Due to its composition and its size, the Cantabrian Holm Oak forest is the main natural forest of Busturia, growing on limestone outcrops with well-drained poor soil. Although the considerable extensions of the Atlantic forests of the past no longer exist, this forest still covers large areas of Busturia.
Holm Oak forests are more frequently found in the Mediterranean climate where rainfall is lighter and temperatures are milder, with this type of perennial forest noted for the number of species competing for the limited available resources.
Nevertheless, the Urdaibai estuary is home to five Holm Oak forests with two in the Busturia area: Atxapunte and Muruetako Atxa. Both these forests share the rich clay found on the Altamira-San Bartolome plain as seen in the abundance of limestone found in the vegetable gardens and the fields. Another one is found on Sandindere Island and in the Kurtzigane area there is a young Holm Oak forest.