EFFECTS OF TRELLISING ON THE ENERGY-BALANCE OF A VINEYARD

Field experiments were conducted in 1992 and 1993 in a commercial vine yard near Lamesa, TX, to evaluate soil and canopy energy balances. In 1992, grapevines were wrapped tightly to trellis wires, creating compa ct hedgerows that were 3 m apart, 1.6 m high and 0.4 m wide with littl e foliage below 1 m above the soil surface. In 1993, vines were allowe d to grow outward and downward from the trellis because of concerns th at excess shading of vines and fruit had occurred the previous year. T his change in trellising created wider, less dense hedgerows that incr eased sunlit leaf area and reduced sunlit soil area from the previous year. Leaf area was also 55% larger in 1993. We examined how the chang e in trellising affected soil and canopy energy balances. The Bowen ra tio method was used to measure the vineyard energy balance including t otal latent heat flux (lambda E), Latent heat flux from the canopy (la mbda E(c)) was determined from sap flow measurements of transpiration. Soil latent heat flux (lambda E(s)) was calculated as the difference between lambda E and lambda E(c). These values were combined with meas urements of soil net irradiance to partition the vineyard energy balan ce into soil and canopy components. The change in trellising in 1993 h ad little effect on vineyard net irradiance (RE) and lambda E(c) but d id alter the partitioning of R(n) and lambda E into soil and canopy co mponents. Canopy R(n) and lambda E were substantially higher for the o pen hedgerows in 1993 whereas soil R(n) and lambda E were lower than f or the dense hedgerows in 1992. Both trellising and leaf area contribu ted to changes in the energy balance. A comparison of lambda E(c) per unit land area with lambda E(c) per unit leaf area suggested that roug hly 60% of the difference in lambda E(c) between years was caused by t he change in trellising.