Skógræktarritið - 15.05.2001, Side 182

SKÓGRÆKT HANDAN SKÓGARMARKA / NSSE
PIRJO WELLING AND KARI LAINE
Regeneration by seed
above the timberline
SAMANTEKT
Jafnvel nú er oft gert ráð fyrir að nýliðun með fræjum sé lítil í fjalla- og
heimskautagróðri. Samt eru margar kannanir sem mæla gegn þessu.
Þéttleiki fræplantna á ofangreindum svæðum er sambærilegur við
þéttleika fræplantna á suðlægari svæðum, og reyndar hafa mælst
nokkur met á heimsvísu í Norður-Finnlandi og á Ellesmere Island.
í fjallsheiðum á sér stað virk uppsöfnun fræforða vegna þess að um-
hverfisskilyrði koma næstum alveg í veg fyrirað plönturnar nái að
hefja vöxt. Aftur á móti í valllendi gengur hratt á fræforðann er fræ-
plönturnar vaxa úrmoldinni. í báðum gróðurlendunum erfræspírun
svipuð í opnum í sverðinum eins og í Iokuðum sverði. Nokkrir eigin-
leikar plantna hafa áhrif á það hvernig nýliðun úr fræi tekst. í valllendi
eiga sér stað breytingar á mikilvægi þátta sem ákvarða vaxtarlag
plantna, leiðir þeirra til fjölgunar með kynlausri æxlun eða kynæxlun,
lögun fræja vegna dreifingar og fræstærð á lífsferlinum. Gróðursam-
setningin breytist því mikið meðan á ferlinu stendur. Þrátt fyrir þessar
breytingar eru plöntur sem fjölga sér á kynlausan hátt ríkjandi í gróðr-
inum.
The significance of regeneration
by seed above the timberline
Even today it is often assumed
that seed regeneration has little
significance in the regeneration
of arctic and alpine plant com-
munities and populations (Archi-
bold 1995). Regeneration should
proceed mainly by vegetative
means in these harsh environ-
ments. Seed production often
fails, and seedling survival is low
(Wager 1938, Billings 1974).
However, the first contradictory
observations emerged at the
same time as those of Wager
(1938).
Söyrinki (1938) found in the
late 1930's that the total seedling
densities are commonly several
hundreds, even thousands of
seedlings per square meter (m2)
in the Petsamo District, subarctic
Finland. Several decades later,
Freedman et al. (1982) and
McGraw and Shaver (1982) also
found high total seedling densi-
ties in the arctic region: at Elles-
mere Island, High Arctic Canada,
13-5916 seedlings / m2, and in
Alaska, 35-3376 seedlings / m2.
Furthermore, the total densities
of seed banks are often several
hundreds or even thousands of
seedlings / m2 (Freedman et al.
1982, Fox 1983, Roach 1983,
Chambers 1993, Semenova and
Onipchenko 1993, Welling and
Laine 2000). Seedling survivor-
ship can be surprisingly high,
more than 50% after two growing
seasons (Chambers 1995). In
Kilpisjarvi, in northern Finland,
the species reproducing mainly
by seed are important in main-
taining the species richness of
alpine vegetation (Welling 2000).
The total seedling densities
found in arctic and alpine areas
are comparable with the seedling
densities found in more southern
areas, and some global maxima
have been recorded in northern
Finland (Söyrinki 1938) and on
the Ellesmere Islands (Freedman
et al. 1982). These records reveal
that seed and seedling dynamics
is worth much attention in arctic
and alpine research programs.
Features of seed regeneration
in alpine plant communities
There are several phases in the
pathway of seed regeneration
(Fig. 1). Several factors affect the
density, quality and species rich-
ness of seed and seedling floras
in different regeneration phases,
and the relationships between
these phases.
The density of seed rain varies
widely from year to year in alpine
areas (Chambers 1993, Molau and
Larsson 2000). The density and
180
SKÓGRÆKTARRITIÐ 2001 l .tbl.