The cold winds howling out of the lower Fraser Valley got me thinking about paper birch trees. When George Vancouver sailed into the Salish Sea he noted the stands of birch trees along the shores of a bay in what is now Whatcom County in northwest Washington and named the bay Birch Bay.
Paper birch are a very common tree of northern North America, but are not common in western Washington. The tree's natural range extends down to Everett, but no further south. The tree is more common as one heads north along the BC coast. It grows in disturbed areas and is then crowded out by conifers.
At the time of Vancouver's exploration, the forests around Birch By would have had disturbances that would allow paper bark birch to thrive. One would be simply humans. Birch Bay was an area with significant First Nations population and they would likely to have disturbed the forest around the bay for fire wood, building materials, and simply moving settlement sites. I have done some geology hazard assessment work around the bluffs of Birch Bay and large middens of shells and other debris are not uncommon along the bluff slopes and are such that a non archaeologist can readily identify them.
The other disturbance would be fire. Fires set by First Nations people for managing the landscape for desired wildlife and/or plants was a common practice. Natural fires may have taken place as well.
Yet another disturbace would be the very high winds that flow out of the Fraser Valley. Evergreens growing on shallow wet soils are susceptible to blow down. During a rather intense outflow windstorm in 1991 hundreds of Douglas fir were blown over in the low land areas of western Whatcom County when high winds arrived suddenly with the ground being wet and soft.
Of course conditions like these exist in other places as well in western Washington, so another factor must be present that limits the range of paper bark birch. One idea that readily comes to mind is temperature. Paper birch is a tree of the north. A wood borer, bronze birch borer, severely damages birch trees and will kill the tree. One of the recommendations for managing the pest when you have birch trees in your yard is lots of water and mulch to keep the roots cool and planting locations on the north and east sides of the house. The tree can fend off the borer longer if it is kept cool. Hence, birch trees will do better in cooler climates. Cold temperatures will not eliminate birch borers and it is not the only controlling factor on birch borers, but it will slow them down. Birch borers are found throughout the paper bark birch range but are more common in the southern edge of the range and appear to be a major factor in the paper birch range (Haak, 1996).
The plunging temperatures associated with Fraser outflow winds, combined cool summers, past and perhaps present day disturbances and a borer may be why Birch Bay is unique in Washington State as having this otherwise common tree.
4 months ago