Monterey Bay reserve may be model for how to replace trees

Fresh tree stumps the size of dining tables line the road at the Elkhorn Slough Reserve, the remains of hulking eucalyptus trees that blanketed the ground in a thick layer of aromatic wood chips. Nothing grows in the wreckage, but about 40 yards away, a grove of young oak trees flourishes.

Mary Paul, a stewardship associate at the reserve just east of Moss Landing at the heart of the Monterey Bay, revels in the contrast. Where the oaks now stand, staff removed a 13-acre grove of eucalyptus in the 1990s and planted well over 10,000 acorns. In a few decades, the roadside site where eucalyptus trees were cut down this summer should match the restored oak savanna beside it.

Paul hopes the restoration efforts at Elkhorn Slough will help quell public opposition to the removal of the eucalyptus — an invasive tree native to Australia that has become a California icon, and a shelter for migrating birds and Monarch butterflies, but must be removed because they crowd out native species, increase fire risk and guzzle the state’s scarce water.

“It’s not just about clear-cutting all the eucalyptus within the watershed, or even within the reserve — it’s about creating healthy ecosystems and healthy balance,” Paul said. “We are trying to balance everything.”

Mary Paul, stewardship associate at Elkhorn Slough Reserve, explains how the oak behind her grew tall and spindly to compete for sunlight with eucalyptus trees that have since been cut to stumps. (Photo by Alix Soliman) 

Returning eucalyptus-choked landscapes to their original glory is a long and arduous process. It involves coaxing the barren earth back to life and dealing with thousands of tons of highly flammable wood.

Also called blue gum, eucalyptus is a fast-growing tree native to Tasmania, an island state south of the Australian mainland. During the California Gold Rush of the 1800s, they were planted for windbreaks, fuel and timber. But when logs came out of the mill, the lumber was of poor quality, and many plantations were abandoned. Thousands of acres are left standing today.

At the Elkhorn Slough Reserve — home to a vast array of birds, marine mammals, amphibians and fish — thirsty eucalyptus trees leave the wetlands more vulnerable to drought and problems with water quality. To save this vital habitat, the reserve has made removing the trees a priority in areas where they are doing harm.

But some community members have objected. In 2015, when the Elkhorn Slough Reserve proposed chopping down more than 1,200 trees across 50 acres by 2025, staff received angry comments from locals, many concerned that important Monarch butterfly and bird habitat would disappear.

Similar objections have erupted around felling other eucalyptus groves, such as the removal of 31 trees on private land around Rodeo Creek in Santa Cruz, which was just completed. “I had no idea how much wildlife lived in the eucalyptus before I moved here,” said Santa Cruz resident Rebecca Wyatt, who opposed the move. “In the evening time before the sun goes down, it’s just magical — hundreds and hundreds of birds are diving in and out … but now, they’re homeless. It’s just heartbreaking.”

Monarch butterflies group together for warmth and safety in a eucalyptus grove at Lighthouse Field State Beach in Santa Cruz in 2022. The monarchs have returned to the area during their migratory journey between California's central coast and Mexico. (Photo by Shmuel Thaler, Santa Cruz Sentinel)
Monarch butterflies group together for warmth and safety in a eucalyptus grove at Lighthouse Field State Beach in Santa Cruz in 2022. The monarchs have returned to the area during their migratory journey between California’s central coast and Mexico. (Photo by Shmuel Thaler, Santa Cruz Sentinel) 

Monarch butterflies migrating from the Rocky Mountains to California’s Central Coast every winter huddle in some eucalyptus groves for warmth. The reserve is also an important spot along the Pacific Flyway, a major bird migration route, where seabirds nest in eucalyptus groves and fatten up in the rich estuary habitat.

“One of the ways that we addressed that concern was pointing out that we were going to leave active bird nesting and butterfly nesting habitat intact, in a part of the reserve where there’s a huge grove of eucalyptus trees,” Paul said, pointing to a dense stand in the distance, beside two stacks of the Moss Landing Power Plant. “We’re not going to remove that at all, ever.”

At another restoration site where eucalyptus trees were downed in August 2022, native plants sprout from holes in a foot-thick layer of eucalyptus wood chips. The scene challenges a common belief that eucalyptus is “allelopathic,” meaning it releases chemicals in the soil that make it difficult for other plants to grow. Scientists disagree on this, however, and no previous studies have looked at planting in eucalyptus mulch as Paul is doing on the reserve’s restoration sites.

The thick layer of mulch blocks sunlight from reaching seeds of invasive weeds such as Italian thistle, poison hemlock and Cape ivy that would otherwise quickly take over.

“Our personal experience on the reserve was that we did not see any effects of allelopathy on our native plants when we planted them with wood chips,” Paul said.

To encourage only native plants to grow, Paul and her team dug pits into the eucalyptus mulch down to the soil layer and planted species grown in the reserve’s native nursery, including California goldenrod and creeping wild rye. The reserve plans to create a pond at the end of the catchment for endangered California red-legged frogs and California tiger salamanders, which rely on bodies of freshwater in the winter to breed.

“There was a historic stream that was present on this site that hadn’t really been seen very much when the eucalyptus were here,” Paul said. After the trees were felled, the stream resurfaced during last winter’s storms.

Even after creating wood chip mulch to help with restoration projects, thousands of tons of wood can be left behind after a eucalyptus grove is cut down. So Paul and her team have experimented with turning it into charcoal.

After felling their first grove in 2018, Elkhorn Slough Reserve staff and volunteers chopped up the trunks and larger branches and built dozens of 4-foot diameter burn piles, lighting them from the top-down to slowly “pyrolize” the wood into charcoal or “biochar”  — rather than burning it from the bottom-up into ash, which would release much more carbon into the air. They then buried some of the charcoal as part of an effort to restore a salt marsh, locking up the carbon the eucalyptus trees had sucked from the air over their lifetime.

But this manual approach to making charcoal proved impractical. “That was such a labor-intensive process that we ended up stopping because we just didn’t have the capacity to do it anymore,” Paul said.

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