Skógræktarritið - 15.05.2001, Blaðsíða 167

succession birch forest (Olsson
et al. 2000). The heathlands in
Sjodalen, Oppland county, is re-
duced by 70% in the same peri-
od, now covered by woodlands
(Olsson et al. 2000). The ongoing
rise in forest-limits around for-
mer summer farms in Norway is
in other words a natural re-
sponse of reduced felling, grazing
and mowing. However, one
should also consider that climat-
ic changes may speed up this
process (Aas and Faarlund 1995).
The large and visible landscape
changes are not the only results of
the abandonment of summer
farms and the following re-growth.
As shown in the Grimsdalen-
study, this process also effects the
biodiversity of the summer farm
landscape. According to Kielland-
Lund (1992), about 350 vascular
plants have their main Norwegian
distribution in seminatural vege-
tation types. Several investiga-
tions also show that summer
farming has increased biological
diversity in subalpine areas
(Olsson et al. 1995, Austrheim
1998, Lunnan et al. 1999, Bryn
2000). Antropochores are found
there either as a response of
active cultivation or random dis-
persal by domestic animals
(hooves, fur or dung) or other
transport mechanisms. Small
alpine species seems to fit into
the subalpine seminatural vegeta-
tion due to lower competition and
abundance of light. The clearing of
the birch forest has probably cre-
ated dispersal-corridors forthese
species from higher altitude down
to the summer farms at lower ele-
vations. Also lowland species exist
in the summer farm landscape,
probably explained by higher radi-
ation, more nutrition and less
competition.
The vascular plants in the sem-
inatural vegetation are usually
adapted to a certain disturbance
regime, and many plants are tol-
erant to trampling, grazing
and/or mowing. Plants with a
short life cycle (Figure 1) are
probably more abundant in the
seminatural vegetation as a re-
sponse of the disturbance regime
and favourable light conditions
(Bryn 2000).
Summer farm landscapes
embodies both natural and man-
induced gradients for environ-
mental factors like light, wind,
radiation, moisture, snow-cover,
nutrients, birch forest succes-
sions, vegetation cover, biodiver-
sity, disturbance, etc. The man-
induced gradients reflect the
decrease in human induced dis-
turbance and influence with
increasing distance from the
summer farm (Vandvik 1995, Bryn
2000). These man-induced gradi-
ents, operating on different spa-
tial and temporal scales, provide
living conditions for several func-
tional groups of organisms nor-
mally not found in undisturbed
subalpine birch forest.
As the data from Grimsdalen
shows, however, the results of bio-
diversity studies depend on both
temporal and spatial scale (Table
1). The a-diversity, i. e. the diversi-
ty on a small spatial scale, is high-
est within the young birch forest,
but decreasing to the lowest level
in old birch forest. The 7-diversity
i.e. the diversity on a large spatial
scale, is highest within the semi-
natural vegetation, decreasing
with proceeding succession
towards old birch forest.
The high a-diversity of the
young birch forest is probably
explained by spreading of „re-
growth-species" and colonization
of the small-plots (Huston 1994),
while some seminatural species
are still existing there. The (3-
diversity is, however, higher in
seminatural vegetation com-
pared with subalpine birch suc-
cession forest (Bryn 2000), indi-
cating higher species turnover
between the small-plots and
thus more heterogeneity in semi-
natural vegetation.
Scaling up the spatial dimen-
sion confirms the impression of
higher diversity and heterogene-
ity in seminatural vegetation. The
7-diversity in the seminatural
vegetation includes 92% of the
species existing in the two forest
succession zones, in addition to
50 plant species exclusively
found in seminatural vegetation
(Bryn 2000). High y-diversity in
the seminatural zone (Table 1)
may also be explained by high
heterogeneity. Seminatural vege-
tation is a result of a complex
combination of biotic and abiotic
factors, management practice,
management intensity and man-
agement continuity, resulting in
a numerous variation of the
species composition (Hughes
and Huntley 1988, Norderhaug et
al. 2000). When for example the
grazing pressure is light, the
domestic animals will graze cer-
tain areas harder than others and
also leave some nearly untouch-
ed (Nedkvitne et al. 1995). This
may give plots with different
species composition even within
small areas of seminatural vege-
tation.
To give a correct picture of the
variation and change in vascular
plant richness after abandon-
ment, it is therefore important to
sample on different spatial
scales. The Grimsdalen-study
shows that light and space are of
great importance for the species
composition of seminatural veg-
etation. The study, however, also
shows that the importance of dif-
ferent environmental factors for
structuring the vegetation de-
pends on the spatial scale. The
importance of environmental fac-
tors and effect of the re-growing
process after abandonment on
rare plants and seminatural
vegetation types was best shown
SKÓGRÆKTARRITIÐ 2001 l.tbl.
165