Náttúrufræðingurinn - 2016, Síða 36

Náttúrufræðingurinn
108
og minnkandi beitarálags vegna
samdráttar í sauðfjárstofninum í
upphafi níunda áratugar síðustu
aldar. Þó er ljóst að fleiri þættir
eru öflugir áhrifavaldar, svo sem
fræuppsprettur, fræframleiðsla og
frægæði annarsvegar og hinsvegar
hentug set fyrir spírun fræs og vöxt
og viðgang smáplantna.
Náttúruleg útbreiðsla birkis er
breytileg frá einu tímabili til annars
og því mikilvægt að kortleggja
útbreiðslu þess með vissu millibili.
Liðin eru rúm 40 ár frá fyrstu
birkikortlagningu á Íslandi á
landsvísu. Hún var síðan lagfærð
fyrir um aldarfjórðungi. Það er
ljóst að á nokkurra áratuga fresti
geta orðið talsverðar breytingar á
útbreiðslu birkis, t.d. við breytingar
á loftslagi og vegna ýmiss konar
landnýtingar. Þær niðurstöður sem
hér eru kynntar verða grunnur
fyrir síðari birkikortlagningu. Það
er þó ekki einungis útbreiðsla
birkis sem þarf að fylgjast með,
heldur verður einnig að vakta
breytingar innan birkisvæðanna,
líkt og gert er í vöktunarverkefni
Rannsóknastöðvarinnar á Mógilsá
þar sem safnað er gögnum á 10
ára fresti í völdum mæliflötum
innan kortlagðra birkisvæða. Með
þeim úttektum fæst yfirsýn yfir þær
breytingar sem eiga sér stað í vexti
og viðgangi náttúrulegs birkis á
Íslandi. Náttúrulegir birkiskógar og
birkikjarr eru meðal mikilvægustu
vistkerfa landsins, ekki síst í ljósi
þess að fá vistkerfi hafa orðið fyrir
jafnmikilli skerðingu frá landnámi.
Því er brýnt að fylgjast vel með
breytingum sem á þeim verða. Þá
getur aukin þekking á nýliðun og
vexti birkis nýst vel við að bæta
aðferðir við endurheimt birkiskógar.
Með endurkortlagningu náttúru-
legs birkis á Íslandi liggur nú
fyrir nýtt stöðumat á útbreiðslu
þess, og er þar með hægt er að
átta sig á þróun síðustu áratugina.
Niðurstöður okkar eru fyrstu
vísbendingarnar um að margra alda
samdráttarskeiði í sögu náttúrulega
birkiskóga og birkikjarrs er lokið
og útþenslustig hefur tekið við, að
minnsta kosti í bili.
SUMMARY
The natural birch woodland in
Iceland – a new assessment on
distribution and state
Downy birch (Betula pubescens Ehrh.) is
the only native tree species in Iceland that
forms woodland. The growing form is of
low stature and its crookedness relates
with the subarctic mountainous downy
birch that is common at the tree line in
Scandinavia, often named mountain
birch and sometimes described as sub-
species of downy birch (ssp. czerepanovii
(N.I. Orlova) Hämet-Ahti). It has been es-
timated that at human settlement in the
ninth century, natural birch woodland
was the dominant ecosystem on Icelandic
lowlands, covering there nearly all min-
eral soil types, together about 20,000–
30,000 km2 (20–30% of terrestrial area of
Iceland). Archaeo logical, palynological
and historical research shows that anthro-
pogenic destruction of the woodland took
place soon after the settlement and was
systematic and intended. The culture of
stationary farming required open heath-
land for grazing livestock. The woodland
was cut down or burned to give space for
grazing. The first thematic mapping of
the natural birch woodland was done in
1972–75, giving the estimated area of nat-
ural birch woodland of 1250 km2. Only
about 5% of the original woodland re-
mained after 1100 years of human influ-
ence. Between 1987–91 a new inventory
was conducted on the natural birch
woodland but with other emphasis.
The Icelandic national forest inventory
(INFI) with systematic sampling plots
started in 2005 with the aim to sample
verifiable information on the effects of
forest and woodland on the greenhouse
gas fluxes. Natural birch woodland was
one of two strata. The other was cultivat-
ed forest consisting mostly of plantations
of both native and introduced tree species.
Maps of natural birch woodlands were
used as a population for sample plots.
Right from the beginning of the INFI it
became clear that the map from the
1987–91 inventory was not showing the
current distribution of the birch wood-
land. The map was too coarse and dis-
torted and often polygons were displaced
mainly due to bad projection. Attempts to
rectify the map did not give satisfactory
results so in 2010 it was decided to re-
map all natural birch woodland in Iceland.
The mapping was done in the field with
SPOT 5 satellite images as a reference.
The woodland area was outlined and
separated into polygons where the main
variable was height class at maturity. The
field work lasted for five summers during
the period 2010–14.
The re-mapping of the natural birch
woodland gave the current total coverage
of 1,506 km2, 1,5% of the total land area
The distribution of the three major classes
of height and maturity of natural birch
woodland can be seen on Fig. 6 and in
Table 1. By analysing the age classes of
mapped areas that were not represented
in the map from the 1987–91 inventory,
we estimated a newgrowth of natural
birch woodland to be 130 km2. As shown
in Table 3 the newgrowth is unevenly
distributed between the five regions of
Iceland. The southern region (Suðurland),
western (Vesturland) and the Westfjord
peninsula (Vestfirðir) have substantial
newgrowth but in the northern and east-
ern regions the increase was less. The
correlation test between newgrowth in
different regions and increase in mean
summer temperature (June–August) did
not give significant linear relations de-
spite a positive trend. Likewise, no corre-
lation was found between the changed
number of grazing sheep in the regions
between 1989 and 2012, although a simi-
lar trend was observed. However, other
research has shown that both factors
(grazing pressure and summer tempera-
ture) play major role in the colonisation of
birch into treeless areas. Other factors, e.g.
the quantity and quality of birch seeds
and favourable microsites for germination,
can play an important part in the colonisa-
tion of birch and should not be ignored.
The distribution of natural birch wood-
land is a dynamic process that changes
from one time to another. Forty years has
passed since the first countrywide map-
ping of natural birch was made. Within
only few decades it will be necessary to
remap the distribution again and hope-
fully build it on more research to gain
better understanding of the dynamics
and ecology of the natural birch wood-
land in Iceland.