Saturday, March 12, 2016

Will the nest trees hold N2B and the Decorah North Nest?

We're getting some concerned questions about the strength of the trees that N2B and especially the Decorah North Nest are sitting in. The North eagles have an extremely large nest by any measure, with an estimated total area of 49 feet and and an estimated weight of around 2100 pounds. Will the trees be able to handle it?

The likelihood of any given tree or branch to break depends on its strength, its size, its shape, its health, and its location, all which affect its capacity to withstand loading by things like wind, ice, and bald eagle nests. In general, thicker trees are stronger than thinner trees, balanced trees are stronger than top or side heavy trees, healthy trees are stronger than sick or dead trees, and trees composed of dense or heavier wood are stronger than trees composed of lighter wood. Location can impact a tree's exposure to lightning and affect how it grows in the presence or absence of light and prevailing winds.

Let's take a look at the strength of five trees: a white oak (the Decorah North Nest), an eastern cottonwood (the Decorah Original Nest), a loblolly pine (bald eagles nest in lobolly pines), a sugar maple, and a shagbark hickory. A few terms before we get started:
  • Specific Gravity: Specific gravity measures the density of a material. In general, the higher the specific gravity, the denser and stronger an object is. However, other mechanical properties also influence a tree's strength.
  • Modulus of Rupture, or MOR: MOR is a measure of the maximum bending load in a material just before it yields or ruptures. 
  • Modulus of Elasticity, or MOE: MOE is a measurement of stiffness that determines a material's deflection from a load.
  • Toughness and Hardness: Toughness is the ability of a material to absorb energy and plastically deform without fracturing, while hardness measures the force required to embed an 11.28 mm (.444 in) steel ball into wood to half the ball's diameter. These two characteristics are often inversely correlated in trees: that is, a hard tree can be less tough than a weak tree. However, toughness is often proportional with density, and trees create areas of denser wood where needed. More on that later! 
Wood Species
Specific Gravity*
Bending Strength (MOR) 
Stiffness
(MOE)
Hardness

kg/m3
MPa
GPa
 lbf
Shagbark Hickory
0.72
139.3
14.90
1,880
Oak, White
0.70
102.3
12.15
1360
Sugar Maple
0.63
109.0
12.62
1450
Loblolly Pine
0.47
88.3
12.30
690
Cottonwood
0.36
58.6
9.45
430

Trees experience dynamic and static loading. Static loads are long-term and don't change quickly. Think of growing limbs or a bald eagle nest. Both of these things place a load on a tree, but they happen slowly and the tree has time to respond to them. Dynamic loads like wind gusts occur immediately and unpredictably, giving a tree no time to respond. Think of N2 - a healthy tree! - snapping during a storm in July 2015.

MOR describes the maximum amount of load that can be placed on a tree before it breaks, while MOE measures its resistance to being deformed elastically. Think of climbing a tree. When you step on a branch, it might bend, but unless you exceed MOE, it will snap back into place once you take your weight off. If you exceed MOE but not MOR, the branch will stay bent once the weight is removed. If you exceed MOR, the branch will break. I hope you are holding on tightly to something else!

Also keep in mind that the measurements for MOR and MOE are mega- and giga- pascals respectively. A pascal is one newton per square metre, or the force needed to accelerate one kilogram of mass at the rate of one metre per second squared. While some of these trees are stronger than others, a lot of force is needed to rupture or displace any of them! A cottonwood has the lowest MOR, but it would still take 58.6 million pascals to break it. That's a lot of static force! The DNN nest might be large, but a sturdy white oak is more than capable of supporting it. If I'm doing the calculation correctly, the weight of a roughly 2000 pound nest converts to 8825 newtons, or .008 MPa, which is nowhere near the weight or force needed to cause static rupture in any of these trees, assuming that they are healthy and the nest is relatively well balanced.

Like people, trees grow and change throughout their lives. Stress effects wood growth or build up: the more stress or compression there is on an area of a tree, the more wood will build up there to counterbalance it, toughening that area of the tree and expanding and pushing back in the opposite direction of the compressive force. You can sometimes see this on the underside of large tree limbs and you can clearly see it in cut wood. Naturalist Bernd Heinrich writes: "In tree limbs the greatest stress occurs in the “hinge” area, at the attachment to the tree. It is here that the growth rings of limbs are thicker on the bottom than on the top. A mechanism of selective wood deposition could help to brace the limbs up laterally." If we sawed through the limbs that supported N2, we might find that they were thicker and had a different pattern of deposition than similar limbs that did not support bald eagle nests. As the tree grows, it responds to the weight of nests and branches by piling on more wood where it is needed - and trees overbuild to begin with!

Of course, limbs and trees do sometimes break. However, bald eagles have a remarkable record of success when it comes to choosing trees.  One study found that, of more than 10,000 bald eagle breeding attempts that were monitored in Virginia since 1962, only one in 850 failed due to tree loss. Nearly half of these losses were due to violent storms that snapped off live trees during the nesting period.  Of the remaining losses, nearly all were in trees that had been dead only one year.

So do eagles somehow know where to place a nest for strength and balance, or is it not possible to begin a nest anywhere that isn't well balanced already? The more questions I ask, the more questions I have! But either way, bald eagles are master builders and the trees we have nests in are more than strong enough to hold them!

Bald Eagle nest in East Central Minnesota
This bald eagle nest is located in a tree about 12 miles from my house. As you can see, it is a big nest in a small tree! Bald eagles have occupied it for roughly four years. During that time, the tree has responded to the weight by deforming and thickening. I suspect this tree will go down some day in a storm, since it is more prone to succumbing to dynamic loading given its top heaviness. However, this photo gives us a great look at just how strong trees really are.


 Things that helped me learn and write about this topic:
This was the most technically difficult subject that I have ever written about. Wood strength and qualities are measured in many different ways, boards are not the same as live trees, and even arborists and horticulturalists think about and measure trees in various ways. Since trees are alive, they also aren't as uniform as, say, a nice pine 2x4. I found all of the articles I read to be fascinating (except the ones I couldn't understand at all) and they helped me to garner a whole new measure of respect and appreciation for the trees that surround us.
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