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Berkeley ESPM C129 - Lecture 26, Leaf Energy Balance, Part I

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Biometeorology, ESPM 129 1Lecture 26, Leaf Energy Balance, Part I November 3, 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 Topics to be Covered A. Introduction a. History of leaf energy balance studies b. Role of leaf energy balance on Evolution and leaf shape c. Resistance/conductance networks B. Leaf Energy Balance: Steady-State Linear Theory 1. Dry leaf a. Tl-Ta, leaf-air temperature differences 2. Wet and transpiring leaf a. Tl-Ta, leaf-air temperature differences b. Evaporation, f(net radiation) c. Evaporation, f(isothermal radiation) 3. Night, dew and frost L26. 1 Introduction and History If one goes to the San Francisco Botanical Garden one can view a wide array of leaf sizes. Large leaves exist that have diameters approaching a meter. Others are tiny and fragile, less than a millimeter. How do all of these plants co-exist? We can address this question and other related ones by examining the energy balance of leaves and by determining the factors that affect its temperature. The temperature of leaves is often different than the temperature of the air. Under some circumstances leaves are warmer than air temperature. Under other conditions they may be cooler. Many biophysical processes respond non-linearly to temperature. Hence we need to evaluate the temperature where the processes are occurring, e.g. the leaf. Leaf temperature affects saturation vapor pressure at leaf surface, it is correlated with isotope fractionation, it affects molecular diffusivity, respiration, photosynthesis, hydrocarbon emission rates, enzyme kinetics. So if we want to evaluate these important processes correctly we must evaluate the temperature where the action is, at the leaf surface.Biometeorology, ESPM 129 2It is evolutionarily imperative for a leaf to manage its temperature to prevent a run away energy exchange that could cause lethal and catastrophic temperatures. A way by which this has occurred is through the evolution of stomata, active pores that regulate the loss of water and diffusional uptake of CO2. Other factors that can contribute to the status of the leaf energy balance include its stomatal density and capacity, transpiration, leaf size, orientation and shape, its spatial position in the canopy, leaf optics, pubesence and roughness. If one examines the plants with huge leaves in the botanical garden, one will notice that they tend to be associated with tropical plants growing in the humid understory of the forest and the plants I saw were growing in pools of water. The subject of leaf temperature was quite controversial in the early 20th Century. Early historical bias and conventional wisdom concluded that leaf temperature was warmer than air (Curtis 1926). But this concept came from studies in the humid east and glass houses. By 1938 Wallace and Clum had reported that leaves could be 7 C below air temperature (Wallace and Clum 1938). However, Curtis refuted these data and argued they were based on poor experimentation and faulty logic (Curtis 1938). He was especially critical of results from wilted leaves with vasoline that prevented transpiration and still resulted in ‘transpirational cooling’. To bring closure to this problem and explain why do large range of values in leaf-air temperatures exist, one must look at theory and wait for proper measurements of leaf energy exchange. Klaus Rasche (Raschke 1960) and David Gates (Gates 1965; Gates 1966) are credited with formulating one of the first models that evaluate the energy balance of a leaf. Raschke’s work was stimulated while stationed in India. There he became curious about correlations between leaf size, climate zone and position in the canopy. He was particularly interested in the relation between leaf size, temperature and transpiration (eg the role of leaf boundary layers and temperature). It was uncertain whether smaller leaves cooled easier, so they were able to conserve water or they possessed a thinner boundary layer, so they were able to transpire more efficiently. Gates was a physicist interested in biophysics of plants. He made early measurements of net radiation and spectral reflectances of leaves and extended these measurements to study leaf energy balance. A thorough evaluation of his work is available in his book Biophysical Ecology (Gates 1980). By the 1970s ecologists were using energy exchange measurements to understand the form and function of leaves (Parkhurst and Loucks 1972; Taylor 1975). In a classic study on optimal leaf form S.E. Taylor (Taylor 1975) cites example of Quercus douglassii , the species I work with, with having larger leaves on sheltered northeast slopes than on sunnier southwest slopes. We also find they have very erect leaves, to minimize the amount of incident sunlight. To better understand how leaves behave and evolved, one needs to consider how the environment has changed over the evolutionary history of a leaf. Several hundred million years ago, CO2 levels were about 2000 ppm. In this environment, temperatures were elevated due to a greenhouse effect, stomatal conductances were lower and solar radiation was less than today. Leaf energy exchange had to be plastic to survive that initial environment, as well as today’s. McElwain et al (McElwain, Beerling et al. 1999) argue that leaf temperatures could have exceeded air temperature by 10C in the elevatedBiometeorology, ESPM 129 3environment of the Triassic-Jurassic period, when CO2 concentrations could have been 2000 ppm plus and air temperature 3 to 4 oC warmer on a global basis. If true this effect could have forced natural selection towards smaller leaves, that would not reach lethal temperatures of 45 oC. It could be a reason why there was a 95% turnover in megaflora in Europe. The reduction in taxa with respect to leaf size at the Triassic-Jurasic boundary was 99, 80, and 60 to 10% for leaves 5, 2 to 3 and 0.5 to 1 cm wide, respectively. Now this is a truly interesting and exciting Biometeorological application!! In our modern era, scientists are able to simulate complex temperature patterns across leaves of varying shapes as was produced recently by Roth-Nebelsick (Roth-Nebelsick 2001). Figure 1 adapted from Roth-Nebelsick 2001. Impact of leaf size on the transient development of


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Berkeley ESPM C129 - Lecture 26, Leaf Energy Balance, Part I

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