By Dr. David Wartinbee, for the Redoubt Reporter
A couple weeks ago I happened upon some aerial photographs I took right after a major portion of the Caribou Hills burned. Many folks lost their cabins to the fire and everyone suffered a drastic scenery change. This weekend, while out in the Caribou Hills, I remembered that a large portion of the fire was in the Deep Creek watershed. That caused me to revisit some of the stream studies that have documented fire-caused changes.
Riparian areas around streams produce leaves, twigs, bud scales, bark, pollen and seeds that fall into the nearby stream. These vegetable materials often are a major energy source for organisms found within the stream. Fires destroy not only the materials that might fall into the stream but also the plants that are producing them every year.
After a forest fire, we can see some major changes in what gets washed into the stream. There may be a short-lived increase in woody materials, like charcoal, being washed into the streams. Plant roots that normally bind the soil together are killed and the soil becomes more fragile. When rain comes, the unstable soil easily washes away. Any woody debris on the surface can be washed into the stream too. Besides the damage caused by increased soil erosion, the burned vegetable material entering the stream is not easily broken down by normal stream processes.
Forest fires usually burn up organic components in the soil. When the soil loses the leaf fragments, bark debris or its rotting tree parts, it changes drastically. It can no longer hold much water and we will probably see increased runoff after a rain. Less water retention and increased runoff will cause the stream to become “flashy,” meaning the stream is more prone to having flooding events. On top of the increased propensity to suffer sudden high-water events, those parts that are left in the soil are more easily washed away. The bottom line is that soils in burned areas can be heavily damaged and will be nutrient-poor for many years to come.
Without the normal vegetation canopy over a stream, we can expect to see some interesting temperature regime shifts after a fire event. Post-fire streams tend to have warmer water in the summer due to the lack of shade. In the winter, without the shrubs and trees that normally act as an insulator, stream temperatures tend to be colder.
The guild of stream insects called shredders normally chop up leaves into tiny pieces that will be a food source for other stream invertebrates. When these leaves don’t appear in a stream after a fire, the shredders suffer faminelike conditions and many die out. Those organisms that rely on fine leaf particles will also have to find another food source. Additionally, organisms further up the food chain, like predatory fish that eat shredders, will have to find another source or they will go hungry, too.
With the loss of the riparian vegetation, the stream is no longer being shaded. With increased light available, we will probably see an increase in photosynthetic algae and diatoms. (Remember how slippery rocks can be when covered with diatoms? Be especially careful in post-fire streams.)
One way to quantify in-stream photosynthetic activity is to measure the amount of chlorophyll A that is present on rocks and stream substrates. We would expect to find a significant increase in the amount of chlorophyll A in a fire-ravaged stream.
Those aquatic insects that prefer to feed on diatoms and algae will suddenly have more food available and they are likely to flourish. As an example, baetidae mayflies that feed on diatoms often increase in streams after a fire. Their predators will now have more potential food and they too will grow in abundance. While there will still be a variety of invertebrates living in the stream, the members of the community will be different than before the fire.
It may appear to be simply a loss of some members and a gain in others, after the watershed is burned. However, we can expect that the total biomass produced within the stream will decline significantly. One might think of it simplistically as a fire causing fewer bugs to exist in the stream. But this biomass decline will also manifest itself with a drop in the number of fish that can be supported by the stream.
When forest fires change the local scenery, remember that the streams and all the organisms therein will also be suffering major changes.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River watershed.