In the secluded woodlands on the slope of the Waianae mountain range, 66-year-old Perry Barker leads a group of seniors on a strenuous trail to replace invasive species with native plants as part of the state’s forest restoration project.

The group of about 10 hikers, ranging in age from 49 to mid-70s, work each week in the nearly 3,600-acre lowland Honouliuli Forest Reserve that is home to dozens of endangered species — more than half of which are only found in Hawaii.

Even with two recent knee replacements, Sandra Klein, 77, followed Barker through the rugged terrain earlier this month to sow 68 native plants including aalii and aweoweo. They are trying to eliminate hundreds of invasive species through their unofficial volunteer forestry group dubbed Senior Day Care. This year the group is planning to plant around 300 native species.

“There’s a special feeling to be able to put something back where it used to be. You look around and things have changed so much, not neccesarily for the better,” said Klein, who has been hiking for more than 50 years. “It kind of gives a peaceful feeling to be able to go up and into the mountains. It’s just us and the plants and nature. It’s beautiful up there. We’re seniors and we all have our various aches and ailments but you kind of forget about those things when you’re out there.”

Barker, who lives in Ewa, has been taking care of the forest, which includes cultural sites and unearthed trails, as well as part of the watershed that feeds the island’s largest drinking water supply. Working twice a week for the past decade, it has become so familiar to him that it’s like his home.

“It is my backyard. If I couldn’t have access here, I’d be a sad person because I love it up here,” said the retired Federal Aviation Administration electronics technician and manager who has been hiking Oahu’s forests since moving here from the mainland in 2006.

Even after having heart valve surgery at the Cleveland Clinic in Ohio in January 2016, he was back hiking within a month.

“Perry has a passion for it. He comes up here sometimes on his own,” said group member Lynette Williams, 68, who formerly worked as program coordinator for the reserve with The Nature Conservancy. “We see a need for this kind of work because these things are being endangered. We’re at a critical point where endangered animals and plants are dying on our watch. We’re not doing much but we can’t just stand by. We have to do something.”

Most of the group members are avid hikers who previously did the work as volunteers for The Nature Conservancy, which formerly managed the property before the state took over the land in 2010.

Many of them started clearing overgrown trees and shrubs with the Hawaiian Trail &Mountain Club and the Sierra Club of Hawaii, removing invasive species by cutting them down with hand saws and hatchets and applying herbicides to kill them.

Following the transfer of the land to the state, the retirees wanted to continue their preservation work in the forest, which is not open to the public.

“We’ve cleared trails so that they’ll be there if it is open to hiking in the future. Most of us were raised at least partially as children in rural environments — some of us were farmers,” said Barker, who was raised on a ranch in Colorado. “We’re a little bit crazy. We all enjoy being out in the forest and we like giving back to the land.”

The volunteers get help from the state with some tools and herbicides and water catchment systems so they can water the plants, but they don’t get paid for the work that begins at about 8 a.m. and continues until 3 p.m. every Thursday.

“It’s a whole day in the forest. Once we finish with our day’s work we go home and take a nap,” Barker said, adding that he also goes to Honouliuli on the weekends to scout things to do and plan for the following week’s activities. He works up to 12 hours a week in the forest.

Instead of spending his golden years in a more laid-back fashion, Barker finds meaning in toiling away at his favorite place.

“The pure enjoyment of watching the improvement to the forest and the plants that we plant seeing those grow and mature, it’s very rewarding,” he said.“But you have to be a special type of person, I know. Most people think of volunteering as taking care of someone. We’re different. You just really feel good when you feel like you can make a difference to the landscape, the plants and all of the forest. There’s always a sense of satisfaction.”