Snow and Box Elder Buds

I was on the phone this morning, listening,  talking and looking out my kitchen window at the goldfinches at my feeder.  I couldn't believe my eyes:  it looked like snow.  It couldn't be cold enough, could it?

I convinced myself that what looked like snow had to be soot from my neighbors trash "burn can."  It wasn't until I checked the weather report that I discovered the truth:  yah, it was cold enough and that was snow. 

Despite the temperature, I went down to the Chippewa River State Trail to walk a mile or so and photograph tree buds.

Acer negundo male flowers

Acer negundo female flowers (and emerging leaves)

female flowers

Box Elder male flowers have been in bloom for several days.  Today was the first time I spotted the green female flowers and leaves.  Box Elders are "dioecious" (die - eee - shus) - each tree is either male or female.  The male trees produce only male flowers and female trees produce only female flowers.

I'll be looking for the new fruits (samaras) over the next couple of weeks.  The female trees have been dropping their old dry fruits over the past couple of months - but many are still hanging on - another example of marcescence.

Falling Samaras

I've been watching two trees that hold their fruits all winter - the ash (Fraxinus) and boxelder (Acer negundo).

Their fruits are similar - both are "winged achenes" known as "samaras,"  distinguished by flattened wings of paper-like tissue that develops from the wall of the ovary.

Botanists describe these fruits as "simple" (the ripened ovary of a flower with a single pistil), "dry" (not fleshy like an apple) and "indehiscent" (they don't open to expel seeds - like a milkweed pod)

When I was a kid we called them helicopters and whirlybirds.   Their winged design helps the wind fly them away from the parent trees.

When do these trees "let go" and drop their seeds?

I could look it up, but it's more fun to wait and see.  I've been waiting and I've been watching all winter.

The dropping of the samaras started last week...

Marcescent Leaves

Marcescent Bur Oak Leaves 

along the Lower Chippewa River in Durand, Wisconsin

When I was a kid, I learned the two kinds of trees:  coniferous and deciduous. 

Coniferous trees, also called evergreens, have needles or scale-like leaves and cone bearing seeds. With the exception of tararack and baldcyress, conifers don't drop their foliage all at once in the fall.

A few deciduous trees -  notably oaks, American Beech and Hop-hornbeam - hold on to their withered, lifeless leaves through the winter.

This has also been observed in young trees.  This phenomena is called marcescence.


I never thought much about it - until recently, when I got interested in prairie vegetation.  I've been looking at Bur Oaks lately:  the corky ridges on their branches, the insect galls, their acorns, their resistance to fire and their marcescent leaves.

Why do they "break the rule" and hang on to their dead leaves all winter?
 
Some scientists have suggested that not dropping withered leaves may protect tree twigs, especially in young trees, from deer and other winter browsers.  

Others suggest a downside:  marcescent leaves, heavy with snow and ice, make tree branches vulnerable to breakage during storms.