The effect of plant competition and simulated summer browsing by deer on tree regeneration.
Abstract
The numbers of deer (Capreolus capreolus, Cervus elaphus and Dama dama [fallow deer]) are rising throughout Britain and population densities frequently exceed those known to have a significant impact on broadleaved trees. This experiment, which took place at four sites in England over a 5-year period, studied the effects of competition from the ground flora and severe, simulated, browsing by deer during summer, on the growth and mortality of naturally occurring Acer pseudoplatanus, Fraxinus excelsior, Fagus sylvatica and Quercus robur seedlings. The seedlings were initially 30-40 cm tall with stem diameters of 2.6-6.2 mm. Competition from the ground flora was removed by maintaining circular areas, 1 m in diameter, free of vegetation using a general herbicide. Browsing was simulated by clipping new shoots that were longer than 1 cm in June and August. The control of the ground flora had little effect on the growth and survival of seedling trees. In general, the number of tree seedlings producing sufficient growth to receive the simulated browsing treatment decreased with time, but for those seedlings that were clipped there were no consistent trends in either numbers or weights of shoots removed. Unclipped seedlings continued to grow. The height and stem diameter increments varied with species and site, but after 5 years unclipped plants were 50-200% taller with stem diameters 20-130% greater than at the start of the experiment. The mortality of unclipped seedlings was low and 80% or more survived. Clipping significantly reduced the survival of Fraxinus excelsior and A. pseudoplatanus, but Q. robur and Fagus sylvatica were unaffected. On an annual basis, there was no relationship between mortality and accumulated number or dry weight of shoots clipped, initial stem diameter or height, and whether or not the seedling was under the overstorey canopy. The results from this study indicate that established seedlings of A. pseudoplatanus, Fraxinus excelsior, Fagus sylvatica and Q. robur that are >30 cm tall can survive several years of severe browsing under the shade provided by overstorey canopy covers of 50% or more. This suggests that management should aim to improve survival during the first 1-2 years of growth. This may be achieved by exclusion of animals by fencing, protection of individual seedlings using tree shelters, or culling to control herbivore numbers.