Live Crown Ratio is a tree measurement common in forestry. It is an indicator of stand density and tree vigor, and foresters use LCR for guiding the application of silviculture prescriptions like pre-commercial thinning and stand release.

Arborists and urban foresters can guide management decisions by utilizing LCR measurements as well. More living branches provide more photosynthetic capacity. Photosynthesis is the basis for energy production, and that energy drives most tree functions. Stored resources derived from photosynthesis are critical during times of seasonal stress like drought, and they can help to navigate insect and disease pressure as well.

Consider, as an example, managing for hemlock wooly adelgid. Many times in integrated pest management there needs to be a threshold in place to make decisions about treatment. LCR is one way arborists can quantify that threshold. Because LCR is measured as a percentage, we can develop a prescription that reads something like “a common threshold of 30% LCR should be present in order for the trees vascular system to have the capabilities to transport a systemic treatment from the root system upwards and into the foliage where the pest is actively feeding.” (https://savehemlocksnc.org/wp-content/uploads/2017/08/Assessing-Hemlock-Health.pdf) That threshold may change for different landscapes, species and IPM managers, but it offers a good illustration of one way that arborists can utilize LCR for applying tree management prescriptions.

From a bio-mechanical perspective, the more live crown a tree has, the greater its dampening capabilities in dynamic loading events like high winds during storms. Energy moves down the stem and is deflected outward along secondary and tertiary branches. Large, low branches in the crown particularly play an important role in dampening events for stability (https://dnrtreelink.wordpress.com/2018/06/11/timely-tree-tips-live-crown-ratio-and-the-value-of-low-branches/).

Tree species plays an interesting role in clearly illustrating LCR. Conifer species, or those trees that we define as excurrent in form (dominant central stem and small secondary branches) are much easier to conceptualize when illustrating a LCR. Decurrent trees tend to have more expansive forms by comparison, and therefore LCR is not as linear in form on decurrent crowns. In decurrent crowns, much of the healthy growth resides in the periphery of the crown, perhaps on several competing leaders. Generally speaking, it is a good precaution to consider species when utilizing LCR to determine management options.

In terms of vascular function, sugars move through the phloem from source to sink. Sugars produced in the leaves can either move upward or downward in the crown to storage sights. Poor LCR ratios make it difficult for trees to deliver sugars lower on the stem and root system. Once again, limbs in the lower crown, for this reason, play a major role for establishing good trunk taper. We see the importance of this especially in developing the crowns of young trees. By maintaining temporary low scaffolding for the first few years of growth on a young tree, we can influence better trunk taper because those branches are such a vital source of energy production. Furthermore, establishing permanent scaffold branches low in the crown will provide not only energy production but also stability.

LCR can be affected by abiotic influences, notably site conditions and exposure. Solitary trees, or trees at the edge of a stand, can be suspect to greater winds and heavy snow loads in colder climates. Their architecture will demonstrate phototrophic tendencies: offsetting or overloading towards the sunlight. A wolf tree found in the forest has a drastically different architecture than the younger stand that developed around it because of the site conditions on which the wolf tree matured for most its life. Trees growing in tight groups (like monocultures) will compete for sunlight, and stand uniformity with a lower LCR will be obvious. Trees on the edge of a stand will exhibit crown imbalances. “Flag” trees can develop on highly exposed sites, their architecture influenced and offset by the prevailing wind. Therefore, site characteristics will  have a drastic impact on LCR.

LCR is an interesting concept because it can guide tree management decisions, but it can also reveal the interesting history of trees or tree communities and the site on which they grow. Trees are products of their environment, and LCR is a clear indication of that.