Terpene biosynthesis and growth allocation of Pacific Northwest conifers

The relationship between ungulates and vegetation is dynamic, with changes in either having potential to significantly alter processes and functions of the other at various temporal and spatial scales. Past research on ungulate browse in the Pacific Northwest focused primarily on morphological impacts of browse on regenerating conifers. However, few studies have explored the influence of both plant mineral nutrition and terpenoid production on ungulate browsing behavior and plant growth patterns. A series of experiments were established for Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), western hemlock (Tsuga heterophylla Raf. Sarg.), and western red-cedar (Thuja plicata Donn ex D. Don ) to address: (i) ungulate browsing behavior relative to foliar nutrition and terpenoid production; (ii) the influence of simulated browse and supplemental mineral nutrition on resource allocation and terpenoid production; and (iii) the genetic and phenological variation of foliar terpene production and emission across varying levels of nutrient availability. In the presence of browse, the benefits of supplemental mineral nutrition differed by species. For Douglas-fir and western hemlock, these benefits were realized post-browse whereby higher nutrient resources provided recovery from browse by augmenting resources to promote growth. Western red-cedar, however, allocated additional nutrient sources toward production of oxygenated monoterpenes (alpha-, beta-thujone), which act to chemically deter browse. Olfactory cues may not come directly from the emission of alpha-, beta-thujone, but possibly originate from the high quantities of the diterpene beyerene being emitted from western red-cedar. Regardless of alterations in plant mineral nutrition and terpenoid composition, however, ungulate behavior in relation to the ever-changing environment has a significant influence on the fate of forest regeneration