Alan Puttock

Eurasian beaver activity increases water storage, attenuates flow and mitigates diffuse pollution from intensively-managed grasslands

Beavers are the archetypal keystone species, which can profoundly alter ecosystem structure and function through their ecosystem engineering activity, most notably the building of dams. This can have a major impact upon water resource management, flow regimes and water quality. Previous research has predominantly focused on the activities of North American beaver (Castor canadensis) located in very different environments, to the intensive lowland agricultural landscapes of the United Kingdom and elsewhere in Europe. Two Eurasian beavers (Castor fiber) were introduced to a wooded site, situated on a first order tributary, draining from intensively managed grassland. The site was monitored to understand impacts upon water storage, flow regimes and water quality. Results indicated that beaver activity, primarily via the creation of 13 dams, has increased water storage within the site (holding ca. 1000m3 in beaver ponds) and beavers were likely to have had a significant flow attenuation impact, as determined from peak discharges (mean 30±19% reduction), total discharges (mean 34±9% reduction) and peak rainfall to peak discharge lag times (mean 29±21% increase) during storm events. Event monitoring of water entering and leaving the site showed lower concentrations of suspended sediment, nitrogen and phosphate leaving the site (e.g. for suspended sediment; average entering site: 112±72mgl-1, average leaving site: 39±37mgl-1). Combined with attenuated flows, this resulted in lower diffuse pollutant loads in water downstream. Conversely, dissolved organic carbon concentrations and loads downstream were higher. These observed changes are argued to be directly attributable to beaver activity at the site which has created a diverse wetland environment, reducing downstream hydrological connectivity. Results have important implications for beaver reintroduction programs which may provide nature based solutions to the catchment-scale water resource management issues that are faced in agricultural landscapes.

Woody plant encroachment into grasslands leads to accelerated erosion of previously stable organic carbon from dryland soils

Drylands worldwide are experiencing rapid and extensive environmental change, concomitant with the encroachment of woody vegetation into grasslands. Woody encroachment leads to changes in both the structure and function of dryland ecosystems and has been shown to result in accelerated soil erosion and loss of soil nutrients. Covering 40% of the terrestrial land surface, dryland environments are of global importance, both as a habitat and a soil carbon store. Relationships between environmental change, soil erosion, and the carbon cycle are uncertain. There is a clear need to further our understanding of dryland vegetation change and impacts on carbon dynamics. Here two grass-to-woody ecotones that occur across large areas of the southwestern United States are investigated. This study takes a multidisciplinary approach, combining ecohydrological monitoring of structure and function and a dual-proxy biogeochemical tracing approach using the unique natural biochemical signatures of the vegetation. Results show that following woody encroachment, not only do these drylands lose significantly more soil and organic carbon via erosion but that this includes significant amounts of legacy organic carbon which would previously have been stable under grass cover. Results suggest that these dryland soils may not act as a stable organic carbon pool, following encroachment and that accelerated erosion of carbon, driven by vegetation change, has important implications for carbon dynamics.

Stable carbon isotope analysis of fluvial sediment fluxes over two contrasting C4-C3 semi-arid vegetation transitions†

RATIONALE Globally, many drylands are experiencing the encroachment of woody vegetation into grasslands. These changes in ecosystem structure and processes can result in increased sediment and nutrient fluxes due to fluvial erosion. As these changes are often accompanied by a shift from C(4) to C(3) vegetation with characteristic δ(13) C values, stable isotope analysis provides a promising mechanism for tracing these fluxes. METHODS Input vegetation, surface sediment and fluvially eroded sediment samples were collected across two contrasting C(4) -C(3) dryland vegetation transitions in New Mexico, USA. Isotope ratio mass spectrometric analyses were performed using a Carlo Erba NA2000 analyser interfaced to a SerCon 20-22 isotope ratio mass spectrometer to determine bulk δ(13) C values. RESULTS Stable isotope analyses of contemporary input vegetation and surface sediments over the monitored transitions showed significant differences (p <0.05) in the bulk δ(13) C values of C(4) Bouteloua sp. (grama) grassland, C(3) Larrea tridentata (creosote) shrubland and C(3) Pinus edulis/Juniperus monosperma (piñon-juniper) woodland sites. Significantly, this distinctive δ(13) C value was maintained in the bulk δ(13) C values of fluvially eroded sediment from each of the sites, with no significant variation between surface sediment and eroded sediment values. CONCLUSIONS The significant differences in bulk δ(13) C values between sites were dependent on vegetation input. Importantly, these values were robustly expressed in fluvially eroded sediments, suggesting that stable isotope analysis is suitable for tracing sediment fluxes. Due to the prevalent nature of these dryland vegetation transitions in the USA and globally, further development of stable isotope ratio mass spectrometry has provided a valuable tool for enhanced understanding of functional changes in these ecosystems.

Dryland, calcareous soils store (and lose) significant quantities of near-surface organic carbon

Semiarid ecosystems are susceptible to changes in dominant vegetation which may have significant implications for terrestrial carbon dynamics. The present study examines the distribution of organic carbon (OC) between particle size fractions in near-surface (0–0.05 m) soil and the water erosion-induced redistribution of particle-associated OC over a grass-shrub ecotone, in a semiarid landscape, subject to land degradation. Coarse (>2 mm) particles have comparable average OC concentrations to the fine (<2 mm) particles, accounting for ~24–38% of the OC stock in the near-surface soil. This may be due to aggregate stabilization by precipitated calcium carbonate in these calcareous arid soils. Critically, standard protocols assuming that coarse fraction particles contain no OC are likely to underestimate soil OC stocks substantially, especially in soils with strongly stabilized aggregates. Sediment eroded from four hillslope scale (10 × 30 m) sites during rainstorm events was monitored over four annual monsoon seasons. Eroded sediment was significantly enriched in OC; enrichment increased significantly across the grass-shrub ecotone and appears to be an enduring phenomenon probably sustained through the dynamic replacement of preferentially removed organic matter. The average erosion-induced OC event yield increased sixfold across the ecotone from grass-dominated to shrub-dominated ecosystems, due to both greater erosion and greater OC enrichment. This erosional pathway is rarely considered when comparing the carbon budgets of grasslands and shrublands, yet this accelerated efflux of OC may be important for long-term carbon storage potentials of dryland ecosystems.This research was conducted while A.M. Cunliffe was in receipt of a NERC Doctoral Training grant (NE/K500902/1) and was supported by the NSF Long Term Ecological Research Program at the Sevilleta National Wildlife Refuge (DEB-1232294) This research was conducted while A.M.C. was in receipt of a NERC Doctoral Training grant (NE/ K500902/1) and was supported by the NSF Long Term Ecological Research Program at the Sevilleta National Wildlife Refuge (DEB-1232294). L.T. was supported by travel bursaries from the University of Sheffield, the Worshipful Company of Farmers, and the Royal Society Dudley Stamp Memorial Fund Award. We are grateful to John Buffington, Jon Pelletier, and two anonymous reviews whose suggestions greatly improved upon earlier versions of this paper.

Sediment and nutrient storage in a beaver engineered wetland: Sediment and Nutrient Storage in a Beaver Engineered Wetland

Abstract Beavers, primarily through the building of dams, can deliver significant geomorphic modifications and result in changes to nutrient and sediment fluxes. Research is required to understand the implications and possible benefits of widespread beaver reintroduction across Europe. This study surveyed sediment depth, extent and carbon/nitrogen content in a sequence of beaver pond and dam structures in South West England, where a pair of Eurasian beavers (Castor fiber) were introduced to a controlled 1.8 ha site in 2011. Results showed that the 13 beaver ponds subsequently created hold a total of 101.53 ± 16.24 t of sediment, equating to a normalised average of 71.40 ± 39.65 kg m2. The ponds also hold 15.90 ± 2.50 t of carbon and 0.91 ± 0.15 t of nitrogen within the accumulated pond sediment. The size of beaver pond appeared to be the main control over sediment storage, with larger ponds holding a greater mass of sediment per unit area. Furthermore, position within the site appeared to play a role with the upper‐middle ponds, nearest to the intensively‐farmed headwaters of the catchment, holding a greater amount of sediment. Carbon and nitrogen concentrations in ponds showed no clear trends, but were significantly higher than in stream bed sediment upstream of the site. We estimate that >70% of sediment in the ponds is sourced from the intensively managed grassland catchment upstream, with the remainder from in situ redistribution by beaver activity. While further research is required into the long‐term storage and nutrient cycling within beaver ponds, results indicate that beaver ponds may help to mitigate the negative off‐site impacts of accelerated soil erosion and diffuse pollution from agriculturally dominated landscapes such as the intensively managed grassland in this study.

The Study of Land Degradation in Drylands: State of the Art

Land degradation is difficult to define because land can only be considered degraded with respect to some use to which it may be put. However, physical and biological properties of the landscape are typically measured to characterize degradation rather than its inherent or potential utility. One approach to characterizing land degradation is by assessing the provisioning of ecosystem services. Most provisioning ecosystem services depend on water, and water management is crucial to maintaining and increasing ecosystem services in arid lands. In contrast, vegetation change has been most commonly employed as an indicator of land degradation. Nevertheless, the close relationship that exists between vegetation and other biophysical processes of the environment means that any change in vegetation will result in a concomitant change to these other processes also. Of particular importance is a change in vegetation distribution since the spatial distribution of associated biophysical parameters controls landscape fluxes, and hence degradation, by controlling landscape connectivity. From a management perspective, an understanding of the degree of connectivity in a landscape can aid in triage of remediation efforts. Areas that are dominated by long connected pathways will not respond to localized, small-scale manipulations because those pathways present inertia that a small-scale manipulation cannot overcome. Two important ecosystem services provided by drylands are grazing land and agricultural land. Both land uses can be drivers of degradation. The role of grazing in land degradation depends on several factors which can be grouped into three categories: number of animals, kind of animal species and grazing system. For agriculture, systematic crop residue removal without fertilisation, poor cultivation practices and extensive soil salinization are examples of mismanagement that may lead to land degradation. Aside from the immediate provisioning of food, drylands provide ecosystem services at a broader scale. Drylands are highly significant to the global carbon cycle. Land degradation in drylands has implications for the effectiveness of carbon sequestration as well as for storage (through soil erosion). Because many dryland soils have been degraded they are currently far from saturated with carbon and as a result their potential to sequester carbon may be highly significant. To understand land degradation better, efforts have been made to develop integrated human-environment research that overcomes the perceived deficiencies of reductionist, discipline-based research. However, much integrated environmental research to-date has resulted in a ‘hierarchical relationship’ between the human and physical components. Three approaches have been advocated to improve human-environment understanding: (a) systems science that emphasises feedbacks between integrated human and natural systems; (b) computer-simulation modelling that explicitly represents the interaction of individual human decisions and physical processes; and (c) participatory research that emphasises engagement with the actors in the region being studied. However, many questions remain open, and advancing beyond narrow scientific disciplinary specialization is vital if the hierarchical relationship in understanding physical and social causes of land degradation is to be broken.

The study of land degradation in drylands

Land degradation is difficult to define because land can only be considered degraded with respect to some use to which it may be put. However, physical and biological properties of the landscape are typically measured to characterize degradation rather than its inherent or potential utility. One approach to characterizing land degradation is by assessing the provisioning of ecosystem services. Most provisioning ecosystem services depend on water, and water management is crucial to maintaining and increasing ecosystem services in arid lands. In contrast, vegetation change has been most commonly employed as an indicator of land degradation. Nevertheless, the close relationship that exists between vegetation and other biophysical processes of the environment means that any change in vegetation will result in a concomitant change to these other processes also. Of particular importance is a change in vegetation distribution since the spatial distribution of associated biophysical parameters controls landscape fluxes, and hence degradation, by controlling landscape connectivity. From a management perspective, an understanding of the degree of connectivity in a landscape can aid in triage of remediation efforts. Areas that are dominated by long connected pathways will not respond to localized, small-scale manipulations because those pathways present inertia that a small-scale manipulation cannot overcome. Two important ecosystem services provided by drylands are grazing land and agricultural land. Both land uses can be drivers of degradation. The role of grazing in land degradation depends on several factors which can be grouped into three categories: number of animals, kind of animal species and grazing system. For agriculture, systematic crop residue removal without fertilisation, poor cultivation practices and extensive soil salinization are examples of mismanagement that may lead to land degradation. Aside from the immediate provisioning of food, drylands provide ecosystem services at a broader scale. Drylands are highly significant to the global carbon cycle. Land degradation in drylands has implications for the effectiveness of carbon sequestration as well as for storage (through soil erosion). Because many dryland soils have been degraded they are currently far from saturated with carbon and as a result their potential to sequester carbon may be highly significant. To understand land degradation better, efforts have been made to develop integrated human-environment research that overcomes the perceived deficiencies of reductionist, discipline-based research. However, much integrated environmental research to-date has resulted in a ‘hierarchical relationship’ between the human and physical components. Three approaches have been advocated to improve human-environment understanding: (a) systems science that emphasises feedbacks between integrated human and natural systems; (b) computer-simulation modelling that explicitly represents the interaction of individual human decisions and physical processes; and (c) participatory research that emphasises engagement with the actors in the region being studied. However, many questions remain open, and advancing beyond narrow scientific disciplinary specialization is vital if the hierarchical relationship in understanding physical and social causes of land degradation is to be broken.