The wavy pattern in tree rings is called “hazel growth”. The causes of this particular growth formation are not yet completely understood. Most likely they are genetically determined. Local growth conditions can also favour hazel growth.
The name "hazel growth" comes from its similarity to hazelnut wood. Spruce wood with hazel growth is particularly coveted because of its balanced and rich sound properties. Musical instrument makers use it for string instruments in particular.
In the 17th century, Antonio Stradivari selected spruces with hazel growth from the Foresta dei violini (violin forest) in the Val di Fiemme in the Dolomites, to make his famous violins.
In Austria, hazel-spruce tonewood know-how was recognised by UNESCO as a national intangible cultural heritage in 2011.