Luc Forsyth can’t decide which memories best illustrate the glorious hardship of his six seasons as a tree planter in Canada.

Maybe it was his first day, when his crew leader told him to “deal with it” after hours of work in new boots had shredded his feet. Maybe it was years later, when his hands were so stiff after seemingly endless days of cold rain that he urinated on them in a desperate attempt to defrost himself.

Or perhaps it was the many raucous nights in the nearest logging town, shooting pool and doing laundry with the rest of the planting crew, a group of strangers bonded by adversity. (Here are five ways people are using trees to save the world.)

“To this day, it’s been the best and worst job I’ve ever had,” Forsyth says.

Though he now lives in Mexico City working as a photographer and documentarian, Forsyth was once among the estimated 6,000 workers across Canada’s vast timberlands who plant around half a billion trees each year. Nearly half of these trees are planted in British Columbia and Alberta, where lumber companies harvest large tracts of forest for wood pulp and construction material.

More than 90 percent of Canada’s forests are publicly owned, and forestry laws requiring replanting were enacted in the mid-1980s out of concern that harvest practices at the time were unsustainable. Laws vary from province to province; in British Columbia, for example, three seedlings must be planted for each mature stem harvested.

Reforesting the Boreal Filmmaker and photographer Luc Forsyth tells the story of how reforestation is happening in parts of Canada through the tireless hard work of young tree planters.

The preferred harvesting method is clear-cutting in staggered blocks—an approach Canadian forestry researchers say emulates natural large-scale disturbances caused by fire, floods, and insects. For example, insects damaged 43.4 million acres of forest in 2015, while timber harvests claimed just 1.9 million acres.

Replanted and naturally regenerated forests are left undisturbed for 60 to 120 years, or until the trees are mature and ready to be harvested again. In recent years, management plans have also had to consider impacts on streams, soil, air quality, wildlife, and the water table.

To comply with the law, harvesters contract with companies that specialize in a unique form of seasonal labor: summer planting camps.

“With all the technology we have, there still isn’t a more effective way to do this job than by giving a 22-year-old a shovel,” Forsyth says. (This teenager is on track to plant a trillion trees.)

In 2016, he returned to a planting camp in Alberta—but with a camera as his tool for documenting the lives of Canadian tree planters. Uniformly young, planters are mostly between the ages of 19 and 26, and they come from as far away as Australia to test their mental and physical fortitude, and to earn some cash.

View Images A tree planter is pelted by hail during a mid-June storm in southern Alberta. Planters work in all weather conditions except direct lighting, including rain, snow, and hail. Photograph by Luc Forsyth

Planters pay for all their own supplies: tent, shovel, sleeping bag, and travel to the remote bushlands where the camps are located. During the day, in all weather except lightning storms, they lug 40 to 50 pounds of seedlings and gear over barren, broken landscapes.

Pay is by the tree—the average is around 10 cents per plant—and competition is fierce. Skilled planters can drop more than 7,000 seedlings in a day, earning hundreds of dollars a day over the four-month planting season. Women make up about half of the seasonal workforce, and they are often among the top planters in the camp groups. (In 2016, people in India planted 50 million trees in a single day.)

For his part, Forsyth says he planted 8,100 trees on his best day, and he estimates that he placed half a million in total. He graduated college debt-free thanks to his summers laboring in the clear-cut forests.

The mix of species planted depends on the region and microclimate, but it generally reflects the previous composition of the forest before it was cut. Regional replanting can include up to 19 native species, from white spruce and jack pine to hemlock and Douglas fir. In some areas, large swaths of monoculture exist naturally and are replanted as such. (A 1,230-year-old pine found in Italy is now Europe’s oldest tree.)

View Images A planter holds a butterfly in his palm during a short break. Photograph by Luc Forsyth

Arriving at camp in good physical condition helps get you over the initial suffering (there are even training and nutrition guides), but being a buffed-up gym rat is no guarantee of success: The strain is more of a mental thing than a physical one.

“You take a 20-year-old economics student from Toronto and throw them into that environment, it either makes them or breaks them,” Forsyth says.

Planters like to equate the difficulty of their jobs to running a marathon, but one study found it’s actually harder: Elite marathoners have a higher working heart rate while running a standard race of 26.2 miles, but planters exert more effort overall—they’re maintaining elevated heart rates for nine hours a day, five to six days a week, for 16 weeks.

“Running a marathon requires about 2,300 – 2,400 calories, while the amount of energy a tree planter goes through in a day is slightly more than double that,” the study notes.

View Images A foreman walks down a muddy logging road as she checks on her planters during a heavy storm. Foremen are responsible for making sure their planters have everything they need to plant trees efficiently and continuously, and a good foreman is constantly on the move. Photograph by Luc Forsyth

So why do it, aside from the pay? While it’s well known as a “cool summer job” in Canada, it’s not necessarily viewed as an environmental one, Forsyth says. Instead, in an on-demand internet age, the sense of accomplishment at the end of a grueling season can be addictive.

“Planting is for people who want to live in a different way, who thrive on pushing themselves in unusual ways. The day-to-day is miserable, but the whole is special,” he says.