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Issue link: https://read.dmtmag.com/i/217788
coinciding events such as what flowers are blooming when this butterfly appears, another aspect of it tries to predict the date on which this will occur. Not only is it difficult to predict an occurrence date across latitudes, it is difficult to predict year to year.Where I live in Minnesota, the spring of 2012 started in mid-March, but in 2013 there was nearly a foot of snow on the ground at the beginning of May. Humans may mark the vernal equinox as the first day of spring each year, but nature's rhythms are less reliable. To try and get a more predictable date, several different phenological indexes track temperature changes and other phenomena, and these can be used to key in when certain events may take place. On the global scale, remote sensing using satellites can give us indications of changes in vegetation at the ecosystem level. The commonly used indicator Normalized Difference Vegetation Index (NDVI) measures the amount of near-infrared (NIR) light reflected by plants. As the cellular structure of leaves naturally bounces NIR light back into space, measuring this can give a pretty good indication of the vegetative growth phase the plants are in. Bud break, vigor/growth, length of the growing period, and the start of fall senescence can all be seen from space at the global scale. Over the course of several years, the NDVI can track these seasonal parameters to illustrate all sorts of climatic changes. Combining this data with historical records, we can see that the spring emergence date for many plant species has been occurring sooner in the year than they were before 1960. The first oak leaves to appear in the spring have advanced eight days earlier than they were 250 years ago, corresponding to the rise in global temperature near the end of the Twentieth Century. Although the NDVI is a great tool for tracking global changes, it is primarily a tool for recording what has already happened, not predicting when events will occur. For that, we turn to a different index. As touched upon earlier, the leaves of oaks are emerging earlier due to a rise in annual temperature. The correlation between ambient temperature and emergence dates can be predicted by tracking the amount of accumulated heat using growing degree days (GDD).The concept is fairly simple and intuitive.Take, for example, a tree that would typically flower in the first week of June during a "normal" year. If we have consistently cool temps in April and May, that flowering time is likely to be later. If we have an abnormally early and warm spring, it can flower sooner. If we have cool temps in April and the first half of May, followed by unseasonably warm temps the last two weeks of May, our tree may flower right on time. It is not the temperature on one particular day that matters, but the accumulation of temperatures over a period of time that makes the difference. That's what is being tracked by the GDD model. How it is calculated is fairly simple as well. First, you start with a base temperature. In most cases, the base temp (or TBase for our equation) is 50 degrees Fahrenheit, because not much plant growth occurs when temps are in the forties, thirties, or colder. Then, on every day you take the high temp (TMax) add the low temp (TMin), and divide by 2 to get an average. Now, subtract our base temp and you have that day's growing degree. The equation looks like this: www.landscapeirrigation.com Phenology starts with observations. TMax + TMin - TBase = GDD 2 Say today's high is 75 and the low is 55. Add them together for 130, divide that by 2, for 65, then subtract 50, and we have a growing degree day of 15. If the weather stayed like this for 5 days in a row, we have accumulated 75 GDD. If it stayed like that for 30 straight days, we would have 450 GDD. The GDD number accumulates for the entire growing season, until the temps stay consistently below the base temp, and then reset at the start of the new year. There are several good resources online for checking what Green industry professionals are utilizing phenology as a tool to assist with marketing their plant health care services. Landscape and Irrigation 13