Predicting Transpiration Response to Climate Change: Insights on Physiological and Morphological Interactions that Modulate Water Exchange from Leaves to Canopies

Leaves are key factors in the global water exchange cycle. As the primary control interface involved in regulating water loss, understanding the relative influence of leaf morphological and physiological transpiration factors is critical to accurate evapotranspiration predictions. We parameterized a three-dimensional array model, MAESTRA, to establish a link from the leaf to canopy scale and attempted to isolate and understand the interplay among variation in morphological and physiological variables affecting transpiration. When physiological differences were accounted for, differences in leaf width (Lw) among Acer rubrum L. genotypes significantly affected leaf temperature and transpiration under slow to moderate wind velocities. In instances, Lw variation among genotypes resulted in a 25% difference in transpiration. This study demonstrates how simple morphological traits like Lw can provide useful selection criteria for plant breeders to consider in a changing climate. The boundary layer governs the diffusion of gases between vegetation and the atmosphere at the leaf and whole crown scale. Although Prandtl is credited with the introduction of boundary layer theory in 1904 (Schlichting and Gersten, 2004), the first leaf model incorporating physically based boundary layer concepts was not introduced until 52 years later (Raschke, 1956). Raschke’s pioneering work identified the influence of leaf size (e.g., leaf width) on the leaf energy budget through sensible heat and water exchange; however, consideration for boundary layer effects on the diffusion pathway of the leaf waxed and waned thereafter (Schuepp, 1993). Nonetheless, leaf size and within-crown spatial distribution affect the boundary layer conductance of the canopy and are a key element in modulating the soil–plant–atmosphere water transport process at the earth’s surface (e.g., Monteith and Unsworth, 2008). The thickness of the boundary layer interacts with stomatal conductance (gs) to control transpiration. At scales larger than the leaf, however, the boundary layer control on transpiration can become more important than genotypespecific stomata expressions. For example, it is commonly accepted that under ventilated conditions at the leaf surface, stomata control transpiration in response to vapor pressure deficit (VPD) (e.g., Bunce, 1985; Katul et al., 2009). In contrast, it is the thickness of the boundary layer that often controls transpiration at the stand scale, a consequence of environment and canopy structure interactions (Hinckley et al., 1994; Jarvis and McNaughton, 1986; Meinzer et al., 1995). Thus, there remains a need to understand the relative interaction between canopy controls over water loss and how leaf morphology and physiology scale up to whole plant and canopy scales (Monteith, 1989). Our objective was to assess the transpiration response among genotypes of a common horticulture tree crop through investigation of the leaf-to-atmosphere microclimate interactions in a changing climate. We used a three-dimensional spatially explicit individual plant process model, MAESTRA (MultiArray Evaporation Stand Tree Radiation A) (Wang and Jarvis, 1990) to investigate and separate the contributions of morphological and physiological transpiration parameters in leaf and canopy scale transpiration predictions. We parameterized the model on a genotypespecific basis from measurements on commercially available Acer rubrum L. (red maple) cultivars. We tested the hypothesis that variation in Lw within red maple considerably modified boundary layer conductance, leaf temperature, and transpiration aside from physiological differences. MATERIALS AND METHODS Site and plant material. Measurements were taken on South Carolina grown red maple cultivars [‘Summer Red’ (SR), ‘October Glory’ (October Glory ) (OG), ‘Autumn Flame’ (AF), and ‘Franksred’ (Red Sunset ) (RS) and one Freeman maple cultivar, Jeffersred (Autumn Blaze ) (AB) and used to parameterize MAESTRA. Plants were grown in 114-L Spin Out-treated plastic pots containing a mixture of 20:1 pine bark to sand (by volume) fertilized with 8.3 kg m of Nutricote 20N–3.0P–8.3K Type 360 (Chisso-Asahi Inc., Tokyo, Japan) on an outdoor gravel pad. Trees were placed in a completely randomized design and spaced 1.5 m center to center in rows 1.5 m apart. The continuous stand consisted of 3 m tall equal age saplings of five genotypes (n = 20 trees per genotype). Each tree was watered three times daily to near container capacity with 360 pressure compensating microemitters (ML Irrigation Inc., Laurens, SC). Substrate volumetric water content was monitored daily in each container at 10 cm and 20 cm below the substrate surface in four pre-drilled locations on opposite sides of the container (Theta Probe Type ML2; DeltaT Devices, Cambridge, UK) to verify that root zone volumetric water content was maintained within a previously determined well-watered range (0.4 to 0.5 m m). Measurements of leaf-level and whole tree morphology and physiology. Leaf physiology was quantified using a portable steady-state gas exchange system (CIRAS-I; PP Systems Inc., Amesbury, MA); cuvette conditions were controlled at 25 C air temperature, 1.3 kPa VPD, saturating photosynthetic photon flux (1000 mmol m s or greater), and varied carbon dioxide (CO2) levels. Photosynthetic activity was assessed by constructing net photosynthesis versus intercellular CO2 concentration curves from which maximum carboxylation rates (mmol m s) and maximum electron-transport rates (mmol m s) were estimated. Leaf reflectance and transmittance were estimated with a SPAD meter for quantum yield calculations (Model 502 Minolta Camera Inc., Ramsey, NJ). Detailed descriptions of leaf-level morphology and physiology have been described in Bauerle et al. (2003, 2007). Whole-tree morphology and physiology are as described in Bauerle et al. (2009). Model description. MAESTRA is a threedimensional process-based model that computes transpiration, photosynthesis, and absorbed radiation within individual tree crowns. Specific to this study, a modified version previously validated to estimate deciduous tree transpiration (Bauerle et al., 2002; Bowden and Bauerle, Received for publication 8 Oct. 2010. Accepted for publication 2 Nov. 2010. We thank Parsons Nursery for donating the trees for this study and the following funding agencies for partial support of this study: USDA-NIFA, Specialty Crops Research Initiative Grant (Award No. 2009-51181-05768) and USDA-FNRI, Cooperative Agreement (Agreement No. 58-6618-2-0209). This paper was part of the colloquium ‘‘Water Management and Plant Performance in a Changing Climate’’ held 4 Aug. 2010 at the ASHS Conference, Palm Desert, CA, and sponsored by the Water Utilization and Plant Performance in a Changing Climate (WUM) Working Group. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 46(2) FEBRUARY 2011 163 2008) and within-crown light interception (Bauerle et al., 2004) was used. Each genotype’s leaf physiological and morphological difference was parameterized with clonal-specific parameters. During the scaling process, MAESTRA scaled up leaf-level biochemical and energy balance properties that were linked with stomatal gas regulation both spatially and temporally (Bowden and Bauerle, 2008). This characteristic was also used to scale up genotype-specific transpiration (E) responses and to analyze the physiological and morphological regulation of E among genotypes. A full description of the model is beyond the scope of this article; however, detailed descriptions and references to model components can be found in Emhart et al. (2007) and Reynolds et al. (2009). Model validation tests. Predicting leaf to whole crown transpiration (Bauerle et al., 2002, 2009; Bowden and Bauerle, 2008), withincrown light interception (Bauerle et al., 2004), and whole-tree CO2 exchange (Reynolds et al., 2009) with MAESTRA has been previously demonstrated in red maple cultivars. Separating the morphologic versus physiologic leaf and canopy transpiration factors. MAESTRA was used to scale up leaf transpiration to the whole crown of a simulated 20-m tall mature deciduous canopy. The canopy was vertically stratified into 10 equal layers (2-m depth per layer) and gs was estimated according to the Leuning (1995) submodel. We evaluated the vertical effects of horizontal wind speed, air temperature, and Lw on canopy transpiration profiles when forced convection is present. We held genotype physiological and morphological parameters constant at the mean (Table 1) and varied Lw. Simulations were conducted using the minimum and maximum Lws (SR: 3.98 cm versus AB: 10.18 cm) at two different growth temperatures (20 versus 35 C) to assess within-species Lw effects on transpiration. In addition, genotype-specific transpiration response curves were created from polynomial regressions over a range of possible broadleaf plant Lws at genotype-specific physiology parameters reported in Table 1.