Nick Thomas, from Natural Resources Wales (NRW), said if soil had been incinerated in upland areas rich in peat, which grows at about a millimetre a year, it could take centuries for the vegetation to come back.
He called 2026 the worst year Wales had ever experienced.
As well as damage to natural habitats and animal life, Thomas warned loss of soil could increase flood risks, because it no longer acted as a "sponge" holding water on high ground.