A Mulroney-Reagan summit

THIS is an opportune moment - with a new year now barely under way - for the United States and Canada to improve their somewhat troubled political relationship. President Reagan is expected to visit Canada this spring for his third annual meeting with Prime Minister Brian Mulroney. The US, in particular, should be taking as many practical steps as it can to underscore the vital importance of its ties with its northern neighbor. During the past year the two sides found themselves squabbling over a number of trade issues: In May, the US imposed a stiff tariff on Canadian cedar and shakes. In October, the US imposed a 15 percent countervailing duty on Canadian softwood, subject to a final judgment at the end of the year.

The two sides finally worked out a joint agreement, by which Canada put a 15 percent export charge on the softwood. Prime Minister Mulroney is now coming under intense criticism within Canada for supposedly ``caving in'' to the US on the softwood lumber issue. Canada's Parliament, which reconvenes next week, will be asked to make the new tax retroactive to Jan. 8.

Finally, later in October, Canada imposed a countervailing duty of about 70 percent on US corn.

The economic difficulties between the two sides, of course, must be seen in perspective. Despite the occasional din over such issues as lumber exports, it is important to remember that the US and Canada remain each other's largest trading partners. Nor are political relations particularly strained, as they were between the two sides under Mr. Mulroney's predecessor, Prime Minister Pierre Elliott Trudeau.

Mulroney was in a sense going out on a political limb by seeking a closer rapport with the US and Mr. Reagan. So far, this has not brought him much reward. After all, here were two men from the ``conservative'' wings of their respective national political spectrums - each something of a political outsider, and both men coming from somewhat similar Irish backgrounds.

The two leaders - and their respective nations - could surely profit from improved ties. The Reagan White House, caught up now in the Iran-contra inquiry, needs to show positive results in the conduct of its foreign policy. In Canada, Mulroney is facing growing voter skepticism over the economy, which has been troubled by continuing high unemployment in the 10 percent range.

Both Washington and Ottawa, expecting another likely spring mini-summit, should now be seeking ways of improving their ties.

Two steps that would be mutually beneficial and are long overdue: an agreement controlling acid rain, and a US-Canadian free-trade pact.