THE RESPONSE OF NUMBERS OF BRAMBLE PRICKLES TO HERBIVORY AND DEPRESSED RESOURCE AVAILABILITY

We investigated the variation in prickliness of bramble (Rubus fruticosus agg.) in relation to browsing intensity by fallow deer (Dama dama) and muntjac deer (Muntiacus reevesi) in an Oxfordshire wood, England. On average, 63% of the leaves of stands of bramble plants in the shaded understorey were browsed by deer while in open, unshaded areas 16% of leaves were browsed. The number of prickles cm-1 was greater on shaded plants compared with plants in unshaded areas. In both open and shaded areas browsing was more intense on the periphery than in the centre of stands. However, stands of bramble in shaded areas were more accessible. Split-plot factorial experiments were carried out with potted bramble canes to determine the effects of simulated browsing, nutrients and irradiance (light) on bramble growth. The resource availability hypothesis predicts that plants occurring in resource limited environments should invest proportionately more resources in anti-herbivore chemical defences than those found in resource rich environments. We expected this to also be the case for physical defences for plants grown under different regimes of resource availability e.g. prickles of brambles. Removing leaves of bramble plants increased the number of prickles cm-1 and also the number of prickles internode-1. Fertilisation decreased the number of prickles cm-1 but light regime had no significant effect. The investment in prickles measured as percent cane dry weight did not differ with light regime or browsing treatment and was approximately 4%, suggesting that prickles do not represent a significant cost to bramble plants. The increased prickliness of plants growing in the understorey of Wytham Wood is best interpreted as a response to increased browsing intensity rather than to shade i.e. a limit in light (carbon) availability. We suggest that experiments on the behavioural response of deer to variation in prickliness of brambles are required to assess the effectiveness of prickles as a deterrent to herbivory.