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More than half of the fresh fruit and almost a third of the fresh vegetables Americans buy now come from other countries. CreditJens Mortensen for The New York Times; Styled by Mariana Vera

It’s obvious to anyone who visits an American supermarket in winter — past displays brimming with Chilean grapes, Mexican berries and Vietnamese dragon fruit — that foreign farms supply much of our produce.

Imports have increased steadily for decades, but the extent of the change may be surprising: More than half of the fresh fruit and almost a third of the fresh vegetables Americans buy now come from other countries.

Although local, seasonal and farm-to-table are watchwords for many consumers, globalization has triumphed in the produce aisle. And despite the protectionist “America First” message coming from the Trump administration, the growth in imports appears likely to continue.

So this is an apt moment to examine how the shift happened, and what it portends — good or ill — for American consumers and farmers.

“I had no idea that more than half our fruit is imported, and it shocks me that this has happened so quickly,” said Michael Pollan, a professor of journalism at the University of California, Berkeley, whose best-selling books have analyzed the tensions between local and global food systems.

The surge in imports, mostly from Latin America and Canada, flows from many other changes during the last 40 years, starting with improvements in roads, containerized shipping and storage technology. Horticulturists developed varieties and growing practices adapted to warmer climates — enabling, say, blueberries and blackberries to be grown in central Mexico.

Growth in American incomes spurred greater demand for fresh produce year-round. Immigrants brought tastes for the foods of their homelands, and in some cases (like avocados and mangoes) these tastes have became mainstream. Foreign growers took advantage of lower labor costs. International trade agreements reduced tariffs and other obstacles to imports, while many American farmers, facing regulatory hurdles at home, have responded by shifting production abroad, mainly to Mexico.

One crucial part of the story is little known: Over the past two decades, the United States Department of Agriculture has issued roughly 100 new rules allowing specific crops to be imported from certain countries — like peppers from Peru. Crops that previously would have not been approved because they might introduce invasive pests and diseases were allowed in through new “systems approaches” that manage those risks by combining methods like orchard inspections, sprays and bagging of fruits.

Many foreign crops have recently been approved for importation using these protocols, including Chinese apples and Colombian avocados. Some are still in the rule-making process (Chinese citrus, European apples), and others are under study (Brazilian citrus, Mexican guavas).

As a result, the proportion of the imported fresh fruit eaten in the United States rose to 53.1 percent in 2016, from 23 percent in 1975, according to the Agriculture Department’s Economic Research Service. Fresh vegetable imports rose to 31.1 percent from 5.8 percent. (Still, the United States remains a net agricultural exporter, with grains, soybeans, meat and nuts accounting for most of the trade surplus.)

Greater availability has led to a huge increase in per-capita consumption of many crops, including mangoes (up 1,850 percent from 1975 to 2016), limes, avocados, grapes, asparagus, artichokes and squash. Yet consumption has fallen for other crops — like peaches, oranges, cabbages and celery — that are still primarily grown in America.

For consumers, the chief advantages of the import boom are the increased availability and variety of fresh produce, particularly in winter, when imported berries, grapes and stone fruit now compete with citrus and stored apples.

“It’s easy to criticize food that comes from far away,” Mr. Pollan said. “But if the question is whether this is good for your health or not, in general it is.”

Many imports cost less than domestically grown equivalents, and competition from imports keeps prices down for domestic produce.

Imported produce is also sometimes fresher than the domestic equivalent. In spring, newly harvested Gala apples from New Zealand may be crunchier than the same variety from American orchards, which were picked the previous fall. And some imports are simply superb, like flavorful pink seedless muscat grapes from Chile, now in season.

But unlike imported furniture or washing machines, produce is perishable and may suffer from transport. It may be picked less ripe. Varieties may be selected for durability at the expense of flavor, and treatments mandated to kill pests (hot water for mangoes, cold temperatures for citrus) can degrade flavor or texture.

In many fruits, acidity drops over time, and off flavors develop; weeks-old cherries, for example, may still look fine but taste flat. Vegetables, too, can decline. Domestic asparagus, grown mostly in California, Michigan and Washington, tends to be plumper, juicier and more flavorful than the more fibrous and rubbery imports from Mexico and Peru.

It might seem logical that older produce is also less nutritious, and for some compounds such as vitamin C, levels do decline with time. But there does not appear to be any evidence that the overall nutrient content degrades significantly. From a public health standpoint, the benefits of increased availability and consumption of imported produce outweigh any such worries, nutritionists say.

It might also seem that imported fruits and vegetables are more likely than domestic produce to cause food-borne illness, but there’s no evidence that this is so. “I don’t think that produce grown outside the United States is less safe,” said Bill Marler, a lawyer in Seattle who often represents consumers in food-borne illness cases.

Of some concern is a 2015 report from the Food and Drug Administration that found that 9.4 percent of imported fruit samples violated federal standards for pesticide residues, compared with 2.2 percent of domestic samples. (For vegetables, the figures were 9.7 percent for imported and 3.8 percent for domestic.) But that’s probably not enough to justify avoiding imported produce.

“‘Eat your veggies’ is good advice no matter what,” said Marion Nestle, a professor emerita of nutrition, food studies and public health at New York University. “The benefits of plant-based diets are better established than the harms of pesticides.”

Reports of fraud from countries like China and Costa Ricahave raised a concern: Is imported produce labeled organic as reliably free of pesticide residues as domestic equivalents?

No published scientific studies have directly addressed that topic. Jessica Shade, director of science programs at the Organic Center, a nonprofit organization that provides information about organic food and farming, said all certified organic produce, foreign or domestic, is subject to residue testing and on-site farm inspections by certifiers.

Even so, Mr. Pollan said, “I generally have more confidence in domestic than imported organic.”

Then there are environmental issues: Because imported fruits and vegetables typically travel farther than domestic produce from farm to table, they cause greater harm from carbon emissions and pollution. That’s especially true for produce arriving by air, which is likely to be fresher and cost more than produce arriving by ship.

But transport miles are just one component of environmental costs, and in some cases fruits and vegetables grown in a suitable climate overseas may require fewer resources for farming and be more generally sustainable than off-season domestic produce — cultivated, say, in heated greenhouses.

Drug cartels looking to launder money and diversify their operations control or extort some of the production and packing of certain export crops from Mexico (the largest exporter of produce to the United States), including avocados, mangoes and limes. Growers and militias have fought back, but the implications of cartel control for the quality, cost and healthfulness of the produce remain unclear.

For American farmers, too, imports have had mixed consequences. The increased international trade in produce has benefited many of them (including growers of Northwestern apples and California citrus) but harmed others (producers of Florida tomatoes and California asparagus).

Most growers’ organizations maintain that trade accords like the North American Free Trade Agreement have helped American produce farmers on balance.

“Nafta over all has been positive, and we oppose U.S. withdrawal from the agreement,” said Ken Gilliland, director of international trade for Western Growers, which represents produce farmers in Arizona, California, Colorado and New Mexico. “Eliminating Nafta and implementing tariffs would have a negative impact on our members’ ability to export.”

Most of the advantage from exports, however, has gone to large growers.

“Clearly the larger shippers have benefited more from the globalization of produce,” said David Runsten, policy director of the Community Alliance With Family Farmers, a California group that advocates for small farms. “Smaller organic produce growers in California are feeling the effects of increasing imports.”

The Trump administration’s crackdown on illegal immigration is likely to worsen the shortage and high cost of labor, a serious threat for many American farmers. “The United States needs immigration reform and a stable labor force if it wants to continue to grow most of its own produce,” Mr. Runsten said.

An onslaught of invasive pests and diseases such as spotted wing drosophilaand citrus greening has cost farmers, and ultimately consumers, many billions of dollars for control measures and lost crops. Did any of those plagues arrive with commercially imported produce, or, as is generally thought, did they come in through other pathways such as smuggling of produce and plants by individuals?

“It is difficult to conclusively determine whether a pest introduction came through a regulated or unregulated route,” Philip Berger, executive director of the Agriculture Department’s Center for Plant Health Science and Technology, wrote in an email. “Compared to other possible pathways of introduction, U.S.D.A.-regulated imports of produce for food use have a lower risk of introducing new pests.”

Increasingly, however, scientific studies have found that trade agreements and resulting increased produce shipments may have contributed to the movement of invasive species into the United States. So far no one has fully connected the dots, or estimated the costs for American farmers of pests and diseases attributable to imported produce.

“It’s not clear that our investment in inspection resources has kept pace with the increase in trade,” said Michael R. Springborn, an associate professor of environmental science and policy at the University of California, Davis, who studies invasive species.

Whatever the drawbacks or advantages, imports are likely to continue growing. According to a recent Agriculture Department report, fresh produce imports will rise 45 percent from 2016 to 2027, implying that a decade from now, three-quarters of our fruits and almost half of our vegetables will be imported.

In other words, we could end up getting our produce as we do our fish — more than 80 percent of which is imported.

“If we can’t grow the products here, we’re going to have to import them,” said Kathy Means, vice president of industry relations for the Produce Marketing Association, which represents both supermarkets and growers. “I’m not uncomfortable with that at all.”

The federal government is focused on the economics. Jason Hafemeister, trade counsel to the secretary of agriculture, said in an interview that increased trade has generally benefited farmers and consumers. The Trump administration seeks to ensure a level playing field in trade for American farmers, he added, but wants to let consumers choose what matters to them in produce, whether price, freshness or origin.

Mr. Pollan, not surprisingly, does worry as imports climb. “I think it would be a tremendous loss if we weren’t growing a significant percentage of our produce, for reasons having to do both with quality, and with the knowledge of the environment that farmers bring to a society,” he said.

Consumers who agree can vote with their dollars by prizing local and domestic produce when available, and staying alert for decreased quality in less-fresh imports. The next decade or two will determine whether Americans can enjoy the advantages of an increasingly globalized supply without losing the very real benefits of domestic produce.

“Don’t underestimate the ritual of eating seasonally, the pleasure one can have as fruit comes into the market,” Mr. Pollan said.

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