If you look at most of the deciduous trees in southern Colorado right now, you’ll notice that the buds are starting to get a little bit bigger. These buds are leaves that are ready to grow once the bud bursts open.
There are two ways trees know when trees wake up for spring. First, they respond to noticeably warmer days after a stretch of cold temperatures in winter. At the same time, they react to a change in light duration, when shorter nights and longer days of sun exposure spur new growth and development.
The U.S.A. National Phenology Network shows spring leaf out happening earlier in the western United States and the Appalachian mountains since 1981 on a national view.
Climate Central, an independent organization that conducts research on the impacts of climate change, in a recent report said that spring ‘leaf out’ is happening earlier in 76% (181) of 239 cities analyzed. The most dramatic changes are observed in Flagstaff, AZ where leaf out is happening 18 days earlier compared to 1981. In Colorado Springs we’ve seen the second greatest change, along with Reno, NV. Both cities are leafing out 17 days earlier over the last 38 years.
The amount of daylight at particular locations on a daily basis doesn’t change from year to year as the tilt of the earth’s axis and journey around the sun is stable and consistent. That means the change in leaf out each spring is changing due to temperature changes during the winter and spring.
In wanting to better understand this data, the trends, and some of the differences across the country, I recently exchanged emails with Theresa Crimmins, USA National Phenology Network director, about the changes in spring leaf-out and their data I presented in the maps above. She says that changes to seasonal plant behaviors can be used to measure how transitions between seasons are moving in time as a response to the observed changes in the earth’s climate.
She shared with me several recently published scientific papers that calculate Spring Leaf Index. In the papers, Spring Leaf Index is a combination of leaf index and bloom index.
Conceptually, the leaf index indicates the first phase of the transition from winter to spring, when shrubs start to become active but before deciduous trees put on leaves. The bloom index occurs later and is indicative of the window of time when lilacs and other shrubs first start blossoming and deciduous trees leaf out. Because the bloom index lags the leaf index by several weeks, it is necessarily more indicative of the weather patterns later in the growing season.
“We calculate the day of year that the Spring Leaf Index was met in each pixel on the map in each year of the study. Then, for each pixel on the map, the analysts calculated a trend over time in the day of year the index was met,” said Crimmins. “This results in an estimate of whether and how much that index, one way to look at the start of spring, has advanced or delayed.”
Leaf out changes across the U.S. for the most part match with winter and spring temperature changes that have been observed across the United States over the last four to five decades.
One area that didn’t appear to match up to me was the Great Lakes area, specifically upstate New York, where temperatures have warmed since 1970 during both the winter and the spring, but spring leaf out seems to have delayed. Crimmins says that data processing and bin ranges for these maps cause some regional specificity to be lost when looking on a national scale.
Data was rounded to the nearest whole number for the maps I’ve got above. Additionally, the relatively large bin of 1-5 days delay becomes a bit misleading for upstate New York.
“In actuality, that region is really not showing very much change; 1/2 day over that period is pretty small,” said Crimmins. Note that 1/2 day will round up to 1 for this national view as the data is cleaned up.
“Further, when we look at what’s been happening in that region over a longer period, say 70-100 years as several other studies have done,” said Crimmins, “the trend actually reverses, and we see an earlier leaf-out in that region, which is consistent with the increasing winter and springtime temperatures we see east of the Great Lakes.”
As a meteorologist who has to take complicated atmospheric concepts and simplify them every day to make my forecasts and the expected impacts of incoming weather understandable, I found that, like my branch of science, climate changes and impacts on phenology are complicated too!
Back to Colorado where leaf out is happening up to two and half weeks earlier than 50 years ago. Trees getting leaves earlier means they are more susceptible to the weight of water-laden snow in late season snow storms. During the “bomb-cyclone” blizzard in March 2019, many tree limbs came down due to the weight of the snow. Downed tree limbs cause problems blocking streets, landing on cars and houses, and pulling down power lines.
Of course, impacts from changes in winter and spring temperatures aren’t limited to just plants. Although warmer winters may sound nice to some, a 2017 study projected that warming could cause winter recreation to shrink by up to 50% by 2050, resulting in tens of millions of forgone visits each year. This would be a significant blow to the local communities in Colorado who depend on winter tourism dollars. In the 2015-16 season, skiing and snowmobiling alone generated $11.3 billion nationally and supported over 191,000 jobs.
Winter cold isn’t just important for recreation—production of fruits such as apples, cherries and peaches contribute $4 billion annually to the U.S. economy and rely on a period of winter chill. Warmer winters also threaten to interfere with other natural rhythms—allowing crop pests to persist for longer and a mismatching in availability of food (and other resources) with important natural events such as migration, hibernation and reproduction.