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Chapter 6 -- Changes and Trends in Stream Habitat and Fisheries (continued)

Trends in Large Woody Debris and Riparian Vegetation

Because riparian vegetation is the source of large woody debris in streams, trends in large woody debris and riparian vegetation are best considered together. Although there are few if any historic descriptions of large woody debris loading in Smith River, the abundance of large woody debris in stream channels was probably high at the beginning of European American settlement. Under conditions prior to European American settlement, the role of fire in riparian forests was probably minor. Catastrophic loss of riparian vegetation by fire was probably rare due to high moisture content and low slope position. This is true today also.

Since European American settlement, the quantity of large woody debris in streams has been, in general, decreasing. This trend is due to a combination of influences. First, people have directly removed woody debris from streams and, in a few cases, directly added large wood to streams. Large woody debris, especially log jams, have been cleared from streams for various purposes including mining operations, misguided stream restoration, and possibly navigation improvement on the lower river. Over the long term, other human activities gradually influence the quantity of large woody debris by reducing rates of recruitment of stream logs.

Logging operations have influenced large wood in streams over both the long and short term. The immediate effect of early logging operations was probably an increase in large woody debris in streams. Following the sudden and massive input of wood during logging, quantities of wood in streams decreased gradually because the riparian canopy was typically eliminated during logging. This resulted in a decades-long period during which there were no large trees nearby that would fall into the stream (Bryant and Sedell 1995). Agriculture, road construction, and the floods of 1955 and 1964 also reduced the size and quantity of conifers in riparian areas, contributing to further reduced loading of large woody debris in streams. Aerial photographs from the 1940s show that riparian canopies had narrower openings over the streams and contained a larger proportion of conifers. Besides "resetting" the riparian vegetation, the 1955 and 1964 floods also redistributed large woody debris in the stream network. Following timber harvest and other disturbances, many streamside forests have regenerated mostly in hardwoods, especially alder. Along the lower Smith River, areas of the flood plain that formerly supported riparian forests are now dedicated to other uses.

Large woody debris loading in streams will be affected for many years and decades by past logging practices, stream clearing activities, and the mid-century floods. Following timber harvest, large woody debris in streams is expected to decrease for 50 to 100 years (Bryant and Sedell 1995). Where redwoods are present in the riparian zone, declines in large woody debris may be more gradual due to rot resistance of this species.

Except in unusual cases when funding and access are available for transporting and adding wood to streams, future trends in large woody debris in streams are tied to future trends in riparian vegetation. Recruitment of large woody debris into streams depends on the growth of suitable trees in the riparian zone. Conifer regeneration is particularly important because conifers provide the largest and longest-lasting woody debris. In riparian areas that are now covered primarily with young hardwoods, natural succession to conifers will take decades. When conifers are allowed to grow to large sizes in the riparian area, large woody debris in streams will gradually increase over decades and centuries. It is difficult to overstate the importance of conifer regeneration in riparian areas.

On both public and private land, fire is always a potential influence on vegetation including riparian vegetation. However, catastrophic loss is unlikely. Another destructive agent is attacking riparian trees in areas of serpentine soils, such as in the North Fork subbasin. An important riparian conifer, Port Orford cedar, is declining in abundance due to root rot disease. Because these riparian forests have an open character due to the harsh soil, the decreased abundance of Port Orford cedar might not produce a significant increase in stream temperatures. However, the decrease of Port Orford cedar will presumably lead to a reduction in the number of large conifers that fall in the stream. This could have a significant effect on stream habitat quality.

On public lands, riparian protection policies of the Smith River NRA and the Northwest Forest Plan will allow large conifers to grow in riparian areas. Over decades, this will renew supplies of large wood in streams. On private timber lands, trends in riparian vegetation and large woody debris depend largely on forest practice regulations. Almost all the private lands in the Smith River watershed are in California and are under the jurisdiction of California Forest Practice Regulations. Because high stream temperatures are not a pressing problem in the Smith River watershed, forest practice rules should place more emphasis on development of future sources of large woody debris and less emphasis on retention of maximum shade.

Even where the riparian canopy is relatively intact and provides shade and nutrients, there are many riparian areas that contain fewer large conifers than prior to European American settlement. This causes a reduced rate of input of large woody debris. This reduction will persist for decades until large conifers regrow and eventually fall in the stream. Decreased amounts of woody debris throughout the Smith River system, especially in the estuary, lower river, and lower tributaries, may be a significant reason for the decline in anadromous salmonid populations from historic levels.


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