INSTREAM WOOD LOADS IN OLD-GROWTH AND NON-OLD-GROWTH MONTANE FORESTS

Instream wood strongly influences physical form and process in mountain streams, trapping sediment and organic matter, increasing flow resistance, local scour and erosion, and altering bedforms, cross-sectional geometry, and channel planform. Instream wood influences stream ecology by creating habitat and providing nutrients. Despite the recognized importance of instream wood, more than a century of intensive resource use in Southern Rockies montane forests has altered streams in this environment. We know little of the natural range of variability in the amount, spatial distribution, and temporal variation of wood.

Old-growth stands have minimal flow regulation and logging impacts, and provide baseline information on instream wood loads and river dynamics in the absence of human manipulations. Quantitative understanding of these streams can provide a point of reference and guidelines for restoration and management in streams with a history of direct and indirect human alteration.

We also have limited knowledge of the effects of ecological disturbances on stream wood loads in montane forests of varying stand age. Over the past century, montane forests in the Rocky Mountains have been influenced by fire and by extensive mountain pine beetle (Dendroctonus ponderosae) outbreaks. Dead and dying trees are likely impacting instream wood supply and distribution, but at present there is no knowledge of these effects. This study is relevant to the Intermountain West, where forests are being altered by a variety of insect infestations, particularly because warming climate is increasing the extent and intensity of insect infestations.

Although several studies document instream wood loads and geomorphic effects in subalpine forests of the southern Rocky Mountain region, little is known of instream wood loads in lower elevation, montane forests of the region. This study compares instream wood loads and associated stream geomorphology between montane old-growth forests and younger forests and stands with greater than 25% of trees infested by mountain pine beetles. Preliminary results indicate that streams in old-growth forests and in stands affected by beetle kill have greater instream wood loads, although streams in old-growth have more structural complexity as a result of larger and more persistent wood pieces.