By Volunteer Mario Mora
We have all seen the bumper stickers stating, “Trees Are The Answer”. Naturally one asks, “Well, what is the question?” With this article we are going to look at the effect of trees have on the well being of salmon. In other words in this article we’re going to ask the question, “Can increasing the amount of trees along stream sides improve the habitat for salmon?”
To start with we have to take a look at the current stream sides (also known as riparian areas) and how much vegetation they have now compared to what was there in the past. The first settlers reported streams and rivers in this area so thick with salmon that one could cross a river by walking on the backs of salmon and not get their feet wet. Thus we can conclude that the riparian conditions that existed in the past were well suited for salmon to thrive. Historically our riparian areas were heavily forested, with gaps in the forest canopy occurring sporadically. The composition of these forests varied in hardwoods and conifers. Some of these historic native species are;
Hardwoods
- Red alder (Alnus rubra)
- Bigleaf maple (Acer macrophyllum)
Conifers
- Douglas-fir (Pseudotsuga menziesii)
- Western red cedar (Thuja plicata)
- Western hemlock (Tsuga heterophylla)
Now if one takes a look at our riparian areas, we can see that houses, office buildings, roadways and other man made construction has taken away much of the trees shading our rivers and streams.
The resulting decrease in tree cover along our riparian areas has a direct influence on salmon. Having tree canopies overhang a stream not only reduces the amount of sunlight hitting the stream during the day, by helping keep the stream cool by absorbing short wave radiation, but also making sure our streams don’t get too cold by emitting long wave radiation (result from being heated during the day) to the surface of the stream. The trees ability to help moderate the stream temperature is exactly what the salmon need. Now generally spawning salmon prefer temperatures from 40° F to 49° F, and the temperatures should not exceed 56° F. And ideally salmon embryos need an average daily temperature at or below 46° F to 50° F, and that the temperature for pre-emergent development should not exceed 54° F.
Trees along our streams also help to stabilize the stream banks. With the decrease in riparian vegetation greater amounts of soil run off into the streams. This results in increased turbidity (i.e. water that looks dirtier), and less available dissolved oxygen, which can also be detrimental to salmon.
Besides all of this, having trees on the property, whether it is yours or public land, is more aesthetically pleasing. The image that a lot of us have that makes streams and rivers appealing to us is visiting a river and having trees provide shade in the summer (with maybe a tire swing attached to jump out into the river), a nesting place for many of our birds, all while standing majestically over the river.
So yes, salmon should care about trees, and the amount of them, along stream-sides and rivers. Or more to the point we should, if we care about salmon.
References
Johnson, S.L., & Jones, J. a. (2000). Stream temperatures responses to forest harvest and debris flows in western Cascades, Oregon. Canadian Journal of Fisheries and Aquatic Sciences, 57 (S2), 30-39
Larson, L. L., & Larson, S. L. (1996). Riparian Shade and Stream Temperature : A Perspective, 18(August), 149-152.
Malcolm, I. a., Hannah, D. M., Donaghy, M. J., Soulsby, C., & Yougson, a. F. (2004). The influence of riparian woodland on the spatial and temporal variability of stream water temperatures in an upland salmon stream. Hydrology and Earth System Sciences, 8(3), 449-459.