Terrestrial Weeds are An Angling Problem

     By now, all anglers should be aware that aquatic weeds represent a direct threat to our fishing by damaging the ecology or by preventing access. However, most anglers don’t realize that terrestrial weeds can cause significant problems for fish and aquatic habitats. Most fishermen have long considered terrestrial weeds to be an agriculture problem and they are certainly a major problem. However, we need to understand that terrestrial weeds can have significant impacts in the water as well.

     Just like with aquatic invasive species (AIS), noxious weeds are introduced from other areas and outcompete our native species. They typically have no natural controls in their new habitats and a variety of strategies give them a distinct advantage over native species. If weeds are allowed to colonize without any control they can completely displace native species and create a monoculture stand where the only plants living are the invaders. To the uneducated angler this might not seem to be a problem. After all, the shorelines are still covered with green vegetation so all seems to be good.

     Unfortunately, just because the invaders provide vibrant green vegetation, they do not provide the same ecological benefits that native plants do. In fact, when the native plants are displaced by invasives there is a distinct possibility that the water will be significantly impacted by the weeds. Let’s take a look at some of the problems these weeds cause.

Increased Surface Water Runoff and Reduced Stream Flow

     The water that flows in our streams and rivers is supplied in several ways. The most obvious is from direct runoff from rain events or melting snows. However, these inputs do not provide the critical base flows that fish and other organisms depend upon. These base flows are typically supplied by springs and seeps that bring underground water to the surface. These ground water sources are critical to the health of a stream. These are the flows that sustain fish in low water periods and keep the stream watered during drought. The ground water sources are recharged when water infiltrates the soil during a rain or snow melt event. The water that is absorbed gradually works its way through the soils until it reaches some underground waterway that ultimately carries the water to the surface. The flows of these springs and seeps are directly related to the amount of water that soaks into the ground during a wet event.

     When noxious weeds displace native vegetation they typically provide a much different ground cover than native plants. Many invaders have mechanisms that discourage other plants from growing close to them and it is common to see weed infested areas with lots of bare ground between the invading plants. These bare ground areas encourage water to run off rather than soak in. Any water that travels away is lost to the ground water and can ultimately result in reduced flow contributions to the stream. Research has demonstrated that this increase in surface runoff, with a corresponding loss of ground water recharge, can be very significant.

     In a 1989 research paper, Lacey, Marlow and Lane investigated precipitation and runoff and determined that knapweed infested lands produce 50% more surface runoff than lands with native vegetation. Their study showed that during a rain event 23% of the water falling on native vegetation converts to surface runoff while 36% of the precipitation on a weed infested plot converts to runoff. This means that during a precipitation event significantly more water will run directly into the stream resulting in less stable stream flows and significantly less groundwater recharge. In watersheds with low summer flows, this lack of ground water recharge can lead to serious habitat decline.

     Terrestrial weeds can also contribute directly to reduced flows by literally sucking the stream dry. This problem has been widely recorded after riparian areas are infested with the invasive salt cedar (Tamerisk sp.). These highly invasive shrub/trees put out a deep tap root that can reach any water available. A mature plant can consume more than 200 gallons of water each day and in many cases waters are bordered by thousands of plants. There are stories of small streams that used to hold fish that are now dry stream beds because the salt cedar have sucked them dry.

Increased Sediment in Streams

     Sediment in streams is a killer. It can smother fish eggs, invertebrates and other living organisms resulting in significant stream deterioration. Many of the worst habitat problems we have are caused by excess sediment in our streams. Of course, we have long identified many sources of sediment and have acted to control them. We encourage ranchers to avoid grazing cattle in riparian areas, we require road construction to mitigate for sediment, we have placed restrictions on mining and other activities and we work to reduce the sediment in our streams in many ways. Unfortunately, we too often overlook the problems that terrestrial weeds are causing.

     We have established that weeds lead to increased surface water runoff and it’s easy to understand that the increased water flows are carrying more sediment with them. In fact, the amount of sediment associated with weed infested runoff is far higher than the increase in water runoff. In the study mentioned above, the researchers found that the sediment yield from weed infested plots was nearly 3 times greater than from native vegetation plots. In their experiments they found that a plot with native grasses would yield 44 kg/ha of sediment during a 30 minute event while a plot infested with knapweed yielded 128 kg/ha during the same test. This is a significant difference and there is no doubt that invaded landscapes have the potential to significantly increase the sediments in our streams.

Changes in Water Chemistry

     Although there are no studies that demonstrate this problem, it is highly likely that some weed invasions are impacting the waters they are adjacent to. The most likely problem comes from compounds that the weeds accumulate in their leaves that are released into the water when the plant loses its leaves in the fall. Spotted knapweed leaves produce the chemical cnicin, which acts as an herbicide to inhibit competition by a wide range of other plant species. This phytotoxic compound inhibits seed germination and can lead to cell death of competing plants by acidification of the cytoplasm. When Knapweed leaves, which are loaded with this chemical, are dropped we can assume they are adding an unwanted chemical to the stream. While there are no aquatic studies of this problem, it is likely that the native invertebrates in the stream do not utilize this vegetation in the same way as they use native plants.

     Salt cedar is another example of a plant that can impact water chemistry. These weeds have the unique ability to remove salts from the soil and accumulate it in their leaves. With their long deep tap roots, they are able to gather salts from deep in the ground and bring them up into the leaves of the plant where they are excreted as fine granules of salt that cling to the leaves and fall to the ground. These plants are so effective at releasing salt that they have the ability to raise soil salinity to the point where nothing else will grow. Of course, we can imagine the impact of having this salt released into a small waterway.

     Other noxious weeds also produce harmful compounds that are likely having a negative impact on water chemistry but additional research is needed to better quantify both the chemical changes and the impacts they trigger.

Loss of Overhead Cover

     Many of the weed species that are invading our riparian areas have significantly different structure than our native plants. In some cases, we can lose significant amounts of overhead vegetation that benefits the stream. One of the most obvious impacts of the loss of cover is an increase in water temperatures. Without the overhead canopy of vegetation to shade the stream the water is exposed to direct sunlight and solar heating. Since many of the hottest and sunniest days correspond with the lowest stream flows the temperature difference can be significant.

     Loss of overhead cover also relates directly on fish behavior. A fish that is comfortable in a spot that is protected by a screen of overhead vegetation will likely migrate away when the security that comes from the cover is removed. The additional erosion and sediment runoff discussed above can create a stream that is broader and shallower so the loss of the security provided by overhead cover can mean that the stream will no longer provide quality fish habitat and populations will decline.

Changes in the Food Web

     Another little researched impact comes from potential changes to the food web as noxious weeds displace native vegetation. As we have already discussed, many weeds produce objectionable compounds. When the organisms at the bottom of the food web seek food do they avoid invasive weeds? Are they keyed in on certain types of native vegetation? If these native plants are gone how will they respond? While these are currently unanswered questions it is easy to imagine that there is at least some level of food web disruption taking place.

     While the impacts of terrestrial weeds on aquatic habitats are not fully understood, we certainly know enough to realize that these terrestrial invaders have a significant impact on what is happening in and on the water. It is not enough for anglers to assume that if it is green on the shoreline it is a good thing. Rather, it is time to teach yourself to identify the weeds that are a problem in your area and take action against them. Here are some suggestions for becoming a weed warrior:

  •    Learn to identify common weeds and avoid spreading their seeds
         In most areas there are only a few nasty weed species and you can easily learn to ID them. Avoid walking through weed patches and always avoid parking in weeds.

  •    Carry a trash bag to carry out weeds or weed flowers that you might pick
         Some weeds will actually do better if they are pulled (new weeds sprout from the roots) so be sure to know which weeds should be pulled. Many weeds can be controlled by picking their flowers and seed pods before the seeds fully develop.

  •    Carefully comb your dog to remove any weed seeds after you are in the field
         It’s easy for a dog to transport weed seeds in their coat so take time to make sure they are weed free and properly dispose of seeds in the trash.

  •    Report new weed sightings to the proper authorities
         Weeds are different from AIS in that they can be controlled after they are found. It is important to quickly report new weed sightings so be sure to watch and report.

     As Clean Anglers most of us are already regularly practicing Inspect, Clean & Dry. Adding weeds to the list of things we are inspecting for is not very difficult and can help a lot. If you want to learn more about the weeds in your area contact your local Extension Office or County weed specialist. Remember that not everything green is good for the shoreline and that weeds are a fishing problem too. Be smart about weeds and together we can make a difference.