Journal of Forest Science, 67, 2021 (7): 307–317 Review
                https://doi.org/10.17221/13/2021-JFS
                Methodological approaches to the valuation  
                of forest ecosystem services:  
                An overview of recent international research trends
                Caterina Patrizia Di Franco, Gianmarco Lima, Emanuele Schimmenti,  
                Antonio Asciuto*
                Dipartimento di Scienze Agrarie, Alimentari e Forestali (SAAF),  
                Università degli Studi di Palermo, Palermo, Italy
                *Corresponding author: asciuto.antonio@gmail.com
                Citation: Di Franco C.P., Lima G., Schimmenti E., Asciuto A. (2021): Methodological approaches to the valuation of forest 
                ecosystem services: An overview of recent international research trends. J. For. Sci., 67: 307–317. 
                Abstract: Forests represent the most important source of ecosystem services (ES) on a global level both for the production 
                of goods and for the provision of services and externalities, nevertheless scientific research in the economic field is lacking. 
                Currently the number of documents relating to ES is 16 673, of which only 1 379 concern the forestry sector. The aim of 
                this study is to provide an overview of scientific research trends in the field of economic evaluation of forest ecosystem 
                services (FES). To this end, an on-line bibliographic survey was carried out on the main scientific search engines, which 
                made it possible to quantify the works and at the same time to detect the main evaluation methods used for the different 
                FES. This survey allowed to collect 93 articles meeting the search criteria: the most active continents were Europe and Asia, 
                whereas most of the articles focused on the joint evaluation of provisioning, regulation and cultural services, even if a good 
                number of them only concerned cultural services. The most widely used valuation methodologies were the contingent 
                valuation among the stated preference techniques and the market price among direct observation criteria.
                Keywords: international classification; forests; methodologies; monetary; valuation
                  Changes made by man to forest ecosystems due          sequently, a new branch of research has been 
                to a growing demand for raw materials (food, fresh      generated in the field of the economic evaluation 
                water, wood, fibre, energy sources, etc.) are recog-    of forest ecosystem services (FES) which has pro-
                nized as the main factor of climate change, deterio-    duced scientific literature that has not been widely 
                ration of ecosystems and air pollution.                 quantified, analysed and organized yet.
                  Cultural growth and a deeper environmental              The main objective of this study is to fill the liter-
                sensitivity of local communities have increasingly      ature gap, coming to the detection of the papers, as 
                brought out the role of forest ecosystems as assets     well as to provide an analysis of the trends of inter-
                of general interest. On the institutional level, this   national scientific research, over the last 22 years, 
                change is identifiable in the current framework of in-  in the field of the economic evaluation of ecosys-
                ternational conventions and is reflected in the politi- tem services (ES) of specific forest sites. They were 
                cal orientation of individual countries that more and   sorted out with reference to publication year, to 
                more recognize the social role of forestry heritage.    country, to valuation methodology which was used 
                  The issue of the valuation of ecosystem services      to estimate each single category of ES.
                has therefore carried out a crucial role in order to      The information thus obtained can contribute 
                acquire knowledge for policy development, eco-          to broadening the knowledge of the economic ap-
                system management and spatial planning. Con-            proach to the valuation of different ecosystem ser-
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                Review                                                      Journal of Forest Science, 67, 2021 (7): 307–317
                                                                                           https://doi.org/10.17221/13/2021-JFS
                vices, and might be used by the scientific community       MEA: Supply, Regulation and Habitat and cultural 
                to undertake sectorial studies, considering the meth-      and aesthetic beauty services (Pascual et al. 2010).
                odologies applied to the various original case stud-         Finally, the CICES, after an initial series of meth-
                ies found through a bibliographic search conducted         odological proposals for the classification of ecosys-
                both on Scopus database and Google Scholar.                tem services (Haines-Young, Potschin 2010, 2011), 
                                                                           has developed a hierarchical structure organized in 
                LITERATURE REVIEW                                          three sections: Provisioning, Regulation and Mainte-
                                                                           nance, Cultural Services; in divisions (main processes 
                  Ecosystem services and their classification. The         or outputs); in groups that distinguish the processes 
                term “ecosystem services” was first introduced by          in biological and physical ones that can be linked to 
                Ehrlich and Ehrlich (1981). The concept was origi-         tangible resources; in classes that identify individual 
                nally intended to arouse public interest and estab-        entities, of which the unit of measurement and indi-
                lish a framework for highlighting the social benefits      cators to measure ecosystem services are provided 
                of ecosystem conservation as the rate of biodiver-         (Version 4.3) (Haines-Young, Potschin 2013).
                sity loss was becoming increasingly evident (West-           In the latest version (Version 5.1) (Haines-Young, 
                man 1977; Pimentel et al. 1980; De Groot 1987).            Potschin 2018), for a better conformity with the 
                Since the 1990s, the study of ES has become a focal        principles of ecosystem accounting (United Na-
                point of numerous research projects.                       tions 2003) and to address the key issues identified 
                  An analysis of their current conditions and the          in the literature, the scope of the CICES focused on 
                consequent effects on the wellbeing of the popula-         the identification of final services and the classifica-
                tions was promoted by the United Nations with the          tion was broadened to cover abiotic aspects.
                Millennium Ecosystem Assessment (MEA 2005)                   Its hierarchical structure allows for an adequate 
                project launched with the aim of identifying eco-          level of detail, in fact, moving from the Section to 
                compatible development strategies and consolidat-          the Division, to the Group and to the Class, the 
                ing the culture of valorisation of “multiple benefits      services are increasingly specific, but the charac-
                provided by ecosystems to mankind”.                        teristics used to define them at the lower levels are 
                  Cultural growth and the increase in the environ-         dependent on the categories above them.
                mental sensitivity of the community have increas-            The structure of the Provisioning section has 
                ingly brought out the role of forest ecosystems as         been modified with “Biomass” and “Genetic mate-
                assets of collective interest. Forests and woods have      rial of all biota” to distinguish the results of biotic 
                been defined as the most important source of ES            ecosystems from abiotic ones at the division level. 
                (FAO 2010), in fact they represent the natural sys-        This section covers all nutritional, non-nutritional 
                tem with the highest content of genetic, specific and      material and energy outputs from living systems as 
                ecosystem, as well as historical and cultural diver-       well as from abiotic outputs (including water).
                sity. The international recognition of the multifunc-        The Regulation and Maintenance section in-
                tional character of forest ecosystems derives from         cludes ways in which living organisms can mediate 
                the joint production of goods (wood and non-wood           or moderate the environment that affects human 
                renewable products), services and externalities. De-       health, safety or comfort, along with abiotic equiv-
                spite this, a number of scientific works concerning        alents. The division covers the transformation of 
                the forestry field is sensibly missing.                    biochemical or physical inputs into ecosystems in 
                  At the international level, three classification sys-    the form of waste, toxic substances and others; and 
                tems have been developed to define the typology of         the regulation of physical, chemical and biological 
                ecosystem services: Millennium Ecosystem Assess-           conditions. Finally, the Cultural section includes all 
                ment (MEA); The Economics of Ecosystems and  non-material, non-rival and non-consumptive out-
                Biodiversity (TEEB); Common International Clas-            puts of ecosystems (biotic and abiotic) that affect 
                sification of Ecosystem Services (CICES).                  people’s physical and mental states.
                  The MEA organizes ecosystem services into four             The economic valuation of ecosystem services. In 
                categories: Support, Procurement, Regulation and           recent decades, the awareness that many of the neg-
                Cultural Ones.                                             ative impacts are at the basis of current global chal-
                  The TEEB proposed 22 ecosystem services divided          lenges (climate change, loss of biodiversity, pollution, 
                into three main categories that revisit those of the       etc.) has prompted the scientific community to start 
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                Journal of Forest Science, 67, 2021 (7): 307–317 Review
                https://doi.org/10.17221/13/2021-JFS
                studies and research also on economic evaluation; in      economic value of ecosystem services that directly 
                fact, different methods have been proposed for the        influence the prices of the real estate market. These 
                economic evaluation of ecosystem services that are        methods are mainly suitable for evaluating regula-
                based on the utilitarian approach, grounded on the        tion and maintenance and cultural services.
                fact that people exploit directly or indirectly the ben-    – Methods based on stated preferences, including 
                efits of ecosystems, according to individual choices.     the contingent valuation which consists in a sample 
                Therefore, all goods and services are traced back to      survey of the reference population, hypothesizes 
                the total economic value (VET) which includes two         development scenarios for a given ecosystem ser-
                components (Gómez-Baggethun, De Groot 2010):  vice, allows to reveal the willingness to pay or the 
                the use value, in turn divided into direct and indi-      willingness to accept a compensation for the loss 
                rect, and the non-use value (Krutilla 1967), generally    of the service itself; the choice experiment meth-
                classified into existence, bequest and option. As to      od (CEM), in which the interviewees must choose 
                the latter, some authors instead place it in the cat-     between baskets of attributes present at different 
                egory of use values, although it can be considered        levels both in quantitative and qualitative terms, 
                as a sort of insurance in anticipation of a possible      which constitute the environmental good, arriving 
                future use of the resource (Gren et al. 1994; Pearce      at the determination of the marginal willingness to 
                2001; Balmford et al. 2002; Silvestri 2003; Turner et     pay for each attribute. These methods are crucial 
                al. 2003; EFTEC 2005).                                    above all for estimating non-use values (bequest, 
                  The use value can be attributed to ecosystem ser-       existence, option).
                vices exploited, directly or indirectly, for reasons of     However, the application of these methods is 
                production or consumption.                                generally expensive both economically and tem-
                  The second, on the other hand, is more difficult        porally; therefore, in the last few decades, vari-
                to assess because it corresponds to what is intrin-       ous scientific studies have made use of the ben-
                sic to a resource and therefore not used directly or      efit transfer (BT) method, which makes extensive 
                considered in a future perspective.                       use of the results obtained with different evalua-
                  To determine the values of ecosystem goods and          tion methods applied at a specific place and time, 
                services, in the literature there are various evalua-     transferring them to environmental goods and 
                tion methods that can generally be divided into the       services of a different place and time (Wilson, 
                following categories (Schirpke et al. 2014; Soraci et     Hoehn 2006; Plummer 2009).
                al. 2016; Pillari 2018):                                    Finally, open source computer models were cre-
                  – Methods based on direct observations for goods        ated that are able to map and evaluate ecosystem 
                that have a reference market and are therefore trad-      services, currently used in many programs and ini-
                able. The direct use value can be defined through         tiatives with both scientific and planning purposes. 
                the market price, which is used when there is an          These models include the Integrated Valuation of 
                immediate relationship between a market good and          Ecosystem Services and Tradeoffs (InVEST) (Sharp 
                the ecosystem service itself; the cost necessary to       et al. 2014), which treats ES in both biophysical and 
                produce the goods, that is, the amount of money           economic terms; the Social Values for Ecosystem 
                necessary to replace or restore an ecosystem ser-         Services ‒ SolVES (Sherrouse, Semmens 2015), 
                vice following damage; the production function  which evaluates and maps the social values of the 
                which relates the good resulting from a production        ESs; the Artificial Intelligence for Ecosystem Ser-
                process with the factors of production used to ob-        vices (ARIES) (Villa et al. 2014), which aims to bal-
                tain it. These techniques are particularly suitable       ance the user’s need for clarity without renouncing 
                for evaluating the services of the provisioning and       at the same time the maintenance of the complexity 
                regulation and maintenance function.                      of the space-time flows of the benefits provided to 
                  – Methods based on indirect observations, used          the community (Pillari 2018). 
                for ESs lacking in a target market, including the 
                travel cost which is grounded on the travel costs that    SURVEY DESIGN
                people incur to reach a site where they can enjoy the 
                desired ES; the avoided cost, that is the cost neces-       In order to quantify and analyse the scientific ac-
                sary to avoid the possible damage deriving from the       tivity related to the economic evaluation of forest 
                loss of a service; hedonic prices used to define the      ecosystem services (FES), a bibliographic search 
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                Review                                                        Journal of Forest Science, 67, 2021 (7): 307–317
                                                                                             https://doi.org/10.17221/13/2021-JFS
                was carried out with a quantitative approach, using         “forest AND ecosystem AND services AND NOT 
                two of the major search engines dedicated to scien-         agricultural”. Overall, there were 703 Scopus docu-
                tific research, namely Google Scholar and Scopus,           ments, of which 636 were published in English; while 
                both created in 2004.                                       the secondary documents were 676. The articles in 
                                                                                                                                    -
                  The time interval analysed is 1997–2019, where            scientific journals were 557, lowered to 495 when ex
                the choice of the starting date is linked to the pub-       cluding those ones written in languages other than 
                lication of the scientific article by Robert Costanza       English.
                “The value of the world’s ecosystem services and              At the same time, another search was carried out 
                natural capital” (Costanza et al. 1997), which intro-       on Google Scholar, directly targeting the FES and 
                duces one of the first definitions of ecosystem ser-        using the same keywords as before. It should be 
                vices, and the volume by Gretchen Daily, “Nature’s          noted that the procedure followed in the latter case 
                services” (Daily 1997), in which the author, in ad-         was very laborious as Google Scholar does not al-
                dition to describing the main ecosystem services,           low filtering and limiting the search field as Scopus 
                emphasizes the importance of their evaluation also          does. The initial number of documents is in fact in-
                in economic terms.                                          determinate and not always totally consistent with 
                  The research on the Scopus database was con-              the entered keywords, so the researcher is forced 
                                      rd 2020 by combining the “eco-        to examine an unlimited list of documents without 
                ducted on March 3
                system AND services” search words both as “title of         any clue on their overall number.
                the article” and as “keywords” to identify the works          Despite the obvious limitations related to the 
                concerning ecosystem services in general; a total of        choice of Google Scholar, this search engine has 
                8 864 documents were present in the database, of            made it possible to intercept scientific works not 
                which 8 195 published in English. In the first step         indexed in the major databases (Scopus, Web of 
                the search also involved secondary documents, i.e.          Science, Science Direct), which instead would have 
                those not indexed in the Scopus database, equal             been neglected.
                to 7 809. There are three possible reasons for their          The initial amount of articles obtained by the two 
                inclusion in the above category: they derive from           search engines was remarkable. To achieve the ob-
                bibliographic references or from citations con-             jective of quantifying the research activity carried 
                tained in Scopus documents; inability to index  out at an international level in the field of the eco-
                them with certainty due to incomplete or incorrect          nomic evaluation of specific forest sites and with ref-
                data; impossibility of finding the text (https://ser-       erence to individually considered FES, we proceeded 
                vice.elsevier.com/app/answers/detail/a_id/11239/            by gradually eliminating all the articles concerning 
                supporthub/scopus/#:~:text=Secondary%20docu-                the mere descriptive aspects relating to the habitats 
                ments%20are%20documents%20that%20are%20                     and those which dealt only with the ES classification; 
                not%20indexed,incomplete%20or%20incorrect%20                this preliminary work made it possible to obtain a 
                data.%20There%20is%20missing%20content.).                   set of articles with an economic angle on which to 
                Overall, the works relating to the ES therefore  focus the analysis. Subsequently, also the articles re-
                amounted to 16 673.                                         lating to payments for ecosystem services (PES) and 
                  Among all the Scopus documents only articles in           those that provided an overall economic evaluation 
                scientific journals were selected which amounted            of the ES or just a part of them without reaching the 
                to 6 192, and in particular those written in English        value of the single ecosystem service were excluded 
                that equal to 5 600.                                        from the subsequent analysis.
                  These figures might seem to be rather low because           After the above described filtering work, the stud-
                the search has been intentionally limited to “ecosys-       ies selected for the subsequent descriptive analysis 
                tem” and “service” keywords referring to the semi-          were found to be 80 on Google Scholar and 28 on 
                nal work by Costanza et al. (1997), while other ones,       Scopus, for a total of 108 articles. After verifying the 
                even if relevant, have not been taken into account,         simultaneous presence of 15 works on both plat-
                such as “multifunctional” and “management”, “envi-          forms, the final number of papers was reduced to 
                ronmental services”, “local services”, etc.., since they    93 (see Electronic Supplementary Material where a 
                were more in use in the previous years.                     complete list of these 93 papers is provided).
                  Subsequently, the works concerning the forest eco-          For each article, the title, the authors, the year of 
                system services were identified using the keywords          publication, the investigated ecosystem services, 
                310