Large Wood Debris Recruitment on Differing Riparian Landforms along a Gulf Coastal Plain (usa) Stream: a Comparison of Large Floods and Average Flows

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 (r1⁄4 0.942) and low terraces (r1⁄4 0.915), but was significant for floodplains (r1⁄4 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. Copyright# 2007 JohnWiley & Sons, Ltd. key words: large wood debris; floods; coastal plain; riparian; geomorphology; climate cycles Received 29 August 2006; Accepted 13 September 2006 INTRODUCTION Throughout North America and most of the world, there has been a long history of wood removal from streams and rivers (Sedell and Froggatt, 1984; Triska, 1984; Gippel et al., 1996; Benke, 2001; Montgomery and Piegay, 2002). In addition, extensive riparian deforestation combined with flow regulation and channelization has reduced overbank flooding and limited potential sources of large wood debris (LWD) recruitment into larger streams and rivers (Sedell and Froggatt, 1984; Gippel et al., 1996; Benke, 2001). Yet, numerous studies (see Gurnell and Sweet, 1998) have documented the importance of LWD in streams and rivers (e.g. habitat for fishes and invertebrates, bank stabilization, sediment retention, altering water flow, structuring instream and riparian habitat, and enhancing productivity). While there is widespread recognition of the role of wood debris, there have been few opportunities to examine its dynamics in an undisturbed setting, particularly in larger streams and rivers. In southeastern Coastal Plain, USA streams, LWD appears to be particularly important in contributing to stream foodwebs. Invertebrate diversity, habitat-specific abundance, biomass and productivity are greater on submerged RIVER RESEARCH AND APPLICATIONS River. Res. Applic. 23: 391–405 (2007) Published online 1 February 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rra.977 *Correspondence to: Stephen W. Golladay, J. W. Jones Ecological Research Center, Rte 2, Box 2324, Newton, GA 39870, USA. E-mail: [email protected] Copyright # 2007 John Wiley & Sons, Ltd. wood debris compared to other habitats (Benke et al., 1985; and reviewed by Benke, 2001). Many streams and rivers of the southeastern Coastal Plain have a long history of human-caused degradation. Beginning in the early 1800s with snagging operations and continuing through the 1900s with river regulation and pollution, streams have been managed for human benefit with little thought to ecological consequences (Benke, 1990, 2001). While human degradation is prevalent, many free flowing streams remain in the southeast. For example, Georgia has five rivers with segments of greater than 200 km that are considered unregulated (Benke, 1990). Many of their major tributaries have little flow regulation on their main stems. Also, in some areas the distribution of human populations has changed dramatically during the past 100 years. For example, since 1920 the population of many rural counties in southwest Georgia has been declining. This has resulted in decreasing numbers of small farms and livestock, particularly along stream corridors (e.g. Craft and Casey, 2000). It has also resulted in the consolidation of farms into large operations concentrated in the uplands (Craft and Casey, 2000). As a result, riparian forests have recovered to some degree from extensive disturbance in the late 1800s and early 1900s (Craft and Casey, 2000) and are approaching maturity (Lynch et al., 1986). As a consequence of changing human land use, extensive and intact riparian areas occur along many streams. These rivers and their tributaries offer an opportunity to examine the recovery of stream/riparian interactions under minimally altered flow regimes. This paper represents a continuation of a previous study examining the influence of riparian geomorphology on wood debris recruitment in a Gulf Coastal Plain stream (Palik et al., 1998). Previous work suggested that debris recruitment was pulsed in nature and associated with a large flood. Floodwaters interacting with spatial drivers (i.e. riparian landform) also influenced recruitment rates and debris characteristics. This study examines debris recruitment across the same landforms but contrasts two large floods as well as years of average flow. The first flood (1994) was the result of an early summer tropical storm (reported in Palik et al., 1998) and the second flood (1998) was the result of above average winter rainfall associated with El Niño conditions in the Pacific Ocean. Specific objectives are (1) to compare debris recruitment rates in average flow years with those with substantial flooding, (2) to compare debris recruitment rates and characteristics across differing riparian landforms and (3) to examine the effects of substantial floods of differing magnitude and antecedent conditions on debris recruitment rates and characteristics. METHODS Study area This study was conducted at the Ichauway Ecological Reserve, an 11 500 ha reserve located in southwestern Georgia, USA, in the Plains and Wiregrass Subsections of the Lower Coastal Plains and Flatwoods Section (ECOMAP, 1993; Keys et al., 1995). The reserve is bisected by 23 km of Ichawaynochaway Creek, a 5th-order black-water stream with headwaters that originate in swamp-forest. Mean bank-full width of the stream is about 30m. Flow in Ichawaynochaway Creek is low and stable in summer and fall. Late winter and early spring storms often result in bank-full discharges, causing flooding of low-lying areas. A long-term USGS stream gaging station is located approximately 18 km upstream from the northern boundary of Ichauway (USGS Milford Gage 02353500). Average annual discharge is 22m s 1 (Hickey et al., 2002). A series of repeating riparian landforms occur along the stream corridor and include sand ridges, terraces and floodplains (Goebel et al., 1996; Palik et al., 1998). Floodplains occur along reaches with broad stream valleys and are low in elevation relative to the base-flow channel. Floodplains support a mixed-hardwood forest community. Sand ridges occur within narrower valley segments and, compared to floodplains, are high in elevation above the base-flow channel. Sand ridges support a sparse canopy of longleaf pine (Pinus palustris Mill.) and scrub oaks (Quercus spp.). Terraces are abandoned floodplains that occur in stream valleys of variable width and elevation. Terraces support longleaf pine forest or mixed-hardwood forest where surface fire has been excluded. The streamside forest on each of the riparian landforms, that is, the forest growing laterally along stream banks and levees (Palik et al., 1998), are mature or climax (Lynch et al., 1986). They support a diverse woody community with up to 47 tree species (stems having a dbh 5 cm). Carolina ash (Fraximus caroliniana, Mill.), eastern red cedar (Juniperus virginiana L.), bald cypress (Taxodium distichum (L.) Rich.) and southern live oak (Quercus Copyright # 2007 John Wiley & Sons, Ltd. River. Res. Applic. 23: 391–405 (2007) DOI: 10.1002/rra 392 S. W. GOLLADAY, J. M. BATTLE AND B. J. PALIK