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.