Look straight up in a dense stand and the crowns rarely meet, leaving a network of bright channels between them. The phenomenon has a name, crown shyness, and explanations have ranged from chemical signalling to light sensing. The plainest candidate is mechanical: in wind, neighboring branches hit each other. Clatterbuck and colleagues set out to measure that. Does it hold up?
The numbers support . was greater for most species on the exposed side of a crown, the side with no neighbor to strike. Accelerometers in the outermost branches showed sway acceleration rising faster with wind speed in the growing season, when leaves are on, than in the dormant season. Combining sway with and mass gives the force available to break a bud.
Trees growing close together collide. Wind swings the crowns, the outer branches strike their neighbors, and the result is mechanical injury: lost branch tips and lost buds, repeatedly, over the life of the stand. That reduces crown size and leaf area, alters growing space, and suppresses the future growth of the damaged branches.
Most previous work on this came from monoculture conifer stands, where crown dynamics influence crown dimensions and stand density, especially once a stand is overstocked. Clatterbuck and colleagues went instead to several deciduous species, and asked what abrasion between neighbors does to stand dynamics.
They measured three things. Branch elongation in crowns where growth was inhibited by an adjacent tree and where it was not. Bud durability by growing season, for species with and indeterminate shoot growth forms, using a pendulum impact tester. And crown movement during wind events, logged by three-axial accelerometers fixed at the outermost points of the crowns.
Branch elongation was greater for most species on the exposed side. Acceleration from gusts rose more steeply with wind speed in the growing season, with leaves on the tree, than in the dormant season. Putting sway together with bud mass and durability gives the force needed to break or abrade a bud at a given wind speed. Because preformed determinate buds are tougher, abrasion helps build mixed species stands, letting slower-growing species with determinate growth stratify above faster-growing trees whose tips keep getting knocked off.
wat. Nobody is being polite. The branches that face a neighbor keep getting knocked back, and the gap is what is left.
wat wat
Crown abrasion turns out to shape which species end up above which, letting slower-growing trees with tough buds stratify over faster-growing ones with fragile tips.
How do we know?
Clatterbuck, Brannon and Yost measured branch elongation on inhibited and uninhibited sides of deciduous crowns, tested bud durability with a pendulum impact tester, and logged crown movement during wind with three-axial accelerometers.
What is still unknown
The breaking force is calculated by combining sway, bud mass and durability rather than observed as a bud snapping in the field. The work covers a set of deciduous species in one region.
Learn something?
Where to next
Why is there a gap of sky between one tree crown and the next?trees
Do trees really help each other underground?forest
What sets the limit on how tall a tree can get?trees