Gardening

Ask the Gardener: Cracking the mystery of falling trees

Extreme heat and stress can lead to structural damage. What you can do to prevent it.

A large tree split in half and landed on a house in Newton. Jessica Rinaldi

During a particularly hot afternoon in June, my staff and I were walking a shaded trail at Garden in the Woods in Framingham when we heard a significant cracking noise, followed by the sound of limbs and branches crashing to the forest floor. I was lucky to glimpse where the tree fell, and even more fortunate that it occurred well away from any pathway or garden visitor.

When we arrived at the location, we discovered a tree trunk that had broken about 20 feet up, along with the remains of what appeared to be a healthy red oak tree tangled on the ground. With no outward signs of decay, there must be some connection between the weather and these sudden failures of branch, limb, or trunk.

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Indeed, several readers have written about cherished trees splitting apart or losing limbs. Today, I will write about the importance of tree risk assessment, particularly during the summer months. I will also discuss some theories about why extreme heat and stress on trees can lead to structural damage, and what can be done to mitigate those events.

First, tree risk assessment is a systematic process that evaluates a tree’s overall health and stability. Performed by trained arborists, this process helps predict the likelihood of a tree falling, causing harm, or damaging property.

It is essential to note that this is a valuable tool in tree, canopy, and forest management, but not a foolproof method for predicting how trees will behave in the landscape. The protocol begins with a visual assessment of the tree structure and extends to account for the surrounding environment and potential targets. Based on the sum of factors, a recommendation will be made for pruning, canopy reduction, or retrenchment, cabling, and in the most risk-prone situations, removal of the tree entirely.

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I strongly advocate for exploring all options before deciding on removal, as the benefits of trees in our landscape are tremendous, not just for the shade and cooling effect they provide, but also for their wildlife value, especially as mature trees are home to thousands of organisms. Be sure to work with certified arborists who are trained in tree risk assessment. Reputable companies will offer a range of solutions unless their assessment indicates removal is necessary.

A note of caution: Unfortunately, there are plenty of outfits out there who see dollar signs when asked to perform risk assessment and will immediately recommend removal rather than risk reduction through pruning or cabling.

Summer is a crucial time to assess trees due to a combination of factors, including high heat and drought stress. Summer storms are often violent, with high winds and heavy downpours. Lastly, the growth of foliage can help hide defects, add additional weight, or torque to limbs, branches, and trunks.

Returning to our downed tree in the forest, a possible explanation is known as sudden limb drop. It occurs on hot, calm days, when, for seemingly no reason, a large branch, limb, or trunk simply breaks off and falls to the ground. It is impossible to predict when it will happen, but regular risk assessments can help lessen the chances.

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The causes of this phenomenon are still hotly debated, but most arborists agree that it is related to humidity levels in the tree canopy. Trees absorb water from the soil into their roots, where it is then transported throughout the trunk, limbs, branches, and branchlets and eventually exits the leaves as water vapor, a process known as evapotranspiration. For more water to be drawn in, it must be released via the leaves, and in the process, it cools the tree down. We release heat and water as sweat, dogs “sweat” from their tongues, and trees “sweat” through their leaves. The theory is that on hot, calm days, the trees can’t “sweat” enough and hold the extra moisture (and therefore extra weight) in their limbs, leading to structural failures.

Basically, sudden limb drop is the tree’s response to a hot, dry environment where transpiration needs exceed vascular capabilities. When it gets too hot to keep all tissue properly circulated, the tree responds with auto-amputation, letting go of a limb.

What can be done? Consistent, regular assessments and pruning, especially of mature trees, are the most effective preventive measures to minimize the risk of sudden limb drop. Even though mature trees are established, additional waterings (if permitted by local ordinances) performed at night to reduce water loss can help trees get through stressful times.

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To bring the discussion back to the trunk, trees are part of the New England landscape, part of our shared natural and cultural heritage. We ought to care for them as an extended member of our family as they give us so much freely, shade from the sun, leaves for our garden beds, and a stage from which songbirds perform for our ears.

Uli Lorimer is the director of horticulture at the Native Plant Trust in Framingham. Send your gardening questions, along with your name/initials and hometown, to [email protected] for possible publication. Some questions are edited for clarity. Subscribe to the Globe’s free real estate newsletter at Boston.com/address-newsletter.

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