As summer leaves behind the memory of warm weather and lake days, fall brings with it an explosion of color. Cool temperatures and warm colored leaves push some people inside to Starbucks for a pumpkin spice latte, but others stay outdoors and observe nature at work.
Phenology is the practice of tracking the timing of seasonal events like birds migrating and flowers blooming, according to emeritus professor in the department of forest and wildlife ecology Stanley Temple.
As the seasons switch, naturalists take to the wild to observe and document the changes they see. Documentation of phenological notes over the last hundred years now informs on how climate change affects the seasons, Temple said.
“Wisconsin has a big tradition and a big stake in phenology — the timing of seasonal events,” Temple said. “It’s a study of the timing of seasonal events, but we’ve played a big role in understanding how climate change is affecting plants and animals because we have such a rich historical sort of baseline to compare with.”
Key to Wisconsin’s influence in the field of phenology is Aldo Leopold. Leopold was a conservationist, author and naturalist, who became a professor at the University of Wisconsin in 1933, according to Temple, and by 1945 he accumulated decades worth of phenological records.
Leopold published a paper on his notes, which included over 10,000 observations and over 300 unique species. Later, his oldest daughter Nina Leopold Bradley retired and published her own findings that certain seasonal events occurred earlier in the year compared to when her father observed them.
These findings were consistent with what climate scientists were observing, Temple said. To this day, phenological records are still a common, important tool for naturalists and climate scientists alike.
Keeping phenological records aids climate science by providing historical markers for when events occurred in the past versus in the present, Temple said.
Climatologists can also use these comparisons to model and predict when events will occur in the future. Additionally, it can elucidate “phenological mismatches” which occur when two linked species, like pollinators and the plants they pollinate, respond to the climate differently. These mismatches could have consequences for those species.
Some of the changes expected in Wisconsin this time of year include seeds falling from plants, tamarack trees turning gold, whitetail deer making scrapes, frogs beginning to hibernate and wood ducks beginning to migrate, according to Temple. All these changes come each year, as plants and animals are adapting to colder temperatures and decreasing daylight. One can see them outdoors daily.
“The fact that it’s a few tenths of a degree warmer this year than it was last year, that’s pretty hard for us to see or appreciate,” Temple said. “But when you can see these things right in your own backyard, in your own neighborhood, shifting, it’s one way that people can become aware that climate change is affecting where I live.”
The UW Arboretum is one place to see phenology in action, consisting of over 1,200 acres of land hosting a myriad of trees and wildlife.
Longenecker Horticultural Gardens Curator David Stevens said the living collection of woody plants at the Arboretum is the largest in Wisconsin and has species from 2,600 different taxa, or groups of species. The collection contains every tree native to Wisconsin as well as woody plants from around the globe capable of surviving the cold winters.
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“We’re a great oasis right here in Madison,” Stevens said. “We do have the ability at least to have a great fall color display.”
Stevens said the Arboretum keeps a long-standing collection of phenological notes that Leopold started in the 1930s. Many of these notes are on spring changes like blooms and bird migration but also changes that come in the fall.
The seasonal changes seen at the Arboretum include the leaves changing colors, birds migrating south, acorns and fruits dropping from trees and deer and other mammals stocking up for the winter and many more, according to Stevens.
The seasons regulate when fall colors start to appear, Stevens said. Moisture and cold temperatures in the spring, as well as drought or storms in the summer, could affect when seasonal changes like frost and color changes occur in the fall.
When trees go from leafy green to the autumn yellows and oranges, this change is dictated by the decreasing daylight, Stevens said. Less daylight causes plants to stop producing chlorophyll, the source of their green color, and start producing carotenoids, the orange pigments in carrots. Trees start to prepare for the winter by internalizing nutrients, like sugar. When the leaves eventually fall off, the trees cover the sites where a leaf once was with a waxy coating.
Phenological records have shown how climate change is affecting the Arboretum, Stevens said. They will see spring and fall alike coming earlier or later than it historically has, which makes planning around those seasons more difficult. The Arboretum used to give visitors fairly accurate estimates of peak fall colors and peak flower blooms, but this is no longer the case, according to Stevens.
When Stevens is not at work “getting paid to geek out about trees and shrubs” he runs an organic farm in Baraboo with his wife. The changing seasons affect both his work at the Arboretum and his farm.
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Stevens’ farm grows a variety of organic herbs like holy basil or lemongrass. He said the year’s first frost can dictate when to harvest his crops. He used to expect the first frost to come in the first week to ten days in October, but in recent years it has come sooner — a phenological indicator of climate change.
“It really shortens the growing season window and makes you have to really scramble,” Stevens said.
The early frost affects Stevens’ farm as well as local vegetable farmers, Stevens said, as they have to try to harvest all the crops that are not cold tolerant and cover those that they cannot harvest.
Phenology is also on full display at the Lakeshore Nature Preserve.
Volunteer and Outreach Coordinator at the Lakeshore Nature Preserve Bryn Scriver said the preserve spans 300 acres — taking up about one-third of campus — and contains lots of diverse ecosystems and wildlife.
“It’s a great place to go to relax, exercise and just have a place for rest and a place to go be in nature,” Scriver said.
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This is the time of year when yellowjackets, a type of wasp, nest in the ground on the preserve. During this process, the yellowjackets become aggressive, according to Striver. Fuzzy caterpillars known as the Wooly Bear Caterpillar begin to appear, flowers like asters and goldenrods are blooming, berries on the woodland plant Jack-in-the-pulpit turn from green to bright orange and fungi like Stinkhorns start to appear.
One event Scriver looks forward to is the first frost of the year, which is when the mosquitos die. She said a killing frost will kill the above-ground parts of many plants and grasses as well as dry out leaves. After a killing frost has occurred, they can begin to do prescribed fires — an important aspect of managing the preserve.
Of course, the seasonal changes are not only seen at the Arboretum or the Lakeshore Nature Preserve. What Aldo Leopold started nearly 100 years ago can be done anywhere outdoors. And it is now a well-studied scientific practice that can be used to predict the natural world.
“Phenology has become sort of a big, big thing,” Temple said. “And it’s sort of transitioned from being this hobby of Victorian naturalists to now being a serious climate science.”
