Early growth performance and structural stability of eight introduced Hevea brasiliensis (Willd. ex A.Juss.) Müll. Arg. clones in Tepi, Southwest Ethiopia

Rubber tree ( Hevea brasiliensis (Willd. ex A.Juss.) Müll.Arg.) is a key industrial crop for expanding Ethiopia’s latex production. Early growth performance and structural stability of eight introduced clones (IRCA230, PB217, RRIC100, RRIC121, IRCA41, PB86, RRIC101, and RRIM623) were evaluated at the Tepi Agricultural Research Station. A randomized complete block design with three replicates was employed. The survival (%), height (m), and diameter (cm) were measured annually for five years (2019–2023). The slenderness coefficient was calculated as the annual ratio of the mean height (cm) to the mean diameter (cm) for each clone to assess structural stability. A bias-reduced (Firth) logistic regression analysis revealed significant effects of clone and year on survival probability over the study period (Likelihood ratio χ² = 177.89, df = 13, p < 0.001). Survival increased significantly after the establishment year ( p < 0.001), averaged across clones and blocks, exceeding 97% from Year 2 (2020) onward and approaching complete stand stabilization by Year 5 (2023). Linear mixed-effects models (individual tree random intercepts) showed that clone and year strongly influenced height and diameter (all χ²s significant, p < 0.001). Significant Clone × Year interactions for both height and diameter indicate that growth trajectories over time differed among clones (height χ² = 426.22; diameter χ² = 1285.73; p < 0.001). By the end of the trial, the RRIM623 was the tallest (11.7 ± 0.7 m), and the RRIC121 had the largest diameter (11.5 ± 1.4 cm). The slenderness value decreased from 140 to approximately 100 after the third year. PB86 had the lowest average slenderness ratio (80), while RRIM623 retained a higher slenderness, suggesting potential structural instability. RRIC121 and RRIC100 combined high early growth and stability, indicating that they are prime candidates for large-scale H. brasiliensis cultivation in similar agroecology. Further studies on latex yield are recommended.