Why do trees break at the same wind speed?
Why do trees break at the same wind speed?


Image source: Wikimedia Commons
Note that in the above image, that almost all of the trees had their tops snapped off. This claim is about tree's being snapped in two and is not about trees falling over or being uprooted.
This claim is approximately valid for many species of trees. Apparently oaks can withstand slightly higher wind speeds than can pines. This claim does not apply to non-woody trees such as palms. It also is not true for species of woody trees that have evolved to live in areas where winds occasionally exceed 42 m/s (the equivalent of a category two hurricane).

I just do not understand what can cause such behavior to be valid for all size and types of trees.

The authors of a recent paper on this subject attribute it to evolution. While winds frequently exceed 30 m/s (126 km/h), they very rarely exceed 50 m/s (180 km/h) in most places. The sweet spot in a tree's ability to sustain winds is between these limits. A tree that is too flimsy will be snapped by a heavy gale, won't live to maturity, and won't spawn offspring. A tree that is too sturdy is wasting precious energy building unneeded structure.
As for why and how, it's a matter of allometry. Trees grow in girth as the age and add to their canopy. Tree allometry investigates the relationship between tree girth at chest height and other features of trees such as height and canopy area. The relationship between girth and canopy area is close to linear.
Another factor is defects, which are places where a tree is most likely to snap in two. The extent of these defects also scales with size. Young trees lose tiny branches; the young tree eventually grows around these, leaving the small defect buried deep inside the tree as it ages. Older trees lose larger branches, leaving behind a potentially larger defect.
References:
Critical wind speed at which trees break.
Trees, regardless of size, all break at the same wind speed. Here’s why.
Urban Tree Database and Allometric Equations.