Thursday, June 5, 2014

Fire-enhanced Forests

Wildfires are a fact of life for many western landscapes.  During summer there is increased fire risk as forests and grasslands dry out. Yet as western coastal communities established more than a century ago, one of the characteristic changes was to suppress both wildfires and human-caused fires. This is critical for property protection. But it’s bad for plant communities that evolved with fire, including some conifer forests, shrub communities like chaparral, and grasslands. Now, for more than a century, fuels have built up in many natural landscapes instead of regularly burning. 

Most of our local conifers are not fire-adapted. Only Douglas-fir has thick bark, evolved to withstand fire and protect delicate cambium layers beneath. Western hemlock, western red cedar, and Sitka spruce are thin-barked species; their bark burns easily, and cambium layers beneath the bark die quickly, killing the trees even in moderately hot fires. 

Shore pine, Pinus contorta var. contorta, is a coastal cousin to lodgepole pine, Pinus contorta var. latifolia. This pine can withstand wildfire only as a large tree. It also has a mix of cones, some which open and shed seeds quickly, and some that stay tightly closed for years, and open to shed seeds after a fire. 

Lodgepole and shore pine both grow in very dense stands when young; these are called doghair stands, where the young trees ‘grow as dense as dog hair.’ In natural self-maintaining ecosystems, fires come through regularly every few years and burn out the accumulated litter, kill small to medium sized trees that are too close together, and leave larger, well spaced trees.  Likewise, chaparral shrubs like manzanita and coyote-bush burn to the ground and then resprout in following years. These fires typically move quickly over the landscape, pruning rather than clear-cutting. 

Regular small fires clear spaces between large trees and make the forest understory open and easy to walk through.  A regularly burned pine stand looks like a city park, with well-spaced large trees, a few shrubs, and turf. Once a fire burns through a stand of trees and opens it up, it takes a few decades for burnable materials to build up and support more than a cool, fast-moving fire. It’s reasonably fire-resistant in the meantime. 

A too-dense stand, on the other hand, burns hot and completely, from the crown of the trees to the ground. Wood logs and branches on the ground ignite, limbs and shrubs carry fire into tree canopies, fire moves from canopy to canopy as well as across the ground, and the result is a hot fire that kills the trees and leaves only ashes.  These stands do not recover. They start over with new seedlings, and for decades are more vulnerable to catastrophic fires, because they no longer have sturdy old trees to weather fires, provide seeds, and offer wildlife habitat. It’s not uncommon for dense pine stands to burn down again and again after a catastrophic fire. 

We live north of the forests that are dominated by fire-resistant trees and shrubs. In our area, when fires burn from dunes into forests, the forests generally burn down. The risky time for our community each year is summer, the dry season.  The solution for homeowners is regular fuel reduction, with thinning, limbing and wide spacing of remaining trees. 

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