Forest Management and Stream Organisms: Role of Trees in Aquatic Food Webs

Linda Benedict  |  6/28/2006 7:26:51 PM

It is imperative that efforts be made to preserve forested buffer zones along Louisiana’s streams. These streamside management zones, or SMZs as they are known, help prevent loss of aquatic biodiversity essential to the well-being of the environment.

Water quality monitoring efforts in Louisiana’s streams focus on the concentrations of sediment, nutrients and other compounds in the water. However, activities within the watersheds of these streams actually play the most critical role in determining stream water quality. The development and implementation of best management practices (BMPs) for all major land uses can contribute greatly to the protection of Louisiana’s streams, rivers and bayous.

In forested watersheds, forest management activities such as road building, tree removal, site preparation, thinning and prescribed burning can result in increased inputs of water, sediment, nutrients and organic matter in adjacent streams. However, implementation of effective forestry BMPs can minimize these inputs.

One forestry best management practice focuses on the maintenance of a streamside management zone, or SMZ, which is a strip of forest along each bank of a stream, known as the riparian zone. An SMZ is typically 30-200 feet wide within which timber harvesting is minimized or prohibited, providing a buffer between the stream and the disturbed area.

Riparian trees have long been known to provide important habitat for forest- dwelling wildlife, particularly birds. From an aquatic standpoint, the importance of an SMZ has been linked to its role as a filter for water moving to the stream. However, maintenance of trees within the riparian zone actually results in other important benefits to the stream and the organisms that live there.

In smaller streams, the riparian forest provides a closed or near-closed canopy that shades the stream and keeps water temperatures low. In Bratton Branch, a small stream in Kisatchie National Forest, LSU AgCenter researchers found that stream temperatures during summer were often 18 degrees F cooler than air temperatures, which often exceeded 80 degrees F. In addition to temperature amelioration, the forest canopy reduces light levels in small streams, minimizing the development of algal communities. Although this might not seem beneficial, loss of the canopy results in large increases in algal growth, particularly in streams with high nutrient loading. This increase in algal biomass results in high dissolved oxygen conditions during the day, but extremely low levels at night. Because of the continual movement and oxygenation of water in most streams, resident invertebrates and fishes are not well adapted to low dissolved oxygen concentrations. Consequently, loss of the riparian canopy can result in low dissolved oxygen levels at night due to algal respiration, and higher water temperatures (which also result in lower dissolved oxygen levels) during the day. Most stream fishes in smaller systems are adapted to cooler waters and moderate to high dissolved oxygen levels, and loss of the canopy can cause sensitive species to move downstream or die, resulting in large changes in the abundance and species composition of the resident invertebrate and fish communities.

As streams increase in size, the canopy opens. The types of organisms inhabiting the stream change, and the importance of the shading function of the SMZ declines. However, riparian trees also play a critical role in determining the shape and habitat structure of a stream, which is closely linked to the diversity of resident organisms. The root structure of riparian trees helps stabilize the bank by resisting erosion during periods of high stream discharge. This in turn helps stabilize the stream channel and reduce sediment inputs that typically increase stream turbidity during storm events. In addition, continued erosion underneath the root mass will form an undercut bank, which is preferred habitat for many stream fishes.

Through time, the undercut bank will collapse, and the tree, now referred to as large woody debris, will fall into the stream. This promotes the formation of upstream slackwater pools, as well as shallow, higher velocity habitats (riffles) as flow is restricted to a smaller part of the channel. Downstream pools are scoured out as water flows over the trunk during storm events.

It is important to maintain natural densities of trees adjacent to streams, creeks and bayous. The sequence of events leading from a growing tree to submerged woody debris is a key link between riparian forests and aquatic habitat diversity. This habitat diversity, in turn, is positively related to the distribution and abundance of invertebrates and fishes in a stream. Some invertebrates, such as larval mayflies and certain fishes like darters and some minnows, are riffle- dwelling species. Snails, crayfishes, sunfishes, bass and topminnows prefer pool conditions. Loss of large woody debris because of the removal of riparian forests often results in long reaches of uniform habitat conditions in adjacent streams, resulting in a concurrent decline in the diversity and abundance of stream organisms.

In addition to large woody debris, riparian trees also provide streams with a continuous supply of organic matter in the form of small sticks and leaves. As stated previously, little sunlight reaches streams that have dense overstory canopies, which results in low levels of algal production in the stream. Consequently, these streams depend on forest-derived organic material to provide the energy that drives the aquatic food web. This material, known as detritus, can fall directly into the stream or move to the stream during riparian flooding associated with storm events and is processed first by bacteria and fungi, which continue the detrital breakdown that began with the leaching of water-soluble compounds.

A group of organisms known as shredders, such as crayfish, amphipods, and the larvae of certain aquatic insects, consume pieces of the detritus and its bacterial and fungal coating, and break it into smaller particles as they feed. These smaller detrital particles are food for other invertebrates and some fishes that collect the particles or filter them from the water column. All of the shredders, collectors and filterers provide food to carnivorous insects, crustaceans, fishes, birds, mammals and reptiles at the top of the stream food web. Thus, contribution of organic detritus to the aquatic food web is another role that riparian forests play in the ecology of adjacent streams. This role again emphasizes the importance of maintaining an adequate diversity and density of trees within the SMZ.

In addition to large woody debris and seasonal leaf fall, riparian forests also provide continual input of small woody debris in the form of sticks and branches. The low-gradient, sanddominated streams characteristic of Louisiana’s coastal plain and upland watersheds have minimal amounts of bedrock or gravel, which are preferred habitats for stream organisms in higher gradient stream systems. As a consequence, small woody debris often provides the only available hard-surface habitats, which are used extensively by a diversity of stream-dwelling invertebrates and fishes as cover, food, feeding sites and reproductive habitat. LSU Ag- Center scientists have found that many Louisiana fishes, such as pirate perch and brown madtoms, are closely associated with accumulations of small woody debris, which provide a refuge from the current, a hiding place to escape predators and an abundance of invertebrate prey. Researchers have also found that invertebrate species that inhabit woody debris are often different from those inhabiting the substrate, so the loss of small woody debris inputs would have significant effects on the community abundance and species composition.

In larger streams with open canopies, submerged woody debris provides a surface for development of bacterial and algal communities, called biofilms, which are important food sources for a group of invertebrates known as scrapers, such as snails. Throughout Louisiana, submerged woody debris accumulations are biological hotspots, and these habitats probably support a majority of the invertebrate and fish production in the state’s stream systems.

William E. Kelso, F.O. Bateman Professor of Renewable Natural Resources, and D. Allen Rutherford, Bryant Bateman Professor of Renewable Natural Resources, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2006 issue of Louisiana Agriculture.)
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