A new study from the research arm of the U.S. Forest Service suggests that leaving behind broken branches and the tips of treetops after logging can help fight invasive species.
Scientists suspected that fir boughs and other logging leftovers known as slash could act like gardener’s mulch and protect the soil.
Tim Harrington, a scientist with the Pacific Northwest Research Station, is part of a group that looks at the long-term effects of logging on soil productivity. Harrington set up a four-year experiment growing Douglas-fir seedlings under varying amounts of cover. He found that young firs grew best when about half of the soil was covered with old branches.
“It’s kind of like a no-till system. Basically, do not disturb the soil any more than you have to, and you will benefit the native vegetation. And you’ll also retain growing space for the seedlings of conifers that we’re trying to grow.”
The research was published in the Journal of Forest Ecology and Management. Harrington says the woody cap helped plant nutrients like carbon and nitrogen to accumulate in the soil. Another benefit: slash helped to protect the study plots from invasive species that often blow in on the wind and take over clear-cuts.
“Scotch broom, we found early in the study, that it was inhibited. Two herbaceous species, woodland groundsel and hairy cats ear, which is actually like a dandelion species, they are just not coming in because they don’t have the exposed mineral soil to colonize.”
Harrington says by limiting the growth of weeds and other smaller leafy plants, the woody debris also allowed more water to remain in the soil.
A Doug fir surrounded by woody debris in a test plot near Shelton, Wash.
So, could leaving behind slash help provide an alternative to spraying herbicides on clear-cuts to control weeds? That depends a little on your perspective.
“I think the slash retention is potentially a helpful thing,” says Scott Holub, a silviculture scientist who works for Weyerhaeuser.
But, Holub says, Harrington’s results showed that the native firs grew the most when slash and herbicides were used together to knock back weeds.
“Really the big driver we find is the weeds and when there’s more weeds there’s less tree growth.”
Holub says after logging, Weyerhauser leaves a significant amount of slash spread across its tree farms in Oregon and Washington. Groups that collect logging debris for biomass energy often only take the largest pieces.
Holob says Weyerhaeuser is conducting its own experiments to explore whether leaving behind branches and needles after clear-cuts will keep the soil healthier in the long run. On high quality soils, like those found in the Cascade foothills where Weyerhauser operates, Holub says so far his results have not shown much of an impact from slash on soil productivity.
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