Shifts in leaf litter breakdown along a forest–pasture–urban gradient in Andean streams

Tropical montane ecosystems of the Andes are critically threatened by a rapid land‐use change which can potentially affect stream variables, aquatic communities, and ecosystem processes such as leaf litter breakdown. However, these effects have not been sufficiently investigated in the Andean region and at high altitude locations in general. Here, we studied the influence of land use (forest–pasture–urban) on stream physico‐chemical variables (e.g., water temperature, nutrient concentration, and pH), aquatic communities (macroinvertebrates and aquatic fungi) and leaf litter breakdown rates in Andean streams (southern Ecuador), and how variation in those stream physico‐chemical variables affect macroinvertebrates and fungi related to leaf litter breakdown. We found that pH, water temperature, and nutrient concentration increased along the land‐use gradient. Macroinvertebrate communities were significantly different between land uses. Shredder richness and abundance were lower in pasture than forest sites and totally absent in urban sites, and fungal richness and biomass were higher in forest sites than in pasture and urban sites. Leaf litter breakdown rates became slower as riparian land use changed from natural to anthropogenically disturbed conditions and were largely determined by pH, water temperature, phosphate concentration, fungal activity, and single species of leaf‐shredding invertebrates. Our findings provide evidence that leaf litter breakdown in Andean streams is sensitive to riparian land‐use change, with urban streams being the most affected. In addition, this study highlights the role of fungal biomass and shredder species (Phylloicus; Trichoptera and Anchytarsus; Coleoptera) on leaf litter breakdown in Andean streams and the contribution of aquatic fungi in supporting this ecosystem process when shredders are absent or present low abundance in streams affected by urbanization. Finally, we summarize important implications in terms of managing of native vegetation and riparian buffers to promote ecological integrity and functioning of tropical Andean stream ecosystems.     

Riparian plant species loss alters trophic dynamics in detritus-based stream ecosystems

Riparian vegetation is closely connected to stream food webs through input of leaf detritus as a primary energy supply, and therefore, any alteration of plant diversity may influence aquatic ecosystem functioning. We measured leaf litter breakdown rate and associated biological parameters in mesh bags in eight headwater streams bordered either with mixed deciduous forest or with beech forest. The variety of leaf litter types in mixed forest results in higher food quality for large-particle invertebrate detritivores (‘shredders’) than in beech forest, which is dominated by a single leaf species of low quality. Breakdown rate of low quality (oak) leaf litter in coarse mesh bags was lower in beech forest streams than in mixed forest streams, a consequence of lower shredder biomass. In contrast, high quality (alder) leaf litter broke down at similar rates in both stream categories as a result of similar shredder biomass in coarse mesh bags. Microbial breakdown rate of oak and alder leaves, determined in fine mesh bags, did not differ between the stream categories. We found however aquatic hyphomycete species richness on leaf litter to positively co-vary with riparian plant species richness. Fungal species richness may enhance leaf litter breakdown rate through positive effects on resource quality for shredders. A feeding experiment established a positive relationship between fungal species richness per se and leaf litter consumption rate by an amphipod shredder (Gammarus fossarum). Our results show therefore that plant species richness may indirectly govern ecosystem functioning through complex trophic interactions. Integrating microbial diversity and trophic dynamics would considerably improve the prediction of the consequences of species loss.     

Land-use effects on structural and functional composition of benthic and leaf-associated macroinvertebrates in four Andean streams

The replacement of native forests by pastures takes place widely in the Andes. The effects of such land-use change on aquatic assemblages are poorly understood. We conducted a comparative analysis of the effects of forest conversion to pastures on the taxonomic, structural, and functional composition of macroinvertebrates (benthic and leaf-associated) in montane and upper montane streams (ecosystem type) of the south Ecuadorian Andes. Taxonomic composition of benthic and leaf-associated macroinvertebrates was different between ecosystem type and land use. Also, major differences in the structural and functional composition of benthic and leaf-associated macroinvertebrates were mainly promoted by land use in both ecosystem types. Forested streams showed higher diversity than pasture streams, sustaining more shredder, scraper, and predatory invertebrates. We also observed differences in the macroinvertebrate communities between benthic and leaf-bag samples. Leaf bags had lower diversity and more collector invertebrates than benthic samples. This study highlights the large effect of riparian forest conversion to pasture land on macroinvertebrate communities, and the importance of using appropriate sampling techniques to characterize aquatic assemblages. We also recommend the maintenance and restoration of riparian vegetation to mitigate the effects of deforestation on stream communities and ecosystem processes.     

Composition of riparian litter input regulates organic matter decomposition: Implications for headwater stream functioning in a managed forest landscape

Although the importance of stream condition for leaf litter decomposition has been extensively studied, little is known about how processing rates change in response to altered riparian vegetation community composition. We investigated patterns of plant litter input and decomposition across 20 boreal headwater streams that varied in proportions of riparian deciduous and coniferous trees. We measured a suite of in‐stream physical and chemical characteristics, as well as the amount and type of litter inputs from riparian vegetation, and related these to decomposition rates of native (alder, birch, and spruce) and introduced (lodgepole pine) litter species incubated in coarse‐ and fine‐mesh bags. Total litter inputs ranged more than fivefold among sites and increased with the proportion of deciduous vegetation in the riparian zone. In line with differences in initial litter quality, mean decomposition rate was highest for alder, followed by birch, spruce, and lodgepole pine (12, 55, and 68% lower rates, respectively). Further, these rates were greater in coarse‐mesh bags that allow colonization by macroinvertebrates. Variance in decomposition rate among sites for different species was best explained by different sets of environmental conditions, but litter‐input composition (i.e., quality) was overall highly important. On average, native litter decomposed faster in sites with higher‐quality litter input and (with the exception of spruce) higher concentrations of dissolved nutrients and open canopies. By contrast, lodgepole pine decomposed more rapidly in sites receiving lower‐quality litter inputs. Birch litter decomposition rate in coarse‐mesh bags was best predicted by the same environmental variables as in fine‐mesh bags, with additional positive influences of macroinvertebrate species richness. Hence, to facilitate energy turnover in boreal headwaters, forest management with focus on conifer production should aim at increasing the presence of native deciduous trees along streams, as they promote conditions that favor higher decomposition rates of terrestrial plant litter.     

Recent land‐use changes affect stream ecosystem processes in a subtropical island in Brazil

Land‐use changes such as conversion of natural forest to rural and urban areas have been considered as main drivers of ecosystem functions decline, and a large variety of indicators has been used to investigate these effects. Here, we used a replicated litter‐bag experiment to investigate the effects of land‐use changes on the leaf‐litter breakdown process and leaf‐associated invertebrates along the forest–pasture–urban gradient located in a subtropical island (Florianópolis, SC, Brazil). We identified the invertebrates and measured the litter breakdown rates using the litter bags approach. Litter bags containing 3 g of dry leaf of Alchornea triplinervia were deployed on forest rural and urban streams. Principal component analysis, based on physico‐chemical variables which, confirmed a gradient of degradation from forest to urban streams with intermediate values in rural areas. In accordance, shredder richness and abundance were lower in rural and urban than in forest streams. The land‐use changes led also to the dominance of tolerant generalist taxa (Chironomidae and Oligochaeta) reducing the taxonomic and functional diversity in these sites. Leaf‐litter breakdown rates decreased from forest to rural and finally to urban areas and were associated with changes in pH, water velocity, dissolved oxygen and abundance of leaf‐shredding invertebrates, although global decomposition rates did not differ between rural and urban streams. Overall, this study showed that land‐use changes, namely to rural and urban areas, have a strong impact on tropical streams ecosystems, in both processes and communities composition and structure. Despite of being apparently a smaller transformation of landscape, rural land use is comparable to urbanisation in terms of impact in stream functioning. It is thus critical to carefully plan urban development and maintain forest areas in the island of Florianópolis in order to preserve its natural biodiversity and aquatic ecosystems functioning.     

The Effect of Macroinvertebrate Exclusion on Leaf Breakdown Rates in a Tropical Headwater Stream1

The role of macroinvertebrates in the process of leaf breakdown is well studied in temperate streams, but less is known about their role in the tropics. We investigated the effect of reducing macroinvertebrate access to leaf material on leaf breakdown rates in a forested headwater stream in the Luquillo Experimental Forest, Puerto Rico. We measured leaf mass loss using fine and coarse mesh bags over 12 weeks for two common riparian species: Cecropia schreberiana (Moraceae) and Dacryodes excelsa (Burseraceae). Coarse mesh allowed freshwater shrimp and other macroinvertebrates to access leaf material, while fine mesh did not. Leaf breakdown rates did not differ between C. schreberiana and D. excelsa in coarse mesh bags (?0.0375/day vs. ?0.0395/day, respectively), but C. schreberiana breakdown was significantly slower than D. excelsa in fine mesh bags (?0.0159/day vs. ?0.0266/day). Additionally, breakdown in fine mesh bags was significantly slower compared to coarse mesh bags for C. schreberiana, but less so for D. excelsa. Breakdown rates for all treatments were fast relative to those in temperate‐zone streams indicating that both macroinvertebrates and macroinvertebrate‐independent processing can strongly influence leaf decomposition in tropical streams. The difference between C. schreberiana and D. excelsa indicates that the effect of macroinvertebrate exclusion can change with leaf type.     

Effects of organic matter and season on leaf litter colonisation and breakdown in cave streams

1. Low organic matter availability is thought to be a primary factor influencing evolutionary and ecological processes in cave ecosystems. We examined links among organic matter abundance, macroinvertebrate community structure and breakdown rates of red maple (Acer rubrum) and corn litter (Zea mays) in coarse‐ (10 × 8 mm) and fine‐mesh (500‐μm) litter bags over two seasonal periods in four cave streams in the south‐eastern U.S.A. 2. Organic matter abundance differed among cave streams, averaging from near zero to 850 g ash‐free dry mass m^?2. Each cave system harboured a different macroinvertebrate community. However, trophic structure was similar among caves, with low shredder biomass (2–17% of total biomass). 3. Corn litter breakdown rates (mean k = 0.005 day^?1) were faster than red maple (mean k = 0.003 day^?1). Breakdown rates in coarse‐mesh bags (k = 0.001–0.012 day^?1) were up to three times faster than in fine‐mesh bags (k = 0.001–0.004 day^?1). Neither invertebrate biomass in litter bags nor breakdown rates were correlated with the ambient abundance of organic matter. Litter breakdown rates showed no significant temporal variation. Epigean (surface‐adapted) invertebrates dominated biomass in litter bags, suggesting that their effects on cave ecosystem processes may be greater than hypogean (cave‐adapted) taxa, the traditional focus of cave studies. 4. The functional diversity of our cave communities and litter breakdown rates are comparable to those found in previous litter breakdown studies in cave streams, suggesting that the factors that control organic matter processing (e.g. trophic structure of communities) may be broadly similar across geographically diverse areas.     

A Comparative Study of Leaf Breakdown of Three Native Tree Species in a Slowly‐Flowing Headwater Stream in the Colombian Andes

The dynamics of leaf breakdown in a headwater Colombian stream were evaluated for the native tree species Myrsine guianensis, Cupania latifolia and Nectandra lineatifolia using coarse and fine mesh litter bags. Ten bags of each species (five of each mesh size) were retrieved from the stream at 1, 8, 15, 30, 60 and 120 days. k values ranged from 0.0008 to 0.0058 day^–1 and density of macroinvertebrates from 35 to 55 individuals per leaf bag, peaking at day 8. Myrsine guianensis degraded more rapidly than the other species for both coarse and fine mesh bags. This species and Nectandra lineatifolia presented differences in k values between coarse and fine mesh bags, suggesting that macroinvertebrates influenced the decay rate. Despite the low densities of macroinvertebrates found, shredders represented 12.7% of individuals and 50 to 68% of the invertebrate biomass in bags, indicating that this functional feeding group was an important component of fauna associated with litter breakdown in this first order tropical stream. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Catchment land-use, macroinvertebrates and detritus processing in headwater streams: taxonomic richness versus function

1. We used a litter bag technique to assess the effect of catchment land-use (forest, wetland, agriculture, urban) on the processing of red maple ( Acer rubrum L.) litter in 17 streams in Maine, U.S.A. Litter processing by fungi was predicted to increase with nutrient concentrations along a gradient of land use, from relatively unmodified to highly modified. Litter processing by litter-shredding macroinvertebrates was predicted to decline along this gradient because of a decline in their taxonomic richness and biomass.
2. Land use was associated with the anticipated gradient in nutrient and macroinvertebrate attributes, and a significant relationship was found between land use and nitrate concentration. There was, however, no significant relationship between land use and soluble reactive phosphorus (SRP) concentration. Similarly, shredder taxonomic richness was significantly related to land use type, whereas shredder biomass showed no significant relationship to land use.
3. Attributes of the shredder assemblage structure and nutrient concentrations were both strong determinants of litter processing. Increasing biomass and taxonomic richness of shredders was significantly related to increasing rates of litter mass loss. Increasing concentrations of nitrate and SRP were significantly related to increasing rates of litter softening below threshold concentrations (approximately 0.20 mg NO₃-N L^–1 and 5 μg SRP L^–1).
4. The potentially additive effects of nitrate and SRP concentrations or shredder richness and biomass on litter processing rates were confounded by the lack of significant correlation between these pairs of variables. Consequently, rates of litter processing (as rates of softening or mass loss) did not vary systematically among different land use regimes.     

Leaf litter decomposition in remote oceanic island streams is driven by microbes and depends on litter quality and environmental conditions

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Stimulation of leaf litter decomposition and associated fungi and invertebrates by moderate eutrophication: implications for stream assessment

1. We investigated the effect of moderate eutrophication on leaf litter decomposition and associated invertebrates in five reference and five eutrophied streams in central Portugal. Fungal parameters and litter N and P dynamics were followed in one pair of streams. Benthic invertebrate parameters that are considered useful in bioassessment were estimated in all streams. Finally, we evaluated the utility of decomposition as a tool to assess stream ecosystem functional integrity. 2. Decomposition of alder and oak leaves in coarse mesh bags was on average 2.3–2.7× faster in eutrophied than in reference streams. This was attributed to stimulation of fungal activity (fungal biomass accrual and sporulation of aquatic hyphomycetes) by dissolved nutrients. These effects were more pronounced for oak litter (lower quality substrate) than alder. N content of leaf litter did not differ between stream types, while P accrual was higher in the eutrophied than in the reference stream. Total invertebrate abundances and richness associated with oak litter, but not with alder, were higher in eutrophied streams. 3. We found only positive correlations between stream nutrients (DIN and SRP) and leaf litter decomposition rates in both fine and coarse mesh bags, associated sporulation rates of aquatic hyphomycetes and, in some cases, total invertebrate abundances and richness. 4. Some metrics based on benthic invertebrate community data (e.g. % shredders, % shredder taxa) were significantly lower in eutrophied than in reference streams, whereas the IBMWP index that is specifically designed for the Iberian peninsula classified all 10 streams in the highest possible class as having ‘very good’ ecological conditions. 5. Leaf litter decomposition was sufficiently sensitive to respond to low levels of eutrophication and could be a useful functional measure to complement assessment programmes based on structural parameters.     

长白山地区溪流冻结初期凋落叶分解与底栖动物定殖的关系

长白山森林源头溪流每年11月至次年4月有约70%的河面被冰覆盖,季节性冻融过程特征明显.为了揭示溪流冻结初期凋落叶分解与底栖动物定殖的关系,在长白山地区1条源头溪流中,利用2种孔径(5和0.3 mm)的尼龙分解袋对色木槭、紫椴、蒙古栎的单一及混合凋落叶进行了为期35 d的分解研究.结果表明: 凋落叶质量损失率在单一树种间差异显著,表现为:色木槭＞紫椴＞蒙古栎,而在4种混合凋落叶间差异不显著;除紫椴和3树种混合凋落叶外,粗、细分解袋间凋落叶质量损失率差异不显著;凋落叶混合效应仅出现在紫椴-蒙古栎混合的粗分解袋内;定殖在不同凋落叶分解袋内的底栖动物群落结构差异较大,但撕食者密度在3种凋落叶间差异不显著,撕食者对凋落叶混合效应的响应也不显著.由结果可知,溪流冻结初期微生物是凋落叶的主要分解者,底栖动物的贡献率较低.虽然撕食者密度较低,但撕食者的活动是凋落叶混合效应出现的必要条件.底栖动物对食物和栖息地有一定的选择性,但由于定殖时间较短,凋落叶对撕食者定殖的影响不显著.本研究对源头溪流生态系统的冬季生态过程研究及生态系统管理具有一定的理论意义.     

Effects of shredding amphipod density on watercress Nasturtium officinale breakdown

Shredding stream invertebrates should have a positive influence on the breakdown rates of leaf litter via direct consumption and particle fragmentation. To determine the effects of shredder density on litter breakdown, breakdown of the emergent stream macrophyte, Nasturtium officinale , was investigated using three litter bag mesh sizes [fine (0.2 mm), medium (1 mm) and coarse (3 mm) mesh] and four stocking densities of the shredder, Gammarus pseudolimnaeus , (0, 4, 8 and 16 per bag). Watercress decayed very rapidly, with breakdown rates ( k values) ranging from 0.075 d^-1 for fine mesh with no shredders to 0.24 d^-1 for coarse mesh. Stocked Gammarus increased breakdown rates significantly in fine mesh bags (p < 0.001), but only marginally in medium mesh bags (p < 0.1). Breakdown rates also increased significantly with mesh size. A regression model showed a significant relation of breakdown rate to Gammarus density and mesh size. These results clearly show that shredders can significantly influence breakdown rates and can account for up to 30% of breakdown, but that mesh size effects such as particle size reduction and loss are also very important.     

Litter processing in diffuse and conduit springs

We used bedrock geology and prior water chemistry data to classify and choose 4 conduit and 4 diffuse flow springs in a karst region of southeast Minnesota. Decomposition processes and chemical/physical conditions were compared between spring classes during two seasons (May and September, 1987).Although large storms and conduit run-in events did not occur during our sampling, baseline water chemistry supported our a priori spring classification. Baseline nitrate and atrazine levels were significantly higher in conduit than in diffuse springs. During a minor run-in event, atrazine levels increased significantly to at least 1.2 µg L^–1 in all conduit systems, but remained unchanged in diffuse systems.Decay rates of the two predominant litter sources in the springs (watercress [Nasturtium officinale R.Br.] in May and box elder [Acer negundo L.] in September) were used to assess differences in biological activity between the spring classes. During May, watercress decayed (1 mm coarse mesh bags) at approximately the same rate in conduit and diffuse systems, k = 0.094 and 0.099 respectively. This result was unexpected since shredder colonization was much greater on litter bags in diffuse systems. In September, boxelder decay rate was significantly higher in diffuse (k = 0.018) than in conduit (k = 0.013) springs; and decay rates seemed to reflect significantly higher shredder colonization on bags in diffuse systems. Microbial activity on watercress and boxelder litter, measured as electron transport system (ETS) activity, was seldom significantly different between spring classes. Also, both watercress and boxelder litter decay rates for fine mesh bags (240µm) were similar between conduit and diffuse springs, suggesting that microbial processing did not differ greatly between spring classes.In conduit springs, low shredder colonization was apparently the result of low benthic shredder densities. Shredders and other macroinvertebrates may be adversely affected by discharge fluctuations from conduit run-in events. We suspect that, during times of low flow, watercress standing stock was also an important influence on shredder abundance.     

Leaf litter quality drives the feeding by invertebrate shredders in tropical streams

Amazon and Cerrado‐forested streams show natural fluctuations in leaf litter quantity along the time and space, suggesting a change on litter quality input. These natural fluctuations of leaf litter have repercussion on the organic matter cycling and consequently effects on leaf decomposition in forested streams. The effects of the quantity of leaf litter with contrasting traits on consumption by larvae of shredder insects from biomes with different organic matter dynamics have still been an understudied question. The Trichoptera Phylloicus spp. is a typical shredder in tropical headwater streams and keep an important role in leaf litter decomposition. Here, we assessed the consumption by shredder Phylloicus spp., from Amazonia and Cerrado biomes, on higher (Maprounea guianensis) and lower quality leaves (Inga laurina) in different proportions and quantities. Experiments were performed concomitantly in microcosms approaches, simulating Cerrado and Amazonian streams. Higher leaf consumption occurred in Cerrado microcosms. Litter quantity influenced negatively leaf consumption by shredders in Cerrado, in opposition to Amazonia, where consumption was not affected by leaf quantity. In both sites, we observed higher consumption by shredders in treatment with only M. guianensis and no difference between other treatments with mixture of leaves. In treatment with litter of I. laurina, we noted the use of substrate for case building (due to the higher leaf toughness), affecting the fragmentation process. Therefore, our results indicate that leaf litter quality drives the preference of consumption by Phylloicus larvae in Cerrado and Amazonia streams.     

Do non-native Platanus hybrida riparian plantations affect leaf litter decomposition in streams?

In forest headwater streams where the riparian canopy limits autochthonous primary production, leaf litter decomposition is a key process controlling nutrient and carbon cycling. Any alteration of the riparian vegetation may influence litter decomposition and detrital food webs. We evaluated the effect of non-native Platanus hybrida riparian plantations on leaf litter decomposition in Mediterranean streams. The experiment was conducted in six headwater streams; three lined by native riparian vegetation and three crossing P. hybrida plantations. We have characterized the processing rates of alder leaves and the assemblages of shredder macroinvertebrates and fungi. Litter decomposition was significantly faster in the P. hybrida than in the reference streams. Although the dissolved inorganic nitrogen concentration was higher in P. hybrida, no significant effect was observed in decomposition rates. Differences in decomposition rates reflected the macroinvertebrate and shredder colonization in alder litter, with higher abundance and richness in the P. hybrida streams. However, aquatic hyphomycete sporulation rate was higher in reference streams, suggesting that the variation in decomposition rates is a direct consequence of shredder abundance. Our findings support part of the substrate quality-matrix quality (SMI) hypothesis, which expects that high-quality litter will show increased decomposition rates in a low-quality litter matrix.     

Leaf‐litter breakdown in pasture and deciduous woodland streams: a comparison among three European regions

1. Human land‐use has altered catchments on a large scale in most parts of the world, with one of the most profound changes relevant for streams and rivers being the widespread clearance of woody riparian vegetation to make way for livestock grazing pasture. Increasingly, environmental legislation, such as the EU Water Framework Directive (EU WFD), calls for bioassessment tools that can detect such anthropogenic impacts on ecosystem functioning. 2. We conducted a large‐scale field experiment in 30 European streams to quantify leaf‐litter breakdown, a key ecosystem process, in streams whose riparian zones and catchments had been cleared for pasture compared with those in native deciduous woodland. The study encompassed a west–east gradient, from Ireland to Switzerland to Romania, with each of the three countries representing a distinct region. We used coarse‐mesh and fine‐mesh litter bags (10 and 0.5 mm, respectively) to assess total, microbial and, by difference, macroinvertebrate‐mediated breakdown. 3. Overall, total breakdown rates did not differ between land‐use categories, but in some regions macroinvertebrate‐mediated breakdown was higher in deciduous woodland streams, whereas microbial breakdown was higher in pasture streams. This result suggests that overall ecosystem functioning is maintained by compensatory increases in microbial activity in pasture streams. 4. We suggest that simple coefficients of breakdown rates on their own often might not be powerful enough as a bioassessment tool for detecting differences related to land‐use such as riparian vegetation removal. However, shifts in the relative contributions to breakdown by microbial decomposers versus invertebrate detritivores, as revealed by the ratios of their associated breakdown rate coefficients, showed clear responses to land‐use.     

Leaf Decomposition in a Mountain Stream in the Sultanate of Oman

Decomposition of Juglans regia leaves was studied in fine and coarse mesh bags in a permanent mountain stream in Oman. A rapid initial mass loss, attributed to leaching, was followed by a more gradual decline. Daily exponential decay rates (k) calculated over 32 days were 0.011 (fine mesh litter bags) and 0.014 (coarse mesh litter bags). The difference between bag types was not significant, suggesting limited impact of leaf‐shredding invertebrates. Ergosterol levels on leaves from fine mesh bags peaked at 0.3 mg g^–1 AFDM after 16 days of stream exposure. During the experimental period, which followed the annual leaf fall, the concentration of aquatic hyphomycete conidia in the stream varied between 82 and 1362 l^–1. Based on the morphology of conidia found in the water column or released from leaves, we identified 14 species of aquatic hyphomycetes. Tetracladium apiense was the most common taxon (62.2% of conidia in water column during the field experiment). Three other Tetracladium species contributed another 8%. Plating out leaf particles yielded common epiphytic taxa such as Alternaria sp., Aureobasidium pullulans and Phoma sp. The measured metrics of leaf decay in this desert stream fall within the range of values observed in temperate and tropical streams, with clear evidence for an early leaching phase, and no evidence of a strong impact of leaf shredders. The community of aquatic hyphomycetes appears impoverished. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Responses in organic matter accumulation and processing to an experimental wood addition in three headwater streams

1. Additions of large wood are being used to restore streams that have been subjected to channelization, wood removal or riparian timber harvest. This added wood potentially increases channel stability, habitat complexity and organic matter retention and improves habitat and productivity of higher trophic levels. However, few stream restorations monitor restoration effectiveness after project completion. 2. We added 25 aspen logs (each 2.5 m length × 0.5 m diameter) to 100‐m reaches of each of three forested headwater streams in the Upper Peninsula of Michigan, U.S.A. These wood‐poor streams drain forests that were completely harvested of timber over a century ago and have been selectively logged for the past 50–60 years. An upstream unmanipulateds 100‐m reach in each stream served as the control. 3. We evaluated responses in organic matter processing by measuring red maple leaf decomposition 1 year before and 2 years after wood addition. We also quantified coarse organic matter standing stocks in the main channel and in debris accumulations associated with large wood. In response to wood addition, we predicted both organic matter standing stocks and leaf decomposition rates would increase, thereby enhancing resource availability to higher trophic levels. 4. Leaf decomposition rates did not change following wood addition. Temporal variation in rates among streams was mostly explained by differences in degree days, water velocity, scour/burial and water column inorganic nitrogen concentrations, but not large wood. Variation within streams across years was explained by differences in degree days, water velocity and shredder biomass. 5. Contrary to our prediction, organic matter standing stocks did not increase significantly at the reach scale. However, the experimentally added wood retained c. 4% of total annual coarse benthic organic matter (CBOM) in the first year and an additional c. 15% in the second year, suggesting accumulation over time in the manipulated reaches. The CBOM held by the new logs may be more biologically available because it is less susceptible to burial and transport than material in the streambed. 6. Some shredding macroinvertebrates responded to changes caused by the wood additions. In particular, the common caddisfly shredder, Lepidostoma sp., increased in abundance in leaf bags following wood addition, whereas the biomass of the winter stoneflies, Capniidae, declined in the first year. 7. Considerable funds are spent to restore in‐stream habitat, but few restorations are monitored, particularly over long periods (>5 years). Our results show that longer‐term monitoring is needed to determine the efficacy of these restorations on ecosystem function; organic matter decomposition in our low‐gradient streams did not respond to a substantial increase in large wood after 2 years.     

Preconditioning of leaves by solar radiation and anoxia affects microbial colonisation and rate of leaf mass loss in an intermittent stream

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