergence. We found that budgerigars ( Melopsittacus undulatus ) maintain EWL constant over a range of relative humidities at three ambient temperatures. EWL, expressed as a function of water vapour pressure deficit, differed from a physical model where the water vapour pressure deficit between the animal and the ambient air is the driver of evaporation, indicating physiological control of EWL. Regulating EWL avoids thermoregulatory impacts of varied evaporative heat loss; changes in relative humidity had no effect on body temperature, metabolic rate or thermal conductance. Our findings that a small bird can regulate EWL are evidence that this is a common feature of convergently endothermic birds and mammals, and may therefore be a fundamental characteristic of endothermy.

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