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The addition of woody debris to streams is a common restoration practice. Woody debris dams were added to four streams at Fort Benning Military Installation in 2003 to mitigate the impacts of military training activities on stream ecosystems. The short-term (3 y) and long-term (14 y) effects of these woody debris dams on ecosystem processes and community structure were evaluated. There were several changes to stream ecosystems after the woody debris additions including increased water residence times, primary production rates, ecosystem respiration rates, rates of nitrogen uptake, and retention of organic matter, as well as changes to macroinvertebrate communities. The woody debris dams were still in place 14 years later, and their long-term effects on the streams’ ecosystems are currently being evaluated.

Related Article(s):


CONTRIBUTOR(S): Dr. Natalie Griffiths, Dr. Jack Feminella, Dr. Brian Helms, Sam Bickley, and Dan Isenberg


Key Resource(s):

Introduction

Headwater streams are important components of river networks. Headwaters are often the most pristine portion of a watershed; however, like most waterways, headwater streams are not impervious to degradation from anthropogenic and natural sources[4]. Because of their close hydrologic connection to the surrounding terrestrial environment, headwater streams can be particularly influenced by land disturbances[5][6][7] et al. 2005). Further, because of the hierarchical nature of river networks, disturbances in headwaters can influence downstream ecosystems[8]. Minimizing human impacts on small streams is important for the protection of these ecosystems and their biodiversity and also because of the multiple ecosystem services provided by headwaters (e.g., water for drinking, bathing, recreation, consumption of fish, etc.).

In-stream restoration is an overarching term used to describe the process of improving the ecological condition of stream ecosystems[9]. Restoration is a common practice that is used around the globe, and billions of dollars are spent on stream restoration projects[10]. However, few restoration projects have been monitored after installation, and fewer still have been monitored for long periods of time[10]. It has been even less common for stream restoration monitoring activities to evaluate both the structure (e.g., water quality, macroinvertebrates) and function (e.g., carbon and nutrient cycling) of streams[11].

One common restoration method is the addition of large wood (i.e., coarse woody debris or CWD) to stream channels. CWD additions are a low-cost restoration method[10]. In practice, nearby riparian trees are felled, cut, and placed into the channel of the stream. Wood additions can have several effects on the stream including altering the morphology and hydraulics of the stream, creating habitat for macroinvertebrates and fish, creating colonization surfaces for algae and microorganisms, and increasing the retention of sediment and organic matter, the latter of which is an important basal resource for many stream food webs[3][12][13][2][14].

Assessment of In-Stream Restoration Practices at Fort Benning Military Installation

Fort Benning Military Installation (FBMI) is located in central-west Georgia, USA, in the Southeastern Plains Level-III ecoregion[15]. FBMI was established in 1918, and military training activities have been ongoing since that time. It is likely that due to the lack of agricultural or urban land use within the boundaries of FBMI, several threatened species, endangered species, and species of special concern persist[16]. An important objective at FBMI is maintaining environmental sustainability while meeting military training objectives (Fort Benning)[16].

Military training activities (i.e., dismounted infantry tactics, tracked vehicle maneuvers) on sandy upland soils can lead to erosion and high sedimentation rates in streams at FBMI[17][6][18][19][1]. Therefore, an experimental stream restoration pilot project was implemented in 2003 at FBMI to determine whether CWD additions would reduce the effects of military activities on stream ecosystem processes.

Two SERDP projects have focused on evaluating the effects of those CWD additions on the streams’ ecosystem structure and function. The first project compared stream ecosystem processes before (2001-2003) and for three years after (2003-2006) the addition of CWD dams to streams at FBMI. The second project is measuring the same ecosystem processes 14 years after the CWD additions (2017-2018) to determine whether there have been any long-term benefits.

CWD Additions at FBMI

CWD additions were installed in 4 headwater streams within Ft. Benning’s boundaries as part of a previously funded SERDP project (SI-1186)[1]. Riparian trees (blackgum [Nyssa sylvatica] or white oak [Quercus alba]) were used for the CWD additions. The trees were felled on-site in August 2003 and cut to the desired length. The trees were left on land to dry for 2-3 months prior to installation in the stream. In October 2003, the cut trees (~10-20 cm diameter, 1-2 m in length) were added in a “Z” design along a 100-150 m section within each stream. Three cut tree sections were used to make the “Z” design, and 10-15 of these “Z”-shaped woody debris dams were installed along each stream length (~10 m apart). The CWD dams were anchored using rebar stakes driven into the streambed. The “Z” design was used to increase the retention of organic matter and slow water flow to maintain a stable streambed. The CWD additions approximately doubled the amount of CWD in the restored streams. The CWD additions themselves increased the coverage of CWD in the streams by 3.1 to 5.2%, resulting in a total CWD coverage (naturally occurring + added CWD) of 6.9 to 12.1%[1].

One issue encountered several months after the CWD additions was excessive sedimentation that led to the burial of much of the added CWD in 2 of the 4 restored streams. Sedimentation and burial was likely exacerbated by high precipitation and stream flow in the post-restoration period, as the summer precipitation amounts in 2003, 2004 and 2005 were among the highest on record at that time[1]. Because of the issue of CWD burial, the original CWD additions were augmented with additional wood in 2004, and the CWD coverage in these two streams was thereby tripled[1]. Burial of CWD dams was assessed in 2005, and ranged from 30 to 75% in the restored streams.

Figure 1. CWD Additions Shortly After Installation in a Stream
Figure 2. Burial of CWD Dams in a Restored Stream (Photo taken in March 2006)

Assessment of Restoration Efficacy

The 2007 SERDP project used a before-after control-intervention (BACI) approach[20] to assess the effects of CWD addition on stream ecosystem structure and function. From July 2001 through October 2003, pre-restoration measurements were made at 8 stream sites on a monthly and seasonal basis. In October 2003, CWD dams were installed at 4 of the 8 stream sites, leaving 4 stream sites to serve as unrestored controls. From October 2003 through November 2006, post-restoration measurements were made at all 8 stream sites on a monthly and seasonal basis, enabling pre- and post-restoration periods to be compared using the BACI method[20].

Beginning in May 2017, 14 years after the CWD additions, monthly and seasonal measurements resumed at 7 of the 8 stream sites (one unrestored site was not included in the reassessment because it was disturbed) to assess the long-term efficacy of CWD additions as a stream restoration technique. A subset of the 2007 study’s ecosystem structure and function measurements was collected from May 2017 through November 2018.

File:Griffiths1w2 Fig3.png
Figure 3. Location of the Study Streams at Fort Benning Military Installation Used in the 14-year Reassessment of CWD Addition Efficacy

Initial Effects of Restoration (2001-2006)

This section describes the main findings from a study that evaluated the initial effects of CWD additions on stream ecosystem structure and function including effects on water quality, nutrient uptake, stream metabolism, benthic particulate organic matter, and macroinvertebrates as these responses were measured for three years immediately after restoration. For a more complete description of these and other findings, please see Mulholland et al. 2007[1].

Water quality grab samples were collected approximately monthly (8x/year) and were analyzed for total and inorganic suspended sediments, dissolved organic carbon, ammonium, nitrate, and soluble reactive phosphorus concentrations. Field measurements of temperature, specific conductance and pH were collected during each site visit. In general, there were no changes to these water quality parameters in restored streams in the 2004-2006 post-restoration period[1].

Seasonal, short-term ammonium releases were conducted to assess the effect of CWD additions on ammonium uptake metrics, which measure the rate at which nutrients (ammonium) cycle in streams[21]. The ammonium uptake rate increased in restored streams immediately (within 1 month) after restoration[2]. There are several reasons as to why the CWD additions may have led to an increase in ammonium uptake rate. First, it is likely that the CWD additions increased the size of the transient storage zone (defined as hyporheic zones and areas of slow moving water) which allowed for increased removal of nutrients via biotic or abiotic mechanisms[2]. Second, the CWD may have acted as a stable substrate to allow algae and microbial biofilms to colonize, and these organisms subsequently took up the added nutrients[2]. Lastly, the CWD may have increased retention of organic matter, leading to increased demand for nutrients by biofilms that colonize that organic matter.

File:Griffiths1w2 Fig4.png
Figure 4. Nutrient Addition at one of the Study Streams

Whole-stream ecosystem metabolism is a measure of gross primary production (GPP) and ecosystem respiration (ER), and represents how organic carbon is produced and consumed by organisms within the stream[22]. Whole-stream metabolism was measured seasonally in each stream in the pre- and post-restoration periods using the one-station diel oxygen change method[18][23]. GPP rates did not change in the first year after CWD additions. However, GPP rates increased in 3 of the 4 restored streams from autumn 2004 through spring 2005, likely due to increased algal growth on CWD. This was corroborated by increased algal biomass (chlorophyll a concentration) in the restored streams in winter 2004Cite error: The opening <ref> tag is malformed or has a bad name. ER rates increased in restored streams for the first 2 years after CWD additions (2004-2005) likely due to the increases in transient storage zone size, increased area for microbial colonization, and increased organic matter retention. By 2006, ER rates in restored streams decreased to pre-restoration levels likely due to burial of the CWD[1].

Benthic particulate organic matter (BPOM) is organic matter (e.g., leaves, wood) that is retained on the surface of the stream bed (i.e., by debris dams) or within the near surface sediments. In sandy-bottom streams, BPOM acts as an important source of nutrition for the primary consumers (e.g., stream macroinvertebrates)[24]. There was no clear effect of CWD additions on BPOM in restored vs. unrestored streams; however, BPOM increased within the CWD additions suggesting that these debris dams increased the retention of organic matter[1].

Macroinvertebrates are a diverse group of organisms, and several macroinvertebrate metrics are used to assess the health of stream ecosystems[25][26]. Macroinvertebrate assemblage responses to CWD additions were variable, with the effects varying by season and by invertebrate metric (e.g., diversity, Ephemeroptera, Plecoptera, Trichoptera [EPT] taxa). EPT density, which is often used as an indicator of stream health, increased in restored streams post-restoration, but only in winter[1]. The percentage of taxa classified as clingers (which attach to surfaces in the stream) also increased in restored streams in the post-restoration period compared to unrestored streams, but only in spring[1]. Clingers require stable substrata and the CWD dams may have provided this stable habitat. The percentage of taxa classified as shredders (which consume organic matter) increased in restored streams in the post-restoration period, but only immediately after the CWD additions (winter 2004). There was no difference in percentage of shredders between restored and unrestored streams when all post-restoration years (2004-2006) were examined. The initial increase in shredders may have reflected increased retention of organic matter by the CWD dams; however, the diminishing effect of the CWD additions over time may have reflected high stream flow and burial of CWD dams. Further, total macroinvertebrate density decreased over time likely reflecting the periods of high stream flow in the post-restoration period. There were no clear effects of restoration on total macroinvertebrate density.

File:Griffiths1w2 Fig5.png
Figure 5. Collection of Macroinvertebrates in One of the Study Streams

Long-Term Effects of Restoration (2017-2018)

From May 2017 through November 2018, post-restoration measurements of stream ecosystem structure and function were made approximately 14 years after the CWD additions in 7 streams (3 unrestored, 4 restored) on a monthly and seasonal basis. This study design enabled the long-term assessment of CWD dam additions as a restoration tool. Water chemistry was measured at each stream monthly, and ammonium uptake rates, whole-stream metabolism estimates, BPOM content, and macroinvertebrates were measured at each stream on a seasonal basis.

Analyses of the long-term responses of stream ecosystems to CWD additions are currently ongoing. The most immediate finding is that the CWD dams are still in place 14 years after their initial installation. Other findings will be posted as they become available.

File:Griffiths1w2 Fig6.png
Figure 6. CWD Dams Immediately After Restoration (from Mulholland et al. 2007[1])
File:Griffiths1w2 Fig7.png
Figure 7. CWD Dams 14 Years After Restoration (Photo taken by Sam Bickley)

References

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See Also

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