Enhancing the Value and Durability of Invasive Tree Species Through Thermal Modification

Invasive tree species remain underused in European forests despite timber demand. Box elder and green ash have potential as alternatives to native hardwoods, but their outdoor suitability remains insufficiently understood. This study evaluated whether thermal modification could improve their aesthetic value, dimensional stability, and durability. Wood was modified at 180 and 200 °C in an oxidative atmosphere and tested for wood–water relations, mechanical properties, color, and durability. Thermal modification caused pronounced darkening with a total color change ΔE* of 37.1–56.3 and a slight density decrease of 3.5–7.2%. Equilibrium moisture content decreased by 43–60%, and anti-swelling efficiency ranged from 18.6% to 45.9% depending on species, direction, and temperature, whereas swelling coefficients and anisotropy remained unaffected. Compressive strength and modulus of elasticity did not decrease, while static bending strength decreased by up to 38% (51% at equivalent moisture content) and impact bending strength dropped by up to 70% (74% after moisture correction) at 200 °C. Fungal decay mass loss dropped to 3.1–12.7%, upgrading green ash to durability class 2 and box elder to class 3 under EN 350. Theoretical estimates suggest that the reduced operating moisture content can effectively counterbalance cell wall degradation, indicating that 200 °C modification successfully enhances the value and outdoor potential of both species.