Live Crown Ratio (LCR) is a simple yet important concept to be aware of when assessing or working with trees.

The LCR for any given tree is derived by dividing the vertical length of the tree’s live crown by the total height of the tree. We then multiply that result by 100 to convert the ratio to a percentage:

(length of live crown/total tree height) x 100 = % LCR

Let’s say your tree is 80-feet tall and the lowest extent of the living tree canopy measures 30-feet up from the ground. This means you have 30 feet of bare (or mostly bare) trunk with an additional 50 vertical feet of live crown above that. In this example:

(50’ / 80’) x 100 = 62.5% LCR

At 62.5%, roughly 2/3 of the tree’s total height is occupied with live crown, which is a good thing. In urban landscapes, we typically want trees to have an LCR of 60% or greater.

Image result for Live Crown Ratio
Image by David J. Moorhead, University of Georgia; taken from http://www.bugwood.org

A simpler but less accurate method is to estimate the percentage of the tree’s live crown by eye. If you can look at a tree and decide where the mid-point of its trunk is—the 50% mark—that gives you a benchmark for visually estimating the live crown ratio.

The lower the live crown is on the trunk, the larger the ratio; the higher the live crown is on the trunk, the lower the ratio.

Trees with a high LCR have relatively more leaves that are producing sugars through photosynthesis. The more sugars a tree can produce in a year, the more likely it is to be healthier, stronger, and more resilient to stress.

Research has shown that trees with a high LCR are also more wind-resistant during storm events.

In high winds, the leaves, twigs, and branches either deflect or absorb and dissipate wind energy. Research shows that trees’ lowest and largest branches play an important role in lowering the tree’s center of gravity and minimizing wind stress on the entire tree. The greater the LCR, the more likely a tree is to have more and larger low limbs that help it cope with wind stress.

Giant sequoias like this one in Bremerton are known for their natural tendency to retain low branches, their high LCR’s, strong trunk taper, and superior wind resistance. Photo by Kitsap Sun.

On a related note, lower and larger branches also contribute to faster growth of the trunk’s diameter compared to sections of the trunk with fewer and smaller branches. This phenomenon results in tapered tree trunks, wider at the bottom and skinnier at the top. Trees with more trunk taper are relatively more wind-firm than those with less trunk taper.

So… the larger the LCR, the more taper your tree is likely to have and the more wind-firm it is likely to be.

Pruning of low limbs impacts the wind-resistance of trees and reduces valuable leaf surface area. In urban landscapes we need to remove low limbs all the time for clearance and visibility, but often this can be achieved while retaining a minimum LCR of 60%.

It’s important to be aware of the LCR concept when assessing trees, and definitely before breaking out the pruning saw.

Here are some links to more information on the topic:

The Urban Tree Foundation: “Raising the Crown”

Deeproot Green Infrastructure Blog: “…Why young trees need lower branches”

USDA Forest Service Field Guide, Chapter on Measuring and Sampling Tree Crowns (see pages 7- 10)