Perceptions of wood in rivers and challenges for stream restoration in the United States

This article reports a study of the public perception of large wood in rivers and streams in the United States. Large wood is an element of freshwater aquatic ecosystems that has attracted much scientific interest in recent years because of its value in biological and geomorphological processes. At the heart of the issue is the nature of the relationship between scientific recognition of the ecological and geomorphological benefits of wood in rivers, management practices utilizing wood for river remediation progress, and public perceptions of in-channel wood. Surveys of students' perceptions of riverscapes with and without large wood in the states of Colorado, Connecticut, Georgia, Illinois, Iowa, Missouri, Oregon, and Texas suggest that many individuals in the United States adhere to traditionally negative views of wood. Except for students in Oregon, most respondents considered photographs of riverscapes with wood to be less aesthetically pleasing and needing more improvement than rivers without wood. Analysis of reasons given for improvement needs suggest that Oregon students are concerned with improving channels without wood for fauna habitat, whereas respondents elsewhere focused on the need for cleaning wood-rich channels for flood risk management. These results underscore the importance of public education to increase awareness of the geomorphological and ecological significance of wood in stream systems. This awareness should foster more positive attitudes toward wood. An integrated program of research, education, and policy is advocated to bridge the gap between scientific knowledge and public perception for effective management and restoration of river systems with wood.

Public perception as a barrier to introducing wood in rivers for restoration purposes

Reintroduction of wood in rivers for restoration purposes is now recognized in a positive way by scientists. Nevertheless, the perception of wood in riverscapes is strongly affected by the socio-cultural environment. This cultural influence might explain why wood reintroduction is accepted and promoted in some regions of the world but not in others, despite the demonstrated ecological benefits. From an extensive student perception survey, we show that most of the groups from nine countries in the world considered riverscapes with wood to be less aesthetic, more dangerous, and needing more improvement than riverscapes without wood. By contrast, this way of thinking was not observed in Germany, Sweden, and Oregon (USA), where the first instances of wood reintroduction occurred.

Impact of Nitrogen and Phosphorus on [ 14 C]Lignocellulose Decomposition by Stream Wood Microflora

Nutritional and physical factors affecting the decomposition of [ 14 C]lignocellulose prepared from Douglas fir ( Pseudotsuga menziesii ) were examined by incubating the labeled substrate with homogenized surface wood scrapings obtained from a Douglas fir log in a Pacific Northwest stream. Incubations were conducted in distilled water, in stream water collected from four different sources, or in a defined mineral salts solution with or without supplemental N (KNO 3 ). Decomposition rates of [ 14 C]lignocellulose, as measured by 14 CO 2 evolution, were greater in each of the four filter-sterilized sources of stream water than in distilled water alone. Decomposition experiments conducted in stream water media with the addition of defined mineral salts demonstrated that [ 14 C]cellulose decomposition was stimulated 50% by the addition of either KNO 3 or KH 2 PO 4 /K 2 HPO 4 and further enhanced (167%) by a combination of both. In contrast, [ 14 C]lignin decomposition was stimulated (65%) only by the addition of both N and P. Decomposition of [ 14 C]lignocellulose was greatest when supplemental KNO 3 was supplied in concentrations of at least 10.0 mg of N liter −1 but not increased further by higher concentrations. The decomposition of [ 14 C]lignocellulose increased as the incubation temperature was raised and NO 3 −1 -N supplementation further increased these rates between three-and sevenfold over the range of temperatures examined (5 to 22°C). Accumulation of NH 4 + (2 to 4 mg of N liter −1 ) was always observed in culture filtrates of incubations which had been supplemented with KNO 3 , the quantity being independent of NO 3 − concentrations ≥ 10 mg of N liter −1 . The role of supplemental NO 3 − in the decomposition of [ 14 C]lignocellulose is discussed in relation to wood decomposition and the low concentrations of N found in stream ecosystems of the Pacific Northwest.

Nitrogen Dynamics in Stream Wood Samples Incubated with [ 14 C]Lignocellulose and Potassium [ 15 N]Nitrate

Surface wood samples obtained from a Douglas fir log ( Pseudotsuga menziesii ) in a Pacific Northwest stream were incubated in vitro with [ 14 C]lignocellulose in a defined mineral salts medium supplemented with 10 mg of N liter −1 of 15 N-labeled NO 3 − (50 atom% 15 N). Evolution of 14 CO 2 , distribution and isotopic dilution of 15 N, filtrate N concentrations, and the rates of denitrification, N 2 fixation, and respiration were measured at 6, 12, and 18 days of incubation. The organic N content of the lignocellulose-wood sample mixture had increased from 132 μg of N to a maximum of 231 μg of N per treatment after 6 days of incubation. Rates of [ 14 C]lignocellulose decomposition were greatest during the first 6 days and then began to decline over the remaining 12 days. Total CO 2 evolution was also highest at day 6 and declined steadily over the remaining duration of the incubation. Filtrate NH 4 + -N increased from background levels to a final value of 57 μg of N per treatment. Filtrate NO 3 − N completely disappeared by day 6, and organic N showed a slight decline between days 12 and 18. The majority of the 15 N that could be recovered appeared in the particulate organic fraction by day 6 (41 μg of N), and the filtrate NH 4 + N fraction contained 11 μg of 15 N by day 18. The 15 N enrichment values of the filtrate NH 4 + and the inorganic N associated with the particulate fraction had increased to approximately 20 atom% 15 N by 18 days of incubation, whereas the particulate organic fraction reached its highest enrichment by day 6. Measurements of N 2 fixation and denitrification indicated an insignificant gain or loss of N from the experimental system by these processes. The data show that woody debris in stream ecosystems might function as a rapid and efficient sink for exogenous N, resulting in stimulation of wood decomposition and subsequent activation of other N cycling processes.

Microbial Decomposition of Wood in Streams: Distribution of Microflora and Factors Affecting [ 14 C]Lignocellulose Mineralization

The distribution and lignocellulolytic activity of the microbial community was determined on a large log of Douglas fir ( Pseudotsuga menziesii ) in a Pacific Northwest stream. Scanning electron microscopy, plate counts, and degradation of [ 14 C]lignocelluloses prepared from Douglas fir and incubated with samples of wood taken from the surface and within the log revealed that most of the microbial colonization and lignocellulose-degrading activity occurred on the surface. Labeled lignocellulose and surface wood samples were incubated in vitro with nutrient supplements to determine potential limiting factors of [ 14 C]lignocellulose degradation. Incubations carried out in a nitrogenless mineral salts and trace elements solution were no more favorable to degradation than those carried out in distilled water alone. Incubations supplemented with either (NH 4 ) 2 SO 4 or organic nitrogen sources showed large increases in the rates of mineralization over incubations with mineral salts and trace elements alone, with the greatest effect being observed from an addition of (NH 4 ) 2 SO 4 . Subsequent incubations with (NH 4 ) 2 SO 4 , KNO 3 , and NH 4 NO 3 revealed that KNO 3 was the most favorable for lignin degradation, whereas all three supplements were equally favorable for cellulose degradation. Supplementation with glucose repressed both lignin and cellulose mineralization. The results reported in this study indicate that nitrogen limitation of wood decomposition may exist in streams of the Pacific Northwest. The radiotracer technique was shown to be a sensitive and useful tool for assessing relative patterns of lignocellulose decay and microbial activity in wood, along with the importance of thoroughly characterizing the experimental system before its general acceptance.