Clonal variation in monoterpene concentrations in Sitka spruce (Picea sitchensis) saplings and its effect on their susceptibility to browsing damage by red deer (Cervus elaphus)

Damage to young trees as a result of browsing by deer, can lead to significant loss of revenue in commercial forestry plantations. This study investigated the chemical and morphological characteristics of Sitka spruce saplings which influenced their susceptibility to browsing. Specifically, we tested whether genetic variability in monoterpene content of saplings affected deer feeding behaviour. Clonal cuttings of Sitka spruce were grown for 5 years following propagation from 15 parent trees growing on the West Coast of Scotland. Monoterpene concentrations and morphological characteristics of individual saplings were recorded prior to offering them to a group of tame red deer hinds in a series of preference trials, conducted within an experimental enclosure on a moorland site in northeast Scotland. Browsing intensity was assessed after each trial by recording the number of distinct signs of damage on each tree. Results showed that a high proportion of variation in browsing damage could be attributed to inter-trial variability, possibly due to increasing familiarity with the Sitka saplings as the experiment progressed. However, individual saplings were found to differ in their susceptibility to browsing according to predictable criteria: height had a positive influence on browsing damage while monoterpene concentration had a negative influence. The different clones also varied in the amount of damage they received, and between-clone variation in monoterpene concentration was an important factor in explaining variation in preference. These results demonstrate the important genetic influence on susceptibility of Sitka spruce to browsing damage by red deer and suggest that foresters may be able to exploit genetic variation in monoterpene concentrations to improve the resistance of Sitka spruce to deer browsing.