The effect of previous browsing damage on the morphology and chemical composition of Sitka spruce (Picea sitchensis) saplings and on their subsequent susceptibility to browsing by red deer (Cervus elaphus)

Three year-old Sitka spruce saplings, which had received one of two levels of fertiliser addition (high or low) in one year, were either browsed by red deer or left undamaged in a subsequent year. After a further year of growth, the selection and browsing behaviour of red deer on damaged and undamaged trees were studied in a controlled experiment. Morphological and chemical analysis of ten trees from each treatment combination showed that damaged trees were significantly smaller after one year than undamaged trees but had longer needles. Chemical composition was not affected by browsing. High fertiliser addition increased most size parameters and increased the nutritive value of trees. The preference for previously damaged and undamaged trees was compared in a series of thirty two trials. On eight separate occasions, four red deer hinds were individually offered the choice of two damaged and two undamaged trees in an experimental enclosure. During each trial, biomass removed, number of bites removed, duration of browsing, intake rate, and mean bite weight at each tree were recorded. Biomass removed, bites taken and time spent at trees were significantly greater for previously undamaged trees than for damaged trees, All three variables were also positively related to the height of each tree and the number of lateral branches on each tree. The greater foraging effort on undamaged trees was not fully accounted for by the morphology of trees or their chemical composition. It is suggested that increased needle length caused animals to stop feeding sooner and hence led to less browsing damage in previously damaged trees. The results imply that previously observed localised browsing damage in forestry plantations is unlikely to be the result of changes to the plant induced by previous browsing damage. (C) 1998 Elsevier Science B.V.