On Friday, June 3rd, the Arrowhead Native Plant Explorers joined Dr. Jessica Savage of the University of Minnesota – Duluth for a nature hike at Tischer Creek in Duluth. Dr. Savage discussed the phenology (the timing of biological events) of native trees in the Duluth-Superior area.
At the beginning of our walk we observed a box elder, which is in the maple genus (Acer negundo). Interestingly, maple species can be either monecious (having both male and female parts on the same tree) or dioecious (having individual trees that are either all male or all female). Box Elder is dioecious.
Along Tischer Creek we observed many willows. Willows are capable of rooting from branches that break off of the original tree. This rooting is a useful adaptation for living in riparian environments, because willows are able to spread their populations as the water transports them downstream. The rooting adaptation is a useful trait in their native ecosystems, and it can also give willows a competitive advantage when colonizing new areas. Willows have a reverse latitudinal gradient, meaning that willow species richness increases as you move farther from the equator.
On the day of our field trip, maples and birches had mostly finished producing flowers and were in the process of producing fruit. Dr. Savage noted that this year there was little differentiation between the phenology of trees in Duluth near the edge of Lake Superior and trees occurring farther inland.
Dr. Savage explained that ash, oak, and walnut trees are typically the last trees to leaf out in the spring. These trees have large pores in their wood. Having large pores can be beneficial because it allows trees to move water quickly, however it can be detrimental when the water in the pores freeze and form large bubbles over the winter. Ash, oak, and walnut trees have to produce all new tissue in the spring to cope with the bubble formation in their wood.
Conifers along Tischer Creek were in the process of producing new needles. Dr. Savage explained that as the conifers produce new needles, the twigs expand and the needles spread outward.
Dr. Savage noted that understory vegetation will typically leaf out before the canopy. Understory plants will take advantage of gaps in the canopy occurring before leafout that allow for more light to reach the forest floor. Spring ephemerals such as Virginia Spring Beauty (Claytonia virginica), White Trout Lily (Erythronium albidum), and Wild Leeks (Allium tricoccum) grow vigorously during this spring period of abundant light and then die back as the canopy begins to mature.
Poplar catkins were abundant in the air and along the Tischer Creek trail.
Dr. Savage noted that red leaves in maple occur twice a year: in the Spring and in the Fall. During the spring, maple leaves can be exposed to large amounts of light, which can actually damage the leaf. The red pigments that occur seasonally in maples, called anthocyanins, can function like sunscreen to help protect the plant from harsh light.
Dr. Savage demonstrated the “levitating dogwood” trick with a Red Osier Dogwood (Cornus sericea). This is where a dogwood leaf will appear to be floating in midair, although the leaf is actually still attached by its vascular tissue. Dogwoods have spiral thickening in their veins.
Lastly, Dr. Savage discussed ways to get involved with monitoring phenological change. If you are interested in volunteering to track seasonal changes around Duluth or in your backyard, you can check out Dr. Savage’s citizen science project, Nature’s Timekeepers. There are three trails at different distances from Lake Superior that volunteers visit periodically and observe the phenology of target plants along the route. The volunteers enter phenotypic data into the Nature’s Notebook app. This data can then be used by scientists to track how the timing of biological events is changing over time.
Arrowhead Native Plant Explorers 