A Synthesis of the Ecology of Headwater Streams and their Riparian Zones in Temperate Forests

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.