Why do branches, trees fall during storms?

We saw it in January 2021, New Year's Eve 2022 and Christmas week 2023. Get ready for a possible repeat if forecasters are correct. Winter weather is about to wallop our urban forest.

During those memorable winter storms, Sacramento got hit by a one-two punch of heavy rain and strong winds. That brought down hundreds (by some counts, thousands) of branches and often whole trees.

Now, a powerful atmospheric river is flowing over Northern California. According to the National Weather Service, Sacramento likely will receive closer to 5 inches over seven days instead of the original 7-inch estimate, but that’s still enough to keep us under a flood watch through Friday.

In addition, strong gusty winds of 35 mph or more are expected to hit Sacramento late Tuesday night and again early Thursday, Christmas Day. Any outdoor decorations will not only be soggy, they may go airborne.

Such storms usually leave us asking: What happened? Was it too much wind? Or rain? Or both?

While the stormy weather plays a key role, two other factors set damaged trees up for problems: Drought and improper pruning.

Years of drought have significantly weakened the roots of many big trees, particularly large conifers (such as redwoods or firs) as well as elms and birches (one of Sacramento’s favorite lawn trees). Drought literally shrinks a tree’s rootball, so they have less support to hold them upright. Other trees never had a good anchor in the first place.

Used to shallow watering from sprinklers, solitary “lawn trees” may never develop those strong deep roots. Just under the surface of the turf, lawn trees’ roots are further weakened during drought when lawn irrigation is cut back.

Prolonged drought also caused tree dieback and dead or weakened branches. Those are the first to fall during a windstorm.

But even well-hydrated trees – especially evergreens – can topple over under the right conditions: Saturated soil and big gusts of wind.

During winter, conifers – redwood, pine, fir, spruce, yew, juniper, cypress and their cousins – tend to be at greatest risk. These evergreen trees retain their foliage year-round and can become top-heavy. When it rains, those needles become saturated with water, holding extra weight on a tree’s branches. That foliage also catches the wind like a huge sail.

In the forest, redwoods and other evergreens grow close together; their roots intertwine and help hold each other up. But in the suburban landscape, these big trees often are planted alone, without that extra support.

How a tree is pruned also can weaken its structure. According to the Sacramento Tree Foundation, a common evergreen problem is “lion tailing”; the foliage is pruned off near the trunk, leaving clumps of leaves at the end of limbs. The result looks like a lion’s tail. But when it rains, that places most of the weight at the end of the branch, making it prone to breakage.

With more rain and wind expected this week, now is the time to access the damage – and maybe bring in a professional arborist.

Be careful of trees that may be leaning. If cracks appear in the ground around a tree’s base, stay away – that’s a major danger sign. Put up barrier tape to keep people from walking around it and call an expert.