Occurrence and infestation status of mistletoe (Viscum album L. ssp. austriacum) in Scots pine forests: The case of Olur Forest Sub-District Directorate

Mistletoe (Viscum album L. ssp. austriacum), a hemiparasitic plant, may induce physiological stress in Scots pine (Pinus sylvestris L.) stands by drawing water and mineral resources from its host. This study aimed to determine the occurrence and infestation status of mistletoe in pure Scots pine stands within the Olur Forest Sub-District Directorate, to examine the relationships of infestation with tree and stand characteristics, and to assess its relationship with diameter increment using different evaluation methods. The study was conducted in 36 sample plots ranging from 400 to 800 m². Infestation severity was assessed using the Six-Class Dwarf Mistletoe Rating System (DMRS). In addition, the Volumetrically Enhanced Six-Class Dwarf Mistletoe Rating System (VEDMRS), which evaluates mistletoe clumps according to their number and size, was applied. Annual ring data obtained from increment cores were used to evaluate diameter increments over the last 10, 20, and 30 years. Mistletoe was detected on 518 (77.1%) of the 672 trees examined, whereas no mistletoe was observed on the remaining 154 trees (22.9%). Infestation was particularly concentrated in moderately closed stands below 1800 m elevation containing trees in the 30–40 cm diameter class, and varied markedly with stand crown closure and elevation. Mistletoe load was not uniformly distributed within the crown, with severe infestation becoming more prevalent in the middle and upper crown sections. Although annual ring analyses indicated relationships between infestation indicators and periodic diameter increments, the independent effect of infestation on diameter increment was not statistically significant after accounting for tree age and current diameter. VEDMRS did not show a statistical advantage over DMRS in explaining diameter increment; however, by incorporating the number, size, and within-crown distribution of mistletoe clumps, it provided a more detailed assessment of mistletoe load. These findings provide a scientific basis for planning silvicultural interventions, monitoring, and mechanical control measures in the region.