Photo 1. The bark of the hop hornbeam tree is smooth on the windward side but cracked on the opposite, leeward side (Photo by M. Randić)

WHY IS A TREE’S BARK SMOOTH ON THE WINDWARD SIDE?

Nature field guides often teach us that, in a pinch and without a compass, we can use moss, the width of rings on tree stumps, or other indicators in nature, to determine the cardinal directions. They also tell us that tree bark facing north is, as a rule, rougher than bark facing south. 

However, on the northern slope of Žbeljac Hill at the foot of Mali Platak, we came across hop hornbeam trees (Ostrya carpinifolia) where the bark was smooth on the north and north-east sides but cracked and rougher on the south side (Photos 1 and 2). This phenomenon was especially obvious on trees which grow along the edge of the stand and are more exposed to the bora wind (Photo 3). At present, we assume that the bora wind is the main “agent” of this phenomenon; perhaps ice crystals or particles of rock or sand, carried by the wind, hit the bark on the windward side and act like tiny projectiles, “polishing” and smoothing the bark.

Marko Randić

Photo 2. Are ice crystals and/or sand particles the reason why bark on the windward, that is, north-east side of the tree trunk is smooth? (Photo by M. Randić)

Photo 3. A stand of hop hornbeam trees on the northern slopes of Žbeljac Hill: It is obvious that the trees and branches are bent in the direction in which the bora wind blows. The smoothness of the bark can best be seen on trees at the edge of the stand. Note the narrow-leaved moor grass growing around the trees – this grass is an indicator of habitats exposed to strong and frequent bora winds; compare:       http://www.ju-priroda.hr/zanimljivosti.asp?id=2zanimljivosti/ljudi/9-simpozij-agronoma.html (Photo by M. Randić)