Large wood debris recruitment on differing riparian landforms along a Gulf Coastal Plain (USA) stream: a comparison of large floods and average flows

Abstract

In southeastern Coastal Plain streams, wood debris can be very abundant and is recruited from extensive forested floodplains. Despite importance of wood debris, there have been few opportunities to examine recruitment and redistribution of wood in an undisturbed setting, particularly in the southeastern Coastal Plain. Following extensive flooding in 1994, measurements of individual downed trees (species, dbh, orientation, distance from base-flow channel and condition) were made across replicated riparian landforms in a Gulf Coastal Plain 5th-order stream. Annually, the fate of these trees was determined and newly recruited trees were noted. More than 300 downed trees have been recorded. Recruitment varied across landforms with more constrained reaches having greatest mortality. Total tree mortality varied substantially across years. Generally, tree recruitment was greatest in years with substantial floods (1994 and 1998). For each riparian landform type, tree mortality was correlated with the maximum daily flow during the period preceding annual debris surveys. This relationship was particularly strong for sand ridges (r 2 ¼ 0.942) and low terraces (r 2 ¼ 0.915), but was significant for floodplains (r 2 ¼ 0.413). Greatest rates of debris recruitment per maximum daily flow were observed for sand ridges followed by low terraces. Flood characteristics also influenced debris recruitment. The 1994 flood was caused by a tropical storm and resulted in a rapid rise in streamflow. Much of the debris recruited during this flood was from toppled trees and was oriented parallel to the stream channel. In contrast, the 1998 flood was preceded by a wetter than average winter with more gradually rising flows and there was no relationship between riparian landform and debris characteristics. These results indicate that wood recruitment dynamics in Coastal Plain streams are complex. Wood recruitment rates are controlled by cyclical variations in climate interacting with riparian geomorphology. Infrequent high flows appear critical in the maintenance of the instream debris pool.

9.11 Wood in Fluvial Systems

“…They estimated that in the most extreme case, recovery of in-channel wood to a steady-state condition probably would not occur for over 1000 years. Other extreme events that have been shown by monitoring or modeling to impact heavily on stream wood storage include stream cleaning and forest harvesting (Bragg and Kershner, 1997), beetle infestation (Bragg, 2000), debris flows Gresswell, 2003, 2004;Reeves et al, 2003;Bigelow et al, 2007), and floods (Palik et al, 1998;Johnson et al, 2000;Hering et al, 2004;Golladay et al, 2007;Oswald and Wohl, 2008). They reconstructed the impact of fire in a pristine catchment close to the foresttundra border in Quebec, Canada.…”

Factors influencing wood mobilization in streams

“…Wood pieces provide stable substrate for invertebrates and biofilms, entrap leaves and other organic matter, afford overhead cover for fish, promote hyporheic exchange flow and transient storage, enhance hydraulic heterogeneity, and encourage pool formation and channel meandering [Angermeier and Karr, 1984;Beechie and Sibley, 1997;Gregory et al, 2003;Johnson et al, 2003;Mutz and Rohde, 2003;Eggert and Wallace, 2007;Stofleth et al, 2008]. The frequency and character of wood inputs varies in space and time [Latterell and Naiman, 2007;Golladay et al, 2007] and is strongly affected by riparian management [Flebbe and Dolloff, 1995;Angradi et al, 2004;Kreutzweiser et al, 2005;Czarnomski et al, 2008].…”

Fluvial processes and vegetation — Glimpses of the past, the present, and perhaps the future

“…Although the impact of LWD in many low-gradient systems is minor (Golladay et al, 2007), the character and delivery of LWD may be an important determinant of riparian-zone processes.…”