1. Introduction
The concept of sustainability in a forestry perspective has a development history that dates over 300 years back in time [
1], without the author himself having supplied any definition of this term [
2]. In some respects, this idea at that time was the first conservation principle ever formulated [
3]. In recent decades, it has spread in an exceptional manner to other areas [
4]. To some authors, the sustainability concept is currently belonging to the field of social ethics [
5]. Nowadays, it continues to be a forceful message with a wide representation in diverse disciplines [
6]. In synthesis, von Carlowitz’s premise promoted the concept of not exhausting resources in order to bequeath them to future generations, safeguarding finite natural resources for future generations [
7].
In order to integrate sustainability into forest management, it has been customary to define a multidisciplinary ensemble of criteria and indicators in order to arrive at a consensus as to what sustainable forest management should be like [
8], although sometimes it is not easy to transfer this idea to a strategic level [
9]. It has been successfully applied on different spatial scales, using more or less aggregated information. Thus, there are indicators for sustainable forest management focused at a national or supranational level [
10,
11], or at a management unit level [
12,
13]. Further, in order to include this concept of sustainability in forest management, tools like certification systems aimed at ensuring sustainable management in forests have been developed. Thus, the systems most used nowadays simply require compliance with a series of indicators [
14]. However, although it is commonly believed that a certification scheme implies a sustainability attribute, this direct relationship is not always clear [
15].
On the basis of the literature consulted, it has been verified that the definition of sustainable forest management was usually of a static nature. That is to say, the studies in question contained only one measurement of the indicators, without taking into account their temporal evolution, in spite of this component being included in the definition of the word sustainability [
16]. However, only in some studies has it been proposed to measure a set of criteria over the entire planning horizon covering forest management in a certain case study [
17,
18,
19]. Given the extensive length of rotations in many forest systems, it is necessary to have a set of indicators available whose values are known in each period or, at least, are quantifiable at the end of the planning horizon.
Given the multidimensional nature intrinsic to the concept of sustainability, and as has been mentioned previously, the need to apply multifunctional management requires the use of Multi-Criteria Decision-Making (MCDM) techniques. These methodologies have been widely employed to solve typical forest management problems [
20,
21,
22]. Moreover, multicriteria methods have been applied prolifically to tackle aspects related to sustainability [
23,
24,
25,
26], even dealing with aggregation problems and dynamic sustainability indicators [
27]. These methodologies are also recommendable for integrating different ecosystem services into the decision-making process [
28,
29,
30]. Finally, it is fitting to insist that these methods permit the solution of the emergent problem of how to aggregate the indicators on which the idea of sustainability is based [
31].
Bearing in mind that sustainability is difficult to define in precise terms [
32,
33], the main objective of this study is to present a flexible model, based on multicriteria techniques. These kinds of models permit the redefinition of sustainability in a forest system, not at one specific moment, but through the evolution of the indicators considered throughout the planning horizon. This new view of sustainability should address the multifunctionality of forest systems, which implies defining a diverse set of criteria and indicators, choosing the best silvicultural alternative for each stand, and enabling the stakeholders involved to have their opinion integrated into it. It should be emphasized that some authors have affirmed that the rigidity of the concept of a normal forest does not ensure the idea of sustainability in force today [
34], which would justify using the approach proposed here in order to adapt this notion to the present-day context. Finally, in the following Sections, we will introduce terms like “criterion”, “objective”, “indicator” and “index”. In order to avoid misinterpretations, and following [
35], under an MCDM umbrella, criteria are the objectives or goals to be considered relevant for a certain decision-making situation. However, if applicable, we have considered a hierarchy between criteria and indicators. Indicators are parameters or sub-criteria which can be measured, and which correspond to a particular criterion. Besides, some sustainability measurements can be defined as a synthetic, aggregate, or composite index, and the value achieved by this index is a proxy of the respective sustainability goodness.
4. Results
Beginning with the pay-off matrix,
Table 1 shows the results of the optimization of the six criteria separately. In
Table 1, ideal values (principal diagonal of the matrix) are shown in bold type, while anti-ideal (or least desired ones) are underlined. The main diagonal includes the maximum values that each criterion can reach, known as the ideal point. Other parts of the matrix contain the anti-ideal points, which would be the worst results obtained for each of the criteria. Finally, to increase the informative character of this matrix, several additional rows were included by measuring the area occupied when each criterion is optimized by each of the silvicultural treatments described, as well as the average rotation.
The results given in this table show the ideal and anti-ideal values reached by the criteria, according to the optimized criterion (per columns). First, it can be seen that some objectives do not reach ideal values (NF, IM, G). Then, the level of conflict existing between the different criteria, mainly between NF and IM, and between C and G, can be observed, as well as the fact that the results are fairly similar in relation to carbon balance, because the percentage for this criterion varies the least compared to the rest. In synthesis, it can be noted that there is no solution corresponding to the optimization of a single criteria that appears to be sufficiently attractive. In short, the results of this pay-off matrix justify the setting up of an MCDM model in order to achieve solutions that are more balanced from the point of view of sustainability.
However, before revealing the results of those models, the result of the first interaction with DM1 to obtain the preferential weights associated with the criteria is shown (
Table 2). Note that the sum of the weights for each priority level is equal to 1. Finally,
Appendix A (
Table A2) contains the weights obtained throughout the second interactive process with DM2 for the seven categories, in which the stands and the silvicultural alternatives associated with them have been grouped.
Next,
Table 3 displays the solutions obtained for the two priority levels proposed. It can be noted that the solutions at the second priority level (
U2) are the same as the values found in the preceding solution (
U1), whereas the values of the three criteria included in
U2 improve in comparison with those obtained in
U1. Further, to facilitate comparisons, the same auxiliary rows have been included. In general, it can also be seen that the areas assigned to each silvicultural alternative are notably modified when moving from
U1 to
U2.
In the auxiliary rows in
Table 3, the area selected by the model for each silvicultural alternative is indicated. Also, this solution of the LGP model can be compared with the value proposed by the DM2 (the area pointed out as being the one preferred for applying each silviculture), information which is available on
Appendix A (
Table A2). In
Figure 2 the difference (in surface) between both areas (applied or selected by the model and preferred) can be noted. However, it should be taken into account that the area preferred exceeds the value of the total forest area, because, for each stand, a set of 2–3 possible silvicultural treatments were proposed.
5. Discussion
The method here proposed for evaluating sustainability in forest management has the advantage of following the evolution of the criteria over the planning horizon [
19], but it could also be adapted when including spatial constraints [
97]. In that case, it could be suitable to design a model simultaneously integrating strategic and tactical planning [
98]. Furthermore, the methodology proposed could be extended to incorporate the preferences of a group of stakeholders for addressing sustainability [
99,
100]. Some authors suggest compiling the forest managers’ preferences on silvicultural aspects in a goal programming model [
101], although the way these individual preferences would be aggregated is still an open question [
102]. Finally, although the inclusion of dynamic aspects could trigger highly specific models, limiting their transference to others [
103], we do not believe that this could happen in the proposal presented in this study due to its potential adaptation to other works, with other criteria and indicators.
The results presented in
Table 3 allow us to observe the differences in relation to the values achieved for the indicators situated at the first priority level, at forest level. Thus,
Figure 3 shows how the values of the criteria
NF,
IM and
G obtained in the pay-off matrix are modified (
Table 1), with respect to those obtained in the solution of the LGP model (
Table 3).
As could be expected, in the comparison between models (
Figure 3), the values of the criteria obtained in the LGP model are lower than the ideal ones reached in the pay-off matrix. These reductions indicate the opportunity cost associated with the simultaneous consideration of the six criteria defined in this model, instead of the values proposed in the individual optimization of each criterion. Thus, taking into account that the
NF and
G values (located at each end of the graphic) are adimensional and that they represent indicators of the less-the-better type, in the LGP solution their values are 2.7 and 1.6 times worse, respectively, compared to that obtained in the pay-off matrix. Finally, the
IM criterion’s value of 5358 ha, which represents the area managed by the most suitable treatment in each management unit (according to DM2), was reduced to 3750 ha in the LGP solution; i.e., about 30% of the area under ideal management (
IM) is taken advantage of in LGP, using a different silvicultural treatment from the one that a priori was preferred for application in these areas.
From a sustainability perspective, it has been implicitly assumed up to now that the criteria selected functioned within weak sustainability parameters [
31]. Namely, a certain degree of compensation between the different indicators was presumed, although the LGP method implies that there is no finite trade-off between the different indicators of the different criteria considered [
89]. However, in the literature, aggregated sustainability indexes based on multicriteria techniques including premises associated with strong sustainability can be found; i.e., it is found that there cannot be compensation between the criteria [
104], nor can a certain degree of compensation between criteria even be chosen [
105,
106].
The methodology proposed here is open to incorporating other silvicultural alternatives. In our case study, GS has been proposed in order to favor the development of multi-aged stands [
79], and GTR has been suggested in order to maintain multiple forest values [
76], including biodiversity conservation [
107], and, especially, to promote silvicultural treatments compatible with the protection of wildlife species [
108]. The percentages of mature forest chosen here are those most frequently used in the literature [
109,
110]. Another silvicultural alternative (TNP) embraces the prohibition of final cuts (this forbiddance is mandatory in all Spanish National Parks). Maintaining this type of protected area is highly beneficial for certain species, and this justifies not employing a conventional silvicultural alternative [
111].
On another side, and as mentioned above, other criteria could be integrated into the model, if considered to be appropriate. For example, we are aware that, unlike other studies [
112,
113], no criterion expressly linked to the social pillar of sustainability has been proposed. Although one study made in the same case study [
114] did suggest two recreation indicators, it has not been possible to transfer this idea to our analysis. Along these lines, some studies point to an underrepresentation of social type indicators as being common [
103].
Again, and aside from the significance of the Gini coefficient proposed here, which has been applied as an indicator of forest structural heterogeneity [
115,
116], we have attempted to define our own biodiversity criterion [
117] that could be integrated into the LGP model. In fact, one of the objectives in the case study [
118] is focussed on the conservation of the black vulture, an umbrella species [
119], which is catalogued as “near threatened” at the global level [
120]. For this reason, we pursue the possibility that, at the end of the planning horizon, there is a multi-aged forest. The inclusion of this criterion (Gini coefficient) under an environmental pillar is justified when the forest managers aim to promote the existence of uneven-aged stands as a biodiversity conservation measure [
63,
79].
However, it has not been possible to model the evolution of this species over 100 years, due to the lack of information available for predicting these values, so it has not been included in the analysis. Furthermore, it would have been necessary to determine the impact of the different silvicultural alternatives on the vulture populations, because it has been demonstrated that they have an influence on some requirements of the nesting of this species [
121]. The lack of consideration of this criterion illustrates the difficulty in finding criteria compatible with the information assembly over the planning horizon, in spite of the availability of suitable methodologies for their possible integration into the model proposed. However, some studies have been developed under the goal programming technique successfully integrating aspects related to biodiversity [
122,
123]. Lastly, the methodology proposed here can also be extended, incorporating other silvicultural alternatives centred on other ecosystem services different from timber production [
124].
Furthermore, the results of the LGP model permit one to obtain an overall value of sustainability derived from the resolution of that model, making it act as an aggregated index of the sustainability of forest management, assimilating the procedure reported in [
125]. Indeed, if the objective is to compare sustainability between similar forests, or between parts of the same one, the solution of the LGP model could allow them to be ranked in terms of greater or lesser aggregate achievement. In [
12,
13], some examples of this extension are shown. Finally, the opportunity cost in terms of sustainability (gains or losses of sustainability) could also be calculated, by obligating a single silvicultural alternative to be used in some part of the case study. This circumstance can be of interest when, as can be seen in this case, the solutions notably vary with respect to the area that each silvicultural treatment has to cover, and this also happens in other studies [
126].
In addition to incorporating new sustainability criteria, one aspect of the methodology proposed here that should be highlighted lies in the combination of flexibility and complexity that can be incorporated, for example, in terms of the silvicultural alternatives considered. The advantage of this flexibility is indispensable for tackling typical contemporary challenges in forest management [
127,
128]. Since there is no single specific definition of the idea of complexity in silviculture [
129], some authors affirm that the impact of certain forest practices on forest system dynamics can compromise their sustainability [
81]. Thus, the consideration of diverse silvicultural alternatives for approaching different criteria would lead to what some authors already call “complex adaptive systems” [
130]. Lastly, this methodology can be applied at different levels (spatial or temporal) at which it is desired to carry out the planning and silviculture [
128], contributing to the flexibility of the proposed methodology.