Effects of trellising on the energy balance of a vineyard

Field experiments were conducted in 1992 and 1993 in a commercial vineyard near Lamesa, TX, to evaluate soil and canopy energy balances. In 1992, grapevines were wrapped tightly to trellis wires, creating compact hedgerows that were 3 m apart, 1.6 m high and 0.4 m wide with little foliage below 1 m above the soil surface. In 1993, vines were allowed to grow outward and downward from the trellis because of concerns that excess shading of vines and fruit had occurred the previous year. This change in trellising created wider, less dense hedgerows that increased sunlit leaf area and reduced sunlit soil area from the previous year. Leaf area was also 55% larger in 1993. We examined how the change in trellising affected soil and canopy energy balances. The Bowen ratio method was used to measure the vineyard energy balance including total latent heat flux (λ E ). Latent heat flux from the canopy (λ E c) was determined from sap flow measurements of transpiration. Soil latent heat flux (λ E s) was calculated as the difference between λ E and λ E c. These values were combined with measurements of soil net irradiance to partition the vineyard energy balance into soil and canopy components. The change in trellising in 1993 had little effect on vineyard net irradiance ( R n) and λ E , but did alter the partitioning of R n and λ E into soil and canopy components. Canopy R n and λ E were substantially higher for the open hedgerows in 1993 whereas soil R n and λ E were lower than for the dense hedgerows in 1992. Both trellising and leaf area contributed to changes in the energy balance. A comparison of λ E c per unit land area with λE c per unit leaf area suggested that roughly 60% of the difference in λ E c between years was caused by the change in trellising.