## 3 Results
### A Traceability System for Timber Supply Chain Monitoring and Compliance  
The model discussed above was deployed to implement a traceability system for the timber sector in the autonomous region of Galicia, Spain, to provide an immutable and transparent record of all transactions in the timber supply chain, from its origin in the forest to its eventual conversion if final consumer products. The main objectives of timber traceability in Galicia are to guarantee a responsible exploitation of forests as a key natural resource and to duly manage CO\({}_{2}\) balances among operators [51]. The technical solution was based on the following elements: An online frontend to provide operators with the required functionality to notarie and track batches, that is, to visualise at different levels of detail the journey of a batch of timber up to the resulting final products (i.e., forward tracking), and the origin of the timber utilised to build some final product (i.e., backwards tracking). Batch tracking is also available to the general public. The frontend also provides utility functions to register operators, and manage operators' profiles and logging permits. A smart contract running in the Alastria consortium blockchain [52] to implement the on-chain traceability logic and data (i.e., traceability-critical data). Alastria is a public-permissioned, multi-sector blockchain platform designed to facilitate the development of decentralised applications and services across various industries by connecting businesses, academia, and public administration within a secure and legally compliant framework. This smart contract implements the traceability logic discussed in this paper. A relational backend for data management, including all off-chain traceability data (i.e., traceability-relevant data and complementary data). With the help of blockchain technology, it was possible to immutably notarie the issuance and exploitation of logging permits. Each logging permit was associated with a record with a unique identifier, including all relevant information, such as the geolocation of the forest, the types of trees authorised for logging, the start and end dates of each permit, etc. A product journey starts with an authorised operator filling some of the trees included in a valid logging permit to become timber. All relevant details of this act are modelled as the first batch of the value chain. Each time an operation is carried out along the timber value chain, a new batch is generated to record all the relevant details, such as the operation's date, place, and time, the outputs from previous operations used as input, the type of processing carried out, the CO\({}_{2}\) balance of the operation, and the identification information of the resulting products. The generation of that batch is also notarised using the blockchain, generating a chain of notarisations that guarantee traceability. Each time timber is transferred from one operator to a different operator, such transfer is also registered in the blockchain as a special batch with outputs equal to its inputs. This way, any operation involving timber with an impact in the carbon footprint and in the overall carbon balance among operators is duly registered, ensuring the integrity of the timber's chain of custody and providing full traceability from origin to final destination. According to the classification of traceability data discussed above, timber traceability data in each batch can be classified as follows:  
1. Traceability-relevant data used to calculate the cryptographic digest: batch ID, product ID, stock-keeping unit (SKU) number, producer's fiscal ID, owner's fiscal ID, production date and time, total units, CO\({}_{2}\) equivalence, and timber species ID. This information is stored in a MariaDB relational database. 2.