Those Other Evergreens

March 8, 2020

When someone mentions evergreens to me, I usually think of conifer trees.  It might be the spire-like subalpine fir I see on the way to Dipper Lake, the bushy lodgepole pine and Douglas fir that carpet much of the park, or even the scattered Ponderosa pine that dot the western side of Sunflower Hill.  While evergreen trees are the most obvious examples of plants that keep their leaves throughout the winter, there are actually more kinds of evergreen shrubs and flowers in the park than there are these larger cousins.  Buried beneath the snow or peeking up through the crust are a couple of dozen species of plants which, unlike most of their neighbours, hang on to their green leaves throughout the coldest months of the year.

Prince's pine (Chimaphila umbellata) is found along most trails in the park. 


Evolution is an amazingly creative process. While most flowers and shrubs in temperate climates shed their leaves in the winter to protect themselves from cold temperatures and water loss, a small number have evolved alternative approaches and thrive despite winter conditions.  Hanging on to green leaves gives plants an advantage in fall and spring when moisture is still available as it lets photosynthesis continue later and start earlier.  And plants that retain their leaves don’t need to give up the investment they have made in energy and nutrients to build those complex structures. 

 Oregon grape (Mahonia aquifolium) with its shiny, waxy leaves. 


Evergreen shrubs and flowers in the nature park tend to have waxy coatings on their leaves to protect them from moisture loss when the ground is frozen and temperatures are low.  Inside the leaves a bunch of biochemical processes kick in when the weather begins to cool down in the fall, and these processes result in an increased resistance to frost damage by the time winter is established.  Sugars are concentrated to lower the freezing point of sap, and water is pumped out of cells into the spaces between them.  Other chemical changes may take place to protect the leaves from solar radiation. In warm times of the year when water is available, much of the sunlight’s energy is converted by photosynthesis into usable sugars. When the ground is frozen and photosynthesis stops, that energy can be damaging to leaves -- it would result in bleaching if other chemical pathways that use xanthophyll compounds were not activated to help dissipate the energy as heat. 

 Kinnikinnick  (Arctostaphylos uva-ursi) hugs the ground and is usually snow covered all winter.

 

Snow cover is probably the most important factor in evergreen shrub and flower survival.  Snow is an effective insulator and also blocks solar radiation.  For the low growing shrubs like kinnikinnick and twin-flower and herbaceous plants like the wintergreens, it would be a rare winter in Kimberley that didn’t have enough snow to fully cover them.  Underneath the snow the temperature would stay warmer in during cold snaps and the number of freeze/thaw cycles over the winter would be much reduced. 

 Yellowstem ceanothus or snowbrush (Ceanothus velutinus) is found on drier sites in the park. 

 

For larger shrubs like Labrador tea and yellowstem ceanothus, protection from snow cover becomes a bit more iffy.  In high snowfall years most of these plants are buried, but in years with less snow the higher branches of the plants are exposed.  We have seen widespread dieback of ceanothus leaves along the top of Sunflower Hill in some recent winters - although the plants, so far at least, seem happy to grow new leaves and have productive summers. 

 

The next time I am out snowshoeing or skiing in the park, I’ll be keeping an eye out for some of these green winter plants and appreciating the diversity of life in the Kimberley Nature Park. 
 

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