The effect of sapling density, heather height and season on browsing by mountain hares on birch.
When mammalian herbivores are present at high densities their browsing activity can have a negative effect on establishment or regeneration of native woodlands. The impact on saplings of browsing by herbivores is known to vary in response to sapling density. Until now, experimental tests of the effect of sapling density have usually involved only one tree species and no alternative forages. In this study we used a planting experiment conducted in the field to test the effects of surrounding vegetation height, season and density of downy birch Betula pubescens on the extent of browsing damage by mountain hares Lepus timidus, in the presence of background vegetation that also represents an acceptable food source. The experiment was conducted on heather Calluna vulgaris moorland between March 1998 and October 1999 in north-east Scotland. Birch saplings (n=6708) were planted in short and tall heather in arrays at low, medium and high sapling density within a 107-ha area from which large herbivores were excluded. The number of saplings browsed was recorded monthly and individual sapling morphological measurements and characteristics of the ground vegetation were noted twice yearly, pre- and post-winter. More saplings were browsed in winter and spring than in summer, especially during the period immediately after planting. Saplings were browsed at a higher rate in short heather, which facilitated easier access by mountain hares. The proportion of saplings that were browsed decreased with planting density in all seasons except winter, when it increased. At the individual sapling level, larger saplings were more likely to be attacked and to have more biomass removed than smaller saplings. Synthesis and applications. This study demonstrates that mountain hares browse birch throughout the year, but browsing is at its most intense during winter and in short heather, and this may potentially suppress tree growth and tree regeneration. This result has important management implications for the uplands of Scotland, where downy birch is used to establish and regenerate native woodlands, often on moorland supporting high densities of mountain hares. Damage to trees by hares can be minimized by planting in tall vegetation where trees are less visible and accessible to hares. Tree planting can be synchronized with temporary hare control; however, effective reduction in hare numbers may be difficult to achieve. Finally, planting density should be low, as during winter, when browsing is most frequent, the rate of browsing increases with increasing tree density.