Bigger and Older Often Means Better Habitat

The Importance of the Individual Legacy Old-growth Tree in the Maintenance of Biodiversity in Commercial Redwood Forests

Photo by Julie Martin
Photo by Julie Martin

Traditionally we think of forest conservation as protection of large areas of land. Is it possible, though, that just one tree could benefit an ecosystem enough to warrant individual protection? Mary Jo Mazurek and William Zielinski report evidence that suggests legacy old-growth redwoods can do just that.

The term “legacy” is applied to old-growth trees that have, for one reason or another, been spared during timber harvest, or have survived stand-replacing natural disturbances such as wildfires. They generally exist amongst smaller trees in managed forests.

In order to better understand what effect legacy redwood trees have on the ecosystems they tower over, Mazurek and Zielinski compared bat, bird, large mammal, and small mammal use of 30 legacy trees to 30 comparable non-legacy trees. They found that a substantially higher diversity and abundance of animals use legacy trees for foraging, breeding, and shelter.

Mazurek and Zielinski attribute this finding to the complex structure of bigger, older trees, which provides essential habitat for the organisms in managed forests. The deeper-ridged bark of legacy trees, for example, harbors a more profitable supply of insects for bats and birds.

Especially important are the rare legacy trees with hollow spaces (known as goosepens) at their bases. The hollows occur only in older trees that have been exposed to unrestricted, repeated natural wildfires. They seem to be the feature that adds the greatest habitat value to legacy trees. Proof of this is the bat guano found on the floor of every study tree’s basal hollow. The whitewash trail bats leave behind tells the researchers not only that hollows benefit organisms by providing shelter, but they benefit the trees in which they are found by attracting animals that richen the tree’s soil with their nutritious feces!

Mazurek and Zielinski feel strongly that given the dwindled existence of old-growth trees and the fragmented nature of mature forests in the redwood region, individual legacy trees may function as ‘mini-reserves’ that promote species conservation. Finding and protecting these legacy trees, reserving young stands to be excluded from harvest in perpetuity, and prescribing repeatedly applied fires to trees that would be set aside for “future legacies” are high priorities for conservation. Mazurek and Zielinski do not believe that any one tree will protect a species, but they are confident that the cumulative effects of preserving grandfather (and grandmother) trees will lead to healthier populations of species in the ecosystems around them.

Mazurek and Zielinski’s report was published in the journal Forest Ecology and Management under the title “The Importance of the Individual Legacy Old-growth Tree in the Maintenance of Biodiversity in Commercial Redwood Forests”

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