The summer's biggest blaze so far was subdued on Friday only after it had raged for a week near the Mediterranean town of Tarragona and destroyed 12,500 acres of woodland. In the densely wooded province of Galicia, 600 fires broke out last Sunday and another 300 during the week, engulfing the port of Vigo in smoke and ash.
Perhaps half are started by accident, like the car crash that sparked the Tarragona blaze. But up to 50 per cent are started deliberately - Galician authorities detained nine suspected arsonists last month - and according to the environmental pressure group Greenpeace, they result in big profits for the powerful paper and pulp industry that buys burned wood cheaply.
Around 15 per cent of timber cut from Spanish forests for industrial use comes from areas devastated by forest fires, Greenpeace says in a report published in Madrid this week. "The burned wood is sold off cheaply to companies that make pulp, chipboard and veneer," said Mario Rodriguez, head of Greenpeace's forest campaign, "Only the outside is burnt. The trunk is intact and a machine cleans it like peeling a carrot."
An intermediary buys the burnt wood at half-price and sells it to the paper companies, forcing producers of "green" or undamaged wood to lower their prices, Mr Rodriguez says. Analysing the correlation between forest fires and the price of wood over the last 10 years, Greenpeace found that the price of the raw material for the pulp and paper industry dropped by up to half in the year following a summer of big forest fires.
"We found that every four or five years - in 1985, 1989 and 1994 - the number of fires rose sharply," says Mr Rodriguez. "And in the years preceding catastrophic fires there was very little wood cut and prices rose.
"We are not saying that the trade in burnt wood is the main cause of forest fires, but it is one cause that should be investigated," he said. "And there should be better control of this trade to prevent speculation."
Greenpeace believes that forest fires mostly affect the plantations of imported pines and eucalyptus cultivated to supply the pulp and paper industry. In the 1950s virgin forests were burned to make way for these exotic fast-growing species which, Mr Rodriguez says, destroyed habitats and enabled pulp companies to control prices.
Government statistics, he claims, do not distinguish between natural forests and cultivated plantations or tree farms, subsuming both under a general category of "tree areas". It was difficult therefore to establish what proportion of forests or plantations were burned. But the plantations now formed more than half of Spain's 12.5 million acres of woodland, he reckons.
"In the 1960s plantations were introduced on a big scale to supply the cellulose industry, and 1975 - when these trees were fully grown - marks the start of an increase in forest fires and the areas burnt," he says.
The Agriculture Ministry issued a report in May which concluded that "forest fires are not motivated by the economic interests of those buying and selling timber". Cattle-raisers seeking to clear land for pasture are the main culprits, it says. But it recommends that chipboard manufacturers should not use burned wood and "a substantial part of the proceeds of the sale of burned wood must go to reafforestation of the affected area".