I am often shocked by autumn in the Boundary Waters. I am used to the dead heat of summer, when the forest seems an uninterrupted sea of evergreen. Friendly firs insulate portages from the rest of the woods, and straight-trunked red pines form cathedral aisles in some campsites. Black spruce and tamarack plunge their roots into bogs, while gangly white pines tower above the forest canopy. The poplar and paper birch would easily melt into the green shoreline if not for their white bark. I sometimes overlook red maples until my canoe drifts close to them.
September touches the Boundary Waters’ green backdrop with vermilion, gold, and flame. These sudden glimpses of color reveal the forest’s previously hidden red maple, paper birch, poplar, and ash trees. More and more deciduous trees, including the coniferous tamarack, lose their green as September continues, and the color riot peaks in mid-September to early October. Eventually, the red, yellow, orange, and even purple leaves lose their festive pigments and turn into dry, brown husks.
But why do leaves go through these changes in the first place? On first thought, it may seem exhausting for leaves to change color so drastically. In fact, the color transformation, known technically as “leaf senescence,” is a fundamental adaptation by deciduous trees to the harsh Minnesota winter. Instead of devoting precious resources to growing winter-resistent leaves like conifers do, deciduous trees simply drop their broad leaves every autumn. The carefully regulated process of leaf senescence allows trees to recover crucial nutrients and energy stored in the leaves before they fall to the ground. Nitrogen and sugars are reincorporated into twigs and branches that remain all winter, to be used again in the spring to fuel the leafing-out process.
As the tree sucks nutrients from leaves to twigs, chlorophyll decays inside the leaves’ cells. Cooler temperatures and reduced sunlight prevents the tree from producing enough replacement chlorophyll, revealing the yellow carotene pigment below. Chlorophyll-less leaves are vulnerable to sun damage, and some trees produce anthocyanin during leaf senescence to block the harmful rays. Anthocyanin washes leaves in scarlet, igniting red maples in fiery displays.
The longer nights and colder temperatures of autumn combine with the natural lifespan of leaves to trigger leaf senescence. Leaves are hardwired in their genes to live a certain length of time, which explains why some tree species turn before others. While weather patterns do not greatly affect the onset of fall colors, they can influence their vividness. Warm, sunny days help the leaves produce more sugar, of which large quantities are needed to produce anthocyanin. Cold nights help destroy chlorophyll more quickly. Slight drought can heighten the reds, while severe drought can overly stress the trees and hasten leaf death. Sudden wind, heavy rain, or early snows can strip trees of their leaves and leave behind a splotchy canvas riddled with missing color.
Next autumn, the onrush of color will surprise me again. Lulled into a false sense of permanence by lush summer vegetation, I’ll start when I glimpse a lone poplar’s bright yellow crown in early September. The first anthocyanin-saturated red maple I discover will hold me speechless for minutes. Soft, golden wisps of tamarack will brush my face as I carry my canoe out of a bog. The colors will mark the end of yet another season in the Boundary Waters, and I feel grateful for them all.