Sitka Spruce Plantation Research Articles

Investigating the abundance and diversity of sawflies (Hymenoptera, Symphyta) within commercial Sitka spruce plantation forests in mid‐Wales

Abstract Sitka spruce is a widely planted and economically important tree species in the United Kingdom; hence, identifying any future biotic or abiotic threats to its productivity is important. Defoliating insects are widely considered to be major forest disturbance agents, and these can include the sawflies (Symphyta), which are well known to reach high population densities that can cause significant outbreak events, leading to tree mortality or impacts on growth. The aim of the current study was to investigate the abundance and diversity of sawflies within stands of Sitka spruce over an elevation range and to evaluate whether there were any potentially important species present. Experimental plots were established in two commercial forest plantations in mid‐Wales, and the adult sawfly community was assessed over a 3‐year period using yellow pan traps. Sixty species of sawfly were caught over the 3‐year period, which accounted for >11% of all known British sawfly species. Results indicated that there were generally greater abundance and diversity of sawflies at lower elevational plots, where temperatures were higher. The most abundant sawfly species captured in the study was the notable defoliator species, the mountain spruce sawfly (Epicenematus montanus), which was found in particularly high numbers in the low‐elevation plot in Hafren forest. A further seven spruce‐associated sawfly species were also caught across the experimental plots. The study highlighted the presence of a number of notable spruce‐associated sawfly species (E. montanus and Pristiphora spp.), some of which have caused significant defoliation of spruce (Picea) species in Europe. The generally higher abundance of these sawflies at lower elevations indicated that Sitka spruce may potentially become more vulnerable to defoliation events caused by sawfly outbreaks in the future, especially when considering current projected climate change predictions.

Modelling the impact of forest management and CO2-fertilisation on growth and demography in a Sitka spruce plantation

Afforestation and reforestation to meet ‘Net Zero’ emissions targets are considered a necessary policy by many countries. Their potential benefits are usually assessed through forest carbon and growth models. The implementation of vegetation demography gives scope to represent forest management and other size-dependent processes within land surface models (LSMs). In this paper, we evaluate the impact of including management within an LSM that represents demography, using both in-situ and reanalysis climate drivers at a mature, upland Sitka spruce plantation in Northumberland, UK. We compare historical simulations with fixed and variable CO2 concentrations, and with and without tree thinning implemented. Simulations are evaluated against the observed vegetation structure and carbon fluxes. Including thinning and the impact of increasing CO2 concentration (‘CO2 fertilisation’) gave more realistic estimates of stand-structure and physical characteristics. Historical CO2 fertilisation had a noticeable effect on the Gross Primary Productivity seasonal–diurnal cycle and contributed to approximately 7% higher stand biomass by 2018. The net effect of both processes resulted in a decrease of tree density and biomass, but an increase in tree height and leaf area index.

Invertebrate predators in Sitka spruce plantations managed by clear-cutting or continuous-cover silvicultural systems

Invertebrate predators of aphids were sampled at ground level and in the canopy in 13 even-aged Sitka spruce Picea sitchensis forest stands managed by clear-cutting and replanting, and in 11 Sitka spruce stands managed as continuous cover forestry using either shelterwood or group selection silvicultural systems. The expectation was that stands managed using shelterwood or group selection systems would support a greater abundance and diversity of invertebrate predators, because of their more complex internal structure, but results were mixed. Hoverflies (Syrphidae), predatory bugs (Hemiptera), ladybird beetles (Coccinellidae) and spiders (Araneae) were more abundant in shelterwood and group selection stands, but soldier beetles (Cantharidae), lacewings (Neuroptera) and harvestmen (Opiliones) were more abundant in mid-rotation and mature even-aged stands, and vespid wasps (Vespidae) showed a similar abundance across all stand types. In contrast, species richness and diversity of the insect predator groups were higher in shelterwood and group selection stands, even for groups that were less abundant in those stands. It was not clear, however, whether the greater species richness of insect predators in shelterwood and group selection was the result of these stands supporting breeding populations of a wider range of species, including less common species, or was because the more open structure of these stands allowed introgression of species from other habitats within the forest.

Forest canopy nitrogen uptake can supply entire foliar demand

Abstract Plant canopies intercept, process and potentially assimilate atmospheric nitrogen (N) additions, but the forest‐scale effects of canopy processes on N cycling and plant nutrition are not clear. Substantial method artefacts and scaling issues exist in previous experimental studies which measure relevant N fluxes either at (a) natural abundance, (b) via a 15N tracer or (c) by incorporation of additional 15N into measured biomass, meaning these processes are often overlooked or discounted. Here, working in a mature Sitka spruce plantation and under ambient conditions, we used all three of these methods independently at stand, tree and branch scale to assess the effects of canopy interception on capture, assimilation and incorporation of inorganic N deposition by canopy biomass. We consistently found that above 70% of N deposition was unaccounted for (i.e. assimilated, processed beyond detectability, volatilised or retained) when passing through the canopy, independent of the assessment method. Using short‐term 15N tracers, we found that this unaccounted N was retained in canopy tissues. Apparent uptake from monitoring of ecohydrological fluxes was sustained and consistent, suggesting that the tree canopies were a sink for N deposition. Seasonal variation in recovery also suggested biological activities influenced fluxes and transformations of this ion within the canopy. The total amount of this canopy N uptake was greater than the mean annual N in litterfall, implying that this flux was supporting the turnover of the canopy. When we used 15N to directly assess uptake into branch biomass we also did not find evidence for immediate relocation to other parts of the tree. Our findings suggest that at our N‐limited site, canopy uptake of N deposition is likely to be as important as root uptake of N but may be limited to supplying N to the canopy rather than the whole tree. Further research in this area is crucial to better understand the interactions of future changes in N deposition on primary production and carbon storage in forests. A free Plain Language Summary can be found within the Supporting Information of this article.

Effects of Sitka spruce masting on phenology and demography of siskins Spinus spinus

Masting behaviour of Sitka spruce Picea sitchensis may influence Eurasian siskin Spinus spinus breeding ecology as breeding siskins specialize on spruce seeds. We caught siskins and other small passerines over 16 years using mist nets adjacent to large plantations of mature Sitka spruce. We sexed, aged, measured and weighed the birds and collected feather samples from fledglings to measure nitrogen and carbon stable isotope ratios. Siskins departed in late summer, and returned, and bred earlier in years of higher cone abundance. Nitrogen and carbon isotopes indicated that siskins fed their chicks on Sitka spruce seeds in most years, and more so in years of high cone production. More siskins were caught following heavy rainfall, when the cones had closed, encouraging the birds to seek alternative food sources. Fledglings were not heavier or larger in years with higher cone crops but were more numerous. However, the age ratio of siskins caught the following year was unaffected by cone crop. Given their reliance on Sitka spruce seeds, climate change may have a major impact on siskin numbers by altering the availability of Sitka spruce seeds, either through changes in masting patterns or cone opening, or due to climate-related changes in forestry practices. Siskins represent a valuable study system to conservation ecology, where a native species is ecologically reliant on introduced taxa.

No effect of warming and watering on soil nitrous oxide fluxes in a temperate sitka spruce forest ecosystem

ABSTRACT Soil fluxes of nitrous oxide (N2O) play an important role in the global greenhouse gas budget. However, the response of soil N2O emissions to climate change in temperate forest plantations is not yet well understood. In this study, we assessed the responses of soil N2O fluxes to experimental warming with or without water addition, using a replicated in situ heating (~2°C above ambient) and water addition (170 mm) experiment in a temperate Sitka spruce plantation forest over the period 2014–2016. We found that seasonal fluxes of N2O during the year were highly variable, ranging from net uptake to net emissions. Seasonal variations in soil N2O fluxes were not correlated with either soil temperature or soil moisture. In addition, none of the individual warming/watering treatments, or their interactions, had significant effects on soil N2O fluxes and N-related soil properties. Overall, our results suggest that despite future increases in temperature, soil N2O emission may remain largely unchanged in many temperate forest ecosystems that are often N-limited.

Application of hierarchical clustering to identify high risk pests to Sitka spruce: Ireland as a case study

Abstract Invertebrate forest pests and pathogens can cause considerable economic losses and modern patterns of trade have facilitated the international movement of pest species on an unprecedented level. This upsurge in trade has increased the pathways available to high risk species, facilitating entry and potential establishment in nations where they were previously absent. To support policy and pest prioritization, pest risk analyses are conducted to decide ‘if’ and ‘how’ pests should be regulated in order to prevent entry or establishment; however, they cannot be carried out for every potential pest. This paper utilizes a hierarchical clustering (HC) approach to analyse distribution data for pests of Sitka spruce (Picea sitchensis (Bong.) Carr.) in order to identify species of high risk to Ireland, as well as potential source regions of these pests. The presence and absence of almost a 1000 pests across 386 regions globally are clustered based on their similarity of pest assemblages, to provide an objective examination of the highest risk pests to Irish forestry. Regional clusters were produced for each taxon analysed including the Coleoptera, Diptera, Hemiptera, Hymenoptera, Nematoda, Lepidoptera and the Fungi. The results produced by the HC analysis were interpreted with regard to biological realism and climate. Biologically meaningful clusters were produced for each of the groups, except for the Diptera and Nematoda, and each of the species analysed were ranked within their group by a quantitative risk index specific to the island of Ireland. The impact of uncertainty in the distribution data is also examined, in order to assess its influence over the final groupings produced. The outputs from this analysis suggest that the highest risk pests for Ireland’s Sitka spruce plantations will originate from within Europe. Ultimately, Ireland could benefit from seeking regulation for some of the higher ranking pests identified in this analysis. This analysis provides the first of its type for Sitka spruce, as well as its application in Ireland. It also serves to highlight the potential utility of HC as a ‘first approach’ to assessing the risk posed by alien species to hitherto novel regions.

Methods for predicting Sitka spruce natural regeneration presence and density in the UK

Abstract: Natural regeneration is crucial for silvicultural approaches based on the continuous presence of a forest cover, or Continuous Cover Forestry (CCF). Sitka spruce (Picea sitchensis) is the main commercial species in the United Kingdom (UK), and its potential for CCF has been demonstrated in various studies. However, there are no quantitative models available to predict its natural regeneration in the country. We describe models for Sitka spruce seedlings presence and density under canopy cover in the UK forests, to be used as a substitution of a regeneration survey. Using a natural regeneration dataset comprised of 340 plots, a Generalized Linear Mixed Model (GLMM) was calibrated to estimate the likelihood of regeneration presence at plot level. Seedling density was simulated in a subsequent step using only the subset of data with regeneration presence (138 plots): we compared methods based on GLMMs calibrated to the observed seedling density, and the simple generation of random numbers similar in distribution to the observed values. We validated the models with a cross-validation method using the calibration dataset and with an independent dataset of 78 plots collected in forests already in the process of transformation to CCF. The best GLMM for regeneration presence included age of the plantation, time after last thinning, favourable ground cover and basal area. After the cross-validation, 73% of the plots were correctly estimated (76% for presence of regeneration and 71% for the absence). After the independent validation process, 82% of the plots were correctly estimated, although 100% for presence of regeneration and only 12% for the absence. Both methods for estimating seedling density had a poor performance, both with the cross-validation and independent validation. The results showed that the tools here described are appropriate for estimating regeneration presence in traditional Sitka spruce plantations. However, alternative methods are required for forests already in an advanced stage of transformation to CCF systems.

Current, emerging and potential pest threats to Sitka spruce plantations and the role of pest risk analysis in preventing new pest introductions to Ireland

Sitka spruce (Picea sitchensis) is the predominant tree species used in Irish forestry, comprising over 50 per cent of the forest estate. The island of Ireland is free of many of the most damaging pests and diseases of spruce present in Europe, and consequently forest stands of Sitka spruce are generally healthy and productive. FORM is a research project with the aim to enhance productivity and pest and disease resistance to key forest species in Ireland. It includes a work package to identify and risk analyse threats to Sitka spruce. A global pest list of spruce was compiled and identified over 1000 pests and potential pests of spruce. Using the pest list as a starting point, this review examines the major pests of Sitka spruce both in its native range in the Pacific Northwest of North America and within Europe where it is grown as an exotic. It also reviews emerging pests and diseases of spruces in general that may threaten Sitka spruce plantations in Ireland. Details of how pest risk analysis of these threats can aid in their continued exclusion are discussed.

Analysis of fungal endophytes in Scottish Sitka spruce plantations shows extensive infections, novel host partners and gives insights into origins

AbstractThis preliminary study identified the fungal endophytes of Sitka spruce (Picea sitchensis)in four Scottish plantations, analysed their recruitment possibilities and community structure across the sites, and identified the possible origins of the endophytes in this non‐native host. Although Sitka spruce is native to north‐western North America, it comprises a huge portion of Scottish forestry and is an economically vital timber tree. Needles from two age classes were collected and cultured for emerging endophyte isolates. Using a combination of morphological features and genetic sequencing of the nuclear ribosomalITSregions of representative morphological types, 57 morphotypes were recorded which represented at least 15 different species, including a hypothesized sister species to the pathogen ofPseudotsuga menziesiiin North America and Europe,Nothophaeocryptopus gaeumannii(previouslyP. gaeumannii) that was present at all sites tested. Diversity, recruitment, site, needle age and needle part effects are discussed. The phylogeny ofNothophaeocryptopusbased on analysis of combinedITSand Tub2 sequence data is given in order to assess the relationship of the unknown endophyte to the Douglas fir pathogenN. gaeumannii.

Flexible foraging strategies in Pipistrellus pygmaeus in response to abundant but ephemeral prey.

There is growing recognition that with sympathetic management, plantation forests may contain more biodiversity than previously thought. However, the extent to which they may support bat populations is contentious. Many studies have demonstrated active avoidance of coniferous plantations and attributed this to the lack of available roost sites and low invertebrate density. In contrast, other work, carried out in plantation dominated landscapes have shown that certain bat species are able to exploit these areas. However, the extent to which bats use plantations for roosting and foraging, or simply move through the plantation matrix to access more favourable sites is unclear. We radio tracked female Pipistrellus pygmaeus over two summers to establish the extent to which individual bats use Sitka Spruce plantations in southern Scotland for foraging and roosting and assess the implications for felling operations on bats. Maternity roosts identified (n = 17) were in all in buildings and most were large (> 500 individuals). We found no evidence of bats roosting in mature Sitka Spruce crop trees, although several bats used roosts in old or dead beech and oak trees as an alternative to their main maternity roost. Home ranges were much larger (mean 9.6 ± 3.12 km2) than those reported from other studies (0.6–1.6 km2), and it is likely that roost availability rather than food abundance constrains P. pygmaeus use of Sitka Spruce plantations. At the landscape scale, most individuals selected coniferous habitats over other habitat types, covering large distances to access plantation areas, whilst at a local scale bats used forest tracks to access water, felled stands or patches of broadleaf cover within the plantation. Sitka Spruce plantations support a high abundance of Culicoides impuctatus, the Highland midge which may act as a reliable and plentiful food source for females during lactation, an energetically expensive period. The use of felled stands for foraging by bats has implications for forest management as wind turbines, following small-scale felling operations, are increasingly being installed in plantations; wind turbines have been associated with high bat mortality in some countries. Decisions about siting wind turbines in upland plantations should consider the likelihood of increased bat activity post felling.

A comparison of Sitka spruce x white spruce hybrid families as an alternative to pure Sitka spruce plantations in upland Britain

The heavy reliance of British forestry on Sitka spruce (Picea sitchensis (Bong.) Carr.) presents a risk in a time of changing climate and increasing incidence of pests and diseases. Three experiments were established in 1987 to examine alternative species including hybrids of Sitka spruce and white spruce (Picea glauca (Moench) Voss.) (SS x WS). After 29 years, the height and diameter growth of the best-performing hybrid families was as good as that of unimproved Queen Charlotte Islands (Haida Gwaii) Sitka spruce at two sites, with slightly reduced diameter growth being the only difference at the third site. The SS x WS hybrid families also outperformed white spruce and Engelmann spruce. Performance of the hybrid families was variable and strongly associated with parentage; selection of both female and male parents should be an important consideration in any future research and breeding programmes. There was no evidence that the hybrid families performed disproportionately well at the drier of the three sites and further work would be required to investigate relative drought tolerance. The results show that the SS x WS hybrid families could be an alternative to Sitka spruce and contribute to increasing forest diversity to develop more resilient forests.

The influence of forest management on the abundance and diversity of hoverflies in commercial plantations of Sitka spruce: the importance of sampling in the canopy

Abstract The majority of Sitka spruce plantations in the UK are managed by clear-cutting and replanting, which leads to the formation of relatively large blocks (5–30 ha) of even-aged, closely spaced trees. These stands are considered to be poor in terms of general biodiversity and vulnerable to outbreaks of insect pests and diseases. In contrast, stands managed using alternative silvicultural systems, such as shelterwood or group selection systems, contain a wider range of tree ages and sizes and have the potential to support greater numbers of other species. If this includes the natural enemies (predators, parasitoids) of pest species, then adopting alternative silvicultural systems might help to reduce current and future pest impacts. To determine whether the greater structural complexity of stands managed using alternative silvicultural systems is associated with an increase in insect diversity and an increase in the abundance of predatory species, we sampled hoverflies (Diptera: Syrphidae) in 5 shelterwood stands, 6 group selection stands and 13 even-aged stands of Sitka spruce at 4 forest sites in mid and north Wales. The larvae of aphidophagous hoverflies are important invertebrate predators in spruce plantations, and hoverfly species richness is often used as an indicator of general invertebrate diversity. Hoverflies were sampled at ground level and in the canopy using yellow pan traps and flight interception traps. 7323 adult hoverflies of 74 species were captured, the majority of these in traps in the canopy. Total hoverfly abundance and the abundance of aphidophagous species were significantly higher ( P

The influence of forest management systems on the abundance and diversity of bark beetles (Coleoptera: Curculionidae: Scolytinae) in commercial plantations of Sitka spruce

Sitka spruce is the most widely planted conifer tree species in the UK, and is generally grown in dense monocultures managed using the clear-cut system. This study investigates the influence of alternative management systems (shelterwood and selection systems) on captures of bark beetles and associated predators. Bark beetle abundance was significantly lower in stands managed by shelterwood and selection systems when compared to mid (21–40years old) and mature (≥41years old) stands managed by the clear-cutting method. The abundance of bark beetles, including individual species, within plots could not be fully explained however, using linear mixed model analysis that not only included the various site characteristics assessed within each plot (numbers of live and dead trees), but also the numbers of predatory beetles (Rhizophagidae and Salpingidae). This suggested that the influence of management type on bark beetle abundance was not entirely due to the bottom-up or top-down forces that were assessed in the study, and that other abiotic and/or biotic factors were likely to be influential. Bark beetle diversity was also influenced by the management system, however higher diversity values within some group selection managed plots were only apparent in circumstances when stands of Sitka spruce had been managed using this system for a considerable period of time, and the habitat had remained undisturbed for a relatively long period of time (i.e. no harvesting interventions). Significantly higher numbers of predatory beetles were captured within the mid and mature clear-cut managed stands. The results from the study indicate that adopting alternative management systems to manage Sitka spruce in the UK could be a potential strategy to mitigate likely increases in bark beetle populations associated with climate change.

Responses of bats to clear fell harvesting in Sitka Spruce plantations, and implications for wind turbine installation

Commercial coniferous plantations are often assumed to be poor habitats for bats. As a result, the impact of forest management practices on bats, such as clear felling, has received little attention, particularly in Europe. However, there is growing evidence from multiple regions that bats do make use of plantation landscapes, and as interest in siting onshore wind turbines in upland conifer plantations grows, there is an urgent need to examine whether felling prior to turbine installation is likely to put foraging bats at risk of collision. In the first study of its kind, we use a “before – after – control – impact” study to explore the short-term impacts of clear fell harvest on bat activity in commercial plantations. Thirty-one mature stands of Sitka Spruce were surveyed using acoustic detectors in three large, upland Sitka Spruce plantations in Britain. Eleven stands were felled between 2013 and 2015, and 26 of the original 31 stands were resurveyed in 2015. The change in total bat activity and species- or genus-specific bat activity was modelled before and after felling occurred at both felled and control stands using generalised linear models. There was no change in overall bat activity at felled sites compared to control sites, but activity of Nyctalus species was 23 times higher following felling. Total Pipistrellus spp. activity doubled at felled sites post-harvesting, although this was mainly driven by increased activity at a few felled sites. When P. pygmaeus and P. pipistrellus were considered separately, activity increased slightly but non-significantly. The size of the felled area influenced activity (for bats overall and Pipistrellus spp.), with 90% higher activity in smaller felled stands (less than 5ha−1) compared to larger felled stands (greater than 30ha−1). For P. pipistrellus, activity in felled areas decreased with the duration since harvesting; the greatest activity occurred in stands felled within two months compared to those harvested more than 16months previously. Higher activity for some groups following felling may occur due to the creation of more edge habitat, which is preferred by both Pipistrellus species we recorded. An increase in activity following the small-scale felling (‘key-holing’) required for the installation of turbines could put foraging bats at risk from collisions with turbines. Further investigation of the influence of both size of clear fell patch, timing of felling and changes in invertebrate abundance due to felling are required to establish the potential risk of key-holing and turbine installation to foraging bats.

Spread of the Introduced Sitka Spruce (Picea sitchensis) in Coastal Norway

Positive and negative effects on ecosystem services from plantation forestry in Europe have led to conflicts regarding non-native tree species. Sitka spruce (Picea sitchensis (Bong.) Carr.) is the most common plantation species in northwest Europe, covering 1.3 Mha. In costal Norway, Sitka spruce was intentionally introduced and is currently occupying about 50,000 ha. Sitka spruce was blacklisted in Norway in 2012, mainly based on the risk for invasive spreading, but little quantitative documentation exists on spread. Here we quantify spread from plantations into abandoned heathland and pastures in thirteen sites where natural regeneration occurs. Spread distances and zero-square distributions related to the nearest edge of the parent stand were fitted by use of Weibull. The median expansion rate was 0.8 m·year−1 in north Norway and 4.4 m·year−1 in west Norway. The maximum establishment distance measured was 996 m. A peak in sapling density occurred within 50 m from the edge, and there was a general decrease in saplings with increasing distance. Conversely, increase in zero-squares percentages occurred with increasing distance. We argue that inclusion of abundance in assessing spread is necessary to define invasiveness. Based on spread models and prevailing forestry practices we recommend that the establishment of new Sitka spruce plantations within 200 m of protected areas should be avoided.

Decomposition nitrogen is better retained than simulated deposition from mineral amendments in a temperate forest.

Nitrogen (N) deposition (NDEP) drives forest carbon (C) sequestration but the size of this effect is still uncertain. In the field, an estimate of these effects can be obtained by applying mineral N fertilizers over the soil or forest canopy. A 15N label in the fertilizer can be then used to trace the movement of the added N into ecosystem pools and deduce a C effect. However, N recycling via litter decomposition provides most of the nutrition for trees, even under heavy NDEP inputs. If this recycled litter nitrogen is retained in ecosystem pools differently to added mineral N, then estimates of the effects of NDEP on the relative change in C (∆C/∆N) based on short‐term isotope‐labelled mineral fertilizer additions should be questioned. We used 15N labelled litter to track decomposed N in the soil system (litter, soils, microbes, and roots) over 18 months in a Sitka spruce plantation and directly compared the fate of this 15N to an equivalent amount in simulated NDEP treatments. By the end of the experiment, three times as much 15N was retained in the O and A soil layers when N was derived from litter decomposition than from mineral N additions (60% and 20%, respectively), primarily because of increased recovery in the O layer. Roots expressed slightly more 15N tracer from litter decomposition than from simulated mineral NDEP (7.5% and 4.5%) and compared to soil recovery, expressed proportionally more 15N in the A layer than the O layer, potentially indicating uptake of organic N from decomposition. These results suggest effects of NDEP on forest ∆C/∆N may not be apparent from mineral 15N tracer experiments alone. Given the importance of N recycling, an important but underestimated effect of NDEP is its influence on the rate of N release from litter.

Geothermal ecosystems as natural climate change experiments: The ForHot research site in Iceland as a case study

This article describes how natural geothermal soil temperature gradients in Iceland have been used to study terrestrial ecosystem responses to soil warming. The experimental approach was evaluated at three study sites in southern Iceland; one grassland site that has been warm for at least 50 years (GO), and another comparable grassland site (GN) and a Sitka spruce plantation (FN) site that have both been warmed since an earthquake took place in 2008. Within each site type, five ca. 50 m long transects, with six permanent study plots each, were established across the soil warming gradients, spanning from unwarmed control conditions to gradually warmer soils. It was attempted to select the plots so the annual warming levels would be ca. +1, +3, +5, +10 and +20 °C within each transect. Results of continuous measurements of soil temperature (Ts) from 2013-2015 revealed that the soil warming was relatively constant and followed the seasonal Ts cycle of the unwarmed control plots. Volumetric water content in the top 5 cm of soil was repeatedly surveyed during 2013-2016. The grassland soils were wetter than the FN soils, but they had sometimes some significant warming-induced drying in the surface layer of the warmest plots, in contrast to FN. Soil chemistry did not show any indications that geothermal water had reached the root zone, but soil pH did increase somewhat with warming, which was probably linked to vegetation changes. As expected, the potential decomposition rate of organic matter increased significantly with warming. It was concluded that the natural geothermal gradients at the ForHot sites in Iceland offered realistic conditions for studying terrestrial ecosystem responses to warming with minimal artefacts.

Base cation budgets under residue removal in temperate maritime plantation forests

The removal of harvest residues—branches, treetops and needles—has the potential to impact the long-term sustainability of forest soils. In this study we used mass balance calculations to predict the impact of harvesting on ecosystem balances of calcium (Ca), magnesium (Mg), potassium (K) as well as on soil acidification (ANCle) for forests in Ireland. Balances were determined for three harvesting scenarios: stem-only harvest (SOH), stem plus branch harvest (SBH) and whole-tree harvest (WTH). The study was carried out at forty sites—selected on a grid across the country. The sites were plantations of Sitka spruce (Picea sitchensis (Bong.) Carr), Norway spruce (Picea abies (L.) Karst.) and provenances of lodgepole pine (Pinus contorta (Dougl. var. latifolia)). Flux calculations were based on site measurements of standing biomass and soil properties as well as regional maps of atmospheric deposition. Under SOH and SBH, inputs were predicted to be sufficient to meet outputs for Ca, Mg and K budgets. Atmospheric deposition was the most important input to Ca and Mg balances. For K, inputs from mineral weathering were as important as deposition. Under WTH, Ca output was greater than input at 19 of the 40 study sites. However, the difference was small relative to the size of soil Ca pools; at these sites, exchangeable pools could support WTH removal for a median of 220years. Magnesium and K removal under WTH was supported by inputs in deposition and weathering. For soil acidification budgets (ANCle), base cation (BC) removal in harvesting under all scenarios was much greater than that BC generated by weathering, suggesting that soils will become acidified over the long term. There was considerable uncertainty around the calculation of fluxes. For Ca balances, confidence intervals spanned positive and negative values at many sites such that it was not possible to predict the balance of Ca budgets. In addition, uncertainty in flux calculations was particularly important for K budget because soil exchangeable pools were small and could be depleted within one to two rotations.

Contrasting impacts of afforestation on nitrous oxide and methane emissions

The impacts of afforestation, with either Sitka spruce [Picea sitchensis (Bong.) Carr.] or European ash (Fraxinus excelsior L.), on nitrous oxide (N2O) and methane (CH4) emissions were assessed using a chronosequence (age-related) approach. Conversion of a semi-natural wet grassland on mineral gley soils to a Sitka spruce plantation drastically increased N2O emissions, whereas the opposite trend was seen for CH4 emissions. Annual cumulative emissions of N2O increased from 0.12±0.17 (±values indicate standard error, SE)kgha−1yr−1 at the grassland site to 10.12±2.19kgha−1yr−1 at the 16-year old forest site in this study, which constituted one of the highest recorded losses for unfertilised coniferous forests in Europe. In contrast CH4 emissions decreased from 7.61±3.49 (grassland site) to 0.49±0.65 (7-year old forest)kgha−1yr−1, with the 16-year old forest site acting as a small sink (–0.80±0.12kgha−1yr−1). The contribution of these gases in terms of carbon dioxide (CO2) equivalents to the total greenhouse gas (GHG) budget showed that they can reduce the global warming amelioration capacity for this type of land use change by 10±2%, although the magnitude of this reduction is associated with significant uncertainty. In comparison the conversion of a managed grassland into an ash plantation on mineral brown earth soil had no clear effects on either N2O or CH4 emissions, with annual cumulative fluxes for N2O of the order of 10 times lower than those from the coniferous stands. A combination of factors could have contributed to these contrasting outcomes, including differences in ecosystem productivity, soil characteristics, management practices, microbial population structure and activity, variations in root activity and vegetation composition, as well as interannual climatic variability, and this is discussed. Overall, the results indicate the difficulty in generalising the impacts of afforestation on GHG budgets in the absence of a better understanding of how tree species, as well as soil and climatic factors, interact to determine trace gas emissions.