Doug Fir and Conifer Pruning

PlantAmnesty   

By Cass Turnbull

Signature Trees If the Pacific Northwest had a signature tree, it would be our towering conifers, mostly Douglas firs (aka Doug firs) and western red cedars, which show up everywhere, in and out of the city. You know you’re home when you look down from your airplane seat and see their dark green cones dominating the landscape below. In the winter they stand stately on hillsides, wrapped in white mists. In the summer, their fallen needles on the campground floor emit a distinctively summer scent.

Blow Down and Limb Shedding Equally iconic in the Northwest is news footage of homes crushed by conifers every time there is a windstorm. Sure, the tree is probably a hemlock, planted on clay soil, topped fifteen years ago, its roots severed when the home was built, and left as a released tree, the sole survivor of a forest that was cut down. But the viewers don’t know that. Suddenly they look at their gentle giants as potential life-threatening organisms. Never mind that one has a better chance of being hit by lightning than being hit by a tree. People are shocked to think that the trees they have considered friendly for so long may not be so friendly after all suddenly, something needs to be done about it. Doug firs are no more likely to blow down than other trees, which is to say that height doesn’t make them more likely to go over. Mark Mead, Seattle Parks Senior Arborist, made this point at a recent PlantAmnesty Symposium, saying that the amount of failure (arbor-speak for fallen trees) of this particular type of tree is directly proportional to the percentage of this tree in the field.

A more common occurrence for homeowners is the shedding of limbs during windstorms. The most telling predisposing factor for limb shedding is a previous history of limb shedding. This usually happens in a storm, so all you do is go out and pick up the limbs the next day. More problematic is when those shedding limbs are within the striking distance of a house or vehicle (targets in arbor- speak).

Edge and Released Trees versus Open Grown Trees The biggest indicator of the likelihood of a conifer blowing down is whether the tree is open grown or an edge or released tree. If your Doug fir appeared in your yard one day and grew up there, all alone, with limbs all along the trunk, it is less likely to pose a risk. It has adjusted its trunk taper and roots to the site and in response to wind conditions. A released tree (or edge tree) is a tree that was once part of a forest and grew up protected from the winds by its neighbors. These trees tend to have skinny trunks, with little taper, and just a fluff of green on the top. Once the land is developed, the newly exposed trees on the edge tend to blow down. Then the next row tends to follow. This is most often seen on the fool ‘em strips or green buffers of trees left by loggers along the road next to where the forest was clear cut. These strips are an accident waiting to happen, with trees predictably falling on power lines and roads—they should be illegal. A released tree is a single tree that was once standing in a forest and now stands alone. I am amazed how many survive and adjust and don’t fall over.

Wind Firming through Topping, Spiral Pruning, Crown Cleaning, Thinning, and Windsailing I just got though reading the results of a wind firming test for trees that are at the edge of a clear cut. It was published in Forestry and written up by Charles A. Rowan, Stephen J. Mitchell, and Hailemariam Temesgen in British Columbia, Canada. Just the term wind firming gets one’s hopes up unreasonably that there might be the magic bullet to secure a safe tree for your home. Whenever a new, interesting sounding term is introduced, a small alarm bell goes off in my head warning me that the term abuse is not far behind. I even bet there will be some following this article! Be that as it may.

In this particular test plot, edge trees were topped (by helicopters!) and spiral pruned by tree workers. Spiral pruning, a term popularized in Canada by Julian Dunster, refers to the removal of limbs, back to the trunk in a spiral pattern, going up the trunk. The results showed that in the following year there was 40% less blow down and 3% mortality. One third to one-half the foliage was removed—a vast amount by arborist standards of pruning. Follow-up studies are not available, but, as an arborist who’s observed trees for 25 years, I would guess that the mortality will increase for 5-7 more years and then taper off. Tree topping and excessive pruning, like cigarette smoking, does not kill right away. It degrades health over time and shows up later as something seemingly unrelated. And let it be known that any sort of topping or reduction (including retrenchment) is worse for the health of trees than any sort of thinning, including spiral pruning, because it allows rot to invade the entire trunk.

This sort of heavy pruning may be appropriate to protect forest stands, but it has no use in urban situations. Dunster remarked that they found that spiral pruning, though lowering the incidence of blow down, increased the likelihood of limb failure. And Mitchell said that topping was not recommended in urban settings because of the likelihood that the new tops that will grow back later are weakly attached and likely to fail in succeeding years. While a 10% mortality rate may be acceptable in the edge of a larger forest stand, it is unacceptable in the urban

setting especially since many of the Doug firs being pruned are open grown, posing no particular threat, but worrying the owner.

All that being said, I acknowledge that dealing with edge and released trees is a tough, a really tough problem. If you take the risky trees out, the next ones are more likely to go down, like a set of dominoes. In these circumstances, a responsible arborist might aggressively thin out as much as 50% of the live crown to prevent blow down. Or the sacrificial tree may be topped. This is a scary solution that should not be done lightly, but rather only after a thorough assessment of the likelihood of failure for it and nearby trees. If the tree is a high-risk tree that stands alone, it should be taken out. If it is a low-risk tree, it should be left alone or perhaps

lightly thinned (10% of the live crown, mostly broken and dead branches). If it is somewhere in between, and especially if removing it will cause the next set of trees to be exposed to winds and likely blow-down, then extreme pruning may be an appropriate measure. Some would argue with me on this point, and with good reason.

Speaking of thinning, also called crown cleaning, a little bit can reduce the amount of limb failure, by taking out dead and weakened limbs. But there are serious concerns that, in some instances, the thinning will cause more limb failure by allowing more wind into the crown of the tree. We all used to promote windsailing (taking the sail out of a tree by thinning it so the wind could pass through) to customers with fears about their trees. Now we are not so certain, and several bad tree services aggressively sell windsailing as the magic cure in our region. There is also research that shows that limbs act as baffles to the wind, relieving stress on the trunk. Take too many off and the trunk begins to sway more, taking the brunt of the wind force. A little may be good, a lot might be bad, like so many things in life. A cone, the natural shape of a Doug fir, is a nice aerodynamic structure, mess with it at your own risk.

Crown Raising, Limbing Up, Elevating, and Skirting Research and recommendations about pruning trees for safety have been all over the map in the last 20 years. For a while there, research indicated that taking off lower limbs was the way to go. But then new research came out that said the opposite was, in fact, the case. Taking off lower limbs is called crown raising, limbing-up, elevating, or skirting.

Generally speaking, trees are well-adapted to losing their lower limbs in nature as they get shaded out when they are small. In general, taking a limb off the lower part of the trunk should be done before that limb reaches one-half of the diameter of the trunk. Making larger cuts allows too much decay to enter the trunk. Crown raising is usually done to provide clearance for cars and houses and to allow in more light. The rule of thumb is that no more than one-third of the trunk should be without canopy at any given time in its life. That means you should slowly remove lower limbs, a few at a time, every year as the tree grows up. The lower limbs feed the lower part of the trunk making it strong.

But Doug firs are often high-crowned, showing as much as two-thirds trunk with only one-third live canopy, which they seem to tolerate surprisingly well. Just know that the principles of engineering are at work here—the higher the crown, the greater the force on the top; the likelihood of stem failure is respectively greater.

Other Causes of Risky Trees Other causes of a conifer becoming dangerous are construction damage, previous mal-pruning, and root rots. Construction damage includes cutting and filling the soil, compacting the soil around trees by driving on it or storing materials there, blading (scraping), trenching for utilities, and paving. Trees’ roots are much closer to the surface than people imagine. If I were

to have a second life to devote to trees, it would be spent trying to end construction damage. Just putting in an irrigation system or a patio can do your tree in. Unfortunately, the tree often takes five-to-seven years to run through its energy reserves or to have the root rots progress to such an extent that the tree actually dies or falls over. By then, so much time has passed that no one connects the effect with the cause.

Our great Pacific Northwest is also home to a huge number of root rots. Of course it is. It’s wet nine months of the year. You got your Armillaria, you got your Phellinus weirii, you got your Heterobasidion annosum, and God knows what else. Sometimes arborists can tell if you have a rot by drilling the root flare or the roots, or by the presence of mushrooms on the trunk, trunk flare, or roots. Call an arborist if you spot mushrooms and things that remind you of mushrooms growing on your tree. Not all of them are bad.

Previous mal-pruning, by which I mean topping or overthinning, especially lion’s tailing (stripping off the inside laterals of branches leaving only some foliage at the very end), can cause trouble. Topping allows rot to enter the trunk of the tree to eat its way down as far as it can before the tree walls it off. This process, called compartmentalization, was thoroughly researched and described by Dr. Alex Shigo, forest pathologist, in the past two or three decades. The other thing that happens after a top job is that epicormic sprouts (like watersprouts) attempt to become the new top. They are likely to be poorly attached in the future, even after they grow large. And the next set of side branches, located just below the topping cut, slowly turn up over the course of several years, sometimes resulting in a candelabra or flame-shaped new crown. These often poorly attached competing leaders are also more likely to break out in windstorms. Many of the tree tops that failed in Seattle’s Inaugural Day storm were those formed after people topped their trees in fearful response to the Columbus Day Storm 50 year earlier. I have often noticed a theme in life: one can create the problem one is trying to avoid by trying to avoid it. Forest fires, for example. So, what do you do if you have tree that has been topped? First, you get an arborist to go up and look it over. How far has the rot progressed? How well attached are the new leaders? If caught early enough, a new leader can be chosen and any others suppressed by removal or shortening (reduction).

End-Weight Reduction One more potential problem is when limbs become over extended (another term prone to misuse). This can happen with open-grown trees and previously topped or lion’s tailed trees. An over extended limb reaches out too far from the basic outline of the tree and is prone to breakage in snow. In this case, responsible arborists may choose to shorten the limb a bit, looking for an outer- facing branchlet to take over as the lead stem, preserving apical dominance. This is done to only about five limbs of a tree (never more than ten— certainly not the entire crown), most frequently at the bottom of the tree, but not always. It is called end- weight reduction. Add alarm bells here.

Risk Assessment Worried? Then get an ISA-certified arborist to do a risk assessment. Hire somebody who is middle-aged. And please know that they can only foretell about 10-20% of the tree failures. Often a tree is just in the wrong place during a high wind. Remember, chances of getting hurt are very slim. If you are arborphobic, sleep downstairs during the storm and recognize that the insurance company will buy you a new roof. Don’t do anything stupid to your tree, like topping or overthinning it, just in case.