PORTLAND, Ore. (AP) _ Louisiana-Pacific Corp. earnings increased 367 percent in the final three months of 1992, partly due to booming sales of a plywood substitute that doesn't require cutting down protected trees.

For the fourth quarter, earnings were $41.1 million, or 75 cents a share, on revenue of $528.7 million, compared with earnings of $8.8 million, or 16 cents a share, on revenue of $405.2 million in the same period in 1991.

Profits for all of 1992 increased 216 percent to $176.9 million, or $3.26 a share, from $55.9 million, or $1.03 a share, in 1991.

The Inner-Seal line of strand board products accounted for more than half of the company's operating profit in 1992, said Harry A. Merlo, Louisiana- Pacific's chairman and president.

Strand board, a plywood substitute, can be manufactured with trees of any diameter, bypassing the need for larger trees, which are increasingly protected by the government as a habitat for the northern spotted owl. The board is made by pressing wood wafers together.

Many of the products the company has developed over the past two decades have gained market share as dwindling timber reserves have forced cutbacks in production of conventional wood products and sent prices rising, Merlo said.

''This scenario is expected to continue,'' he said. ''Even under the most conservative economic forecasts for 1993, industry analysts foresee demand for construction panels exceeding supply.''

Louisiana-Pacific's stock closed at $70, up 25 cents on the New York Stock Exchange.