Disturbance & Succession Dramatic Change & Slow Recovery
Disturbance & Succession Objectives
- List examples of natural and human-created disturbances.
- Describe different ways trees can survive fires.
- Explain what succession is and provide examples.
Smokey Bear has appeared repeatedly in the course as an icon of reducing accidental human impact on ecosystems. This section will focus on sudden disturbances, including wildfires.
A disturbance is a rapid change in an ecosystem, and it can be natural or caused by human activity.
An example of a natural disturbance is the eruption of Mount Saint Helens in 1980.
Source: USGS Archives
An example of a human-induced disturbance is the construction of a dam.
Foster Reservoir created by the Foster Dam on the South Santiam River, Oregon
If you live in the Western United States you may have personally experienced the impacts of the numerous recent wildfires, particularly forest fires.
We took these photos in our yard in September 2020 at the middle of the day with no filter.
The temperature dropped from 93 degrees Fahrenheit to 71 degrees in a few hours, the color was due to the high quantity of particles in the air. The effects of the fire disturbance were just starting locally, over 45 miles from the edge of the nearest fire.
With a washcloth we quickly got an idea of how much was settling out of the air.
By nightfall, high winds were whipping particles through the air.
Air quality alerts became part of the weather forecast. Not only did this threaten human respiratory health, livestock, pets, and wildlife were at risk.
Ash covered our ponds and the goldfish were dead within a week, possibly because they were eating the large ash pieces like fish food.
Lower light levels, lower temperatures, and ash cover decreased plant yield in local gardens and delayed Willamette Valley harvest.
This is not even taking into account the areas directly impacted by the fires. The human and environmental cost have been significant. As of fall of 2021, many of the families who were displaced by the 2020 fires still do not have new homes. Road repairs and land clearing efforts have been gradual and costly. Many of the burnt habitats are not expected to fully recover this century.
Pine Fire Strategies
We will look at how coniferous tree species survive fires and than the process of succession that occurs after a fire. Forest fires clearly have a major impact on tree survival. This video compares the structures of Ponderosa Pine and Lodgepole Pine. How do their structures impact fire survival?
Unless controlled by humans, fires frequently burn through forests, degrading leaf litter, burning off lower branches, and thinning trees.
Small and frequent fires harm organisms but may overall be a benefit to the forest ecosystem. Small fires can remove the ground debris of leaves and branches, which can accumulate over time and lead to larger fires. Small burns break down materials into smaller form so they can more rapidly return to producers. Some pest populations, particularly invasive species, cane be reduced in size by frequent fires.
Some trees survive forest fires with thick fire-resistant bark and long needles that protect growth tips.
Ponderosa Pine is highly fire resistant.
Lodgepole Pine has thinner bark and shorter needles.
This species has serrotinous cones; cones that are closed and protecting the seeds inside, until the high heat of a passing fire opens them up. This way the seeds are dispersed immediately after a fire.
Succession
From earlier in the course, a community is made up of different s___s.
After a disturbance, many species are impacted, some more than others. The process of community building that follows is called succession. Succession refers to the change in a community over time following a disturbance.
Often after a disturbance, the land is scoured of life, or in the case of cooling lava, new ground is created. The conditions can be dry, windy, and have extreme temperatures. Little, if any, topsoil exists.
Some organisms can exist in these harsh conditions, including lichens. The initial organisms create microclimates for other organisms.
Wind-pollinated and wind-dispersed plants like fireweed can spread into new areas and grow in a range of conditions. Many plants with the common name “-weed” are capable of establishing in disturbed areas like roadsides or newly tilled garden soil.
The succession of species that followed the eruption of Mount Saint Helens has been extensively studied and you can learn more if you visit the monument.
Researchers have also been collecting information on smaller natural disturbances, like the creation of ponds that occurs when beavers build dams across a stream. We will have more on beavers and their ecosystem impact in Guide 7B.
If you want to see succession in action, and have an excuse to drive to the Cascades, check out the resource page.
This is the end of the Short-Term Guide. Material from this guide and corresponding guide, along with the previous Organism Care Guide are assessed on the weekly quiz.
Check your knowledge. Can you:
- list examples of natural and human-created disturbances?
- describe different ways trees can survive fires?
- explain what succession is and provide examples?
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Conditions
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