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WSU/Pend Oreille Extension introduced the Sense of Place series in 1999, with a focus on local landscape and natural history of Pend Oreille County, Washington. A partnership with the Kalispel Tribe of Indians Natural Resources Department has allowed us to expand this program through EPA funding to include more classes, a newsletter and this website.

Weeds and Disturbance

Disturbance is one of the constants of change in an ecosystem. Natural disturbances in our forest include fire, flood or wind-throw; human-caused disturbances include roads, trails, introduced stock and logging. Is there a difference in weed response to a human versus natural disturbance? To date, this is not an area that anyone has completed investigating.

Disturbances allow for shifts in availability of limiting resources, such as water, nutrients or sunlight. One of the advantages a weed has is a greater ability to garner these resources. When weeds are dependent upon disturbance, they prefer an intermediate level as too great of a disturbance often results in the loss of a limiting resource, such as nutrients. Weeds like a rich environment too.

Looking more specifically at disturbances in the forest, we find a tangled web of interrelationships. Our forest is prone to small mosaic disturbances such as wind-throw, disease or moderate frequency, low intensity fires. Fire suppression, practiced in the area over the past century, creates its own type of disturbance.

Other human caused disturbances, especially roads which slash the forest wide open, give weeds the opportunity to colonize into other areas that are disturbed, including from the road shoulder or other edges, even under an undisturbed canopy. Some weeds are better adapted to take advantage of these disturbances and pathways of introduction. Successful species include those that are shade tolerant, or that depend on birds for seed dispersal, or those with sticky seeds that are prone to catching in tire tread or on clothing or equipment.

Where weed seed is present, the disturbance of a fire may provide ideal conditions for rapid population expansion. Weeds may contribute to the intensity of a fire, and through fire, can alter the nature of the ecosystem.

Many of our invasive weeds evolved in areas with a high fire frequency and so although they may burn (some of them explosively) fire does not kill them. But their presence may change the nature of the fire. Along the coast of Southern Oregon in the 1980’s an infestation of gorse (Ulex europaeus) burned with great intensity, threatening a nearby town. Eucalyptus forms a high percentage of the urban forest in Berkley and has fueled at least three fires there. A study following the 1992 fire concluded that this exotic tree contributed 70% of the energy released from the vegetation that burned.

Most ecosystems that evolved with a high fire frequency lack fuels that would cause the fire to climb into a tree canopy. Invasive weeds can alter these conditions. In Florida, an exotic tall grass has invaded the open pine forest and has changed fire intensity, killing the recruitment saplings. Over time, as the mature trees die, this diverse forest ecosystem with all its attendant wildlife will convert to a grassland ecosystem.

Ecosystems that evolved with a low fire frequency, such as deserts, lack fire resistant species. These native arid species are widely spaced and so do not carry a fire easily. Much of our desert land has become invaded with annual grasses that fill the vacancies between the shrubs. They carry a fire quickly through these shrubs, often leaving them unable to regenerate. The grass seed bank allows quick regeneration, converting the shrub-steppe ecosystem to a grassland.

What implications are there for our forests and weeds? This remains to be seen as studies emerge specific to our ecosystem.

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