Cross section of cork tree shows the bark that is stripped off about every nine years for use in wine bottles and other light wood applications.

“We don’t know yet if it is a disease or if the mortality is caused by climatic factors,” World Wildlife Fund’s Portugal forest officer Luis Silva told the Lusa news agency.

The conservation group held a joint conference on the problem along with the U.N. Food and Agriculture Organization and Portugal’s agriculture ministry.

Portugal is the world’s leading exporter of cork, followed by other Mediterranean nations, which also attended the conference.

Cork trees can live up to 500 years and can be stripped for their bark every nine years.

Forests of cork cover about 6.7 million acres (2.7 million hectares) across Portugal, Spain, Algeria, Morocco, Italy, Tunisia and France.