Nature Note - Trees and Rain

A majestic bur oak in Shelburne Bay Park. Photo by Lori Fisher.

Fall rains bring out our least curious side. To avoid chill winds and wet clothes, we huddle under umbrellas, raincoats, and any handy cover up, and hurry between home and car, car and other buildings. We walk with quick steps, our eyes cast down to watch for puddles. But through these efforts to avoid chilly downpours, we miss one of nature's small-scale spectaculars: the response of trees to rain.

While some plants in warmer regions use their leaves as rain collectors, leaves in our region, where the availability of water rarely limits tree growth, have not evolved to gather water. Indeed, if rain did not drain from the surface of leaves in our region, they would become less efficient collectors of sunlight, and, due to the weight of the collected water, both leaves and limbs would be more prone to damage. The impact of thousands of pounds of water on trees is dramatically illustrated by ice storms, during which branches (and the needles of conifers) fail to shed water, and limbs and even entire trees crash to the ground under weight of the frozen water.

It should come as little surprise then that leaves often have adaptations to shed the rain that falls on them. These adaptations are largely intended to overcome surface tension -- the attraction between water molecules that gives drops of water their cohesiveness and allows one to add pennies to a full glass of water without causing a spill. Surface tension also causes drops of water to stick to surfaces, such as leaves. Water that does drain to the edge of a leaf will hang there as pendulous drops, the attraction of water molecules for each other and the leaf fighting off the forces of gravity.

If the edge of the leaf were entirely smooth and round, several drops might form at different spots around the leaf. But if one looks at leaves, one finds that many come to sharply pointed tips. In our region, basswood, cottonwood, aspen, beech, birch, hornbeam, musclewood, juneberry, cherry, red oak and a variety of other trees and shrubs all have such sharply pointy tips. Other trees, such as oak, maple, hickory, ash and buckeye have more than one tip on each leaf. During a rain, water droplets accumulate at these tips, gradually forming larger drops that fall more easily than would several smaller drops. The tips thus help trees to shed water.

The response of the leaves to rain in turn affects what we experience when we shelter under trees. Because the drops from leaf tips must be heavy enough to overcome surface tension, one finds fewer, but larger drops falling under trees during prolonged rain than in the open. When wind blows through trees, it can break free drops that are not yet large enough to fall on their own, producing a shower of drops on passersby below. And, as one would expect, trees continue to drip for some time after a rain stops, as drops gradually make their way to the tips of leaves, and breezes shake them free.

Not all of any given leaf slopes toward the tip, however. Many leaves form a shallow arc, with areas far from the base sloping down to the tip, and areas near the base sloping down to the petiole, which attaches the leaf to the branch. Drops therefore also accumulate where the leaf joins the petiole, and even along the petiole, and when surface tension is overcome these drops roll down the petiole. They may continue down branches, and eventually the trunk, forming what hydrologists call stemflow. On a smooth-trunked tree, stemflow may be quite significant. During a rainstorm, a finger placed against the trunk of a beech, birch or young basswood tree will rapidly develop a small puddle above it. On rougher barked trees more water is needed to wet the bark before stemflow can occur, and once stemflow begins, the water often drips off of bumps and rough spots before it reaches the ground.

Even with these various mechanisms for directing water off their leaves, trees still catch a substantial portion of the water falling on them. Between 15 and 25 percent of the annual precipitation falling on a mixed forest of conifers and hardwoods will remain on the leaves and eventually evaporate back into the atmosphere. These scientific data confirm our intuitions; sheltering under trees from the rain does indeed make sense. One gets less wet overall, unless, that is, a friend or the wind shakes the tree, releasing that stored water in a sudden flurry of drops.

The interactions of trees and rain also helps protect water quality. In a forest, layers of vegetation, each intercepting and shedding water falling from above, serve to soften the fall of rain. Water first reaches the tree tops, drips to lower branches, then shrubs, ferns and small plants on the forest floor. By the time water reaches the dead leaves on the ground, drops may have fallen only a few feet or inches. This means that each drop hits with less force, which reduces erosion and allows percolation. Even during a heavy rain, it is uncommon to see water running across the forest floor.

On streets and fields the situation is quite different. Drops hit the ground after dropping thousands of feet, and release the kinetic energy of their fall in a sudden splash. In fields, this splash can mobilize soil particles, making them available for erosion; on streets or roads, the splash may mobilize grease and dirt from cars. The difference between a drop falling from a fern leaf to the ground 18 inches below and a drop falling to the ground from a cloud a mile up is much like the difference between a gentle spray and a high pressure shower massage.

In the fields, the soil particles kicked up by falling rain may simply rest where they fall, as water percolates into the soil. On streets, water cannot percolate into the soil, and instead moves readily down even a slight slope, pouring into storm sewers or ditches, and from there into rivers and streams. Within minutes the bulk of water falling on a city street is gone, in the process carrying a variety of pollutants into our streams, rivers and lakes. Water, like people, moves more frenetically in cities and towns than in the forest. 

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Where and when to look:

The behavior of trees in a rainstorm is far easier to catch than many of nature's delights. Trees can't run or fly, aren't very good at hiding, and can be seen without a long trek up a mountain. Warm clothes and raingear are recommended. It may also prove drier to watch leaves on shrubs or small trees than to crane one's neck to see the uppermost leaves of a large tree. If a short thunderstorm seems to be the order of the day, it can be interesting to see how much time it takes for a tree to begin dripping to the ground below and for how long after the storm it continues to drip.