1 Transitions between major vegetation types, such as the tree line, are useful systems for monitoring the response of vegetation to climate change. Tree lines have, however, shown equivocal responses to such change. 2 Tree lines are considered to be primarily thermally controlled, although recent work has highlighted the importance of biotic factors. Dispersal limitation and the invasibility of the tundra matrix have been implicated and here we propose herbivory as an additional control at some tree lines. 3 We propose a conceptual model in which differing relative impacts of foliage consumption, availability of establishment sites, trampling, dispersal and seed predation can lead to very different tree-line responses. 4 The presence of large numbers of small trees above the current tree line at a site in northern Sweden that experiences limited reindeer (Rangifer tarandus) herbivory suggests range expansion. Other locations in the same region with higher reindeer populations have considerably fewer small trees, suggesting that range expansion is occurring much more slowly, if at all. 5 The use of tree lines as indicators of climate change is confounded by the activity of herbivores, which may either strengthen or nullify the impacts of a changed climate. Similar arguments are likely to be applicable to other ecotones.