| Introduction |
Nothofagus pumilio
occurs from 35o35 to 55o S in the Andean Range and the Coastal
Range of Chile and Argentina. It ranges in elevation from sea level
to 2000 m above sea level. North of 41o it occurs only
in the subalpine zone and commonly forms the upper tree limit. In
the southern part of its range it occurs at both high and low elevations.
In the center of the distribution mean annual precipitation is more than
5000 mm on the western side of the Andes and only 800 mm at the easternmost
localities in Argentina. N. pumilio occurs at cooler and drier sites
than the other members of Nothofagus genus (Veblen et all. 1996).
Nothofagus pumilio and N. antarctica, with the exception of
some Alnus species, are some of the rare deciduous hardwood species
that form upper treelines in the Southern Hemisphere. N. pumilio
is long lived and forms annual rings making it well suited to dendrochronological
studies. However, the regeneration dynamics of N. pumilio are not
well known. Seed production and viability is highly variable from
year to year. Percentage viability was near zero during the years with
low production and ranged from 7 to 22% during the years of high production.
Survival of seedlings highly depends on the light conditions in the gap,
following the literature nearly all of the seedlings died due to low light
conditions in a stand gap. Fallen logs seem to be very important for the
establishment and survival of seedlings (Veblen et all. 1996).
On the upper timberline, vegetative reproduction is very important. Adventitious
roots sometimes formed where branches touched the ground, but usually new
stems relied on the roots of the original tree (Veblen et all. 1996).
The upper treeline is usually formed by N. pumilio, either as an
erect form or light krummholz, or by N. Antarctica, which forms
the typical krummholz upper timber line. The timber line in the southeastern
Andes is generally between 1700 m and 1750 m, but it could be lower due
to strong winds, high snow cover, avalanches, poor drainage and volcanic
ash. N. pumilio (and N. Antarctica) prefer wet and
cool site conditions which are found in the western and southwestern Andes
and on eastern outliers of the Andes surrounded by steppe. In the lowest
part of the elevation range of this forest type, the understory is dominated
by bamboo (Chusquea culeou, C. montana) up to four
meters tall which gradually disappears towards the upper treeline.
|
| Goals |
The goals of the study were
to:
-
determine how Nothofagus
pumilio establishes in forest gaps.
-
determine how browsing effects
Nothofagus pumilio forest regeneration
-
date a forest gap and describe
the affects of forest gap formation on surrounding trees.
-
build a Nothofagus pumilio
chronology for upper timber line.
|
| Methods |
Study site.
The research plot was located in the Chapelco ski resort (San Martin de
los Andes, Neuquén, Argentina). The elevation of the site
is about 1550 m a.s.l. with a north-northwest exposure, the soil is deep
and rich with volcanic minerals, and the bedrock is basalt. The plot was
located in a fairly undisturbed area. It was difficult to find an
ungrazed plot since the whole region is heavily influenced by cattle, goat,
sheep and horse grazing, and by human influences related to the ski slopes.
Treeline chronology construction.
Two cores were collected from each of 17 Nothofagus pumilio trees
just below treeline. Cores were glued into wooden holders, dried and sanded
using up to 400 or 600 grit sandpaper. To improve the visibility some of
the tree ring cores were also stained with blue color and the contrast
was enhanced using chalk. Tree-rings were examined using a binocular microscope
and measured using a Velmex measuring machine connected to a personal computer
running MeasureJ2x program. Measured series were synchronized and
averaged into a local chronology for the upper timber line (UTL). For dating
and synchronizing, skeleton plots and COFECHA were used.
Forest gap study site.
We located a forest gap that was not disturbed by cows and grazing (a difficult
task) in the upper part of the N. pumilio forests near the timber
line. Within this gap we established a study plot to sample N. pumilio
regeneration. The study plot was 2x15 m and was located along a big broken
branch that caused the gap (see figure 1). The plot was divided into subplots
of 1x 2 m and all the trees in the plot were sampled by first determining
the mode of establishment (vegetative or seed) for each plant and then
taking basal cross-sections of the stem. These cross-sections were
sanded and the age of each stem was determined in the lab.
All trees surrounding the gap were cored. One core was taken from the adjacent
gap side of the tree and the other was taken from the side opposite the
gap. Cross-sections from dead trees within the gap were also collected
in order to help date the age of the gap. Additionally one N. pumilio
tree from lower elevation was cut down to determine the rate of browsing.
All browsing scars were counted and the age of the tree was determined.
|
| Results |
a. Nothofagus pumilio
chronology for upper timber line
A 162 year long chronology was built. The sample depth varied between 1
and 11. Period 1922-1999 is covered by all cores. Eight out of 17 trees
could not be dated and synchronized. Main problems that appeared during
the tree-ring width processing was that annual rings in N. pumilio sapwood
were not easy to detect. The tree-ring boundary is due to small differences
in the densities of early- and latewood. Tree rings in the heartwood
are clearly visible. Density fluctuations can be observed regularly. They
could vary from hardly visible, to very distinct ones which look like real
tree rings. Missing rings could not be detected - further studies should
be done.
b. Regeneration of N.
pumilio in a gap
Following our results, the gap established 15-18 years ago. The majority
of the samples (6 out of 11) exhibit abrupt growth changes in the period
1982-1986. Vegetative reproduction predominates and seedlings are rare.
The power of vegetative reproduction is very high. We found broken branches
with diameters more than 5 cm that have made roots and lived on, even one
branch collar was able to make roots and continue to grow. This high reproductive
power was connected to favorable climatic conditions - high air humidity
and frequent precipitation, as indicated by the great abundance of lichens
(Prot-Usnea ssp.) on the trees.
The effects
of snow should not be neglected. The weight of the snow pushes down on
young plants and causes the bending of young stems - krummholz appear and
formation of the erected form of the trees is prevented.
_______________________________________________________
Table 1: Age and mode of establishment
of the samples collected along a transect in a gap
_______________________________________________________
No. Sample Age
Mode of establishment Remark
1 P4V1A
77 vegetative
2 P7V4
45
3 P12V1
22
4 P7V2
32
5 P2V1A
51
6 P4V1B
20
7 P7V2
79
8 P4V2A
100
9 P14S1
14
10 P3S1
81
Older than the gap
11 P7V1A
30
Release 17 years ago
12 P4V1C
18
Release 10 years ago
13 P2V2C
52
Release 17 years ago
14 P2V1B2
Last 10 years growth release
15 P2V1B1
16 P3B1
impossible to measure
Average
53
10 P14S1
14
seedling
11 P3S1
81
older than the gap
_______________________________________________________
_______________________________________________________
Table 2: Proportion between vegetative
and seedlings regeneration
_______________________________________________________
Number % Average age
Growth release in..
Vegetative
9 82
Seedlings
2* 18
15
1985
Total
11 100
* one sample was older than the gap
_______________________________________________________
c. Effect of browsing
on N. pumilio
We analyzed one tree that was browsed by domestic animals. The age of the
analyzed tree was only 10 years (aged at sample taken at the height of
5 cm), and was browsed 198 times in the last two years. This obviously
shows us that domestic animals are the most important factor affecting
N. pumilio forest regeneration. The majority of trees have been
grazed and stunted in growth. In the long term this represents a big problem
in the regeneration of the N. pumilio forest and since the dieback
of this forests is a fact, the regeneration of the forest will be questionable.
|
Conclusions
|
In the gap
-
Vegetative reproduction predominates.
-
Seedlings are rare and suppressed.
Gap age
-
The age of the gap is between
15 and 18 years years old.
-
Seedlings in the gap are approximately
15 years old.
-
A growth release could be observed
in all of the trees surrounding the gap between 1981 and 1983.
Upper timber line
-
A 162 year long chronology of
Nothofagus pumilio of the upper timber line was constructed. It
had sufficient sample depth to adequately cover the last 77 years.
Browsing
-
Grazing pressure might be an
important factor affecting forest regeneration
|
Literature
Cited
|
Veblen T.T., Hill R.S.,
Read J., 1996: The Ecology and Biogeography of Nothofagus forests. Yale
University Press, New Haven and London, 403 p. |
