Crop yield response to water - Cra

Orchard transpirationTree Tr is determined by the amount of radiation intercepted by the tree canopy and by thebehaviour of stomata. The degree of stomatal aperture is influenced by the climate driversmentioned before: radiation, temperature, humidity and wind. The stomatal behaviour of treeleaves is complex; it reflects a trade-off between maximizing the uptake of carbon dioxide andminimizing Tr loss, and is affected not only by environmental, but by internal tree factors as well.While individual leaf Tr depends on its stomatal conductance and on the environment aroundit, tree Tr depends on the number and behaviour of all individual leaves and on their dispositionin relation to the incoming radiation. The integration of the conductance of individual leavesover the whole tree canopy, yields the canopy conductance, a useful concept to understandthe Tr process. In herbaceous crops, the canopy may be considered as a ‘big leaf’; for trees andvines, while the canopies are more complex because of their three dimensional nature and thegaps between individual trees, the concept of canopy conductance is also valid to represent thebehaviour of the whole tree. It is important to characterize canopy size because it determinesthe amount of solar radiation that is intercepted, which is directly related to Tr. Box 3 shows howto estimate some parameters that relate to canopy size.box 3 How to characterize the size of tree canopies.Tree canopies may be characterized using two parameters: canopy volume (m 3 of treevolume/m 2 of ground surface) and leaf area density (m 2 of leaf area /m 3 of tree volume).The first one may be estimated easily with a measuring rod once the tree shape has beenapproximated as a sphere, an ellipsoid, or a truncated inverted cone. However, the secondparameter is much more difficult to assess and requires specialized instruments. As analternative to the measurements or calculations of the radiation actually intercepted bythe tree, a simple parameter that is easy to determine is the degree of ground cover. Theground cover (normally expressed in percentage) is obtained by measuring the shadedarea outlined from the horizontal projection of the tree canopy (See Figure below).d 1d 2Percent Ground Cover2π d 4Shaded area = A = (m 2 )4APercent Ground Cover =tree spacingd 3Example :Shaded area diameter = d 4 = 1.5 mTree spacing= 5 x 2 md 4A = 1.77 m 2Percent Ground cover = A x 10 = 17.7%1 d21d23Canopy Volume: π d 2 + + d 1d 33 4 4Yield Response to Water of Fruit Trees and Vines 257