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Death and Taxus: the high cost of palatability for a declining evergreen shrub, Taxus canadensis

Holmes, Stacie A., Webster, Christopher R., Flaspohler, David J., Froese, Robert E.
Canadian journal of forest research = 2009 v.39 no.7 pp. 1366-1374
Taxus canadensis, plant growth, deer, vertebrate pests, herbivores, browsing, shrubs, woody plants, light intensity, canopy gaps, shade, ecological restoration, environmental factors, tree mortality, vigor, Odocoileus virginianus, Michigan
Taxus canadensis Marsh., a shade-tolerant evergreen shrub, is in decline as a result of past anthropogenic disturbances and increased cervid browsing. To identify factors that may be influencing establishment for this species, we planted 1080 T. canadensis cuttings in a hemlock-hardwood forest in the spring of 2004. Cuttings were planted across a gradient of light environments, consisting of artificial canopy gaps (61-441 m2) and closed-canopy conditions, and half were protected with deer exclosures. Following four growing seasons, T. canadensis survival in exclosures was 92% compared with 75% in controls. A mixed-effects binary logistic regression analysis revealed that deer exclusion had the most influence on the probability of survival; all else being equal an individual was nearly seven times more likely to survive if planted inside an exclosure (P < 0.001). Increasing light intensity had a counterbalancing effect on growth and survival; light increased leader growth (P = 0.019) but reduced an individual's probability of survival (P < 0.001), possibly owing to enhanced herbivory and elevated moisture stress. Consequently, enhancing resource availability and growth may not decrease the impact of biomass loss to herbivory. These results suggest that even with relatively low herbivore densities, exclosures are required for the recovery of highly palatable species.