Thursday, October 15, 2015

Why Are Our Evergreens Turning Brown?

Thursday, October 15, 2015
3:52 pm

Every autumn, our beloved deciduous trees shed their leaves, creating an annual economic boom for children everywhere as parents and neighbors pay them the proverbial quarter to rake excess leaves off their lawns. Of course, once these upstart entrepreneurs have reaped the monetary rewards of their labor, they get to romp around in the leaves until they are spread all over the yard again. I would give anything to go back to those days. But since I can't, I'll bore you with a blog on tree biology.


These types of trees are called "deciduous" trees, while those that keep their leaves all year long are called "evergreens." However, even evergreens do a little shedding, and this autumn, they appear to be more ambitious than usual.

I happen to have a pretty sizable Western Red Cedar in my front lawn, and this bad boy is as brown as I have ever seen it. I took this picture on Monday the 12th, and it really makes you question whether this cedar tree is deserving of the title "evergreen."

Flagging on a cedar tree in my front yard. Taken October 12, 2015

So what's with the brown "flagging" on this Cedar tree? Why are the evergreens doing this so much this year, and will there be any green left for our winter? The answer is yes, and there are two main reasons why.

First, we had a hot and dry summer. Cedars and many other conifers (trees with needles, like Firs, Spruces, Pines etc.) have relatively shallow root systems. This makes them vulnerable to drier and hotter-than-normal summers, as the upper soil responds more quickly to precipitation and heat than the soil deeper down.

A Douglas Fir pulled straight out of the ground at Avery Park, Oregon due to the Hanukkah Eve Storm of 2006. Note how shallow the roots are. Credit: Wolf Read's Storm King Website

This summer, soil moisture was significantly less than average both in the lowlands and mountains, which did not have the benefit of a large winter snowpack to keep the ground moist well through spring. The majority of water loss from a tree occurs through its foliage, so it makes sense that the tree would kill some of these needles to conserve water. If water was low but the tree kept all of its leaves, it would lose even more water, potentially killing the tree. Cedars always flag in the autumn, so the fact that this summer was much hotter and slightly drier (our heavy rains in August made up for much of our deficit) than normal explains why the flagging this year is more prominent than years past.

Second, conifers shed old limbs so they can devote more resources and water to growing new ones, meaning that even in the wettest of years, cedar trees still undergo flagging. On average, Seattle only gets 37 inches of precipitation a year, which is approximately the same amount that Dallas gets. If our trees tried to equally distribute water, they would grow very slowly. Trees in the Hoh Rainforest tend to experience less flagging because of the wetter climate, but needles that are several years old still die every year as the tree focuses on new growth, allowing them to grow to unbelievably massive sizes.

"Big Cedar Tree" with a girth of over 66 feet in Olympic National Park. Credit: Tom and Dianne's North American Adventure

Although the term "flagging" is mostly reserved for cedars, most conifers shed old needles in the autumn, particularly after a hot, dry summer.

There are a number of conifers that actually are deciduous. Most of these are Larches, but some Cypress trees and even the Dawn Redwood have this quality. Meanwhile, there are some non-coniferous trees and shrubs that are evergreens. For example, our state flower, the Pacific Rhododendron, is an evergreen shrub.

A Pacific Rhododendron in Olympic National Park. Credit: Walter Siegmund

Hope you enjoyed a different type of weather blog for today! It's fun to mix things up. Keep an eye out for WeatherOn's winter forecast, which will be released on November 1st. Right now, it looks like we are going to be warmer and slightly drier than normal, but nowhere near as warm as last year. I'd still hold off on a seasons pass until the 2016-2017 winter though.


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