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Berkeley ESPM C129 - Lecture 29 Stomatal Conductance part 2, Observations

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Biometeorology, ESPM 129 1 Lecture 29 Stomatal Conductance part 2, Observations November 10, 2010 Instructor: Dennis Baldocchi Professor of Biometeorology Ecosystem Science Division Department of Environmental Science, Policy and Management 345 Hilgard Hall University of California, Berkeley Berkeley, CA 94720 Email: [email protected] Phone: 510-642-2874 Fax: 510-643-5098 Topics to be Covered: A. Environmental Biology, Continued 1. Response of stomata to environmental and physiological forcings a. humidity deficits b. temperature c. relation to hydraulic conductance d. tree age/height. e. soil moisture B. Diurnal variations of stomata a. ample soil moisture b. soil moisture deficits C. Dynamic Responses L29.1 Temperature and Humidity Stomatal conductance responds to temperature in a complex and curvilinear manner. Conductances are greatest at an optimal temperature (To), which is typically between 20 and 30 C. Conductances are restricted by either extremely low (Tn) or high (Tx) leaf temperatures, which are approximately 5 and 45 C, respectively (Jarvis 1976). The temperature dependency of stomatal mechanics has two origins, enzyme activity/metabolism and the saturation vapor density.Biometeorology, ESPM 129 2If one increases leaf temperature, one will also increase the humidity at the leaf surface, which increases exponentially according to es(Tl). Many papers show that stomatal conductance of many plant species decreases as the vapor pressure difference between the leaf surface and its interior increases (Schulze 1986). Simple linear models are often used to describe the effect of humidity differences on stomatal where ke is a constant. Figure 1 Leaf conductance and leaf-air humidity differences. Adapted from Hall 1983. The role of humidity on stomatal conductance is more complicated, than is suggested above, due to feedback and feedforward control loops between transpiration, leaf temperature and vapor pressure gradients (Farquhar 1978, Schulze 1986, Buckley 2005). In system analysis feedbacks relate to the return of an output on the input. The following text describes the interactions between humidity, leaf water potential and stomatal conductance derived from cuvette studies. 1. An increasing vapor pressure gradient between the stomatal cavity and leaf surface increases peristomatal transpiration. 2. the water status of the guard cells and the stomatal apparatus is affected, causing a lowering of the water potential of the guard cells which produces partial closure of the stomata. 3. Feedback control is evident when this response is followed by a reduction in transpiration and a correction of the water deficit in the guard cells, as when E is directly related to changes in gs. 4. Feedforward control occurs when transpiration remains limited in spite of any correction of the water deficit that may have occurred via feedback effects (Farquhar 1978)). The role of a feedforward mechanism, according to JonesBiometeorology, ESPM 129 3(1983), is less certain, yet Losch and Tenhunen (1981) report that 70 species exhibit feedforward properties. 5. Humidity control on stomatal action is simultaneously modified by a temperature feedback. A reduction in transpiration, associated with partial stomatal closure, can increase the leaf temperature, affecting the temperature control loop and the saturation vapor pressure at the leaf surface. Figure 2 Transpiration and stomatal conductance responses to leaf-air humidity differences. After Jarvis (1980) This sequence does not explain conditions where transpiration falls with respect to humidity deficits, for the above case causes stomata to 'reopen'. On the other hand, if stomata respond directly to D, then a feedforward system can be achieved. This will ensure that stomatal closure will continue in dry air, even if the bulk leaf water status is improved. To address this issue experimentally, Mott and Parkhurst (Mott and Parkhurst 1991) conducted some clever experiments where they exposed leaves to a treatment that consisted of a mixture of helium, oxygen and CO2. The molecular diffusivity of HELOX allows water vapor to diffuse 2.33 times faster than it does in air (effectively changing the boundary layer conductance). This allowed the experimenters to alter the leaf-air water vapor difference and transpiration rates independent of one another, while maintaining a similar leaf temperature. They found that the stomata responded to transpiration, a measure of leaf water loss, rather than the humidity at the leaf surface. They conclude that stomatal responses to humidity may involve two steps.Biometeorology, ESPM 129 41. At low values of leaf-air humidity deficits, the standard hydraulic feedback is in play, greater humidity deficits promote transpiration. This loss of water reduces guard cell turgor and stomatal conductance. 2. At higher humidity deficit values high transpiration rates may cause local water deficits and patchy stomatal closure. If we couple water and carbon exchange to understand stomatal operation we arrive at a different understanding (Cornic 2000). Work by Thomas and Eamus (Thomas et al. 1999) on savanna yield interesting insights on vpd-gs-Ci interactions. In principle reductions in Ci promote stomatal opening. On the other hand, increasing vpd, under ambient CO2, encourages stomatal closure, and simulataneously leads to reduced Ci, which promotes stomatal opening. Stomata therefore receive conflicting signals to open and close with increasing vapor pressure deficits. Three response phases of gs to vpd are noted. 1. low vpd, E is limited by vpd and not gs. As vpd increases, E increases so stomatal conductance does not limit transpiration 2. at mid range values of VPD, stomata close as VPD increases. E remains constant as the increase in the driving force is equal to the change in conductance 3. At high VPD, feedforward effects are noted, causing E to decline as V increases more. Hydraulic limitations limit the supply of water es(Tl)-ea (Pa)0 1000 2000 3000 4000LE (W m-2)050100150200250Ta = 25 CTa = 30 C Figure 3 Model calculations of interactions between leaf evaporation, stomatal conductance and vapor pressure deficit.Biometeorology, ESPM 129 5es(Tl)-ea (Pa)0 1000 2000 3000 4000 5000gs (m s-1)0.0000.0020.0040.0060.0080.010 Figure 4 Computation of stomatal conductance, with changing vpd Classic studies,


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Berkeley ESPM C129 - Lecture 29 Stomatal Conductance part 2, Observations

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