Barbara Vinceti
email: b.vinceti@cgiar.org

Mattia Manica

Nina Lauridsen

Pieter Johannes Verkerk

Marcus Lindner

Bruno Fady

Barbara Vinc

Managing forest genetic resources as a strategy to adapt forests to climate change: perceptions of European forest owners and managers

Keywords: Forest genetic resources, Forest management, Forest reproductive material, Climate change, Adaptation

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Recent attempts to assess the impact of observed climate change on forest ecosystems in Europe highlighted changes in growth, in mortality associated with drought and in the distribution range of many species [START_REF] Lindner | Climate change and European forests: What do we know, what are the uncertainties, and what are the implications for forest management?[END_REF]. Predicting climate change impacts on forest ecosystems remains very challenging, given the level of uncertainty linked to the numerous simplifications applied in the modeling of biological and climate processes, and socioeconomic pathways (e.g., [START_REF] Dyderski | How much does climate change threaten European forest tree species distributions?[END_REF][START_REF] Reyer | Are forest disturbances amplifying or canceling out climate changeinduced productivity changes in European forests?[END_REF][START_REF] Reyer | Forest productivity under environmental change-a review of stand-scale modeling studies[END_REF]. Climate change projections for Europe indicate an increase in duration of drought periods in Southern and Central Europe; however, past climate scenario projections have systematically underestimated observed trends in warm extreme events [START_REF] Lorenz | Detection of a climate change signal in extreme heat, heat stress, and cold in Europe from observations[END_REF]). In the Mediterranean region, forest fires and droughts are projected to increase, while the boreal region is expected to experience enhanced Communicated by Oliver Gailing.

Electronic supplementary material

The online version of this article (https ://doi.org/10.1007/s1034 2-020-01311 -6) contains supplementary material, which is available to authorized users.

1 3 forests growth and increasing risk of forest pests. Based on projections of the European Environment Agency (EEA 2017), Northern Europe is expected to face an increase in summer and winter precipitations and an increase in occurrence of extreme events, such as, for example, flooding and late frosts. In mountain regions, an upward shift of plant species is foreseen. On average, cold extremes are expected to decrease, but their intensity and duration might remain the same or increase under future climatic conditions [START_REF] Rummukainen | Changes in climate and weather extremes in the 21st century[END_REF].

Disturbances from abiotic and biotic pressures (windstorms, bark beetles and wildfires) increased during the twentieth century in Europe and are projected to become more frequent in the next decades [START_REF] Seidl | Increasing forest disturbances in Europe and their impact on carbon storage[END_REF][START_REF] Seidl | Forest disturbances under climate change[END_REF]. Plant diseases are largely influenced by environmental conditions, so environmental changes are likely to impact on pathogens, on their hosts, and on the interaction between them [START_REF] Sturrock | Climate change and forest diseases[END_REF].

Forest responses to environmental changes and adaptive processes are species-specific. They depend on the phenology and ecology of species and ecotypes, on the stand characteristics, on the competition regime, on forest structure [START_REF] Reyer | Climate change mitigation via afforestation, reforestation and deforestation avoidance: and what about adaptation to environmental change?[END_REF][START_REF] Bolte | Adaptive forest management in central Europe: climate change impacts, strategies and integrative concept[END_REF]) and on the characteristics of the distribution range of a species [START_REF] Kremer | Climate change and forest genetic diversity-implication for sustainable forest management in Europe[END_REF]. Tree species with a wide range in Europe will unlikely face extinction at species level, while some local tree populations could be lost, particularly those in marginal ecological sites and at the rear edge of the distribution [START_REF] Fady | Evolution-based approach needed for the conservation and silviculture of peripheral forest tree populations[END_REF]). Tree species with scattered or limited distributions generally appear to be more vulnerable and may face important threats at the species level [START_REF] Hubert | The role of forest genetic resources in helping British forests respond to climate change[END_REF].

Managing genetic diversity has a key role in fostering tree adaptive responses to environmental changes and in mitigating the effects of pests and diseases [START_REF] Alfaro | The role of forest genetic resources in responding to biotic and abiotic factors in the context of anthropogenic climate change[END_REF]Fady 2015); thus, it could contribute to increasing overall resilience of forest ecosystems. This has been recognized by FAO, which launched a global plan of action for the conservation, sustainable use and development of forest genetic resources (FGR) in 2014 (FAO 2014).

The perception of forest owners and managers vis-à-vis the challenges posed to forests by abiotic and biotic pressures is critical to understand what strategies and approaches are actually being adopted to mitigate their effects and what options could be proposed when no action is being taken [START_REF] Blennow | Adaptation of forest management to climate change among private individual forest owners in Sweden[END_REF][START_REF] Blennow | Climate change: believing and seeing implies adapting[END_REF][START_REF] Sousa-Silva | Adapting forest management to climate change in Europe: linking perceptions to adaptive responses[END_REF].

Projected climate changes appear to be greater in magnitude than the climate variability experienced by forest managers in the past, generating pronounced uncertainties. Evidence-based traditional forest management approaches may not propose effective future management solutions rapidly enough. Future climatic conditions may produce novel system states and dynamics, never experienced before; therefore, past practices would neither be applicable nor provide guidance [START_REF] Stainforth | Confidence, uncertainty and decision-support relevance in climate predictions[END_REF]). Furthermore, climate scenarios can be partly contradictory, and predictions are more robust at global or regional than local level. Forecasts are often presented in a format that is not easy to translate in precise recommendations for management at stand level [START_REF] Lindner | Climate change and European forests: What do we know, what are the uncertainties, and what are the implications for forest management?[END_REF].

In decision-making processes about forest management, factors such as values, knowledge, trust, risk perception, desired outcomes, past hardships and beliefs come into play [START_REF] Aubin | Why we disagree about assisted migration: ethical implications of a key debate regarding the future of Canada's forests[END_REF][START_REF] Hajjar | Exploring public perceptions of forest adaptation strategies in Western Canada: implications for policy-makers[END_REF].

A good number of studies have focused on understanding the views of forest-dependent communities and forest owners about climate change and their readiness to introduce changes in their forest management to foster adaptation of forest ecosystems to future climate conditions, especially in North America [START_REF] Grotta | Family forest owners and climate change: understanding, attitudes, and educational needs[END_REF][START_REF] Hajjar | Opinions on strategies for forest adaptation to future climate conditions in western Canada: surveys of the general public and leaders of forest-dependent communities[END_REF][START_REF] Lenart | Perceptions on climate change correlate with willingness to undertake some forestry adaptation and mitigation practices[END_REF][START_REF] Rodriguez-Franco | Understanding climate change perceptions, attitudes, and needs of forest service resource managers[END_REF] and Europe [START_REF] Williamson | Perceptions of climate change risk to forest ecosystems and forest-based communities[END_REF][START_REF] Blennow | Climate change: motivation for taking measure to adapt[END_REF][START_REF] Blennow | Climate change: believing and seeing implies adapting[END_REF][START_REF] Lawrence | Is the private forest sector adapting to climate change? A study of forest managers in North Wales[END_REF]. Despite a different level of awareness about climate change between the two regions, some general findings emerge from the existing literature. Personal belief in climate change and a personal experience of its effects appear to be critical factors that stimulate adoption of adaptive measures in forest management [START_REF] Blennow | Climate change: believing and seeing implies adapting[END_REF][START_REF] Williamson | Perceptions of climate change risk to forest ecosystems and forest-based communities[END_REF]. Forest owners, managers and forestry professionals seemed not inclined to make significant changes to their management in anticipation of climate change, but looked eager to learn more about ways this could affect their forests [START_REF] Grotta | Family forest owners and climate change: understanding, attitudes, and educational needs[END_REF]) and understand better the benefits to be derived from active management [START_REF] Lenart | Perceptions on climate change correlate with willingness to undertake some forestry adaptation and mitigation practices[END_REF]. In some cases, other threats, such as pests and diseases, were felt as more pressing than climate change [START_REF] Lawrence | Is the private forest sector adapting to climate change? A study of forest managers in North Wales[END_REF]; in other cases, lack of financial support [START_REF] Laakkonen | Forest owners' attitude towards pro-climate and climateresponsive forest management[END_REF] or lack of information and knowledge were presented as main disincentives and constraints [START_REF] Sousa-Silva | Adaptation of forest management to climate change as perceived by forest owners and managers in Belgium[END_REF], 2018). In the case of private owners of small forest properties non-continuously managed by experts, it has been found that the prevailing view is to leave natural dynamics in forest ecosystems guide selfregulatory processes, without additional interventions [START_REF] Mostegl | Targeting climate change adaptation strategies to small-scale private forest owners[END_REF].

In order to promote the uptake of adaptive measures to climatic changes by the forestry sector, policy-makers should consider several aspects in addition to information sharing: in particular the socioeconomic context that determines what is economically relevant in a specific system, the institutional and policy contexts and the development of techniques and tools to support a transition in forest management toward the implementation of adaptive measures [START_REF] Andersson | Adaptation to climate change? Why business-as-usual remains the logical choice in Swedish forestry[END_REF].

3

A large part of published research on forests and climate change has focused on the potential shifts in tree species distribution, on the vulnerability of individual species, on the expected impacts of climate change on these and less on the specific adaptive measures recommendable for specific contexts [START_REF] Keenan | Climate change impacts and adaptation in forest management: a review[END_REF]. Evidence collected in the USA showed that the most accepted adaptive practices to climate change were those tested over long time, such as thinning and prescribed burning in highly dense forests [START_REF] Lenart | Perceptions on climate change correlate with willingness to undertake some forestry adaptation and mitigation practices[END_REF].

Tree species selection is widely recognized as the main critical choice through which the capacity of forest ecosystems to withstand climatic changes is directly influenced [START_REF] Blennow | Climate change: believing and seeing implies adapting[END_REF][START_REF] Lawrence | Is the private forest sector adapting to climate change? A study of forest managers in North Wales[END_REF][START_REF] Yousefpour | Forestry professionals' perceptions of climate change, impacts and adaptation strategies for forests in south-west Germany[END_REF][START_REF] Sousa-Silva | Adapting forest management to climate change in Europe: linking perceptions to adaptive responses[END_REF]; in addition, silvicultural treatments that enhance complexity of a stand and modify its age and structure have an influence on how forests respond to climate change [START_REF] Jandl | Forest adaptation to climate change-is non-management an option?[END_REF].

Despite the large amount of evidence on the critical role of FGR to foster adaptation in forest ecosystems [START_REF] Alfaro | The role of forest genetic resources in responding to biotic and abiotic factors in the context of anthropogenic climate change[END_REF], very few studies have examined the readiness of forest owners and managers in Europe to actively manage FGR, beyond tree species selection, to actively contrast the effects of environmental changes. Genetic adaptation to climate change is influenced directly by decisions on the type of regeneration approach used and the geographic origin and genetic composition of the planted material selected [START_REF] Lefèvre | Human impacts on forest genetic resources in the temperate zone: an updated review[END_REF][START_REF] Finkeldey | Genetic implications of silvicultural regimes[END_REF][START_REF] Hosius | Genetic aspects of forestry in the Central Europe[END_REF][START_REF] Ratnam | Genetic effects of forest management practices: global synthesis and perspectives[END_REF]Fady et al. 2015).

Depending on the local context, different alternatives for regeneration can be considered: a) the promotion of natural regeneration where the pace of climate change is not drastically changing site conditions; b) the planting of best-adapted varieties together with those currently used; c) the use of assisted gene flow and assisted migration (as presented by [START_REF] Aitken | Assisted gene flow to facilitate local adaptation to climate change[END_REF], which implies, respectively, the movement of forest reproductive material (FRM-seeds, plant parts such as cuttings and scions, and plants raised by means of seeds or propagated in vitro) within a species range or outside a species range, introducing planting material selecting from provenances and species adapted to the future climatic conditions of the planting site (Huber and Cottrell 2007).

Different forms of assisted migration can be implemented based on what are the goals [START_REF] Williams | Preparing for climate change: forestry and assisted migration[END_REF] and considering the boundaries of existing regulations. Substantial uncertainties are associated with this approach [START_REF] Mclachlan | A framework for debate of assisted migration in an era of climate change[END_REF], due to lack of clarity about future climate scenarios, especially at the local scale, lack of robust seed transfer guidelines, risks of maladaptation and genetic pollution, potential disturbances to indigenous flora and fauna, risks of introducing new pathogens or exposing newly introduced species to new pathogens. The debate on assisted migration is mainly centered around ecological risks and benefits; however, divergent opinions may be related to fundamentally different views on nature, and more specifically, to the ethical aspects associated with the question of whether a deliberate management of natural systems or their selfadaptation should be encouraged [START_REF] Aubin | Why we disagree about assisted migration: ethical implications of a key debate regarding the future of Canada's forests[END_REF]). Thus, opinions on what type and quality (e.g., origin, degree of improvement) of forest reproductive material should be used can be quite different and dependent on the socioeconomic context.

An assessment of these different views was carried out by [START_REF] Hajjar | Opinions on strategies for forest adaptation to future climate conditions in western Canada: surveys of the general public and leaders of forest-dependent communities[END_REF] who examined the opinions expressed by leaders of forest-dependent communities and the general public in two provinces of Canada, about different reforestation strategies in the face of climatic change; the strategies differed in their degree of reliance on biotechnology. It turned out that the views were aligned around a low acceptance for a passive strategy, which meant no planned action to buffer climate change effects. Replanting with local seeds was considered highly acceptable, while other strategies based on breeding and movement of seed outside of a species natural range were considered less acceptable. Solutions based on genetic engineering obtained the lowest acceptance. Examples of similar investigations are currently missing for Europe.

We tried to fill this gap by exploring the extent to which forest owners and managers are actively managing FGR as a strategy to adapt forests to climate change. We tried to understand what were the main forest threats of concern for forest owners and managers across Europe and then looked at the following aspects: a) What is the general knowledge of forest owners and managers about FGR? b) What is their attitude toward actively managing FGR to contrast the effects of climatic changes and of other threats? c) How do forest owners and managers consider genetic resources in their choices about forest regeneration?

Methods

We developed a survey targeting forest owners and managers across Europe. Respondents had the possibility to identify themselves or to answer anonymously. To maximize the rate of response, the survey was available in multiple European languages (English, Danish, Finnish, French, German, Italian and Spanish). The survey was created online using Surveymonkey (https ://www.surve ymonk ey.com/) and was accessible from October 17, 2016, to January 31, 2017. The initiative was widely advertised through posts on websites, newsletters and social media accounts of institutions, programmes and research initiatives related to conservation and sustainable management of forests and FGR. Associations of 1 3

forest owners and managers were also contacted. Reminders were sent at regular intervals.

Respondents had to provide information about their role, experience, level of activity, from local to international and type of entity represented. The content of the questionnaire is in the Supplementary Material.

Forest owners and managers were asked to indicate their perception about the main threats affecting forests today and in the future. For this, they had to choose among the following pre-defined threat categories: drought, fire, freezing rain/ ice storms, overexploitation of forests, pests and diseases, wind storms. More than one threat could be identified, and a possibility was given to add other, unlisted types of threats. In addition, respondents were asked to define the type of management intensity they were referring to in their answers using pre-defined forest management categories, described in the text of the survey (see Supplementary Material). Various questions referred to the degree of consideration of FGR in management decision, in particular with regard to choices around forest regeneration options, sources and quality of FRM. Follow-up questions regarding the sources of planting material were reserved to those who indicated artificial regeneration as a prevalent management choice.

We used descriptive statistics to examine responses to each question, looking at overall patterns emerging at the regional (European) level. A multiple correspondence analysis (MCA) was performed to reveal the underlying structure of the data. Moreover, a correspondence analysis (CA) was applied on contingency tables of countries to investigate the association between the perception regarding current and future threats to forest and the geographic location of respondents. Subsequently, a hierarchical clustering on principal components (HCPC) was applied on the results of the CA to identify groupings of countries with similar responses for each question in the survey. Conditional inference trees (CITs) were applied to individual questions to reveal patterns in responses. They perform recursive partitioning of the sample, based on statistical significance tests. Missing values were discarded from the analysis (14% of the records in the original dataset had to be removed due to missing information; the sample used for the analysis is presented in Table 1) Statistical analyses were performed using the statistical software R (R Core Team, 2017) and the FactomineR, factoextra and partykit packages [START_REF] Le | FactoMineR: an R package for multivariate analysis[END_REF][START_REF] Kassambara | factoextra: extract and visualize the results of multivariate data analyses[END_REF][START_REF] Hothorn | Partykit: a modular toolkit for recursive partytioning in R[END_REF].

Results

A total of 204 forest owners (98) and managers (106) from 15 European countries responded to our survey (see Table 1). Most respondents presented their personal views, and a minority represented the view of associations or other entities (e.g., national forest services, other public bodies, forest companies). Responses mainly referred to the local and regional (sub-national) levels (72%). Only a few responses (ca. 4%) referred to the international level. The majority of respondents had a long-term experience (> 20 years) (56%) and were from the private sector (80%) (Table 1). The prevalent form of management intensity (see definitions in Supplementary Material, in question 3 of the survey) applied by respondents (indicated by 72% of all respondents) corresponded to a combined economic objective/site-adapted forestry (see definition in Supplementary Material), followed by 'Intensive even-aged forestry' (indicated by 14% of respondents). The following categories were represented in much smaller proportions: 'Close-to-nature forestry' (9.6%), 'Unmanaged forest nature reserve' (1%), 'Short rotation forestry' (1%), other (2.5%; this category refers to cases of mixed approaches). We received most response from Austria (52), France (64) and Norway (31), followed by Germany (12), Italy (8) and Spain (9). From other countries (Belgium, Denmark, Finland, Iceland, Portugal, Romania, Slovenia, Sweden and the UK), we received five or less responses.

A MCA applied to all the survey questions combined did not reveal any clustering of respondents. The first two axes 1 3

only explained 11.3% of the variation, and the cloud of individual respondents clustered at the center. Discarding from the analysis, the facultative questions with very low frequency of responses (that could undermine the analysis) did not improve the results. Consequently, individual questions were analyzed separately, looking for prevalent answers.

Based on the perceptions of survey respondents, the top-ranking threats to forests were considered to be pests and diseases, both today and in future (34.8% and 32.6%, respectively), windstorms (29.1% and 23.7%, respectively) and drought (14.4% and 19%, respectively), followed by fire with much lower percentages (4.9% and 7%, respectively). The category 'Other threats' included threats indicated by respondents in addition to the pre-defined categories; most respondents referred explicitly to climate change as a current and future threat.

The HCPC on the factor map detected three main clusters of countries with a similar profile in terms of perception of threats. They are formed, respectively, by: (1) Spain-Italy-Portugal, (2) the UK-Iceland-Sweden-Finland and (3) all other countries (Fig. 1).

Results from the CA indicated that the two first axes explain 56.4% of the cloud total variability, so they represent a good part of the variability (Fig. 2). The distance between any row points or column points gives a measure of their similarity, so threats that have a similar profile among countries are positioned close to each other. The biplot reveals that there is a clear association between current and future perception of the importance of each threat, so the ranking of importance attributed to each threat is not changing in order of magnitude over time in the view of respondents.

Mediterranean countries such as France, Italy, Spain and Portugal perceive fire and drought as critical current and future threats. Slovenian respondents highlighted freezing events as a major threat (probably relating to the major ice storm that occurred in 2014 and has affected about 500,000 ha of forests). Norway, Germany and Belgium reported windstorms as a threat, while the category 'other threats' appeared associated mainly with northern European countries such as Finland, Iceland, Sweden and the UK. In most cases, people who chose 'other threats' specified 'climate change.' Pests and diseases were perceived as current and future threats across countries, with no particular geographic pattern.

A total of 86% of the respondents expressed the view that management of FGR could mitigate some of the future threats affecting forests, in particular pests and diseases and droughts. Other future concerns that could be addressed by proper use of FGR, according to the respondents, were climate change and issues related to low potential resilience of forest stands with limited genetic diversity (including breeding populations). The responses also indicated some awareness among respondents about the linkages between different quality of wood and non-wood products and their genetic origin. A total of 75% of the respondents stated explicitly that they took into consideration genetic aspects in their forest management.

Respondents applied artificial regeneration (43% of respondents) and natural regeneration (41% of respondents) to a similar extent. Coppicing was indicated by 6% of the respondents, while 10% of the respondents indicated using a combination of artificial regeneration, self-seeding and coppice, according to characteristics of the species considered, of the planting site and climatic conditions. The responses revealed a geographical pattern, with a group of countries, mainly although not exclusively from Northern Europe (Denmark, Finland, Iceland, Norway, Sweden) where artificial regeneration is largely prevalent, and a second group characterized by a more widespread use of natural regeneration and coppice (Figure S1 in Supplementary Material).

When opting for artificial regeneration, respondents indicated that aspects of genetic quality of FRM (attention to best origin) were usually taken into account for both conifers and broadleaves across all countries. After excluding missing values, 97% respondents said yes or often for conifers and 74% said yes or often for broadleaves.

We then examined the attitude of forest owners and managers toward moving FRM in order to optimize the matching between future site conditions and the selected FRM. The survey questions on this subject were addressed to all respondents, regardless of what forest regeneration approach they were adopting, in order to understand their general inclination. The proportion of answers falling in the different categories of responses is not randomly distributed. Responses are concentrated around a middle position ('partially positive') for both the movement of tree species and the movement of tree provenances within the same species (Fig. 3), with a significantly higher value assigned to moving a tree species than moving provenances. Respondents rated the option of 'using a tree species not present before in the planting site' as partially positive in ca. 45% of the cases, versus 'using a provenance not present before in the planting site' as 'partially positive' in 32% of the cases. However, by adding up the categories 'rather positive' and 'very positive,' the option of 'using a provenance not present before in the planting site' scored slightly higher than 'using a tree species not present before in the planting site' (40% vs. 35%). By combining all three categories, 'partially positive,' 'rather positive' and 'very positive,' the option of 'using a provenance not present before in the planting site' received more support than the alternative option, with about 10% more votes.

Fig. 2 Results from the correspondence analysis (CA) on current and future forest threats (red color) in the different countries (blue color) to which the survey responses refer. The distance between any row points or column points gives a measure of their similarity (or dissimilarity). Threats that have a similar profile among countries are close to each other. In the same way, those countries positioned close to each other have a similar threat profile. Both current and future threats are projected on the plane. Other threats = respondents expressed independently additional concerns; these mainly referred to climate change as an element of current and future threat. (Color figure online) 1 3

We repeated the analysis above comparing responses of those currently favoring natural regeneration versus those using artificial regeneration. We found that those already making use of artificial regeneration as a prevalent approach had a higher positive attitude toward moving both species and provenances (for respondents operating at the local level, 'very positive' and 'positive' responses were four times more numerous than those of respondents making use of natural regeneration) and also a significantly lower negative attitude toward these two options, compared to forest owners and managers who privileged natural regeneration.

Most forest owners and managers (75%) indicated that they were informed about the characteristics and benefits that can be derived from the use of improved FRM. 1 The majority of the forest owners and managers (73%) indicated that they were aware of the existence of some good local provenances of species of interest for them, from native seed stands. Furthermore, aspects of quality of provenances Fig. 3 Perception of forest owners and managers (expressed as % of respondents; 204 in total), about a using forest reproductive material of a tree species not present before in the planting site; b using forest reproductive material of a tree provenance not present before in the planting site 1 3 turned out to be commonly considered in the choice of FRM (Fig. 4), both for conifers and broadleaves.

Regarding access to FRM, a total of 65% of the respondents indicated they had access to improved FRM; 24% of the respondents did not have access to improved FRM, while the remaining fraction was not informed.

With regard to choices of different types of FRM, local material was generally preferred over 'non-local material.' A 2-sample t test did not reveal any preference on whether 'improved material' was preferred over material not improved.

The majority of forest owners and managers (83%) who opted for artificial regeneration had preferences for some provenances they wished to plant. However, in half of the cases, it was the regulatory framework that defined what could be planted; personal choices came into play in 37% of cases, while other factors played a role in 13% of cases. These included cases of a selection of provenances guided by professional experts, by scientific guidelines or based on a combination of different actors or/and conditions.

Discussion

We asked forest owners and managers about their perception about main threats to forests and their responses reflected well the distinctive predominant threats at regional level. The most highly ranked threats in our survey were pests and diseases and windstorms, followed by drought and fire. Available data on the relative areas of forest affected by different types of disturbance show that pests and insects have an influence on the largest proportion of the forest cover (ca. 1.1%), followed by storms, wind and snow (0.6%) (data from FOREST EUROPE 2015 presented in the European Environmental Agency report of 2016). Fires affect ca. 0.1% of the forest cover in Europe; however, 85% of fires are in southern Europe, so their occurrence has a strong regional pattern. The fact that the option 'climate change' was not available among the closed answers (to avoid the complexity otherwise associated with the concept of climate change) has likely limited the choice of this response. Nevertheless, some respondents explicitly referred to climate change (reporting this under the option 'Other') and the types of threats most cited (e.g., increases in pests and diseases and to increased frequency of fires and droughts) are largely associated with climate change.

The large majority of respondents (more than two thirds) indicated that they are aware of the potential role of FGR management in addressing climate change and mitigate other future threats for forests, in particular the influence of pests and diseases and of drought. This level of awareness may be the result of a close interaction, in some European countries targeted by our survey, between scientists involved in research on FGR and regional and national platforms of forest owners and managers. However, even excluding some countries where the dialogue between scientist and managers is most active (e.g., France, Austria, Norway), results do not change. This may therefore be a reflection of the work of initiatives such as the European Programme on Forest Genetic Resources (EUFORGEN), which has been running for more than 25 years and has focused significantly on knowledge sharing and awareness raising across European countries. For what regards motivations to adopt adaptive measures, a recent study carried out targeting small-scale private forest owners in Austria revealed the motivations of forest owners to adopt adaptive management measures, their perception of climate change and their adaptability to it; the objective was to develop appropriate information campaigns, advisory services and monetary incentives [START_REF] Mostegl | Targeting climate change adaptation strategies to small-scale private forest owners[END_REF]. Results showed that climate change was a fairly well-known issue, and slightly more than half of the respondents were convinced that adaptation measures had to be promptly set in place. However, small-scale private forest owners turned out to be a very inhomogeneous group, with three distinct clusters identified, who reacted differently to the management options and incentives proposed: 'utility oriented,' 'recreation oriented' and 'tradition conscious' forest owners. Our data did not reveal any underlying structure that could lead to identify distinct typologies of forest owners and managers; however, this was probably due to the broad diversification of responses associated with the multicountry nature of our survey. Anyhow, most findings point to the need to design locally adapted solutions, including tailor-made advisory services. The biological and socioeconomic context (including the industrial sector) defines what management solutions are locally preferable to enhance the resilience of forest stands in light of climate change. For instance, planting a better-adapted tree provenance may be a more efficient solution than changing the tree species to be planted; in other cases, specific forest management practices applied after the stand establishment are equally critical in ensuring long-term resilience as the modifications in species and provenances composition. Examples of adaptive management practices are a reduced planting density to mitigate moisture stress, an increased thinning and use of prescribed fire to reduce the risks of insect and diseases outbreaks in overstocked stands, an increased thinning in areas closer to urban settlements to avoid risk of fire. Mixed-species forestry is an important option to adapt forests to climate change [START_REF] Bauhus | Ecological stability of mixed-species forests[END_REF]. Even if mixtures do not provide an effective insurance in all situations, they contribute to reduce the impact of disturbance agents and help in preventing complete losses. However, distributing risks across several tree species may not be necessarily a successful strategy in areas with high tree species diversity as a starting condition or where climate and soils may be suitable only for a very limited range of tree species. What matter are the specific traits of the individual species mixed and the nature of the disturbance. How a mix of provenances would perform versus a mix of tree species depends largely on the local environmental conditions and choices are bounded by management objectives. The identification of most viable solutions requires a careful assessment of projected climate changes and vulnerabilities, and a discussion with stakeholders about most promising options [START_REF] Seidl | Adaptation options to reduce climate change vulnerability of sustainable forest management in the Austrian Alps[END_REF][START_REF] Halofsky | Adapting forest management to climate change: the state of science and applications in Canada and the United States[END_REF].

We examined the characteristics of the pool of respondents from 15 countries in Europe, to assess whether the sample was balanced. The respondents over-represent private versus public ownership, but this is in line with the characteristics of forest ownership across Europe: 13 out of the 15 countries sampled in this study are from Northern Europe, Central-West Europe and South-West Europe (based on the FOREST EUROPE clustering of countries), where private ownership of forests ranges from 60 to over 70% of the total forest area (FOREST EUROPE 2015). A constraining element in the analysis is the unbalanced number of respondents per country, which limited the possibility to look at individual countries, so most results are aggregated. The overall analyses revealed some grouping of responses by countries with regard to perception of threats to forests, but the analysis of all individual responses to all questions failed to show a structure in the data. This means that the grouping of similar responses varied among different questions.

In our survey, forest owners and managers were asked to indicate their prevalent approach for forest regeneration and responses showed a balance between natural and artificial regeneration. Considerable differences between individual countries and regions emerged, as reflected in forest statistics. Nowadays, regeneration by planting dominates in Northern and Central-East Europe, while natural regeneration is prevalent in the other European regions; about 68% of the total European forest area is regenerated naturally or expands spontaneously (FOREST EUROPE 2015). Coppicing is common only in Southern European countries. According to the Forest Resource Assessment released by FAO (FRA 2015), in the countries represented in our survey, artificial regeneration is used for 40-50% of the forest area in Northern and Central-West Europe, and for less than 15% in South-West European countries. Indeed, choices around forest regeneration methods and, in case of artificial regeneration, around sources of FRM, are highly influenced by the characteristics of the forest ecosystem considered and the role of forestry in the economy of a particular country.

In case of use of artificial regeneration, it is interesting to see in our respondents a prevalent positive inclination toward moving FRM (both species and provenances) to match expected future climate conditions, with a slightly higher overall preference for moving tree species rather than provenances. This positive attitude toward moving FRM was more pronounced in those forest users and managers already making use of artificial regeneration. However, this finding is partly contradicted by the responses to a question on a related topic provided by those respondents relying on artificial regeneration; when asked to indicate what FRM they favored, the largely preferred choice of FRM was for local material over non-local material (both improved and 1 3 not improved). The most selected choice in absolute terms was for improved, local FRM, which indicates a lower appreciation for foreign material. The only results that could be compared to these findings derive from the study by [START_REF] Hajjar | Opinions on strategies for forest adaptation to future climate conditions in western Canada: surveys of the general public and leaders of forest-dependent communities[END_REF] in western Canada, focused on assessing perception of threats posed by climate change, targeting leaders of forest-dependent communities and the general public. The findings from [START_REF] Hajjar | Opinions on strategies for forest adaptation to future climate conditions in western Canada: surveys of the general public and leaders of forest-dependent communities[END_REF] revealed a low acceptance for a passive approach (i.e., no intervention) and a high acceptance of replanting with local seeds. Acceptance of strategies that involve more manipulation, such as breeding, use of non-local seeds, moving of seeds outside of a species' natural range was lower.

Scientists are increasingly testing and proposing new management solutions to increase the adaptability of forests to climate change. Among these, the option to plant tree species or provenances in areas where they are not occurring today, but that are expected to become suitable in future, is increasingly considered. This strategy is called assisted migration or assisted gene flow, depending on whether a species is planted within its current distribution range or outside [START_REF] Aitken | Assisted gene flow to facilitate local adaptation to climate change[END_REF], and it is based on evidence generated by genomic studies that show adaptation to particular environmental conditions (e.g., drought or heat) at local to regional scales [START_REF] Aubin | Traits to stay, traits to move: a review of functional traits to assess sensitivity and adaptive capacity of temperate and boreal trees to climate change[END_REF]. Our results show that despite an increasing awareness on the role of FGR management and a stated positive inclination for FRM movement as a measure to foster adaptation in forest ecosystems, there is a certain resistance to introduce foreign planting material and a preference for FRM of local origin (Table 2).

Indeed, forest managers may need to see more research evidence of potential benefits to be obtained from moving FRM, before taking a more proactive attitude in this regard. As observed by other authors [START_REF] Lenart | Perceptions on climate change correlate with willingness to undertake some forestry adaptation and mitigation practices[END_REF], widespread support tends to the be reserved to those adaptive management practices tested over decades, while those that most directly affect the expansion of plant species outside their current range are less supported. Nevertheless, FRM has been moved in trade considerably in the past [START_REF] Koskela | Utilization and transfer of forest genetic resources: a global review[END_REF][START_REF] Jansen | Crossing borders-European forest reproductive material moving in trade[END_REF]). However, tracking and recording of performance were missing in many cases, so evidence on outcomes is not available.

With regard to practice, the large majority of respondents to our survey indicated that, in case of artificial regeneration, they pay attention to the selection of tree species provenances with optimal characteristics, for both conifers and broadleaves across all geographies, so this is another indication that awareness on the importance of FGR is growing. Access to improved material did not seem to be a major constraint, especially for conifers. According to a recent modeling study, the economic benefits of using improved FRM from seed orchards greatly outweigh the additional cost of the planting material due to a significant increase in productivity across diverse site conditions. However, in contexts where forests mainly provide services other than production, the cost of high-quality FRM may be excessive compared to the profits that can be obtained from forest products, and this may be an obstacle for the active implementation of practices to foster adaptation to environmental changes.

Other constraints may derive from a lack of specificity in the recommendations guiding use of FRM in light of climate change and their potentially limited predictive capacity about performance of the FRM transferred, due to different reasons. FRM transfer guidelines have been mostly based on present or past climatic conditions and, in addition, a great amount of uncertainty characterizes future climate projections; habitat similarity for the transferred FRM is assessed mainly in relation to climatic factors, neglecting other important parameters, such as soil characteristics and interactions with other species and provenances; phenological responses may be very different across tree species in reaction to the same climatic change and research has not investigated sufficiently if large differences in response exist also among genotypes within the same species; heritable variation in adaptive traits is partly related to epigenetics [START_REF] Konnert | Use and transfer of forest reproductive material in Europe in the context of climate change[END_REF]. Furthermore, FRM is increasingly traded across countries, but national laws focus only on its production and commercialization and do not regulate its use, so they do not assist in defining where the imported FRM should be planted. There are exceptions in some countries where national laws provide recommendations to forest owners on the use of FRM. These recommendations are all compliant with the Council Directive 1999/105/EC, and in some countries, they include more restrictive rules for FRM transfer into and between ecoregions (e.g., in Bulgaria, Cyprus, Croatia, Estonia, France, Lithuania, Norway, Slovakia, Slovenia, Sweden, the UK, Serbia, Poland); in others, recommendations define what FRM can be used based on species adaptation to site conditions (Belgium, Bulgaria, Finland, France, Greece, Lithuania, Slovenia) [START_REF] Konnert | Use and transfer of forest reproductive material in Europe in the context of climate change[END_REF]. However, in all these cases, recommendations tend to be based on provenance regions whose boundaries were defined through research in provenance trials established Expanding the view to adaptive management measures in a broader sense, a recent systematic review of the scientific literature by [START_REF] Hagerman | Responding to climate change in forest management: two decades of recommendations[END_REF] supports the view that the majority of recommendations on how to respond to climate change with forest management are too generic and vague, therefore difficult to put into practice in contexts with specific socio-ecological characteristics. The authors thus flagged the need to develop recommendations tailored to local needs, taking into account the specificities of the contents for which they are proposed, focusing on management solutions that enable to deal with uncertainty and incorporate different perspectives.

Conclusions

The European forest owners and managers targeted in this study perceived pests and diseases, windstorms and drought as the main threats of concern for forest ecosystems, with significant regional differences. Choices about FRM are very critical, and aspects of quality (best provenances) are considered by forest owners and managers in their selection of planting material, for both conifers and broadleaves. We found a generally positive attitude in moving FRM that is new to the planting site, in order to match the predicted future climate conditions, for both tree species and tree provenances. However, despite the stated positive inclination to moving FRM, the preferred choice of FRM was for local material over non-local material (both improved and not improved). This indicates a trust that local FRM will perform well under future climate conditions. This expectation may be over-optimistic, if global temperatures pass the 2 °C threshold of the 2015 Paris Agreement. Forest managers may need to be exposed to more research evidence of potential benefits deriving from the management of genetic diversity in planting material, both from local and foreign sources, before undertaking active adaptation and mitigation measures.

Fig. 1

 1 Fig.1Hierarchical clustering on factor map (HCPC). Country profiles are visualized in a space defined by different forests threats. Countries closely positioned on the factor map and belonging to the same cluster share a similar profile in terms of perception of threats expressed by individual respondents

Fig. 4

 4 Fig. 4 Extent to which aspects of quality (best provenances) are considered in the choice of forest reproductive material for conifers and broadleaves. Not applicable: the context does not enable to carry out a selection

Table 1

 1 Description of the sample of respondents to the survey *Other entities = representative of forest service, legal representation of regional association of forest owners/land owners, representative of interest group of forest/land owners, forest service, operations manager for a forestry company **Other = representative of stakeholder organization (e.g., forest owners), researcher in forest management, consultant, advisor, extension agent for private landowners. NB. All respondents without a specified association with a country location were removed

	Professional profile	Forest owner	98 48.0%
		Forest manager	106 52.0%
		Total	204
	Representation	Individuals	180 88.2%
		Associations	15 7.4%
		Other entities*	9	4.4%
	Scale of activity	Local level	75 36.8%
		Regional (within-country)	71 34.8%
		level
		Country level	52 25.5%
		International level	6	2.9%
		Countries represented	15
	Years of experience	< 5 years	10 4.9%
		5-10 years	29 14.2%
		10-20 years	45 22.1%
		> 20 years	120 58.8%
	Ownership of forest land Privately owned/managed	164 80.4%
		Public land	20 9.8%
		Other**	20 9.8%

Table 2

 2 Preferred options in the selection of forest reproductive material (FRM) (more than one option could be chosen). The highest values (corresponding to FRM of local origin) are marked in bold

	Type of FRM	N. respondents	% respondents
	Local	87	39.91
	Non-local	10	4.59
	Local improved	96	44.04
	Non-local improved	25	11.47

Improved forest reproductive material has been tested and selected in order to obtain benefits, such as increased productivity, improved timber quality, better resilience to climatic conditions, pests and diseases. In particular, genetic improvement can vary from provenance selection, seed stand establishment, establishment of seedling or clonal seed orchards with controlled pollination, up to advanced breeding techniques and genetic engineering.

Acknowledgements This research was part of the GenTree project which has received funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No. 676876. We acknowledge the contribution of Michele Bozzano, Ewa Hermanowicz and Joanne Fitzgerald in the design of the survey.

Author's contribution BV, NL and PJV contributed to the design and set up of the survey. MM, BV and NL performed data cleaning and analysis. BV wrote the manuscript with the help of PJV, BF and ML.

Availability of data

The data that support the findings of this study are available from the corresponding author [BV], upon reasonable request.

Compliance with ethical standards

Conflict of interest The authors declare that there is no conflict of interest regarding the publication of this article.

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A Falch

V A Badets

C Labrugère

R J Kriek

Co-sputtered Pt x Pd y Al z thin film electrocatalysts for the production of hydrogen via SO 2 (aq) electro-oxidation

Keywords: SO2 electro-oxidation, hydrogen production, electrocatalyst, high-throughput screening

A co-sputtered PtxPdyAlz ternary system was investigated for potential use as anode catalyst for the electrooxidation of aqueous sulphur dioxide (SO2), a key reaction in the hybrid sulphur (HyS) process for splitting water into hydrogen and oxygen. Combining the noble metals Pt and Pd with Al resulted in no significant improvement in onset potential, however, current output was improved for the majority of the electrocatalysts evaluated. Of these electrocatalysts only a single ternary composition exhibited improved stability when compared to pure Pt. It was found that a combination of Pt40Pd57Al3 (annealed at 900 °C) exhibited superior performance when compared to pure Pt and the previously determined best binary electrocatalyst, i.e. Pt3Pd2. Current density (mA.mg Pt -1 ) increased from 108.11 to 181.21 and finally to 396.73 for Pt, Pt3Pd2 and Pt40Pd57Al3 respectively, indicating an increase in activity that correlates with a decrease in Pt content. Atomic force microscopy (AFM) revealed an increase in surface roughness for Pt, Pt3Pd2 and Pt40Pd57Al3, while the occurrence of metal interaction and certain degrees of Al migration (a result of annealing) was confirmed for Pt40Pd57Al3 by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction analysis (XRD).

Introduction

Hydrogen, the only non-carbon containing energy carrier, is the most abundant atom on earth 1 and is considered as the ultimate clean energy carrier to be generated from renewable resources 2 . The range of hydrogen production processes can roughly be divided into hydrocarbon-based (steam-methane reforming and coal gasification), nonhydrocarbon based (water electrolysis and thermochemical water decomposition) and integrated (steam-methane reforming linked to non-hydrocarbon based) 3 . As hydrogen is only found as part of compounds on earth, it is fairly energy intensive to separate hydrogen into its molecular form 1 , and goes hand in hand with huge amounts of environmental pollutants being emitted to the atmosphere. If, however, hydrogen can be returned/produced as part of a remediation process of these detrimental environmental pollutants, a portion of the energy input for the remediation process can be returned in the form hydrogen. This is exactly what the hybrid sulphur (HyS) cycle, a thermo-electrochemical water splitting process, accomplishes, through the electrochemical oxidation of aqueous sulphur dioxide (Figure 1). Thermochemical processes produce hydrogen through a series of chemical reactions with water-splitting being the net reaction. Since hydrocarbons are not used in these thermochemical processes no harmful emissions such as carbon dioxide (CO2) are produced, with the hydrogen produced being extremely pure 4 . There are various thermochemical cycles (a combination of thermochemical and electrochemical reactions 5 ) that can produce hydrogen from water with the HyS cycle (reactions 1 -4) being the most promising 4,[6][7][8]

Net cycle reaction: The HyS cycle undergone extensive development in the 1970s 9, 10 and 1980s [START_REF] Gorensek | Nuclear hydrogen production handbook[END_REF][START_REF] Lu | A study on the electrolysis of sulfur dioxide and water for the sulfur cycle hydrogen production process[END_REF] . For the subsequent 20 years, further development nearly ceased as advanced nuclear technologies gained increased interest [START_REF] Gorensek | Nuclear hydrogen production handbook[END_REF] . After this period, interest in the HyS cycle revived and the HyS cycle was subsequently rated first among 115 thermochemical cycles considered [START_REF] Brown | Initial screening of thermochemical watersplitting cycles for high efficiency generation of hydrogen fuels using nuclear power[END_REF] . Interest in the HyS cycle results from the fact that, whereas the anodic reaction for regular water electrolysis occurs at a standard potential of 1.23 V (SHE) [START_REF] Weast | CRC Handbook of Chemistry and Physics[END_REF] , the anodic reaction for SO2 depolarised electrolysis (SDE) occurs at a standard potential of 0.17 V (SHE) [START_REF] Struck | [END_REF] , which translates into an energy gain of more than one volt 7,15 . In SDE the rate determining step occurs at the anode, which results in the anode attributing to the inefficiencies of the SDE 16 . To this regard, an optimal operational target of 500 mA.cm -2 , at a cell potential of approximately 0.6 V, has been proposed for SDE by Gorensek and Summers 7,[START_REF] Gorensek | Hybrid Sulfur Process Reference Design and Cost Analysis -Final Report[END_REF] . This target can potentially be achieved through the development of an effective electrocatalyst for the anodic electro-oxidation of SO2 and to that regard the properties of the anode is regarded as pivotal for improving the entire HyS process [START_REF] Xue | [END_REF]19 .

H 2 O → H 2 + ½O 2 (4)
While platinum is generally considered the standard anode catalyst for the SDE 8,16 , efforts are underway to identify an electrocatalyst other than platinum (see 20,21 for a detailed list of investigated catalysts). As Pt is costly the development and implementation of a catalyst with increased or equal catalytic activity and reduced cost (less Pt) will be advantageous. Platinum content could be reduced by alloying/combining it with either a more cost effective noble metal or base metals in an effort to still maintain acceptable catalytic activity 20,21 . An earlier investigation by us (employing sputtered thin films) has shown that Pt content can indeed be reduced by combination with Pd, i.e. PtxPdy, without sacrificing electrocatalytic activity 21 . A thin film combination of Pt3Pd2 has specifically been found to exhibit competitive performance compared to pure Pt for the electro-oxidation of aqueous SO2 with annealing further improving performance 21 . For this study aluminium (Al) was introduced as a third component in an attempt to lower the content of both Pt and Pd (both noble metals). Al is considered a metal of great electrochemical potential 22 and a study by Lee 23 on Pt and Al combinations proved that only small amounts of Al is effective in improving catalytic performance towards aqueous SO2 electro-oxidation. This study by Lee was brief and no subsequent investigations were pursued 24 . For this investigation we employed combinatorial evaporative sputtering and high-throughput parallel screening in search of PtxPdyAlz electrocatalysts for the electro-oxidation of SO2. Identified compositions that merit more thorough investigation were evaluated employing conventional methods.

Experimental

Preparation, deposition, screening and annealing of thin films

The wafer and glassy carbon preparation, thin film deposition and electrochemical testing have previously been described in-depth 20,21 . Employing a built-in quartz crystal microbalance (QCM) 20 and Pt, Pd and Al targets, all with a purity of 99.99 % (ACI alloys), the conditions for physical vapor deposition (PVD) were derived for the chosen compositions. The thin films (~60 nm) were subsequently deposited in triplicate by PVD, onto an annular arrangement (Patent Application No. 2015/08424) of 64 square working electrode pads (0.09 cm 2 each), developed through photolithography 20 (Figure 2). The electrode pads were connected by a network of traces to individual rectangular pin pads on the outer perimeter of the wafer. The network of traces were provided with reinforced bending points to prevent braking of the lines/traces. For the conventional electrochemical testing glassy carbon inserts (0.196 cm 2 each) were employed as substrate onto which the thin metal films were deposited.

Electrochemical testing was performed on the wafer containing the various thin film compositions, in a dedicated electrochemical cell 20 , with each working electrode connected individually to a multichannel potentiostat (Arbin MSTAT) for high-throughput screening while conventional testing commenced in a standard three electrode setup 21 . Preconditioning was conducted in degassed 0.1 mol.L -1 HClO4 (Merck), scanning from 0.041 V vs SHE to 1.441 V vs SHE starting at Eocp (open circuit potential) for 25 cycles, followed by 3 cycles up to 1.741 V vs SHE and finally the first step was repeated but only for three cycles, employing a scan rate of 50 mV.s -1 respectively. Linear polarisation (LP) was conducted in 1 mol.L -1 H2SO4 (Merck, pH = 0.55) in the potential window 0.2 V vs SHE to 1.441 V vs SHE at a scan rate of 10 mV.s -1 . This solution was degassed with N2 for 15 min prior to linear polarisation. A stock solution of 1 mol.L -1 Na2SO3 (Sigma Aldrich) was used as an in-situ SO2 source 20 . These electrochemistry testing parameters were identical for the thin films deposited onto the wafer and the glassy carbon inserts. In addition internal resistance (i-R) drop was measured to be 46 Ω for the conventional electrochemical setup employing the various combination of thin films sputtered onto glassy carbon inserts. i-R compensation was intentionally not applied to LP scans as it damps current oscillations (as is observed for our catalyst (not shown)) which is specific to catalytic activity and relates to mechanistic phenomena 25 (mechanistic investigation was not part of the scope). Subsequent to the deposition of the selected thin film combinations onto the glassy carbon substrates (designated with letters of the alphabet as a means to ease comprehension throughout the article), some were kept as fresh (as-deposited) thin films while the rest of the thin films were annealed ex-situ under an Ar atmosphere, employing a rapid thermal annealing oven (Accuthermo AW610), with the maximum temperatures ranging from 600 °C to 900 °C with intervals of 100 °C 21 .

Physical characterisation of thin films

Energy-dispersive X-ray spectroscopy (EDX, employing INCA software) analysis, in moving average mode employing a collection time of 100 s and a 15 keV electron source, was conducted in triplicate before and after electrochemical testing, so as to evaluate thin film composition. Atomic force microscopy (AFM, Nanoscope V Multimode from Veeco) was used to investigate surface roughness, deriving various surface roughness parameters by employing Nanoscope Analysis (Veeco) software. In addition, surface properties of the thin films were characterised employing X-ray photoelectron spectroscopy (XPS). A Thermo Fisher Scientific K-ALPHA spectrometer was used for surface analysis with a monochromatised Al Kα source (hν = 1486.6 eV) and a 200 micrometer spot size. A pressure of 10 -7 Pa was reached in the chamber when transferring the samples. The full spectra (0-1100 eV) were obtained with a constant pass energy of 200 eV and high resolution spectra (C1s, Pd3p-O1s, Al2p-Pt4f, Pd3d, Al2s) at a constant pass energy of 40 eV. Charge neutralisation was applied during analysis. Sputtering was performed with Ar + ions at 500 eV. High resolution spectra were fitted and quantified (Scofield sensitivity factors used for quantification) using the AVANTAGE software provided by Thermo Fisher Scientific.

Results and discussion

Combinatorial sputtering and high-throughput screening of the PtxPdyAlz system

With the focus on identifying a possible ternary anode composition for application in the electro-oxidation of SO2, PtxPdy compositions were evaluated previously 20,21 and characterised so as to establish a standard for comparison with ternary PtxPdyAlz anode compositions 26 . The initial binary combinations investigated include a wide compositional range (PtxPdy) that spanned from pure Pt to pure Pd using mainly 10 % content increments (refer to reference 20 for in-depth details). Pt3Pd2 was identified to be the optimum binary composition and extensive characterisation was conducted on it 21 , with annealed Pt3Pd2 (at 800 °C) proving to be a worthy contender to compete with pure Pt for the electro-oxidation of SO2. In producing a ternary electrocatalyst the combination of Pt and Pd with a base metal will result in an electrocatalyst containing less noble metal with Al making up the balance. Due to the vast number of electrocatalyst combinations which can statistically be formulated using the three components, Pt, Pd, and Al, our selection of electrocatalysts to be subjected to high-throughput screening was aligned with the high-throughput experimental planning categorized by R. Potyrailo 27 . This includes hierarchical developing the experiment with increasing focus covering a space of interest 27 . Employing the finding of our previous work, that a Pt3Pd2 annealed at 800 °C exhibited competing performance, as a guideline for a possible ternary anode composition which contains a base metal, ternary compositions were mainly focused but not limited to the area were Pt is less than 60 %, Pd less than 40% (i.e. Pt3Pd2) and random Al content. Al, however, could not be increased above 60 % as (i) Al deposits at a much lower rate in contrast to Pt and Pd resulting in very long deposition times, while operating the PVD apparatus at its limit, (ii) Al is not very resistant in strong acidic environments with degradation being almost immediate in sulphuric acid, and (iii) a report found in literature, on Pt and Al combinations, proved that only small amounts of Al is effective in improving catalytic performance 23 . Linear polarisation response of the various compositions were obtained through high-throughput parallel screening, which is a simple means of quickly identifying an active composition that may work well in the SDE environment. The average of the polarisation graphs (conducted in triplicate) were used to determine the onset potential for each ratio employing the method reported by Cooper & McGinn with the current density based on (a) geometric area 20,26,28 , and (b) mass Pt present in the thin film.

Before and immediately subsequent to electrochemical testing each of the catalyst pads were visually evaluated for signs of corrosion. Some of the compositions did corrode during electrochemical testing (Figure 3 In the event that the greatest portion of the pad revealed corrosion, it was not included in the results, as it would in essence be the gold circuit that participated in the electro-oxidative reaction. Gold shows a high onset potential for SO2 electro-oxidation, hence obtaining low onset potentials eliminated the possible interference of gold. Onset potential, as a means of evaluating catalyst activity, is a suitable measure in the event of possible corrosion as it is not influenced by any change in surface roughness 26 . As only some catalyst pads revealed severe signs of corrosion, which were subsequently eliminated, current density based on geometric area and mass of Pt present in the thin film were also employed to evaluate thin film combinations. Current density, in contrast to onset potential, is influenced by both surface roughness and composition and by applying both these parameters to identify thin film candidates during the screening phase, limited any systematic errors made. Screening of the various thin film combinations, for the electro-oxidation of SO2, are summarised in Figure 4 as ternary contour plots in terms of (a) onset potential (V), (b) current density (mA.cm -2 ), and (c) mass-specific current density (mA.μg Pt -1 ). Employing these criteria eight thin film composition, three based on lowest onset potential value, three based on highest geometric current density (normalised by geometric area), and two based on highest massspecific current density (normalised by Pt mass), were selected for additional testing employing conventional testing techniques with glassy carbon inserts as substrate.

Composition of the thin films prior to conventional electrochemical testing

The bulk stoichiometry of the eight best electrocatalysts was investigated in triplicate by EDX, both before and after annealing. EDX analysis revealed that the expected theoretical compositions, predicted by employing the built-in QCM, correlate well with the actual sputtered bulk stoichiometry (Figure 5). No difference between asdeposited thin films and their corresponding annealed states were observed.

Conventional electrochemical evaluation

Linear polarisation and subsequent EDX characterisation

Employing physical vapour deposition and high-throughput screening accelerates the complex process of new catalyst design and improves performance identification. However, it is not sufficiently suited for in-depth investigation and characterisation, and conventional methods, such as rotating disk electrode (RDE) linear polarisation, are more suitable. Hence, Figure 6 exhibits the electrocatalytic performance of the eight selected thin films (in the as-deposited (AD) and annealed state (on glassy carbon as substrate)) that did not show signs of corrosion (mainly in the form of delamination) during/after electrochemical testing. (I)-700 °C, referring to Pt, and previously identified (J)-800 °C, referring to Pt3Pd2, are taken as the baseline to which all ternary combinations are compared against. In general, it is evident that no significant improvement in onset potential was achieved, with the majority of electrocatalysts exhibiting similar onset potentials (within error), when compared to (I)-700 °C and (J)-800 °C. However, (E)-AD, 700 °C, 800 °C and (D)-AD performed well. As the results obtained from onset potential are not sufficient for identifying an alternative electrocatalyst, mass-specific current density was also investigated. From Figure 6 it is evident that the majority of the ternary electrocatalysts exhibits an increase in current density. Compared to the baseline, i.e. Pt and Pt3Pd2, Pt40Pd57Al3 (annealed at 900 C) performed the best in that current density (mA.mg Pt -1 ) increased from 108.11, to 181.21 and finally to 396.73 for Pt, Pt3Pd2 and Pt40Pd57Al3 respectively, indicating an increase in activity that correlates with a decrease in Pt content (Figure 7). Subsequent to electrochemical testing (Figure 6) the thin film electrocatalysts were analysed by EDX so as to evaluate any change in composition that could not be visually detected (an indication of short term stability).

Overall, all the catalysts exhibit a slight change in compositional ratio with no specific trend (Figure 8), when compared to the compositional ratios directly after sputtering (Figure 5), with (B)-AD, (F)-AD, (H)-600 °C and (H)-700 °C showing the largest deviation in compositional ratio. This is however not sufficient in identifying a stable electrocatalyst, and additional evaluation on the stability is required. In an attempt to identify a stable electrocatalyst, in conjunction with having a low onset potential and acceptable current output, consecutive cycling in a sulphuric acid environment was conducted (Figure 9). For all the electrocatalysts under investigation, the only electrocatalyst that exhibited improved stability, compared to (I)-700 °C and (J)-800 °C, was (B)-900 °C, lasting about 1.4 times longer. Taking all three parameters (onset potential, mass-specific activity, and stability) into consideration that are indicative of an improved electrocatalyst, (B)-900 °C can therefore be considered the ternary candidate exhibiting the most potential in competing with (I)-700 °C and (J)-800 °C. To put (B)-900 °C further to the test, it was subsequently evaluated under an increased electrolyte temperature of 60 °C (using a SCE reference electrode above this temperature is not advised, due to the instability of Hg2Cl2) with the results also being depicted in Figure 6 and Figure 9. At this elevated temperature the same phenomena was observed for (B)-900 °C, (I)-700 °C and (J)-800 °C as in the case of 25 °C, with no significant difference in onset potential having been observed. However, an improved performance was evident in terms of current output and durability. The (B)-900 °C thin film (at 25 °C) furthermore exhibited totally different behavior in terms of visible signs of corrosion when compared to all the other tested electrocatalysts. This was evident in that the (B)-900 °C thin film (at 25 °C) dissolved gradually into solution (visually monitored until the film was so thin, that glassy carbon started to appear), keeping its active surface (Figure 10a) as only the thin film thickness decreased, while all the other electrocatalysts, including the target electrocatalysts 21 , clearly delaminated. (B)-900 °C (at 60 °C), in contrast, only started to exhibit the normal signs of delamination nearer to the end of its stability lifetime, with the last cycle subsequently being lower in the hydrogen adsorption/desorption region than the first cycle (Figure 10b), as a part of the thin film delaminated from the surface.

Further characterisation and discussion

AFM and XPS analysis were conducted on the most promising binary 21 and ternary thin film electrocatalysts in competing with pure Pt for SO2 electro-oxidation, with the aim being to gain more insight into the physical and electronic properties of these thin films. Pt thin films (as-deposited and the best performing annealed form) were also included in these analyses, serving as the primary reference/target electrocatalyst.

AFM analysis

AFM allows, among other things, meticulous observation and evaluation of the morphological and topographical characteristics of thin film surfaces. Three-dimensional (3D) AFM scans were recorded over a substrate area of 2 μm x 2 μm, for each sample, at three different locations on the thin film. Roughness parameters were calculated from 2 different positions within each of the three individual locations so as to provide reliable average roughness values representative of the whole thin film surface. From the 2D and 3D AFM images shown in Table 1, it is clear that the thin films in the as-deposited form all exhibit a fine distribution of metal particles, adapting the profile of the underlying glassy carbon surface, with unavoidable grooves clearly visible resulting from polishing. Upon annealing the as-deposited thin films evolved from exhibiting a finely distributed appearance to clearly exhibiting different degrees of crystal formation with the least amount of change being visible for (I)-700 °C. (J)-800 °C transformed into a surface with clearly defined grain boundaries, evident of a polycrystalline nature 21 . (B)-900 °C exhibited crystal growth of a totally different nature when compared to (J)-800 °C, with a type of random agglomerated distribution of small 'sheets' of crystals varying in orientation with sharp corners emerging from the surface. It is remarkable how only ~3 % Al metal content can induce such an extreme difference in surface/bulk structure upon annealing. This type of crystal growth might be one of the reasons for observing a prominent improvement in catalyst performance for (B)-900 °C, especially the stability performance observed for this electrocatalyst in a sulphuric acid environment for both 25 °C and 60 °C (Figure 9), when compared to (I)-700 °C and (J)-800 °C.

Table 1: 2D and 3D AFM images for electrocatalysts (I)-Pt, (J)-Pt3Pd2, and (B)-Pt40Pd57Al3

2D

As-deposited Annealed In addition to 2D and 3D imaging, the following surface roughness profile parameters (Figure 11) were acquired: (i) average roughness (Ra), which is based on mean height roughness, (ii) vertical distance between the highest and lowest points (Rt), and (iii) root mean square (RMS) of the distribution of surface height making it a bit more sensitive than Ra 29 . In general, as is evident from Figure 11, (I)-700 °C and (J)-800 °C exhibited similar values for all the roughness parameters with (B)-AD showing a smoother surface (lower values) overall. As for the different annealed forms of the individual thin films, the roughness parameters all increased from the as-deposited state to the specific annealed form, with (B)-900 °C being the most influenced by annealing treatment and also exhibiting totally different crystal growth compared to (I)-700 °C and (J)-800 °C (Table 1 2D). Overall, (B)-900 °C exhibited the highest roughness values for all three roughness parameters acquired, which has a direct and positive effect on the higher current output observed when compared to (I)-700 °C and (J)-800 °C. The rapid thermal annealing of catalyst (B)-900 °C has a positive influence on the performance and stability of this catalyst towards SO2 electrooxidation in a sulphuric acid environment.

XPS

In an attempt to further elucidate the observed electrocatalytic performance of the most promising binary 21 and ternary thin film electrocatalysts, in competing with pure Pt for SO2 electro-oxidation, XPS surface analysis and depth profiling on the thin film samples were conducted so as to gain information regarding their composition and chemical environment. Knowing the overlapping that occurs between Pd3p3-O1s and Al2p-Pt4f peaks, high resolution spectra were collected to ensure that the best fitted quantified results are obtained, especially for materials J and B (cf. Electronic Supplementary Information (EIS) Figure S1 and S2 for fitted XPS spectra after 10s Ar + etching for (B)-AD and (B)-900°C). The fits were conducted subsequent to linear background subtraction using a product of Gaussian and Lorentzian functions, as defined in the AVANTAGE software. Figure 12 reveals comparative information for the AD and annealed states with regard to the extreme surface (top outer layer) of the thin films for Pt4f, Pd3d and Al2s. Figure 13 exhibits compositional information (derived from the C1s, O1s, Pt4f7/2, Pd3d3/2 and Al2s peak areas), related to the depth of the metallic Pt and Pt-based thin films deposited on glassy carbon as substrate. It must be kept in mind that (i) XPS remains a semi-quantitative analysis to follow atomic compositional change with depth (not to be directly correlated with previous EDX results), and (ii) XPS has an accuracy within 5 to 10 % of absolute atomic % compositional information [START_REF] Silva-Bermudez | Bio-tribocorrosion in biomaterials and medical implants[END_REF] .

In the as-deposited state, no shift in the Pt4f7/2 peak at ~71.15 eV was observed for (I), (J) and (B), which is a direct indication that no significant change in the chemical environment occurred during/as a result of physical vapour deposition. Upon annealing, again no shift in the Pt4f7/2 binding energy was observed for (I)-700 °C and (J)-800 °C, however a clear change was observed for (B)-900 °C, exhibiting a slight negative shift in the Pt4f7/2 (Figure 11d) and Pd3d5/2 peaks ( Figure 11e), which could possibly be ascribed to the occurrence of metal-metal interactions as a result of annealing temperature [START_REF] Lopes | [END_REF] . In addition, (B)-900 °C (Figure 12f) exhibited an increase in Al content at the surface (15 at. %) when compared to (B)-AD (only 2 at. %), which is expected if explained in terms of fundamental properties of monometallic metals (atomic radius) 32 . For the metals present in (B)-900 °C, the order of decreasing size with regard to atomic radii is Al (143 pm) > Pt (139 pm) > Pd (137 pm) [START_REF] Weller | Inorganic chemistry[END_REF] . In general, lattice strain is expected to contribute to the segregation/migration of the atom with the largest atomic radius to the surface, which in the case of (B)-900 °C is Al. This supports the observation obtained from XPS data that Al2s mainly appears on the extreme surface subsequent to annealing (Figure 11f), which clearly points towards the migration of Al to the surface upon annealing. While the previous XPS measurements (Figure 12) focused on the surface, depth profiles of the Pt and Pt-based samples (Figure 13) were obtained by combining a sequence of Ar + ion gun etch cycles with XPS measurements from the current exposed surface. The objective of obtaining depth profiles was to validate that the observed electrocatalytic performance was not as a result of a 'skin' formed, as a result of severe segregation/migration to the surface, but rather being the result of a homogeneous composition throughout the depth of the thin film. The first observation for all the electrocatalysts, in both the as-deposited and annealed states, is (i) the presence of unescapable C, N, Si and Cl impurities, due to ambient pollution on the metal surface layer, disappearing after only 10 s of Ar + etching, and (ii) a constant ~55 nm thin metallic layer (validating the calibrated sputtered layer of ~60 nm) recorded by XPS etching for all the analysed thin films. Subsequent to Ar + etching through the thin metallic film, it is clear that at a depth of ~40 nm carbon from the substrate appears. Carbon, however, is present almost throughout the depth of the annealed surfaces (especially sample (B)-900°C), which can be explained either by possible diffusion of the substrate through the Pt/Pd/Al thin film at high temperature (which is unlikely as these specific glassy carbon inserts are stable under high temperatures) or as a result of XPS detecting possible 'exposed' substrate carbon as a result of the roughness of the (B)-900°C thin film (Rt around 30 nm, Figure 11). It is furthermore evident that contraction of the different sputtered layers occur upon annealing with increased contraction as the temperature increases. For (B), Al seems to be in the oxidised state throughout the entire thickness of the layer (118.5 eV binding energy Figure 12f, only data for the top layer is shown). As Al gets oxidised easily in air, it is very probable that the samples were oxidised from the moment when the thin films were exposed to air. In addition, XPS depth profiling for (B)-900°C supports the previous observation (Figure 12f) of a degree of migration of Al towards the surface after annealing and that all three metals are present throughout the depth of the thin film. Concerning Pd3d5/2 peak fitting (cf. EIS Figure S1 and S2), the absence of PdO bonding usually located at 337 eV is confirmed [START_REF] Garrido | [END_REF] as the maximum component is Pd metal at around 335 eV, while another 1 eV higher (~336 eV) could be attributed to Pd-O-X bonding namely Pd-O-Al.

As thin film (B), i.e. Pt40Pd57Al3, presented itself as being a competitive electrocatalyst for SO2 electro-oxidation, measured against Pt and the previously identified Pt3Pd2 composition, XRD spectra were furthermore recorded for (B)-AD and (B)-900°C (cf. EIS Figure S3). (B)-AD is concluded to consist of a PtPd cubic phase, whereas (B)-900°C consist of a possible Pt2.84 Al1.16 cubic phase that may support the slight negative shift observed for the Pt4f7/2 (Figure 11d) and Pd3d5/2 peaks (Figure 11e), which are indicative of alloy formation.

Conclusion

As is evident from literature, only a limited number of surfaces have been investigated thus far for catalysing the electro-oxidation of aqueous SO2. It is furthermore clear that widespread disagreement and non-reproducible behavior exist 24 , as this reaction is very sensitive and highly influenced by the prevailing conditions (electrode material, pH, temperature, preconditioning, pressure, applied anodic potential, etc.). From our investigation, identifying a possible alternative anode catalyst that can compete with pure Pt, it is clear that a lot of scope and room for improvement still exist for improving the catalytic activity and stability of the electrocatalyst. In general, it is primarily the surface properties of a catalyst in conjunction with the secondary bulk effects, which are credited for any observed catalytic improvements/weakening. We have highlighted the influence that a secondary/tertiary metal, in conjunction with annealing, can have on the catalytic performance of the thin film by effecting only slight changes in composition and temperature treatment.

The addition of a third component to the noble metals Pt and Pd, in the form of Al, i.e. PtxPdyAlz, enhanced the electrocatalytic performance towards the electro-oxidation of aqueous SO2 with the Al content kept below ~3 %.

In excess of ~3 % Al the electrocatalytic performance was severely hampered. Overall onset potential, however, did not improve significantly for any of the evaluated electrocatalysts, while improvements in current output and stability (lasting ~1.4 times longer than both pure Pt (the (I)-700 °C sample) and Pt3Pd2 (the (J)-800 °C sample)) was achieved specifically for Pt40Pd57Al3 (the (B)-900 °C sample), both in 25 °C and 60 °C testing environments.

We have shown that electrocatalytic activity for the electro-oxidation of aqueous SO2 can be achieved by substantially lowering the Pt content in that current density (mA.mg Pt -1 ) increased from 108.11 to 181.21 and finally to 396.73 for Pt, Pt3Pd2 and Pt40Pd57Al3 respectively, indicating an increase in activity that correlates with a decrease in Pt content.

Furthermore, additional information obtained from XPS analysis has shown that Pt40Pd57Al3 ((B)-900 °C) exhibited an increase in Al content at the surface (~15 at. %) when compared to (B)-AD (only ~2 at. %) although no sign of a distinctive skin of one metal (or more in the case of Pt40Pd57Al3) was present near the surface for the Pt3Pd2 and Pt40Pd57Al3 thin films (as-deposited and annealed). This confirmed that the observed activity of the thin films was a result of all the metals being present in the thin film surface. XRD analysis of the annealed Pt40Pd57Al3 thin film ((B)-900 °C) has highlighted the presence of some degree of alloy formation.

The power of alternately employing combinatorial and high-throughput methods with conventional electrochemical and characterisation methods, for electrocatalysts development, has clearly been shown, and resulted in the identification of a catalyst that contains a lot less platinum while exhibiting increased catalytic activity and stability. Combining, among other things, the sensitivity of this reaction to the surface where oxidation occurs, with the mechanistic complexity of this reaction 35 ( proposed mechanisms include, indirect oxidation via (i) a dithionate intermediate [36][37][38] or (ii) involving bulk SO2 and an unspecified S(VI) intermediate 39 , and direct oxidation via adsorbed reactant 37 ), a specific active catalysts of interest will be better understood by in depth mechanistic investigation taking into account various factors such as temperature, acid concentration, sulphur coverage etc. It is however relevant to mention, that the majority of mechanistic studies on SO2 oxidation were conducted on platinum and single component electrode materials. Based on the amount of effort devoted to the mechanistic understanding of Pt catalysing SO2 oxidation, do these studies reveal the complexity of this reaction, proving that the necessary mechanistic characterisation of this identified multi-component Pt40Pd57Al3 electrocatalyst evident to be challenging and will require meticulous and repeatable evidence to be fundamentally sound.

Figure 1 :

 1 Figure 1: The thermo-electrochemical HyS cycle for the splitting of water.

Figure 2 :

 2 Figure 2: Annular arrangement of 64 square working electrode pads (Patent Application No. 2015/08424).

Figure 3 :

 3 Figure 3: Examples of varying degrees of corrosion occurring after linear polarisation for (a) Pt34Pd22Al44, (b) Pt43Pd46Al11, and (c) Al.

Figure 4 :

 4 Figure 4: Ternary contour plots for (a) onset potential, (b) current density normalised by geometric area, and (c) massspecific normalised current density, towards SO2 electro-oxidation for the PtxPdyAlz system.

Figure 5 :

 5 Figure 5: EDX after sputtering verifying that the actual sputtered ratios correlate with the expected sputtered ratios of (A)-Pt42Pd23Al35, (B)-Pt40Pd57Al3, (C)-Pt33Pd40Al27, (D)-Pt43Pd46Al11, (E)-Pt48Pd23Al29, (F)-Pt60Al40, (G)-Pt23Pd26Al51, and (H)-Pt23Pd58Al19.

Figure 6 :

 6 Figure 6: Onset potential values (a) and mass-specific current density values (b) for (B)-Pt40Pd57Al3, (C)-Pt33Pd40Al27, (D)-Pt43Pd46Al11, (E)-Pt48Pd23Al29, (F)-Pt60Al40, (H)-Pt23Pd58Al19, (I)-Pt, and (J)-Pt3Pd2.

Figure 7 : 3 Figure 8 :

 738 Figure 7: Correlation of current density (mA.mg Pt -1 ) with Pt content (%).

Figure 9 :

 9 Figure 9: Consecutive cycling of thin film electrocatalyst for stability evaluation with (B)-Pt40Pd57Al3, (C)-Pt33Pd40Al27, (D)-Pt43Pd46Al11, (E)-Pt48Pd23Al29, (F)-Pt60Al40, (H)-Pt23Pd58Al19, (I)-Pt, and (J)-Pt3Pd2. Conditions: scan rate of 100 mV.s -1 and potential window of 0.041 V vs SHE to 1.441 V vs SHE at 25 °C and 60 °C.

Figure 10 :

 10 Figure 10: Consecutive cycling of (B)-Pt40Pd57Al3 annealed at 900 °C in H2SO4 at (a) 25 °, and (b) 60 °C.

Figure 11 :

 11 Figure 11: Surface roughness profile parameters of (I)-Pt as-deposited (I AD) and annealed at 700 °C, (J)-Pt3Pd2 asdeposited (J AD) and annealed at 800 °C, and (B)-Pt40Pd57Al3 as-deposited (B AD) and annealed at 900 °C.

Figure 12 :

 12 Figure 12: Pt4f, Pd3d and Al2s XPS spectra on the extreme surface, i.e. the top outer layer, of both the as-deposited (AD) (black curve) and annealed states (red curve) of (I)-Pt (a),(J)-Pt3Pd2, (b, c) and (B)-Pt40Pd57Al3 (d, e, f).

Figure 13 :

 13 Figure 13: Depth profiles of (I)-Pt, (J)-Pt3Pd2 and (B)-Pt40Pd57Al3, for both the as-deposited (AD) and annealed states. The etch-rate was estimated 0.2 nm/s on each material.

  .

	Thermal decomposition: H 2 SO 4 → SO 2 + ½O 2 + H 2 O	(1)
	Anode reaction:	SO 2 + 2H 2 O → H 2 SO 4 + 2H + + 2e -	(2a)
	Cathode reaction:	2H + + 2e -→ H 2	(2b)
	Net electrolysis reaction: SO 2 + 2H 2 O → H 2 SO 4 + H 2

Acknowledgements

The financial assistance of the National Research Foundation (NRF) towards this research is hereby acknowledged (UNIQUE GRANT NO: 92704). Opinions expressed and conclusions arrived at, are those of the authors and are not necessarily to be attributed to the NRF. The assistance of Dr. L. Tiedt with EDX, Mr. H.S. Kotzé with annealing and Me. L. Swartzberg with conventional electrochemistry runs, is gratefully acknowledged. In addition we want to express our gratitude to Mr. E. Lebraud (engineer, ICMCB UPR 9048, 87 av. Albert Schweitzer, 33608 Pessac Cédex France) for assistance with XRD.