The travel forecast for 1991 is clouded by the gloomy threat of a full-fledged recession, rising fuel costs, the dollar's shrinking buying power abroad and fear of war in the Middle East.

Nevertheless, the travel industry appears to be smiling - if somewhat warily - in the face of adversity, in the belief that Americans will merely adjust their travel plans rather than eliminate them altogether.Some travel agents are reporting their clients are traveling less this winter than during the same period last year. And travelers who are planning winter vacations during 1991 are seeking to stretch their travel dollar as far as possible.

Interest in all-inclusive vacation packages, cruises, more domestic, rather than international, travel and trips to relatively inexpensive destinations, such as Mexico, were reported on the rise in a survey of 200 managers of American Express travel agencies taken in October. In the same survey, half the agents said travel was down from this time last year.

In a survey of 100 members of the American Society of Travel Agents (ASTA) conducted early in December, 45 percent pinpointed Mexico as offering the best vacation values for the winter season. The hottest destination in terms of bookings was Cancun, they reported. Good value also can be found on cruises, particularly in the Caribbean, and in Hawaii and Florida, the survey found. (ASTA is a trade organization of travel agents and suppliers.)

If people are, in fact, traveling less, the impact hasn't been felt in a major way, yet. The U.S. Travel Data Center, a travel-industry research group, reports that travel was up 3 percent in the first eight months of 1990 over the same period in 1989. However, those figures don't reflect the substantial increases in air fares of the last few months.

"We don't see the travel industry affected that much," said spokesman Scott Dring. "But that's not to say it won't be in coming months."

The dramatic growth in business travel by air leveled off in 1990 to just 2.5 percent over the previous year, according to a study conducted by the USTDC for Official Airline Guides. (OAG is a travel information company that maintains schedules and processes fares of the world's airlines.)

The survey also indicated a trend toward longer trips, as businesses attempt to control transportation costs. The number of overnight trips dropped from 15 percent to 12 percent, while trips of five nights and more increased from 18 percent to 23 percent. The percentage of trips of two to five nights remained virtually the same as last year.

The OAG study draws figures from the first seven months of 1990 - before the Iraqi invasion of Kuwait - and, therefore, does not reflect the impact of rising air fares on business travel. But it's almost certain companies will continue to seek ways to save money on travel in the coming year.

Air fares continue to be extremely volatile. Before the Iraqi invasion of Kuwait, OAG processed an average of 70,000 fare changes a day (out of about 10.8 million total fares). Since the invasion, OAG has been tracking about 1.5 million fare changes daily and sometimes as many as 2 million changes, said spokeswoman Sally McElwreath.

Discounted fares, which more than 80 percent of all passengers take advantage of, will continue to be offered as long as airlines are competing for passengers, she added. However, once regular, non-discounted fares go up, they never come down.

Developments in the Middle East also will have a bearing on the number of vacationers who take to the highways this year. Despite hikes in gasoline prices, the American Automobile Association (AAA) expects small increases in holiday car travel during the 1990 season over 1989.

Traditionally, in periods when auto travel has tapered off, it has been due to the threat of low supplies, not to higher prices, said Geoff Sundstrom of the AAA. "And, thus far, we have no indication we'll have (supply problems). As people are more concerned over higher fuel costs, they seem to be opting for automobile travel rather than air travel. With higher ticket prices, people are continuing to travel but they're looking for more economical means."

Year-end tourism tallies for California aren't available, but preliminary indications are that hotel occupancies and attendance at major attractions were down or flat in 1990, said Fred Sater, spokesman for the state office of tourism.

Economic woes are cited for the weakness in tourism in some areas of the state. Additional factors, such as pollution and crime, further weakened tourism in Southern California.

"In light of the economic situation here, we'll probably see more in-state travel, people doing more in their own back yards," Sater said of the coming year.

Travel experts believe that although an economic slowdown might mean a change in the way Americans travel, it won't mean an end to traveling.

"My advance bookings are not showing there's a recession coming. I hope it continues," said Sheila Hyman, owner of a travel agency in San Bruno. "Travel is no longer a luxury. It's a necessity."

The sentiment is echoed by Diane Polomski, assistant manager for public affairs at American Express Travel Related Services.

"I think people have started to budget for travel," she said. "Even in hard times, (they) want to build in some vacation. It's the one thing they can look forward to."

Travelers may be forced to change their habits, however. Taking shorter trips, heading to destinations closer to home and combining business and pleasure trips are other trends likely to surface in shaky economic times.

At Martins Travel and Tours and CruiseMasters travel agencies in West Los Angeles, agents are seeing a record number of last-minute bookings. Susan Tanzman-Kaplan, co-owner of the agencies, surmises that's because travelers are "waiting to see how much they've been hit. On the West Coast, I don't think we've been hit like they have on the East Coast."

Cruises are becoming more popular because costs are fixed, there's a wide range of price and destination, and they free travelers from worry about unstable political situations. Almost half the American Express travel agents surveyed reported an increase in 1990-91 cruise bookings over last year.

The top five U.S. destinations for winter vacationers, cited in the American Express study, were Orlando, Fla., Los Angeles, San Francisco, New York and Honolulu.

Top foreign destinations in the study were the Caribbean, Mexico, England, Germany and France.

Tanzman-Kaplan points to Canada "as a tremendous buy for Americans," despite the new Goods & Services Tax, which goes into effect Tuesday.

Although some U.S.-based tour operators have expressed fears that the 7 percent tax will make Canadian travel too expensive for Americans, other travel agents believe the country will remain a good travel value.

The same cannot be said for Europe, where the value of the U.S. dollar fell last summer between 17 percent and 25 percent. U.S. travelers to Europe returned with tales of outrageous prices, even in countries formerly regarded as economical for American vacationers.

Though final figures aren't yet available, the European Travel Commission projects there were 7.3 million U.S.-citizen visits to Europe during 1990, making it the third consecutive record year. A commission spokesman said they foresee American tourism to remain about the same in 1991, despite the decreased buying power of the dollar.

"So far, I don't see them coming out of the woodwork," one agent said of clients making European bookings for 1991. "The dollar is terrible, prices there are outlandish, so people are taking a wait-and-see attitude."

Travelers planning a trip to Europe can reduce costs by taking advantage of package deals and lower, off-season air fares, and by staying away from large cities and visiting less expensive outlying areas, instead.