Few meals are more American than a burger and fries, the combo McDonald’s made globally famous.

But few Americans probably realize how much the affordability of that classic meal depends on a free-trade agreement.

French fries from Canada. Tomatoes from Mexico. Beef sourced from a supply chain that crosses all three countries.

When it comes to dinner, there’s plenty at stake in the North American Free Trade Agreement, whose future was being negotiated at a sixth round of talks in Montreal last month.

Negotiators from Mexico, Canada and the United States expect to spend several more months working to revise the treaty after President Donald Trump repeatedly threatened to withdraw. NAFTA has allowed billions of dollars of agricultural commodities to travel each year among the three countries.

Without NAFTA, many economists say, the price of some consumer goods would probably go up.

Those include a handful of foods that could face tariff increases, supply chain disruptions and new protectionist trade barriers. While no one can predict exactly how these mechanisms would play out, they underscore the enormous complexity of the North American food system.

To understand the range of factors at play, consider the hamburger.

There’s a reason Canadian politicians have hyped the burger as an example of NAFTA’s universal benefits: Ground beef is the end product of a highly efficient, integrated international system.

U.S. farmers ship corn for cattle feed to Mexico and Canada. Mexico and Canada ship cattle — 1.7 million in 2016 — to the U.S. for slaughter. And the U.S. ships finished steaks and burgers back to its neighbors.

While the U.S. both produces and processes most of the beef in this system, the supply chain is highly specialized by country and attuned to changes across the market.

That’s why economists fear that if NAFTA ends, the system that gives us ground beef could be disrupted by tariffs.

Karen Hansen-Kuhn, the director for trade and global governance at the Institute for Agriculture and Trade Policy, points to one figure: Under its commitments to the World Trade Organization, Mexico can tax imported corn at rates of up to 37 percent — an increase that could echo throughout the system.

You can’t have a burger without fries, and Canada produces tons of them. While the U.S. grows and processes most of the potatoes it consumes, the country has grown dependent on its northern neighbor to back-fill the growing domestic market.

U.S. demand for fries is increasing, said Kevin MacIsaac, the head of the United Potato Growers of Canada.

At the same time, domestic potato production is down 2 percent, and processors have been hampered by the delayed opening of several new processing plants.

But Canadian fries may grow more expensive should NAFTA end and the U.S. falls back on the tariff rates it charges other members of the WTO. That rate is set at 8 percent for frozen potatoes.

Condiments could also face tariff increases.

When it comes to tomatoes, avocados and other fresh Mexican produce, the issues under NAFTA grow more complex.

While imported fruits and vegetables don’t face high tariffs in the U.S., there’s some anxiety among U.S. importers and Mexican farming groups that the country could impose new anti-dumping and countervailing duties on them.

These tariffs are meant to raise the price of imported foods that U.S. officials think are being sold below their fair-market value.

NAFTA includes special mechanisms for resolving anti-dumping conflicts and avoiding duties, said Cullen Hendrix, who heads the Project on Environment, Food and Conflict at the University of Denver — but without the agreement, Hendrix said, U.S. growers could push for measures that protect their crops against Mexican competition.

Ultimately, this could affect the price and availability of Mexican produce, especially during certain seasons.

“We are concerned and worried about any disruption to the supply chain,” said Ramon Paz-Vega, the chairman of Avocados from Mexico, a Mexican trade association. “We believe NAFTA is an exemplary example of the benefits of trade to everyone.”

Last but not least, think about the mushroom.

The U.S. grows most of the mushrooms it consumes, but Canada is also a major supplier of commodity mushrooms, such as buttons, creminis and portobellos. Should the U.S. withdraw from NAFTA, those mushrooms could become subject to a tariff of more than 20 percent.

“Nearly 15 percent of the United States’ fresh mushrooms come from Canada today,” said Aaron Hamer, the chief executive of Ontario-based Highline Mushrooms, the largest exporter to the U.S.

“With the addition of a tariff, we expect consumers would face higher prices and lower supply,” particularly for organic mushrooms, he said.

Like many in agriculture, Hamer is watching the NAFTA renegotiations closely; they could have a major impact on his business.

But he also thinks that the outcome of the talks will have effects for consumers, and he is hopeful that U.S. voters are beginning to realize the potential fallout of a withdrawal.

It’s not merely mushrooms, Hamer pointed out. Without NAFTA, the future of many supply chains could be uncertain.

“I can only hope,” he said, “that now that (consumers) see there are consequences that will negatively impact their everyday life, they are advocating for moderation.”