Introduction
Nature-based tourism is responsible for travel to natural areas and seeks to preserve the environment and welfare of local communities. In general, this type of tourism can play an important role in the economic development of the regions. In order to take advantage of the benefits of tourism, one of the primary and basic steps in nature-based tourism planning is identifying capabilities, determining capacity and ranking areas for the development of this type of tourism. Lorestan province is known as one of the high-potential regions in the field of nature-based tourism in Iran, whose capacities have not been well used. The purpose of this study is to first identify and determine the criteria and indicators for ranking nature-based tourism capacities based on previous research and their localization and then their use in ranking the capacity of nature-oriented tourism in the county of the eastern region of Lorestan province. Finally, several solutions have been proposed to improve the spatial structure of nature-oriented tourism in the study area.
Methodology
This research is applied and its approach is descriptive-analytical. The data required for this research have been collected by documentary and field methods (questionnaire). Criteria and indicators required for ranking the studied counties have been obtained by reviewing the literature in domestic and foreign research and the opinion of local experts. The best-worst technique has been used to weigh the selected criteria (Rezaei, 2015). The best and worst criteria have been determined by 15 experts and by comparing the criteria according to Saatychr('39')s 1-9 scales and then the final weight of the 12 selected criteria from the expertschr('39') point of view has been calculated using Lingo software. Finally, ARAS multi-criteria decision making method was used to rank the nature-based tourism capacity of the counties in the study area. This technique is one of the multi-criteria decision making techniques that was first proposed by Edmundas Kazimieras Zavadskas  and Zenonas Turskis in 2010.
 
Results and Discussion
The results obtained from the weighting of the selected criteria show that the number of nature-oriented tourist attractions (0.235), altitude (0.092) and protected areas (0.077) are ranked first to third. Accommodation centers (0.077), safety (0.072), tourism growth rate (0.070), quality of communication routes (0.067), distance from health centers (0.066), existence of relief groups (0.063), different methods of transportation (0.061), distance of attractions from the city (0.048), the size of cities (0.042) and distance of attractions from cultural centers (0.030) are in the next ranks, respectively. Also, the results obtained from using the ARAS method show that the counties of Khorramabad (0.616), Aligudarz (0.544) and Boroujerd (0.500), Dorud (0.450) and Azna (0.406) are in the first to fifth ranks respectively in terms of nature-based tourism capacity.
 
Conclusion
The purpose of this study was to rank the counties in the eastern region of Lorestan province and to determine the tourism routes and to present the proposed special organization for the development of this region based on nature-based tourism. For this purpose, a number of domestic and foreign researches in the field of nature-oriented tourism were reviewed and the criteria were extracted for ranking the nature-based tourism capacity of the study area. The weight of the criteria was determined by using the paired comparison method and best-worst technique using Lingo software. Then, using one of the multi-criteria decision-making methods) ARAS), the eastern counties of Lorestan province were ranked based on the capacity of nature-based tourism. The results show that Khorramabad, Aligudarz and Boroujerd counties were in higher ranks, respectively. The counties of Dorud and Azna were in the lower ranks, respectively, due to the weakness of their public tourism infrastructure, despite having numerous and unique nature-based attractions. Overall, the results of this study can provide a better understanding of the nature-based tourism, capabilities of this region for planners, decision makers and stakeholders and private investors (domestic and foreign). Providing infrastructure, facilities and equipment are essential in order to optimally exploit the potential of nature-based tourism in the region and also play a more effective role in regional development.
The following suggestions are offered in order to improve the spatial structure of nature-based tourism in the study area:
 - Establishment of eco-tourism resorts in the place of nature-based tourist attractions,
- Establishment of protected areas in the oak forests of the region,
- Definition and determination of hiking trails in nature between attractions that are close to each other,
-Creating nature-oriented adventure recreation sites in the region,
- Improving the existing accommodation and reception centers as well as creating new centers in the study area.
This research has faced some limitations. One of the most important limitations of the research was the lack of cooperation of some organizations and also the lack of a suitable and relevant database in Lorestan province and these restrictions eventually led to the elimination of the three criteria of security and the amount of governmental and non-governmental investment.
 
Finally, in order to complete this research, future research is suggested as follows:
 - Identifying weaknesses, strengths, opportunities and threats in the field of nature-based tourism, and strategic planning for the development of this type of tourism in the study area and Lorestan province,
- Identifying the potential capacity of various sub-sectors of nature-based tourism (sports, adventure, etc.) in the study area and Lorestan province and planning to implement them.