Faced with the worst summer fire season in 10 years, Gov. Jerry Brown is proposing broad new changes to California’s logging rules that would allow landowners to cut larger trees and build temporary roads without obtaining a permit as a way to thin more forests across the state.

The proposal — which has the support of the timber industry but is being opposed by more than a dozen environmental groups — would represent one of the largest changes to the state’s timber harvesting rules in the past 45 years.

The legislative session ends for the year next Friday. On Thursday, the details were still being negotiated by legislative leaders and the governor’s office behind the scenes and had not yet been formally introduced in a bill or put up for a vote.

“They are trying to get to some kind of a deal,” said Rich Gordon, the president of the California Forestry Association, a timber industry group. “They are looking at what can get done politically.”

Under Brown’s proposal, private landowners would be able to cut trees up to 36 inches in diameter — up from the current 26 inches — on property 300 acres or less without getting a timber harvest permit from the state, as long as their purpose was to thin forests to reduce fire risk. They also would be able to build roads of up to 600 feet long without getting a permit, as long as they repaired and replanted them.

Timber industry officials say the changes are needed to cut red tape and increase incentives for landowners, particularly in the Sierra Nevada, to thin pine and fir forests that have become dangerously overgrown after 100 years of fire fighting.

Before the Gold Rush in the 1850s, forests burned naturally every few decades in California from lightning strikes or Indian tribes’ burning. That cleared dead wood and left mostly larger, healthy trees. But without those fires for the past century or more, today’s forests are much more dense, with up to 10 times as many trees per acre in some places. The dense brush and increased numbers of small, spindly trees cause fires burn much hotter now and climb more easily into the tops of trees, creating massive blazes that burn for months.

Thinning forests often is a money loser, however, because taking out the small brush and diseased or insect-ridden trees costs money but brings little or no economic return. Allowing landowners to cut some larger healthy trees, which can be turned into lumber, provides them a return, supporters say.

“How do you get people to go in and take out stuff that has no value?” said Gordon. “If you allow them to get some value — to break even — on the cost of thinning, you can get more thinning projects done.”

Gordon also noted that under the governor’s proposal, a formula would remain in place that requires the overall diameter of trees in a forest to be larger after a thinning project than before.

“If you take out a 36-inch tree, you have to take out a heck of a lot more small trees in order to get the diameter to work in the equation,” he said. “If you take out a 26-inch tree you don’t have to take out as many smaller trees. The administration is arguing if we bump it up we’ll have more thinning.”

Environmentalists say they support some relaxation of the logging rules to make thinning overgrown forests easier to limit fire risk. But they worry that Brown’s proposed changes, which are expected to be introduced into a bill by a legislative deadline Tuesday, would allow loggers to cut large redwoods on the coast in wetter forests that don’t burn often, and other trees — some over 100 years old — without enough protections.

“We acknowledge there is a problem,” said Kim Delfino, California program director for Defenders of Wildlife, an environmental group. “The idea of trying to get a handle on it is a good thing. But this is an over reach. You don’t need to be putting such large trees on the chopping block.”

Delfino noted that scientific studies show that larger trees are the most fire resistant. She said the state should be offering more incentives, like using proceeds from its cap-and-trade auctions, to help landowners defray the costs of thinning projects.

Further, when landowners get a timber harvest permit to do logging on their property, which has been state law since the 1970s, they are required to draw up a plan that shows where they plan to cut, how many trees they want to remove, on what kinds of slopes and which species, like endangered salmon or other wildlife, will be affected. That plan is a public document and the subject of a public hearing.

Timber harvest permits, which can be hundreds of pages and cost tens of thousands of dollars for a professional forester to prepare, also must be approved by the state Department of Fish and Wildlife and the State Water Resources Control Board, who look at the logging impacts on erosion, streams and wildlife, and try to reduce the impact of logging on the environment.

The Brown administration is proposing that its changes to the law be inserted into AB 425, a bill written last year by Assemblywoman Anna Caballero, D-Salinas.

Caballero, whose district runs from Morgan Hill south to Big Sur, and includes Gilroy, Salinas and the Salinas Valley, said Thursday that while the governor’s proposal goes further than she originally intended with the bill — which proposed to ease road-building rules on forest thinning projects — she supports the intent.

“My interest is getting that dead fuel out of there and doing it in a way to generate some revenue and have people working,” she said. “I’m not interested in clear-cutting. This wouldn’t do that. There’s got to be a happy medium.”