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Skyscrapers and Plants

Early spring brings with it the possibility of high winds, and even tornadoes, as Nashville experienced this past week. In my last blog post, we discussed keeping in mind the extreme weather and natural events typical of your region when designing your landscape. As important as initial plant selection might be to natural disaster prevention, it is more important how you arrange the various design elements in the landscape and how you foster (maintain) the landscape post initial building cycle. 

All this talk of high winds got this hip, non-hipster, plant geek to thinking about plants and their natural responses/adaptations to wind. Sexy topic, eh? Well I thought so, thus a trip to the scientific literature, to see what it has to say, was warranted.

Think about plant forms in habitats where frequent high wind velocity is the norm, such as windswept grasslands, high altitude mesas or remote oceanic islands. What characteristics allow plants to exist under these “harsh” conditions? The place to begin for me was looking at tall building design. Think about a tall, square, block building. Winds moving around the shape cause the block to rock back and forth due to eddy action on the leeward side. Imagine how quickly a similarly shaped plant would uproot with that type of motion. Building engineers and architects use several techniques to reduce wind’s forceful influence, including: reconfiguration, flexibility, tapering, and porosity.

Skyscraper design has been reconfigured over time (rounding corners, for example), and different shapes have been tried, as greater understanding and innovation has allowed. Studies repeatedly show plants change their shape to differing degrees, as a way of reducing drag, as winds speed increases. Grasses seem to have an almost limitless ability to reconfigure, which suggests why herbaceous plant forms tend to dominate many windy habitats. A correlating example is the incredible flexibility submerged aquatic plants show in changing current velocities. Why don’t submerged aquatic species uproot and wash away as easily as their land lubber cousins? Incredible flexibility is the answer! Woody terrestrial plants, such as tall canopy trees, do not have the advantage of water to support their structure, so they must rely upon the complex of wood fiber molecules to fight the downward force of gravity and shearing force of wind.

I am not sure if a lot is known as to what part different wood fiber/xylem configurations play in a plants ability to respond to wind, but at least one study looked at the effect of straight grained versus spiral grained branches. Spiraled branches break under the same amount of force as straight grained branches, however, a spiral grained branch is able to more evenly distribute the force along all sides. In contrast, a straight grained branch feels the pinch, so to speak, more on one side. Being able to twist with the wind is like a running back twisting away from the force of the tackle as a means of reducing impact. Even with different adaptations for flexing, wood has greater flexibility limits than their aquatic relatives, so trees and shrubs must utilize other engineering principles to survive.

Skyscraper designers have used “setbacks” for a century, to taper building profiles as they  reach toward the clouds. Not only does this make a more stable structure, wider at the bottom, it also reduces the rocking motion of wind vertices. The apical dominance of most young tree species is a similar natural pattern. Many tree species lose the advantage of tapering as they develop mature, rounded crowns. I would hypothesize these mature tree species rely upon their extensively wide anchoring root base in the absence of tapering.

Lastly, and more recently, architects have tried to make their structures more porous, finding ways to allow the wind to pass through the building, reducing drag and wind shear. Porosity may be one of the main means of woody plants being able to limit the force of wind. Several studies demonstrate that a plant becomes more “porous” with increasing wind velocities up to species specific thresholds. A plant might open “holes” in its canopy by curling the leaf surface, completely folding up as a compound leaf may. Arborists similarly reduce drag in canopy trees via thinning out a portion of inner branches. 

Understanding tree canopy effects on wind velocity is helping soil conservationists reduce wind erosion of soils and pollution from airborne particulate matter. It helps gardeners know how to prepare for the inevitable spring wind storm too. There now; was that long-winded enough for you?

Josh Steffen, Horticulture and Facilities Manager

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