A Welcome to Warmer Weather: Sap flows in trees as they awaken from their restful winter sleep

Trees work hard in the growing season: they pull the sun’s energy and make sugar to use and store throughout the tree. They earn their winter’s rest. After their leaves fall off, trees go into a kind of hibernation, moving the sugar to their roots. In The Hidden Life of Trees, Peter Wohlleben compares trees to grizzly bears, who feed on blueberries or salmon to lay down the fat that will nourish them in winter.

Trees are tough. They don’t get a beach vacation. Trees hunker down, weathering the worst of wind, ice and snow.

Come spring, trees wake up from their long winter’s nap. They welcome warmer weather, ready to flourish. Writing in Northern Woodlands magazine, Li Shen notes that, “the arrival of warm temperatures in April, more than increased day length, induces trees to open their buds.” Each tree starts to move sugar, dissolved in tree sap, up from roots to branches, to provide the energy the tree needs to grow new foliage. Wohlleben writes of water pressure in trees in spring: “if you place a stethoscope against a tree, you can actually hear it. The trunk is full to bursting.”

The tree at this point functions as a kind of a pump. On a cold night, air and other gases in a tree shrink, creating suction that pulls water into the tree. The warm sun of morning causes positive pressure to build. For maple syrup producers, this pressure presents opportunity. The syrup makers scurry to drill small holes in the trunks of sugar maple trees. From these holes they capture some of the sugary sap on its way up the tree. Boil this, and the result is maple syrup.

Pruning trees is possible in spring, when sap movement is at its highest, but it is important to be selective in the species and number of pruning cuts you make. Excessive pruning on the wrong tree can lead to a significant loss of that important sugar needed to grow flowers, leaves, and new branches.

Be careful at the base of trees in spring, too. Melting snow turns the soil around trees to mud. Ruts caused by a truck, tractor or ATV can alter the natural pores in the soil leading to soil compaction (hardening), and damage the feeder roots that trees use to suck in water and nutrients. The ruts might also alter drainage patterns and encourage erosion.

In spring, most trees burst into flower, the first step in providing the seed that will grow their next generation. Tree flowers provide a crucial early source of pollen; bees kick into action to move pollen from flower to flower. Along with the flowers of apple, cherry and pear trees, for example, willow, maple, horse chestnut and linden trees offer important sources of spring pollen. As you can picture, a big tree, with its spreading branches high above the streets and houses, offers a copious feast for the bees.

As the days get longer, songbirds return from the south; robins, sparrows and warblers build their nests in the branches of trees. Chickadees, nuthatches and woodpeckers snack on insect eggs and larvae in tree bark.

Sap flows to branch ends, and buds open; the unfurling leaves begin to soak up energy from sunshine, and use this energy to combine their sap with carbon from the air, to produce sugar. The trees’ rest is over; it is time to go to work.

Bibliography

Nearing, Helen and Scott. The Maple Sugar Book. Chelsea Green Publishing Col, White River Junction, Vt., 1950.

Wohlleben, Peter. The Hidden Life of Trees. Greystone Books Ltd. Vancouver/Berkeley 2015.

Bonnevay, Emilie, Why Does Sap Rise in Spring? Woodland Trust, 2018.
https://www.woodlandtrust.org.uk/blog/2018/05/why-does-sap-rise-in-spring/

Shen, Li. How Do Trees Know When to Wake up? Northern Woodlands magazine: https://northernwoodlands.org/outside_story/article/how-do-trees-know-when-to-wake-up

Kidd, Russell. Michigan State University Extension Note: Protecting Your Forest Trees and Soil During Spring Thaw.
https://www.canr.msu.edu/news/protecting_your_forest_trees_and_soil_during_spring_thaw

Going Buggy: Insect-eating Birds https://journeynorth.org/tm/spring/InsectEaters.html