Narrow spacing affected berry temperature by potentially reducing the duration of berry exposure due to shading from neighboring vines. Compared to the wider row spacing, the berries in narrow row spacing in N-S rows on a flat terrain intercepted up to 36% less sunlight and reduced elevated berry temperatures on the west-facing side . In the E-W row orientation, the number of hours with berry temperatures greater than 35◦C was also reduced with the narrow row spacing due to the shading from neighboring vines . The most balanced sunlight exposure and growing degree hours between each side of the vine was achieved in the N-S row orientation, although notable hourly berry temperature differences were present for both narrow and wider row spacing. For example, west-facing berries exceeded35◦C for about 1 hour for the narrower row spacing and about 3.6 hours for the wider row spacing .The efficacy of shade cloth in reducing or equalizing berry temperature strongly depended on the row orientation and slope aspect . In general, adding shade cloth to the side of the row with partial or full south or west exposure tended to produce a significant reduction in berry temperatures and heat accumulation, as is to be intuitively expected. Adding shade cloth to sides of the vine with partial or full north or east exposure typically had weaker effect, large round garden pots and could actually increase temperatures on north-facing berries due to trapping of energy transmitted from the south.
While avoiding fruit overexposure reduces fruit temperature, in some cases, controlling the amount of direct radiation received by berries with shade cloths consistently maintained the berry temperature below 35 ◦C. For instance, in vines oriented N-S and NW-SE with wider rows, 50%and 70% shade cloth significantly reduced the time berry temperature was above 35◦C late in the afternoon in west facing berries . It was possible in several cases to achieve near-equal heat accumulation between sides of the vine while also minimizing berry temperature extremes by applying shade cloth to one side of the vine. For example, applying 70% shade cloth to the SE side of the vine in NE-SW oriented rows on flat terrain effectively balanced heat accumulation while also eliminating berry temperatures above 35◦C. E-W oriented rows always had high imbalance in heat accumulation regardless of shade cloth density or slope aspect.Comparisons between measured and modeled berry temperature indicated that the model is able to reproduce general spatial and temporal patterns of temperature, and can capture the additional effects of shade cloth. This is in addition to prior validation efforts demonstrating excellent model performance in the absence of shade cloth. Experimental validation of 3D, spatially explicit models is complicated by high sensitivity of localized model predictions to specifics of the canopy geometry. However, overall close agreement between measurements and model predictions in an average sense suggested that the model is robust to variation in vineyard architecture, topography, and the addition of shade cloth.
For model validation purposes, local measurements of ambient berry microclimate were used to drive simulations. Effects of large-scale microclimatic variation was not included within this model, which could affect the predictive ability of the model as large-scale features are varied such as topography. Variation in topography could induce changes in wind speed or sensible heating of the air independent of vineyard structure, which was not represented in the model. However, radiation exposure is the primary driver of berry temperature deviations from ambient, and other microclimatic effects due to large-scale topography are likely to be secondary and establish the baseline temperature state similar to changing weather.The results of this study for flat terrain largely confirmed conclusions of previous work regarding design of vertically-trained vineyards for berry temperature management, but revealed some additional trade-offs for consideration. Similar to previous findings, the NE-SW row orientation on flat terrain is likely to be the best compromise between canopy and berry light interception, reduction of elevated berry temperatures, and balancing of heating between opposing sides of the vine, which was also argued by Tarara et al.. A trade-off of this vineyard design is that it modestly reduces overall vine light interception relative to the more common N-S row orientation. Additionally, there are still significant differences in berry heat accumulation and exposure between sides of the vine in a NE-SW row orientation. However, for VSP vineyards on flat terrain with no shade cloth, the NE-SW row orientation appeared to be the best overall at equalizing exposure between sides of the vine and reducing berry temperature extremes.
For N-S oriented rows on flat terrain, previously well-documented imbalances in berry temperature between sides of the vine were also observed. It is intuitive to understand that the higher air temperatures and lower humidity that occur in the afternoon, when combined with berry exposure to the west sun, creates higher berry temperature than in the morning when ambient conditions are cooler. There is strong evidence that the accumulation of berry anthocyanin is a function of temperature and light Buttrose et al., Downey et al., Hunteret al., Spayd et al. and that the temperature difference between sides of the vine can create imbalance in the mass of the berries, as well as on tritable acidity, pH and phenolic compounds. If row access by mechanical equipment is not a concern, decreasing row spacing could offer some protection against berry temperature extremes, although this is not effective at balancing opposing sides of the vine. Interestingly, results indicated that although there was high berry temperature imbalance localized to the afternoon, daily integrated metrics such as daily growing degree hours and daily berry light interception were almost perfectly balanced between sides of the vine in N-S rows. However, it is possible this was coincidental, or that abnormal diurnal temperature fluctuations such as that caused by clouds could break this symmetry. The NW-SE row orientation on flat terrain resulted in the most elevated berry temperatures. Berries on the southwest side of the vine spent nearly 4 hours above 35◦C, and shade cloth did little to mitigate these temperatures because the sun was nearly perpendicular to the shade cloth at the hottest time of day. Most previous work examining the effects of shade cloth does so for a single site and vineyard design , but results indicated that details of topography and vineyard architecture can have a significant effect on shade cloth performance. In N-S oriented rows on flat terrain, smaller row spacing relative to canopy height significantly reduced the hours of berry exposure to direct sunlight in the east and west side of the vine due to shading from neighboring vines. While berry temperatures were reduced in vineyards with narrower row spacing, grape and wine quality could decline at some point when row spacing is reduced due to excessive berry shading . Mechanical equipment access may be impeded below some threshold row spacing. Full-size equipment generally requires a minimum row spacing of around 3 m for single canopy systems. Thus, large round pots depending on the availability of equipment for mechanization and the vineyard design, shade cloth appeared to be a viable option for mitigation of berry overexposure in widely spaced rows. This study considered only VSP trellis systems at a single fruiting height, which resulted in the potential for high fruit exposure. Other trellis systems that reduce berry exposure are becoming more popular in warm climate regions. However, since it is usually undesirable to completely shade clusters because of its negative effect on berry quality, it is still necessary to understand the interaction effects between canopy architecture and berry exposure. While the results of this work can provide some initial guidance in this regard, future work analyzing different trellis types is still needed. Because of the spatially explicit nature of the model presented in this work, it is likely that only minimal adjustments to the model are needed to accommodate different trellis types.For most cases, it was observed that planting on a slope fully or partially facing south or west increased berry exposure and elevated temperatures relative to north- or east-facing slopes or flat terrain .
Furthermore, a west-facing slope tended to increase temperatures more relative to a south-facing slope. This is intuitive given that the sun spends most of the day to the south, and the sun is to the west during the warmest time of a typical day. In several cases, slope had the negative effect of increasing the imbalance in heat accumulation between sides of the vine. This was especially true for the E-W row orientation, which caused very large imbalance that could not be effectively mitigated by shade cloth. For N-S and NE-SW oriented rows, the impact of slope on the berry temperature metrics was generally small. Shade cloth was able to mitigate the negative effects of slope in many cases. Applying 70% shade cloth in the sloped cases achieved excellent balance in heat accumulation between sides of the vine with N-S, NW-SE, and NE-SW orientations. The 70% shade cloth was also able to reduce the time above 35◦C to 1 hour or lower in all but the case with N-S rows on a west-facing slope, and NW-SE rows on flat terrain.The 3D model developed in this work was able to represent the effects of shade cloth on berry temperature and, thus, provided a viable tool for quantification of interactions between hypothetical vineyard designs and shade cloth on metrics related to berry temperature. The tool could also be effective in performing model-based vineyard designs in which the optimal design is determined under a set of constraints such as slope aspect or minimum row spacing. Although this means that the optimal design is likely to be case-specific, the tool was used to examine general trade-offs in various designs, which are summarized below. While the N-S row orientation on flat terrain was effective in balancing daily berry light interception and heat accumulation between opposing sides of the vine, it is also susceptible to temporally localized berry temperature spikes on the west side that could be managed by applying dense shade cloth. For cases with no shade cloth, the NE-SW orientation was likely the best compromise between berry temperature reduction and balance between opposing sides of the vine, although it still had significant imbalance in heat accumulation and extreme berry temperatures. Addition of sloped terrain tended to exacerbate berry temperature extremes and imbalance when the slope was facing south or west, which in several cases could not be well managed using shade cloth. The shade cloths were more effective in reducing berry temperatures in cases with greater row spacing relative to plant height because adjacent rows could potentially provide their own shade. The simulation experiment in this work used the new modeling tool to examine general trends in berry temperature and light interception as vineyard architecture and shade cloth density were varied. Because of the challenge in concisely presenting results of a very large number of simulated cases, results are limited to a limited number of architectures, a narrow time period, a single latitude, and site-specific weather conditions. As such, care should be taken in direct application of the simulated values for vineyard design, as they may change for a certain site or design. However, the model itself provides a tool that could be used to provide quantitative guidance for vineyard design or management at a specific site.Together, Chapters 2-4 motivated and carried out the development of a modeling tool that can be used to identify strategies for mitigating the effect of excess sunlight and unfavorable temperatures on grape berries. The tool was then used to study how vineyard design and management strategies related to berry shading interacted to influence berry temperature and light interception. Chapter 2 evaluated widely used assumptions when modeling solar radiation interception in plant canopies, namely assumptions of vegetation homogeneity and isotropy. Because of their simple, tractable form, one-dimensional turbid medium models of radiation interception that assume homogeneity or isotropy are used across a broad range of disciplines. However, it is relatively well-known that with varying levels of vegetation sparseness and preferential leaf orientations , the implicit assumptions of vegetation homogeneity and isotropy in this simple class of 1D models are frequently violated. Yet it is not well-known how this assumption violation translates into model errors in a given situation. Results of this work provided quantitative guidance for when a simple 1D model can be appropriately used to estimate light interception.