In the past two years we have recorded approximately 900 and 1000 degree days between May 1st and September 30th. There are actually a few more degree days each year, somewhere in the neighborhood of 25-50 but these fall in the months of April and September and are pretty much outside of the growing season. Those that accumulate after the first good fall frost are really of no importance because the growing season is done after that anyway. However, those accumulated in April can have some impact on the season. Lots of heat in April even 15-20 degree days impact the season in terms of early or late bud break.
Presently our site is completely surrounded by forest. The area that has been cleared is approximately 75 x 105 feet (approx. 0.17 acre) and inside that the vineyard is fenced off at 55 x 100 feet . As mentioned in previous blogs I believe there is somewhat more heat in the centre of the vineyard than in the areas along the forest margins where the temperature data logger is placed. Moving the logger to the centre of the vineyard will give a more accurate picture of the degree day accumulations.
While the site is pretty as it is enclosed by trees, this promotes several detrimental effect on climate of the site including; lowering the potential heat accumulation, increasing the potential for frost due to the cold air dam created by the forest and in turn a slower rise in the soil temperature in the spring.
There is a body of research on these topics, many conducted in British Columbia forest regions. The research has been focused on the local changes in climate in forest vs. clear cut area. The clearings ranged in size from 0.03ha 0.13ha to 1.0ha (approx 0.07 acre to 2.25 acres). The research has revealed that the clear cut area had significantly higher degree day accumulations than the forested areas. This was also related to the clearing size - the larger the clearing the more heat (significantly more heat). They found that over a 5 year period the tree seedlings in the largest opening had the greatest growth. They also found that the winter snow melted up to 5 days sooner on the clear cut areas than forests and quickest in the largest clearings. Some studies also found the soil temperature increased earlier in the spring in the clearings (fastest in the largest clearings) resulting in earlier plant biological activity. While the clearings allowed for more heat to escape the soil at night, there incidence of frost did not change as the slope allowed the cold air to drain away. Other similar research has shown that on flat ground without a slope the clearing can be subject to more frost events. Here is some of the studies;
http://www.forrex.org/publications/FORREXSeries/ss1/paper16.pdf
http://treephys.oxfordjournals.org/cgi/reprint/26/1/105.pdf
Improving Air Drainage
Clear cut opening size increases heat units
We've known about this research and what it could mean to our site but before we cleared any more land at our site we wanted to be sure the ground could sustain the plants. This is now evident and we plan to increase the size of the clearing this year to nearly 1 acre. This will take advantage of the slope of the land and the subsequent cold air drainage. It is also hoped that expanding the size of the clearing will increase the degree days accumulation of heat. More importantly the clearing will take advantage of the slope of the land and eliminate the cold air dam that currently exists at the vineyard.
Reducing the night time low temperatures caused by the forest damming up the cold air can change the degree days significantly over the growing season. Raising the night time low as little as 1.0 degree on average at our site would add about 75 more degree days over the period of May to September. This may also help mitigate potential frost damage in spring and fall. Together, the increase in day time heat and raising the night time low air temperatures over the growing season should help the overall degree day accumulations.
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