Material and methods
The study area is located in NW Finnish Lapland at a transitional area between boreal mountain birch forests and treeless oroarctic and arctic tundra heaths at higher elevations (Virtanen et al . 2016). The study site lies at an elevation of 550–570 m a.s.l. (68.92° N, 20.97° E). In this landscape, tundra-like vegetation occurs also in valleys with frost-hummocky dwarf birch (Betula nana ) heath with some willows and mountain birch (Betula pubescens ssp.czerepanovii ), and moss and lichen dominated ground-layer. In the study area, tea-leaved willow (Salix phylicifolia ) is a common shrub that is generally regarded as palatable, and is browsed by several vertebrate species. Willow ptarmigan (Lagopus lagopus ) browses willows primarily in winter when willow shoots and buds reach above snow level. Reindeer (Rangifer tarandus ) browse willow by ripping leaves and shoots primarily in early summer. In the study area, reindeer are semi-domesticated and have had relatively high stocking rates of c. 10 ind/km2. The study area was a summer reindeer range until 2008, after which reindeer were moved to another area for the summer season. Therefore, in 2008–2018 only solitary reindeer were seen in the area in summer-time. Mountain hare (Lepus timidus ) browsing has been very infrequent in the study area, as well as that of Norway lemmings (Lemmus lemmus ) and voles (Microtus oeconomus , Myodes rufocanus . Peaks of vole and lemming populations occurred in 2009–­2010 and 2014–2015. In our study area we have observed two main folivorous insect herbivores that defoliate leaves partly or completely. One is autumnal moth (Epirrita autumnata ) that feeds on the willow during population outbreaks and can cause complete defoliation of willows. Outbreaks of autumnal moth defoliating willows occurred in 2003–­2005 and 2011–2013. The other are folivorous beetles (mainly Gonioctena spp.; Chrysomelidae) that have been observed in most years.
Experimental design
Eighty willow genets in two forest-tundra ecotone sites were selected for the experiment in 1997. In these sites, the genets were selected within areas of c. 50 × 200 m and 50 × 250 m. Genets were 30–50 cm tall and consisted of 10–25 ramets each. All genets were initially accessible to both reindeer and ptarmigan. The genets were then assigned into blocks of four genets. In 1997–1998, two random genets within each block were rejuvenated by pruning ramets five cm above the ground level. The pruned genets produced sprouts that initially grew vigorously for 1–3 years and within a few years reached the size of unpruned genets. Thus, each block consisted of two old genets and two rejuvenated genets. Within each block one of the old and rejuvenated genets were protected by reindeer fences (2×2 m area). In 2000, reindeer fences were replaced by ptarmigan proof fences in half of the blocks, selected at random. The experimental design from 2000 onwards included treatment combinations of reindeer exclusion and reindeer+ptarmigan exclusion on both old and rejuvenated genets (see Table 1, Supporting Information).
Table 1. The experimental design of the long-term browsing experiment. Reindeer exclusion (–) consists of experimental units with reindeer fence, whereas ptarmigan and reindeer browsing exclusion treatments (–) have fences with 3.5 cm mesh size. Rejuvenated (+) treatments were initially pruned above ground level.