Nature Notes

Patterns and Exceptions

In a shallow lake like Green Heron Pond, water is mixed by the wind and temperatures remain consistent from top to bottom.

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By Greg Lecker

On either side of the Arboretum entry drive are wetlands that lie at a lower elevation than the parking lots and buildings. To the west, several leafless tamaracks border the low grassland vegetation.

Wetland West of Entry Drive

Presently under construction, the Highway 5 Regional Trail project will traverse across the grounds of the Arboretum and wetland on the south side of Highway 5.  This trail project removes a trail gap in the trail system and provides a connection to the Arboretum for visitors arriving on foot, rollerblade or bicycle.  Just past the entry gatehouse is an impressively tall Scotch pine.

Scotch Pine and Nest

The upper tree bark peels, flakes and glows orange; while the lower bark is found in loose plates and reddish-brown in color.  The single trunk is often crooked, spreading irregularly into multiple spreading branches.  Because of the twisty nature of its needles, trunk, and branches, the species name of “sylvestris” is especially apt – it means “wild”. 

It’s the exception in the pattern that attracts attention.  Near the crown is a large nest. My gut tells me it’s the nest of a hawk – either red-tailed or Cooper’s hawk – where the hunter can have a commanding view of the landscape.  The other possibility is a squirrel’s nest – but those nests are typically assembled nearer to the forks of a tree rather than in the exposed outer branches. 

Hoarfrost

Hoarfrost highlights the grasses and bulrushes surrounding Green Heron Pond.  A similar process forms both hoarfrost AND dew. Overnight, temperatures fall enough that water vapor condenses on surfaces.  This assumes that the air is initially damp enough. Dew forms in the warmer months.  In the cooler months, ice crystals form on objects exposed to the free air, such as grass blades, tree branches, or leaves – as well as roadway bridge decks.

Ice Forms on Ponds and Lakes

Following the autumn turnover of the deeper lakes, ice is now forming on lakes large and small!

Ice-In on Green Heron Pond

In a shallow lake like Green Heron Pond, water is mixed by the wind and temperatures remain consistent from top to bottom.  In larger deeper lakes like the 70-foot-deep Lake Minnewashta, just across Highway 5 from the Arboretum, water forms layers defined by temperature differences. Water turnover occurs as water cools in the autumn and warms in the spring. Denser cooler water drops to the bottom of the lake; thus displacing warmer water which moves towards the top of the lake.

Water is a unique compound because its solid state is less dense than its liquid state.  This quality allows ice to float atop lakes rather than forming a solid mass starting at the bottom and moving to the top!  Once water reaches its freezing point, it expands as it transitions to a solid; and thus it floats rather than falls.

For some time yet, lake ice will not be safe for walking upon!  For now, we must enjoy the patterns of the ice and frost from the shore.

Greg Lecker is a Minnesota Master Naturalist Volunteer.

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