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Abstract:
Headwater streams and their riparian areas, i.e., headwater systems, differ from larger streams in a number of fundamental ways that shape their characteristic biological communities. We focus on three distinguishing characteristics of headwater systems in temperate, forested landscapes, particularly in the North American Pacific Northwest, in contrast to larger streams. Small channel size and closed canopy create a physical template of reduced light inputs, strong local microclimate gradients, higher input rates of organic matter, and low primary production. The predominance of organic matter inputs as the primary source of biologically available energy promotes detritus-based communities. Second, headwaters are either fishless or have smaller fish populations (numerically and/or size-wise), leading to reduced effects of fish predation and fewer, if any, piscivores along headwaters. A third difference is the dominant disturbance regime associated with loss of surface flow. Other disturbances include infrequent and large mass failures, e.g., channelized debris flows, but low flows are a characteristic of headwaters and not larger streams. There have been few designed comparisons of headwater systems with larger channels. Still, evidence indicates that headwaters form distinct systems contrasted with larger channels such that management should not treat them simply as big streams writ small.
FOR. SCI. 53(2):131-147.
Keywords: amphibians, fish, invertebrates, periphyton, bryophytes, riparian, streams, disturbance
HEADWATER STREAMS are the channels that occur at the fringe of any fluvial network. These streams are often underappreciated and consequently are controversial regarding the management required to ensure their conservation as part of sustainable forest practices. Headwater streams may compose almost 80% of total stream length in many drainage networks (Sidle et al. 2000, Meyer and Wallace 2001, Naiman et al. 2005). These channels may be overlooked because they are so small and common, or because they are often unmapped (hidden beneath forest canopy or at very low flow in summer when mapping is done). They may also be subject to the passive neglect that comes from not understanding sufficiently what these areas provide in terms of habitat, ecosystem services, and water quality in natural and managed landscapes.
We will refer to headwater streams and the adjacent terrestrial areas as headwater systems. Along a headwater stream the interaction with the surrounding landscape is strongly coupled between the stream and riparian area, and the...