Fall is one of New England’s most spectacular seasons, but what makes the leaves turn such brilliant shades of red, orange, and yellow? And how is a changing climate affecting when—and for how long—we experience peak foliage?

We asked Richard B. Primack, professor of biology and an expert in plant ecology, to explain the science behind fall foliage, what he expects for this year’s season, and how readers can observe the changes happening around them.

People often try to predict whether it will be a “good” fall foliage year. What determines how vivid—or disappointing—the colors will be, and what are you expecting this year?

The best years for impressive fall foliage are years that are not too dry or too wet. This should be a great year for fall foliage because the rainfall over the last few months has been slightly below average and temperatures have been mild. This combination will give leaves the right conditions to change color slowly and produce strong colors. There should also be an extended fall foliage season with warmer-than-average temperatures predicted for October, delaying the first frost. All in all, we have great conditions for this year’s fall foliage season.

Many people assume cold weather causes leaves to change color. What is really happening inside a leaf when it turns red, orange, or yellow?

The combination of shorter days, cooler night temperatures, and sometimes drier soil induces trees to start breaking down chlorophyll in leaves, exposing their hidden yellow and orange pigments. Some people don’t realize that the yellow and orange fall colors are there all the time; they’re just hidden by the leaves’ green chlorophyll. However, the leaves of certain species, especially maple trees, cherry trees, and sumacs, often produce additional new red pigments in the autumn. The periods of warm sunny days and cool nights that we often experience during our New England falls are considered to be the reasons why our region has world-famous fall foliage.

You have used observations recorded by Henry David Thoreau and other naturalists to study seasonal changes. What can those historical records tell us about how fall in Massachusetts has changed?

On the basis of historical records and modern observations, we know that a warming climate has shifted the fall foliage season to happen a few weeks later. In the past, the best viewing season was late September and early October. Now it typically occurs in mid-October. The season is also more spread out, as the first frost is several weeks later than it used to be.

If readers wanted to observe fall like a scientist, what should they watch for in their yards, neighborhoods, or local parks—and what might be happening that most of us overlook?

People should try to look at a wider variety of plants, rather than just one or two types of trees. Dozens of species of trees and shrubs have beautiful fall foliage, and each one exhibits unique combinations of colors. It is also enjoyable to observe one particular plant each day, or several times a week, and note how the colors emerge and change over time and then how it gradually loses its leaves. Some of the best plants to observe are red maples, honey locusts, cherry trees, and ginkgoes, which are commonly planted around the BU campus and surrounding areas.

Other great places to seek out beautiful fall foliage are found along the Charles River (especially in Newton and Watertown), along the Fenway and Muddy Rivers, at the Arnold Arboretum in Jamaica Plain, in the Mt. Auburn Cemetery in Watertown, and in the Webster Woods and around Hammond Pond in Newton (near the Chestnut Hill T station).