Ungulate herbivory of regenerating conifers in relation to foliar nutrition and terpenoid production

• Published in: Forest ecology and management 2011-11, Vol.262 (9), p.1834-1845
• Main Authors: Burney, Owen T., Jacobs, Douglass F.
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Browse preference; browsing; Conifers; Dynamics; Ecosystems; Fertilization; fertilizer rates; forest ecosystems; forest trees; Forestry; Forests; Fundamental and applied biological sciences. Psychology; herbivores; Management; microbial activity; monoterpenoids; nitrogen content; Nutrition; Pseudotsuga; Pseudotsuga menziesii; Pseudotsugamenziesii; seedlings; slow-release fertilizers; Synecology; Terpenes; Terrestrial ecosystems; Thujaplicata; Trees; Tsuga heterophylla; ungulates
• ISSN: 0378-1127; 1872-7042
• DOI: 10.1016/j.foreco.2011.07.035

► We examined ungulate browsing of three Pacific Northwest conifers as affected by fertilization. ► Higher likelihood of browse occurred for fertilized western hemlock. ► Lower likelihood of browse for fertilized western red-cedar was due to increased oxygenated monoterpenes. ► Fertilization aides in recovery from browse for Douglas-fir and western hemlock through stimulated growth. ► Fertilization affects browse potential through changes in foliar nutrition and terpene composition. Ungulate browsing greatly influences regeneration dynamics of some forest ecosystems, yet the relationship between browse susceptibility and foliar chemistry of forest tree seedlings is not well understood. We applied field fertilization (15N-9P-10K controlled-release fertilizer at 0, 20, 40, and 60 g per seedling) and investigated how subsequent changes in terpenoid production and foliar nutrition influence ungulate browse preference for Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco), western hemlock ( Tsuga heterophylla Raf. Sarg.), and western redcedar ( Thuja plicata Donn ex D. Don) seedlings across four sites in northwestern Oregon, USA. Fertilization increased foliar N concentration of all three species, but above-ground growth of only Douglas-fir and western hemlock. Foliar monoterpene concentrations for western hemlock and western redcedar also increased at higher fertilization rates, while Douglas-fir monoterpene production was not affected by fertilization. Regardless of monoterpene levels, ungulate browse preference was greater for fertilized western hemlock seedlings. The opposite response, however, was observed for western redcedar at two of the four study sites where the likelihood of browse was greater for non-fertilized than fertilized seedlings. Differences in browse preference among species may depend on the type and amount of individual monoterpenes manufactured in response to fertilization. Western redcedar produce α- and β-thujone, oxygenated monoterpenes known to promote gastroenteritis and possibly inhibit microbial rumen activity, which were absent from Douglas-fir or western hemlock. Higher concentrations of α- and β-thujone associated with increasing fertilizer rate provide a plausible explanation as to why ungulates preferred non-fertilized western redcedar. Our results illustrate species-specific adaptation in browse avoidance and selective ungulate browsing behavior of individual trees as linked to foliar chemistry.