Urban forests are facing health threats

Illegal immigrants are destroying our forests. Ok, now that I have your attention, I should clarify that the immigrants in question, are not of the two-legged variety. I’m talking about invasive nonnative plants. These seedy foreign invaders are among a host of pressures threatening urban forests in Federal Way and around the Puget Sound region.

It used to be thought, that forests within our cities could be protected by preserving the property they stood on. That notion has been rubbished in recent years. Communities around the region are coming to grips with the fact that urban forests are under assault from invasive species, fragmentation and other facts of modern, urban life.

Even those forests that have been preserved, like our own West Hylebos Wetlands, are faced with forest health problems that, if left untreated, could destroy them. It turns out that life just ain’t easy for a tree in the city.

In today’s column, I’ll address the threats to urban forests. Next week, I’ll wrap up the discussion by describing what’s being done to protect the forest at the West Hylebos Wetlands Park.

Urban forests are only a remnant of a vast forest system that covered the region. Prior to the era of widespread urban growth, these forests were self-sustaining. As trees aged and then died, they were replaced by seedlings that filled in the gap created by the fallen tree. This cycle of regeneration has been broken in urban areas.

In many forests found within Puget Sound cities, invasive nonnative plants have become established. These seedy foreign invaders out-compete native tree seedlings and prevent them from taking their rightful place in the chain of forest succession. That means that when the older trees fall, what replaces them will not be fresh new hemlocks and Douglas-firs, but a monoculture of Himalayan blackberry, bitter nightshade or some other leafy immigrant.

In the Puget Sound Region, Seattle has led the way in studying and addressing urban forest health. Seattle foresters found that the city’s 3,200 acres of forestland are dominated by older trees nearing the end of their lives. With invasive plants filling in the forest understory, it’s estimated that approximately 70 percent of Seattle’s forests will become “ecological deserts” within two decades. As invasive weed monocultures become the dominant habitat, native birds and mammals will be forced out.

While some invasive plants short circuit forest succession, English ivy and other aggressive vines directly attack and kill trees. Introduced by nurseries and gardeners, English ivy escaped from yards years ago and has aggressively spread through urban forests.

This woody vine grows aggressively, covering the forest floor and climbing into the canopy of native trees. The vines steal the trees’ sunlight and nutrients, strangling and killing the tree over several years. Trees weakened by years of ivy strangulation often collapse under the weight of the foliage. Serious English ivy infestations can deforest an urban forest like a slow motion clearcut.

English ivy is a huge problem in Seattle’s forests. We don’t see much of it in the Hylebos Watershed, at least not yet. Other areas of Federal Way, particularly along the shoreline, are already in advanced stages of English ivy infestations.

Urban forests also suffer from fragmentation caused by development. As privately owned forests are converted to housing or commercial uses, the adjacent publicly owned forests are more exposed to damage from wind and urban air pollution. The remaining forests also become less hospitable to birds and other native species, as their habitat shrinks and becomes increasingly surrounded by roads and homes.

Decline of our urban forests is not inevitable. In fact, the techniques for ensuring forest health are well known and technically easy to implement. Starting with Seattle in 2003, a growing number of Puget Sound cities have been developing 20-year forest health plans to assess the health of their forests, and map out plans for getting control of the invasives and replanting native forests.

Here in Federal Way, we have our own mini-forest health plan, developed specifically for the crown jewel of Federal Way forests, the West Hylebos Wetlands. In the next column, I’ll delve into the details of that plan as an example of what it takes to protect our urban forests.

Chris Carrel is a Federal Way resident and executive director of the Friends of the Hylebos. Contact: chinook@hylebos.org.

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