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Understorey succession in Nothofagus forests in Tierra del Fuego (Argentina) affected by Castor canadensis

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Question: Does understorey richness, cover and biomass change during succession in abandoned Castor canadensis impoundments of riparian Nothofagus forests? Location: Magellanic Nothofagus forests at Tierra del Fuego National Park (54°50′32.4″ S, 68°32′11.5″ W), Argentina. Methods: Five meadows of different time since abandonment (1, 5, 6, 9 and 20 years ago) and two controls (pure N. pumilio and mixed N. pumilio - N. betuloides forests) were sampled. Understorey variables (species richness, cover and biomass) in beaver meadows were measured at eight plots, as well as sapling and seedlings age and height. In control treatments, ten plots on each forest type characterized forest structure. Results: Beavers alter vegetation dynamics, modifying biomass and composition of the original forest communities. Richness, cover and biomass were significantly modified when compared to the original understorey. Ferns are the most affected group, while grasses became more abundant. Many species established in the impacted sectors, which did not grow in primary forests. Trees did not regenerate in impacted areas for long periods, and many understorey original species are missing. Nothofagus forests are not adapted to support a long-term beaver impact. Conclusions: Beavers modify the original ecosystem from closed forest to a grass- and sedge-dominated meadow, due to the lack of adaptive regeneration strategies in the Nothofagus forests. The maintenance of the present level of the beaver population is not sustainable over time, due to utilized and impacted tree biomass which could not be replaced by the natural dynamics of the forest ecosystem. Abbreviations: FA = Forest stand; GIS = Geographical information system; LF = Nothofagus pumilio forest; LGF = N. pumilio/N. betuloides mixed forest. Nomenclature: Correa (1969–1998); Moore (1983).
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- UNDERSTOREY SUCCESSION IN NOTHOFAGUS FORESTS AFFECTED BY CASTOR CANADENSIS - 143
Applied Vegetation Science 9: 143-154, 2006
© IAVS; Opulus Press Uppsala.
Abstract.
Question: Does understorey richness, cover and biomass
change during succession in abandoned Castor canadensis
impoundments of riparian Nothofagus forests?
Location: Magellanic Nothofagus forests at Tierra del Fuego
National Park (54°50'32.4'' S, 68°32'11.5'' W), Argentina.
Methods: Five meadows of different time since abandonment
(1, 5, 6, 9 and 20 years ago) and two controls (pure N. pumilio
and mixed N. pumilio - N. betuloides forests) were sampled.
Understorey variables (species richness, cover and biomass)
in beaver meadows were measured at eight plots, as well as
sapling and seedlings age and height. In control treatments, ten
plots on each forest type characterized forest structure.
Results: Beavers alter vegetation dynamics, modifying bio-
mass and composition of the original forest communities.
Richness, cover and biomass were significantly modified when
compared to the original understorey. Ferns are the most
affected group, while grasses became more abundant. Many
species established in the impacted sectors, which did not
grow in primary forests. Trees did not regenerate in impacted
areas for long periods, and many understorey original species
are missing. Nothofagus forests are not adapted to support a
long-term beaver impact.
Conclusions: Beavers modify the original ecosystem from
closed forest to a grass- and sedge-dominated meadow, due to
the lack of adaptive regeneration strategies in the Nothofagus
forests. The maintenance of the present level of the beaver
population is not sustainable over time, due to utilized and
impacted tree biomass which could not be replaced by the
natural dynamics of the forest ecosystem.
Keywords: Beaver; Diversity; Exotic species; Meadow;
Nothofagus betuloides; Nothofagus pumilio; Dynamic cycle;
Regeneration.
Nomenclature: Correa (1969-1998); Moore (1983).
Abbreviations: FA = Forest stand; GIS = Geographical infor-
mation system; LF = Nothofagus pumilio forest; LGF = N.
pumilio/N. betuloides mixed forest.
Understorey succession in Nothofagus forests
in Tierra del Fuego (Argentina) affected by Castor canadensis
Martínez Pastur, Guillermo1*; Lencinas, M. Vanessa1; Escobar, Julio1; Quiroga, Paula2;
Malmierca, Laura3 & Lizarralde, Marta1
1Centro Austral de Investigaciones Científicas (CONICET), cc 92 (9410) Ushuaia - Tierra del Fuego, Argentina;
2Laboratorio de Ecotono (CRUB) Unidad Postal Universidad (8400) Bariloche - Río Negro, Argentina; 3Administración
de Parques Nacionales, San Martín 1395 (9410) Ushuaia – Tierra del Fuego, Argentina;
*Corresponding author; Fax +54 2901430644; E-mail cadicforestal@cadic.gov.ar
Introduction
The invasion and spread of intentionally introduced
exotic species has a major impact on natural ecosystems
and is one of the most difficult problems in nature
management (Vázquez 2002). Invasion of vertebrates
may lead to a reduction in biodiversity and ecosystem
functioning that both impoverishes and homogenizes
the world’s biota (Sala et al. 2000). Tierra del Fuego
(Argentina) has not escaped the problems of numerous
introductions since the colonization of the region at the
end of the 19th century. The most harmful species for
the biodiversity of the native forests in the region are
Mustela vison, Pseudalopex griseus and Castor cana-
densis (the American beaver) which was released in
1946 (Lizarralde 1993).
Beavers are ecosystem engineers that directly or
indirectly control the availability of resources for other
organisms by causing changes in the physical state of
the ecosystem, primarily by mechanical means (Jones et
al. 1994). They are, in a way, allogenic engineers be-
cause they are not necessarily a permanent part of the
system (Jones et al. 1997) and, as a result, patterns
created by beavers can persist after the animals have left
the area (Wright et al. 2004). The modification of both
upland and riparian environments leads to flooding
associated with dam building activity, canal digging
and selective foraging of woods within the riparian
zone (Terwilliger & Pastor 1999). Through the crea-
tion of ponds, the structure and functioning of both
meadows and stream communities structure can be
changed (Johnston & Naiman 1987). Such impacts
have been well documented, notably on riparian trees
(Barnes & Dibble 1986; Johnston & Naiman 1990a;
Breck et al. 2003), biogeochemical characteristics of
stream water and soil (Naiman & Melillo 1984; Naiman
et al. 1994; Johnston et al. 1995; Lizarralde et al. 1996),
riparian plant communities (McMaster & McMaster
2000; Wright et al. 2002) and vegetation succession
144 MARTÍNEZ PASTUR, G. ET AL.
(McMaster & McMaster 2001). Following abandon-
ment, beaver ponds drain and often form persistent
moist patches with graminoids in the forested landscape
with variable hydrologic regimes and vegetation com-
position (McMaster & McMaster 2000; Wright et al.
2002). Novel habitat types can be created in the riparian
zone when species capable of exploiting the resources
provided by these engineered patches are present (Wright
et al. 2002). This depends on the importance of local
and landscape-level controlling mechanisms on plant
species richness (Wright et al. 2003).
Sub-Antarctic ecosystems appear to be very vulner-
able to invasion by introduced species, and beavers are
a great threat to the largely primary forests in one of the
world’s largest and most pristine remaining wilderness
areas (Mittermeier et al. 2001). Beaver impacts on the
forest ecosystem include, according to Morris et al.
(1993) (1) alteration in microclimatic conditions; (2)
introduction of further exotic species; (3) suppression
of native species; (4) changes in natural dynamics; (5)
removal of nutrients and (6) changes in soil physics
(Naiman & Melillo 1984; Francis et al. 1985; Pinay &
Naiman 1991). There are few studies on the impact of
Castor canadensis (beavers) on primary Nothofagus
forests in Tierra del Fuego (Sielfeld & Venegas 1980;
Lizarralde 1993; Lizarralde et al. 1996, 2004; Coronato
et al. 2003). Consequently, little information is avail-
able on the impact on forest components such as under-
storey diversity, structure and natural dynamics in the
flooded zones of abandoned ponds. Therefore, the ob-
jective of the present study was to evaluate changes in
understorey richness, cover and biomass during succes-
sion in formerly beaver-influenced, but now abandoned,
impoundments of riparian Nothofagus forests in Tierra
del Fuego (Argentina).
Study system: Magellanic forests
Nothofagus is the main component of the South
Patagonian forests, with a wide range of natural distri-
bution from 36°50' S to 55°02' S. The southern forests
are predominantly deciduous, with N. pumilio and N.
antarctica as dominants, but there are also some
evergreen forests, consisting mainly of N. betuloides
(Barrera et al. 2000). There are also forests with a
mixture of these main species, called mixed magellanic
forest. In Tierra del Fuego, forests are used mainly for
timber, cattle grazing and tourism. The management of
this natural ecosystem is primarily economic (Martínez
Pastur et al. 2000), while conservation is of secondary
importance (Martínez Pastur et al. 2002a).
The genus Nothofagus contains heliophytic species
which are medium shade-tolerant (Donoso 1993) and
which pass through gap regeneration cycles (Veblen
1989; Holz & Veblen in press; Urlich et al. 2005).
Where a canopy tree was felled, groups of Nothofagus
individuals regenerate, due to the increase in light.
However, these forests have problems regenerating
under systematic or permanent disturbances because
of ungulate grazing and the impact of beavers (Raedake
1980; Lizarralde 1993; Pulido et al. 2000; Kitzberger
et al. 2005). The understorey of the deciduous forest is
poor in species, and consists mainly of the genera
Osmorhiza, Dysopsis and Cardamine (Moore 1983;
Roig 1998). The understorey of the evergreen forest
contains more shrubs, e.g. Berberis ilicifolia, Maytenus
disticha and Ribes magellanicus, as well as herbaceous
plants, e.g. Blechnum, Gunnera and Acaena.
Fig. 1. Location of the
study area.
- UNDERSTOREY SUCCESSION IN NOTHOFAGUS FORESTS AFFECTED BY CASTOR CANADENSIS - 145
Study area: Beaver meadows and primary forests
This study was conducted in the southern part of
Grande Island of Tierra del Fuego. The study site was a
20 ha Nothofagus forest in Tierra del Fuego National
Park (54°50'32'' S, 68°32'11'' W), situated 15 km west of
Ushuaia city (Fig. 1). The area belongs to the southern
anti-boreal zone as defined by Tuhkanen (1992), the
climate of which can be assigned to the domain of sub-
polar climates in the southern hemisphere with short,
cold summers and long, snowy and freezing winters.
This area is subjected to maritime influence, with per-
manently cloudy days with a high humidity, and the
absence of a freezing season. Summer temperatures
vary between 4 °C and 15 °C, while winter temperatures
range from –3 °C to 6 °C. Predominant winds are from
the SW with a mean speed of 6 - 7 km/h and maximum
speeds of over 100 km/h on unusually stormy days.
Mean annual rainfall is 530 mm, homogeneously dis-
tributed throughout the year, with abundant snowfall
(160 mm) that covers the forest floor from May to
September. The forest floor freezes during winter (May
to August) from 10-20 cm down to ca. 1 m deep.
The history of beaver activities in the study area are
well known, as the Administración de Parques
Nacionales and Centro Austral de Investigaciones
Científicas (Tierra del Fuego – Argentina) monitor
them. This area has numerous abandoned beaver ponds,
that were naturally drained to become wet meadows
during the last 20 years. Analysis of aerial photographs
(Instituto Geográfico Militar de Argentina, Buenos
Aires, AR, February 1970, 1:20 000) and satellite im-
ages (SPOT, February 1995) were useful to identify
and compare beaver impoundments, and to evaluate
their colony dynamics. In the riparian area, beavers
were responsible for the change of 2.53 ha of original
forest to meadows, which were used by several beaver
generations along a small, east to west 800-m long
stream. Data from January-February 2001 were in-
cluded and analysed into a GIS (Geographic Informa-
tion System) program. Five meadows were selected
according to the time since abandonment: 1 year (2000
- A1; 0.251 ha); 5 years (1995 - A5; 0.430 ha); 6 years
(1994 -A6; 0.256 ha); 9 years (1991 - A9; 0.123 ha)
and 20 years (1980 - A20; 0.260 ha). The width of
beaver impact along the stream is between 15 m and 95
m (Fig. 2) from riverside to the closest primary forest
without flooding impact surrounding the affected area.
Topography and soil properties were originally not
different along the study area.
Two representative Nothofagus primary forests sur-
rounding the beaver impoundment (Fig. 2) were se-
lected to evaluate the original forest structure before
the introduction of beavers. These were considered as
control situations and were compared with influenced
areas. One of these (LF), a 2-ha old growth N. pumilio
forest, was located upstream in the eastern area. The
other stand (LGF), a 3 ha, mixed old growth N. pumilio
- N. betuloides forest, was located downstream in the
western area.
Fig. 2. Forest stands (LF and LGF) and beaver meadows (A1 to A20) selected along the Cruz stream in Tierra del Fuego National
Park. Dotted line is the National Route 3 and grey lines indicate old dams and boundaries of the flooding areas.
146 MARTÍNEZ PASTUR, G. ET AL.
Methods
Forest structure characterization and understorey
sampling
Forest structure in the control treatment was char-
acterized using angle count sampling method (BAF 8)
(Bitterlich 1984), by ten plots randomly located in
each forest type. DBH, basal area, number of trees and
total bark volume estimated by Martínez Pastur et al.
(2002b) were obtained. In addition, dominant height
(mean of three close dominant trees) was measured
using an Impulse 200 Laser Tech. Inc. clinometer and
site index was defined according to Martínez Pastur et
al. (1997). Understorey variables in beaver meadows
were measured in eight plots randomly located in each
treatment. Every plot was a combination of four 0.25-
m2 subplots, 5 m from the central point. Subplots con-
stituted the unit of statistical analysis. Central plot
points were located in the field using a GPS (± 5 m),
once defined by a polar co-ordinates system. This
consisted of randomly selected azimuth (0° to 359°)
and distance (0 to the maximum ratio in m of each
treatment) starting from the central point of each treat-
ment. The central co-ordinates for the forest stands
(LF and LGF) and beaver meadows (A1 to A20) were
determined into the GIS (Fig. 2).
All vascular plants (Dicotyledonae, Monocotyl-
edonae and Pteridophytae) in the plots were identified
at species level, the lower plants were identified ac-
cording to the main taxonomic groups (mosses and
liverworts). Individual age of tree saplings and seed-
lings was estimated by counting bud scales and density
(plants/m2) by counting individuals within the plot;
plant height was measured with a cm tape. Forest floor
cover (understorey, woody debris and bare floor) was
registered with the help of a grid of 100 points/m2 in
every subplot. All live above-ground plant material
was collected and separated into groups (tree regenera-
tion, other dicots, monocots and lower plants) for bio-
mass determination, using a drying oven at 70 °C until
constant weight was achieved using a precision scale
(± 0.01 g). Biomass was expressed in dry weight (g.m–2).
During sampling, plant specimens were collected to maxi-
mize the range collected. Each species was classified
as sampled when it was found in the plots, or detected
when it was found in the sampled stand or meadow, but
not in the plots. A herbarium was made, and was
annexed to the Herbario de Tierra del Fuego’ at the
Centro Austral de Investigaciones Científicas (CADIC-
CONICET) in Ushuaia, Argentina.
Data analysis
Treatment comparisons were carried out by
ANOVA (F-test) and means were separated using Tukey
significant difference test (p < 0.05). When variance
heterogeneity (Cochran test) suggested lack of nor-
mality, the non-parametric Kruskal-Wallis test was
used, through means separated by the confidence
interval for the median test (p < 0.05). Complete link-
age amalgamation rule and Euclidean distance meas-
urement, based on presence or absence of the species,
were used to group the treatments. Then, an overlap-
ping analysis was carried out using species richness
information, between forests, recent installed mead-
ows and abandoned ponds of more than five years
old, to evaluate the influence of time. In addition,
DCA (Detrended Correspondence Analysis) was car-
ried out, using PC-ORD (McCune & Metford 1999),
with richness and frequency data, with weighting of
rare species.
Finally, indexes for !-diversity (community
species richness), "-diversity (qualitative Jaccard in-
dex) and #-diversity (based on total species richness)
permitted characterization and comparison of treat-
ments. The Jaccard index was calculated as
IJ = c / (a + b – c) (1)
where a is the species number of treatment A, b is the
species number of treatment B, and c is the number of
common species between A and B.
The "-index follows from
" = $ qj (Sr – Sj) (2)
Here qj is the proportional area of the community j;
Sr is the total species number over all communities
and Sj is the species quantity in community j.
The #-index was obtained by addition of mean !
and ".
Table 1. Characterization of the structure of the forest stands
by one-way ANOVA. LF = Nothofagus pumilio forest; LGF =
mixed forest of N. pumilio and N. betuloides; N = tree density;
BA = basal area; TOBV = total over bark volume. Site index
according to Martínez Pastur et al. (1997), with I-II corre-
sponding to the best site quality index and IV-V to the worst
site quality index. F = Fisher test, P = probability.
Variable LF LGF F-value P
Site index I-II IV-V 111.15 0.000
DBH (cm) 41.3 21.4 15.25 0.003
N (trees/ha) 457 1281 8.38 0.016
BA (m2.ha–1) 61.3 48.0 2.50 0.144
TOBV (m2.ha–1) 903.1 373.9 24.71 0.001
- UNDERSTOREY SUCCESSION IN NOTHOFAGUS FORESTS AFFECTED BY CASTOR CANADENSIS - 147
Fig. 3. A. Cluster analysis of forest stands and beaver mead-
ows, based on a species presence-absence matrix. B. Species
numbers overlapping among forest stands and beaver mead-
ows. LGF = mixed forest of Nothofagus pumilio and N.
betuloides; LF = N. pumilio forest; A1 to A20 = beaver
meadows abandoned one to 20 years ago; FA = forest stands.
Species richness was highest in A5 (36 species) and
lowest in A1 (eight species); and between beaver and
non beaver situations the influenced sites were richer
(55 species in A5 to A20). "-diversity in the forest
stands was 0.42, against 0.47 - 0.62 in the older mead-
ows (A5 to A20). A1 was significantly different from
the others, with values from 0.09 to 0.23. Recent and
older meadows were more similar amongst them-
selves (0.40) than forests compared to impoundments
abandoned 5 to 20 years ago (0.14), or to A1 (0.12).
Species richness of the landscape (#-diversity) was
68.3, based on ! = 28.7 and " = 39.6. The #-index
calculated on the basis of environmental groups was
61.5 (34.3 + 27.2). These indices highlight the results
shown in Fig. 3 and 4, where numerous species were
non specialists, being found in all study environ-
ments, but many others needed particular conditions
to live.
Results
Primary forest structure in control treatments
Forest structure presented significant differences
(Table 1), except for the occupation degree (basal area)
where both possessed a complete density. Nothofagus
pumilio forest, LF, showed a regular age structure with
a bell shaped distribution in only one tree layer. On the
other hand, the mixed forest (LGF) was uneven-aged,
reverse j-size distribution, with more than one tree layer
and numerous individuals with small diameters. Moreo-
ver, LF had better site quality, fewer individuals, higher
mean DBH and higher volume per ha.
Beaver effects on vegetation diversity
In the whole study area, 68 plant species were iden-
tified: 43 dicots, 22 monocots and three ferns, of which
seven dicots and one monocot are introduced species
(App. 1, Tables 1 and 2). Primary forests had many
dicots (28 species), few monocots (nine species) and
ferns (three species). Of these, 7.5% are introduced, e.g.
Poa pratensis in LF, and Taraxacum officinale and
Cerastium fontanum, found in LGF.
After one year following beaver abandonment (in
A1), meadow formation was initiated: eight native spe-
cies were observed (three monocots and five dicots),
and three of these were not identified in the primary
forest stands: Carex curta, Juncus scheuchzerioides and
Ranunculus biternatus. The 20-year abandoned pond
A20 had 33 dicots, 19 monocots and three ferns, many
of them not found in the primary forests, including 11
native monocots and 14 dicots. In A20, 13 native species
of the original forests did not return to these sectors.
These were the grass Bromus araucana, forest orchids
Codonorchis lessonii and Gavilea lutea, the tree Drimys
winteri, shrubs Maytenus disticha and Berberis ilicifolia
and herbaceous species Adenocaulon chilense, Dysopsis
glechomoides, Galium aparine, Macrachaenium gracile,
Senecio acanthifolius, Viola magellanica and V. reichie.
On the other hand, mosses and liverworts with many
species in the primary forests, were observed one year
after abandonment, and they developed faster under the
shelter of the other species.
Forests without impact were very similar (Fig. 3A),
but were different from the understorey of the beaver
meadows, which was related to the time of abandon-
ment; 13 species disappeared after the introduction of
beavers, and only grew in closed canopy forests (Fig.
3B). Further, only five species were found in all study
sites. This was also observed in the ordination (Fig. 4),
where many species were only found in primary forests
(LF and LGF) or related environments (A5 to A20).
148 MARTÍNEZ PASTUR, G. ET AL.
Fig. 4. Ordination of environmental parameters and species by DCA. FA = forest stands; A1 to A20 = beaver meadows abandoned
one to 20 years ago. Species codes appear in Tables 1 and 2 of App. 1.
Beaver effects on vegetation cover and biomass
Beaver activity and pond building produces a di-
rect impact on under and overstorey, with complete
removal of the forest. Ground cover in beaver mead-
ows showed significant differences, related to the
time since abandonment (Table 2). The forest floor
cover was 28% - 46% of litter in primary forests, and
increased to 92% one year after drainage (A1). As time
went by, many plants occupied beaver meadows, reach-
ing the bare floor minimum value (13%) in 20 year
abandoned ponds. Total vegetation cover showed an
inverse trend as the litter floor (46-62% in forests and
83% in 9- to 20-year old beaver meadows). However,
vegetation composition varied depending on the species
richness and their re-colonization dynamics. Ferns in
the primary forests were important in the understorey
cover with 16% - 34%, followed by the dicots (10% -
18%). Monocots (1% - 3%) and mosses + liverworts
(1% - 8%) were less important. In the abandoned
ponds, monocots acquired a major importance starting
with a substantially higher cover (7%) than in primary
forests, which increased over the years to 53%-64% in
the oldest meadows. On the other hand, dicots in-
creased slowly reaching only 27%. Ferns had very low
cover in the sectors under impact (< 0.5%), while
mosses and liverworts reached variable values, but
similar to those found in the primary forest (2% -
15%).
Vegetation biomass changed along the studied en-
vironments; in the beaver meadows this was related
to the time since abandonment (Table 3). Primary
forests produced more than 190 g.m–2 of above-ground
biomass. Beaver meadows had a sparse biomass (22
g.m–2) one year after pond abandonment, but the old-
est meadows duplicated the values of primary forest
(386 - 406 g.m–2). Lower plants and ferns represented
the major percentage of understorey biomass in forests
without flooding impact (46% in LF and 20% in LFG),
followed by dicots (13%) and monocots (3%). In the
one-year-old abandoned pond, lower plants (69%) and
monocots (27%) were the most important components.
As time passed, grass¡ increased in percentage of bio-
mass (from 25% to 69%). This left the lower plants in
second place in A1, but these were replaced by dicots
(33% in A20) in the following years. Finally, ferns
diminished their relative importance in beaver mead-
ows compared to primary forests, to less than 1% of
total biomass.
Forest regeneration dynamics
Primary forests presented a seedling bank within
the understorey (10.5 trees/m2, SE = 7.8), with a spe-
cific composition related to the forest type (100% of
Nothofagus pumilio in LF; 72% of N. pumilio and 28%
of N. betuloides in LFG). The mean age of the seedling
bank was 4.8 years with a maximum of 40 years (trees
under 1.3 m total height), and the mean height was 15.5
cm with a maximum of 92.0 cm. Regeneration indi-
viduals made up 1% - 12% of the forest understorey
cover, and contributed 18% to the total biomass (4.8 -
67.6 g.m–2) (Tables 2 and 3).
Through the impoundments following the building
- UNDERSTOREY SUCCESSION IN NOTHOFAGUS FORESTS AFFECTED BY CASTOR CANADENSIS - 149
of dams by beavers, all the Nothofagus individuals in
the upper and lower stratum (trees, saplings and seed-
ling bank) were killed. Re-colonization started after
one to three years, and occurred mainly near the dam of
the abandoned ponds. In the one-year-old abandoned
pond no seedlings were observed, but in five-year-old
beaver meadows 1.33 seedlings/m2 (SE = 1.86) oc-
curred with an age of 1.5 years and height of 2.6 cm;
13% of the seedlings were N. pumilio and 87% N.
betuloides. In A6, 0.18 plants/m2 (SE = 0.40) were
found (means of 3.0 years and 9.0 cm height); these
were all N. antarctica (three years old and 10 cm
maximum height). In A9, there were 0.33 seedlings/m2
(SE = 0.81), 6.5 years old and 55.0 cm mean height, all
of them N. antarctica (8 years old and 80 cm height
maximum). Finally, no regeneration was detected in
the pond that was abandoned 20 years ago. The rege-
neration represented 0.1% and 2.0% of the meadow
vegetation cover, contributing less than 4% to their
total biomass (less than 11.5 g.m–2) (Tables 2 & 3).
Discussion
Old-growth Nothofagus pumilio and N. betuloides
forests comprise of mature trees, with a high crown
closure, high basal area, low density of large trees and
high stand volume (Martínez Pastur et al. 2002a), and
possess either an irregular or a regular structure. The
irregular structure is found in many overlapping even-
aged patches, which originate from gaps (Veblen 1989).
Large-scale disturbances such as fires, wind-throw,
avalanches or landslides, lead to even-aged forests
(Donoso 1993; Rebertus et al. 1997), which are more
homogeneous but conserve many characteristics of the
uneven-aged forests (high proportion of mature indi-
viduals, stocking levels and high crown closure). Ac-
cording to their structure, old-growth forests selected
as reference sites correspond to the two typical forest
types of the southern zone of Tierra del Fuego,
although the N. pumilio forests we studied have
better site quality than most on the Island (Martínez
Pastur et al. 2000; Collado 2001). These structures
could be analogous to the previous forest before
beavers arrived, with a comparable composition of
the understorey.
Beavers manipulate their native ecosystems by re-
moving trees to create dams (Crooks 2002). They also
invade forest streams, flooding lands by pond forma-
tion and felling trees for food and building material.
Above the dams, soils are saturated, stream velocity is
reduced and sediments accumulate. Below the dams,
discharge rates decline and sediment loads are reduced
(Naiman et al. 1988). Additionally, beavers create a
patch structure around the pond where they harvest
trees and shrubs, the magnitude of which is determined
by its geomorphology (Johnston & Naiman 1987;
1990b). They often cut all stems growing near their
winter food caches, dams and lodges, but become more
selective as foraging distances increase (Baker et al.
2005). The amount of biomass harvested by beavers
exceeds by far the total actually ingested, and is much
greater than any other herbivore harvests (Johnston &
Naiman 1990a). Simultaneously, beaver activities al-
ter ecosystem processes, due to changes in the nutrient
cycles and dynamics of the decomposition (Naiman &
Melillo 1984; Francis et al. 1985; Naiman et al. 1988;
Johnston et al. 1995; Johnston 2001). All riparian
beaver effects are the result of their ‘engineering’
activities, with the possible exception of changes in the
composition of riparian trees, partly due to their herbi-
vory (Wright et al. 2004).
In Tierra del Fuego, beaver biological invasion
alters the physical structure of the ecosystems, the
community level responses to engineer induced altera-
tions of habitat complexity and the ecosystem level
Table 2. Characterization of the ground cover of the forest stands and beaver meadows. LF = Nothofagus pumilio forest; LGF =
mixed forest of N. pumilio and N. betuloides; A1 to A20 = beaver meadows abandoned one to 20 years. Means followed by different
letters are significantly different at P < 0.05.
Treatment Litter Debris Total Seedlings Other dicots Monocots Ferns Other lower
floor(%) (%) understorey(%) and saplings (%) (%) (%) (%) plants (%)
Kruskal 26.23 18.75 28.51 26.86 23.08 33.82 33.26 20.80
Wallis test (0.000) (0.005) (0.000) (0.000) (0.001) (0.000) (0.000) (0.002)
LF 27.9ab 9.70ab 62.4b 12.5b 10.3b 1.4a 34.1b 4.2a
LGF 45.9b 8.1ab 46.0b 1.1a 18.5bc 2.7a 15.7b 8.0ab
A1 91.9c 0.3a 7.80a 0.0a 0.5a 7.1a 0.0a 0.3a
A5 19.4ab 22.4b 58.2b 0.1a 7.9ab 35.0b 0.0a 15.3b
A6 25.3ab 2.2a 72.6bc 2.0a 8.2ab 60.1b 0.0a 2.2a
A9 15.3a 1.7a 83.0c 0.2a 16.0b 63.7b 0.0a 3.1a
A20 13.0a 3.7a 83.3c 0.0a 27.0c 53.5b 0.5a 2.3a
150 MARTÍNEZ PASTUR, G. ET AL.
Table 3. Biomass understorey of the forest stands and beaver meadows. LF = Nothofagus pumilio forest; LGF = mixed forest of N.
pumilio and N. betuloides; A1 to A20 = beaver meadows abandoned one to 20 years. Means followed by different letters are
significantly different at P < 0.05.
Treatment Total Seedlings and Other dicots Monocots Ferns Other lower
(g.m–2) saplings (g.m–2) (g.m–2) (g.m–2) (g.m–2) plants (g m–2)
Kruskal 28.51 24.83 27.53 33.73 33.46 20.10
Wallis test (0.000) (0.000) (0.000) (0.000) (0.000) (0.003)
LF 196.5b 67.6b 10.1b 1.3a 58.8c 58.8ab
LGF 193.4b 4.8a 40.0bc 8.4a 20.3b 119.9c
A1 22.4a 0.0a 0.7a 6.2a 0.0a 15.5a
A5 354.7bc 0.1a 25.1bc 100.0b 0.0a 229.6c
A6 318.2bc 11.5a 44.7bc 218.9c 0.0a 43.0ab
A9 405.6bc 8.9a 82.7c 232.7c 0.0a 81.3ab
A20 385.6c 0.0a 126.0d 169.9bc 2.2a 87.5ab
(Mc Master & Mc Master 2001; Wright et al. 2004). In
torrential streams of Tierra del Fuego, the occupation
depends on water availability, which becomes sea-
sonal in small watercourses. Once abandoned, dams
disintegrate causing ponds to drain and expose accu-
mulated sediments (Wright et al. 2003). Eventually
(e.g. when impoundments flood roads), man elimi-
nates beaver colonies, even though these places should
be settled again in the future (as A1). The re-coloniza-
tion depends on the availability of suitable trees as pro-
viders of food and building material (Basey et al. 1988),
preferably less than 15 cm DBH, and of the geo-
morphologic conditions of the environment (Johnston &
Naiman 1987; Mella 1995; Coronato et al. 2003).
Vegetative succession following abandonment and
pond drainage may follow multiple pathways influ-
enced by factors such as surrounding vegetation, hy-
drology, geomorphology, herbivory and re-coloniza-
tion by beavers (Naiman et al. 1988; Terwilliger &
Pastor 1999). Often, grass- and sedge-dominated bea-
ver meadows persist in the landscape long after aban-
donment, because succession in flooded drained ponds
appears to follow an altogether different pathway than
those of other disturbance caused openings (Terwilliger
& Pastor 1999). Nutrient and carbon accumulation
processes during stream impoundment produce long-
term effects on soil structure and functioning, even a
long time after the pond has been drained, resulting in
productive wetland and meadow patches in an other-
wise nutrient impoverished forest (Pinay & Naiman
1991; Johnston et al. 1995).
Changes in nutrient cycles and decomposition dy-
namics (Naiman & Melillo 1984; Francis et al. 1985;
Naiman et al. 1988; Johnston et al. 1995; Johnston
2001) modify the biomass and composition of the new
communities (Barnes & Dibble 1986; Johnston &
Naiman 1990a). Plant species richness typically in-
creases after a patch is created, but later declines due to
formed by a mosaic of different environments, where
closed stands of different species are mixed with others
(wetlands, riparian forests, rangelands or peat-lands).
These forests support a relatively low number of plant
understorey species (Moore 1983; Roig 1998), and
almost all of them are generalist, with many introduced
plants in anthropized environments. However, each
forest type possesses particular biotic and abiotic com-
ponents that generate different ecological conditions,
which allow the installation and survival of distinctive
species. For example, stream shores and wetlands in-
side the forests have a higher species richness and
biomass than the other forest environments (Lencinas
2005). Riparian ecosystems with an original floristic
composition are scarce in Nothofagus woodlands, due
to the current levels of beaver populations (Lizarralde
1993). Beaver activities change the narrow stream
corridor among the forest matrix into a pond by dam
creation, which has different properties than the re-
placed forest ecosystem, as was cited by Johnston &
Naiman (1987) for Populus tremuloides woods. Bea-
ver engineering is responsible for the loss of many
species (secondary trees, shrubs, orchids and many
herbaceous plants) along the shores of rivers and
streams, which do not establish in the meadows. Stream
bank impact could be different depending of geomor-
phology, because wetlands and floodplains are already
under saturated conditions before impoundments, and
ecosystem level processes are principally anaerobic
(Johnston & Naiman 1987).
Beavers mainly invade initially all downstream ar-
eas and eventually upstream ones; each colony can
build more than one pond (Johnston 1994). Usually,
colonies occupy areas for five to 15 years (Lizarralde
1993; Wright et al. 2003), and then migrate to another
site when food supply dwindles, ponds have become
filled with sediments, or when the animals have been
removed by trapping, natural predation or disease
- UNDERSTOREY SUCCESSION IN NOTHOFAGUS FORESTS AFFECTED BY CASTOR CANADENSIS - 151
the dominance of competitively superior species
(Wright et al. 2003). However, beaver meadows in the
study area presented higher species richness in older
ponds than in the youngest. This increase is thought to
be the result of the accumulation over time of poorly
dispersing, late successional species that temporarily
co-exist with better dispersing early successional
species (Wright et al. 2003). Nevertheless, there will
be a positive relationship between habitat complexity
and biotic diversity and abundance (Crooks 2002),
which could be related to spatial heterogeneity, which
increases over time, since unconsolidated sediments
erode to a micro-landscape of pits and hummocks,
particularly in older meadows (McMaster & McMaster
2001; Wright et al. 2002, 2003). On the other hand, soil
moisture also has a large effect on herbaceous plant
species richness and composition (Wright et al. 2003).
Old, drier meadows should have greater species rich-
ness than young, wetter meadows, as was observed in
the study area.
Large numbers of plant species settled in aban-
doned ponds, many of them that do not grow in the
primary forests, because species able to live in the
invader modified habitat take advantage of the ecosys-
tem engineering, while those living in the unmodified
habitat are inhibited (Crooks 2002). Lizarralde et al.
(2004) indicate that beaver altered sites in Tierra del
Fuego have higher levels of organic and inorganic
nitrogen. Some plants are characteristic of associated
environments, such as rangelands, peat bogs or N.
antarctica forests, and develop vigorously, while block-
ing the establishment of regenerating species, which
should restore the ecological characteristics of the
flooded areas. In addition, changes in the environment
opens possibilities for introduced plant species, while
these early invaders can modify the habitat and facili-
tate further invasions (Crooks 2002). There are few
introduced species in the primary forests, but these
species establish abundantly after any impact, e.g.
during harvesting (Martínez Pastur et al. 2002a). Some
of these invaders possibly arrive through roads or
cattle, e.g. Poa pratensis and Taraxacum officinale, or
by dispersion along riversides, e.g. Cerastium fontanum.
Nevertheless, not only is the understorey richness modi-
fied, significant changes also appear in cover, bio-
mass, frequency and abundance in the newly settled
communities, compared to the original old-growth
forests.
At a scale that encompasses both patches affected
by ecosystem modification and patches of unmodified
habitat, there is an increase in regional species richness
(Jones et al. 1997). In the Huntington Wildlife Forest,
Wright et al. (2002) estimated that beaver-modified
patches may contribute as much as 25% of the total
herbaceous plant species richness of the riparian zone.
It is expected that Nothofagus will colonize aban-
doned ponds after a short time, because these species
are adapted to catastrophic disturbances (Donoso 1993;
Rebertus et al. 1997). In this way the forest can be
restored in a wide range of other natural conditions
(Martínez Pastur et al. 2002a; Cuevas 2000). However,
the flooding associated with the dams drowns all the
seedlings and trees in a few years (McMaster &
McMaster 2000). In addition, deposited sediments com-
pletely cover the original forest floor and impede seed
germination, making new input of seeds to the aban-
doned ponds necessary. For this reason, forest species
do not completely regenerate in meadows for a long
time (more than 20 years) related to the beaver occupa-
tion period. In the absence of reproduction from relict
seeds or vegetative propagules such as stumps and
roots, invasion of woody plants requires the presence
of viable seeds in the disturbed area and the successful
germination, growth and competition of the resulting
seedlings (Terwilliger & Pastor 1999). New seeds must
reach the abandoned ponds, and the close proximity to
mature, seed productive stands is essential. In the
study meadows, only A5 has N. pumilio and N.
betuloides seedlings and saplings, which are located
close to the LF stand (Fig. 2). However, N. antarctica
occurs in older ponds (> 6 years), these are not present
in the studied area as tree dominant species, but grow
around peat bogs 1 km upstream. This species is con-
sidered as a pioneer of non-forested environments
(rangelands and borders of peat bogs), creating the
necessary conditions for the establishment of other
Nothofagus species. On the other hand, other factors
could influence the establishment of seedlings, such as
the annual variation in mass seed production of Notho-
fagus (Schmidt & Urzúa 1982), the lack of ecto-
mycorrhizal symbionts (Terwilliger & Pastor 1999)
and the fast development of vegetation, which in-
creased in cover from 46% to 83% in our study. In the
Hutcheson Memorial Forest, Myster (1993) found that
conifer invasion into grass-dominated meadows is
slowed down by the physical effects of grasses, where
the few seeds which make contact with the soil are
shaded out by rapidly growing grasses. In addition,
seedlings must survive resource limitations, anoxic
soils, frost heaving, physical damage and herbivores in
the formed meadows (Terwilliger & Pastor 1999;
McMaster & McMaster 2001).
The population size of American beavers in Tierra
del Fuego is estimated at 35 000 - 50 000, which have
apparently colonized all streams in the Andean and
extra-Andean areas of the island (Lizarralde et al.
2004). This population could potentially exert a strong
impact on the species richness at the landscape level.
152 MARTÍNEZ PASTUR, G. ET AL.
Beavers can create landscapes dominated by aban-
doned patches, and can produce large numbers of
colonizers by rapidly depleting the resource levels and
leaving abandoned patches that recover very slowly
(Wright et al. 2004). The flooded areas represent ca.
3.0% of the total woodlands (Collado 2001); they are
more extensive in some forests of the Chilean sector of
the Island (3.8% Vicuña and 28.8% in Río Bueno)
(Mella 1995). In the forests where the beaver is a
native species, the foraging alters larger areas (e.g. 12-
15% in Voyageours National Park, United States)
(Naiman et al. 1988).
Regeneration strategies of Nothofagus are the main
reasons for a lack of adaptation of these forests to
long-term beaver impact. Beaver activity modifies
the original Nothofagus ecosystem from a closed
forest to a grass and sedge dominated meadow. This
agrees with Naiman et al. (1988), who suggest that the
long time impacted sectors for beavers reaches a
different stable condition from the original one in
Canada. These areas could have returned to their
original structure, but the beavers modified the hy-
drology and vegetation patterns. Longer periods of
time than for other impacts, e.g. a harvested or wind-
blown forest in south Patagonia (Schmidt & Urzúa
1982; Rebertus et al. 1997), are necessary for the
forest to recover. In North America, meadows can
persist for over 70 years once formed and rarely, if
ever, revert back to the original forested riparian zone
(Wright et al. 2003). For these reasons, the mainte-
nance of the present levels of beaver population in
Tierra del Fuego is not sustainable in time, due to the
utilization and impact on tree biomass which could
not be replaced by the natural dynamic forest ecosys-
tem (Johnston & Naiman 1990a).
Acknowledgements. The authors gratefully acknowledge field
assistance by Patricia Ríos, and helpful comments provided by
Esteban Carabelli, Robert Naiman, James Hellfield and
Christopher Anderson.
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... The invasive North American beaver (Castor canadensis) thus has significant consequences for freshwater ecosystem function in these wilderness areas. Past research has established that the presence of non-native beavers changed the structure and composition of riparian forests (Anderson et al. 2006;Martínez Pastur et al. 2006), fish growth and abundance Arismendi et al. 2020), aquatic macroinvertebrate community structure (Anderson and Rosemond 2007), and periphyton community structure (Rodríguez et al. 2020), thus altering entire food webs of invaded streams (Anderson and Rosemond 2010 The largest influence of beavers on hydrogeomorphic processes is through dam construction (Westbrook et al. 2013). In Tierra del Fuego, invasive beavers colonize low-order (1-2 Strahler order) streams and rivers, and seasonally, secondary branches in large and steeper rivers that are too wide or dynamic for beavers to build dams across (Coronato et al. 2003;Westbrook et al. 2017). ...
... When beavers abandon a site, the pond disappears as the water is no longer retained. The area previously occupied by the impoundment is colonized by plants, converting the former pond area into a meadow inhabited mainly with grasses whilst trees do not regenerate for long time periods (Martínez Pastur et al. 2006). Nonetheless, the impacts of beavers on ecosystem metabolism remain unquantified. ...
... The negative consequences of beaver activities in the pristine forest from Tierra del Fuego has been widely described (e.g. Martínez Pastur et al. 2006) as well as beavers' impact in water hydrology and the benthic habitat (Lizarralde et al. 1996, Anderson & Rosemond 2007. The effect of beaver activity on river ecosystem metabolism has received less attention despite metabolism being an integrative measure of the processes controlling organic matter dynamics and nutrient cycling in streams (Tank et al. 2010). ...
Article
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Beavers can modify the hydrology, morphology, chemistry, and biology of ecosystems, though we have limited understanding of how beaver activity alters whole-ecosystem functions. We analyzed the effect of beaver activity and beaver dams on ecosystem metabolism in sub-Antarctic streams and rivers, where beavers are a non-native species. We characterized ecosystem metabolism (gross primary production and ecosystem respiration, GPP and ER) during 1–4 days in six streams and rivers with current beaver activity and dams (active) and three streams with no dams and no beavers (abandoned) as follows: Current beaver activity enhanced metabolism; GPP and ER were higher in sites with beaver activity than in more heterotrophic, abandoned sites. Beavers affect whole-ecosystem metabolism despite no detectable effects on physical and chemical variables in sub-Antarctic streams and rivers.
... While the propagating effects of beaver activity can be beneficial, careful consideration must be given to whether the history of a given watershed is naturally "tuned" to beaver ecology. For example, in areas where beaver has been introduced, trees often lack defensive mechanisms and reproductive strategies that occur in forests that are regularly subject to beaver activity because they do not share a common evolutionary history (Basey et al., 1988;Martínez Pastur et al., 2006;Anderson et al., 2009). Introduced beaver may be particularly devastating for systems invaded by other non-native species, as novel disturbance has the potential to drive feedback loops that favor survivorship of other invaders, promote new introductions or spread, and overwhelm any biotic resistance that might be perpetrated by native species (i.e., invasional meltdown: Simberloff and Von Holle, 1999;Crooks, 2002;Maret et al., 2006;Braga et al., 2018). ...
... For example, introduced beaver in Tierra del Fuego have positive effects on the growth of non-native Brown trout (Salmo trutta) (Arismendi et al., 2020), a voracious predator known for its top-down effects on trophic structure and capacity to become invasive (Hansen et al., 2019). In this same system, beaver also promote invasion pathways for non-native plants because native forests (Nothofagus sp.) are unable to regenerate in the aftermath of beaver disturbance (Anderson et al., 2006;Martínez Pastur et al., 2006). ...
... Beaver also drive successional shifts in vegetation communities through selective harvesting of trees and by altering floodplain dynamics (Naiman et al., 1988;Anderson et al., 2006;Martínez Pastur et al., 2006). Despite their preference for cottonwood, aspen, and willow (Populus or Salix spp.; McGinley and Whitham, 1985;Johnston and Naiman, 1990), beaver harvest a wide range of trees and shrubs and move up to ∼30 m from a stream body to do so (Allen, 1983;Baker and Hill, 2003;Gallant et al., 2004;Anderson et al., 2006). ...
Article
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We discuss the problems associated with beaver disturbance and its effects on conserving the region's native fauna and flora. We refute arguments underlying the claim that beaver is native to the region, and review paleontological, zooarchaeological, and historical survey data from renowned field biologists and naturalists over the past ~160 years to show that no evidence exists that beaver arrived by any means other than deliberate human introduction.
... Many of these alterations can have permanent legacies, and require active interventions to restore the natural values and services [5][6][7]. One of the most dramatic alterations to southern forest ecosystems of the last century is related to the invasion of North American beavers (Castor canadensis) in the Tierra del Fuego Archipelago [8], which extensively alter riparian forests and have become the most relevant issue of conservation concern in southern Patagonia to maintain the provision 3 of 17 regeneration recruitment and survival, generating a reduction in the eco-physiological performance of the seedlings (e.g., [17,24,28]). Beside this, we expect that an early succession tree species, such as N. antarctica, can present a better performance in these modified environments due to its eco-physiological characteristics (e.g., [25]) allowing to a quick recovery of the impacted areas (from meadows to forest environments), and their ecological functions and ecosystem services. ...
... These forests naturally regenerate from seedlings, which could survive many years in the understory until microclimatic conditions facilitate their height growth. The understory plant diversity is poor and has low cover (<40%), being inhabited by few shrubs (<50 cm height), and several herbs and grasses [4,6,8,23,28,29]. ...
... At each study area, we studied four zones ( Figure 1C,D) based on previous studies [4,8]. Three located in the beaver meadows: (i) front (FRO, area just upstream of the old dam), (ii) tail (TAI, area where stream enters beaver meadow), and (iii) cut (CUT, area harvested, but not flooded, by beavers); and one located in one adjacent unimpacted old-growth forests (OGF) acting as a control forests (OGF). ...
Article
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North American beavers (Castor canadensis) are responsible for the major changes in the Tierra del Fuego Archipelago, altering riparian forests for the long-term. Passive restoration of the areas affected was ineffective in the medium-term (up to 20 years), being necessary active strategies. Plantations in abandoned ponds were made with Nothofagus pumilio and N. antarctica tree species across Tierra del Fuego island (Argentina). In the first experiment, we analysed the influence of biotic and abiotic factors in three micro-habitats in the impacted areas: front and tail of ponds, and cut not-flooded forest areas. Five-years-old N. pumilio seedlings had 39% survival in front, 21% in tails, and 46% in cut areas at year-3 of the restoration experiments, being negatively influenced by plant cover and soil moisture. Lower growth was recorded during year-1 (0.7–0.9 cm yr−1), but increased on time (1.9 cm yr−1 front, 1.6 cm yr−1 tail, 4.3 cm yr−1 cut areas). A second experiment explores the alternative to substitute the tree species to face the harder conditions of the impact and climate change. For this, we conducted a new plantation at four locations across the main bioclimatic zones, where 10–40 cm N. antarctica plants attained 17% survival in meadows (front and tail) and 30% in cut areas, being higher with larger than smaller plants (25% vs. 18%), and where they are mainly influenced by rainfall (4% in sites <400 mm yr−1 and 41% in >400 mm yr−1). The main damage was detected in the above-ground biomass due to dryness, but root survival allowed the emergence of new shoots in the following growing season. It is necessary to monitor different Nothofagus species across natural environments in the landscape to determine the feasibility and effectiveness of different strategies in restoration plans, considering the selection of climate-resilient tree species.
... Beavers are known as extraordinary ecosystem engineers (Wright et al. 2002). They use woody material inputs (mainly branches and trunks of trees) to build their lodges and dams, changing the environment to fulfill their life cycle (Lizarralde 2008), which means important ecological alterations, e.g., modification of the hydrology of watercourses by dams (Naiman et al. 1988;Martínez Pastur et al. 2006;Lizarralde et al. 2008). Studies about the impact of beavers on ecosystems have been documented, e.g., in riparian forests, they erode riverbanks and increase the surface of slow aquatic environments (Lizarralde et al. 1996;Anderson et al. 2009;Garcia and Rodríguez 2018). ...
... pumilio) and/or mixed with guindo or coihue de Magallanes (N. betuloides), the flooded areas near to a dam are converted into long-term stable meadows (Martínez Pastur et al. 2006;Henn et al. 2014), where the trees cannot survive (e.g., due to excessive soil moisture) (Toro Manríquez et al. 2018). In many parts of the archipelago, the ecological impact of the beaver is striking due to the trees that have died standing because of drowning when the beaver dam is built. ...
... represents the ideal ratio of ES for the people water purification (Anderson et al. 2009;Simanonok et al. 2011;Martínez Pastur et al. 2016) and cultural services such as aesthetic values, ecotourism, and spiritual values ). However, the beavers could alter the composition of several plant assemblages (Martínez Pastur et al. 2006;Henn et al. 2016), which play a crucial role in the supporting and regulating ES, e.g., the herbaceous plants are angular organisms in forests and grasslands since plants contribute to biodiversity in general (Lencinas et al. 2008, interact with the initial phases of the installation and growth of tree species (Beckage et al. 2000) and thereby determine much of the energy flow and cycling of nutrients (O'Brien et al. 2007), and respond in complex ways to natural and anthropogenic disturbances (Gilliam 2007;Huertas Herrera et al. 2018). ...
Chapter
North American beaver is the most iconic and studied invasive species of Tierra del Fuego. The scientific literature has focused on highlighting the beaver negative impacts on the ecosystems, which have resulted in reduced actions such as eradication. However, the beaver problem in relation to ecosystem services (ES) has been little described or analyzed. This chapter provides a beaver habitat suitability map and discusses the potential positive and/or negative relationships between beaver invasion and ES (provisioning, cultural, supporting and regulating). We found that ES approaches can provide evidence of the beaver role for the people at different archipelago’s vegetation zones. We conclude by outlining how the comprehensive assessment of ES can be a tool that allows to understand and to guide decision-making for the beaver issue.
... Terrestrially, once dams breach and impoundments drain due to abandonment or beaver removal, a successional process begins. In forested regions of the TDFA, however, succession may not yield a return to riparian forest, at least within ~20 years (Martínez Pastur et al., 2006;Wallem et al., 2010 Much is known about invasive beaver impacts on streams and riparian vegetation in the TDFA, but despite a call for research on birds-the most diverse vertebrate class in the TDFA (Rozzi & Jiménez, 2014)-knowledge is lacking about beavers' effects on local avifauna. Evidence from their northern hemisphere range indicates that beaver engineering increases avian abundance and diversity (see Stringer and Gaywood, 2016 and references therein), but documentation of this effect in their invaded southern hemisphere range is limited. ...
... Considering the implications of these findings for restoration efforts that seek to return ecosystems to pre-beaver conditions, it appears that pond drainage without reforestation measures would not return the avian community to an intact forest community in the short to medium term. Alongside beaver removal, dam breaching and pond drainage, active reforestation over decadal time-scales (particularly with N. antarctica) might help regenerate forest ecosystem conditions (Anderson et al., 2006;Henn et al., 2014;Martínez Pastur et al., 2006) that should decrease patch-level abundances of pond-and meadow-associated taxa and increase abundances of forest-associated species. These designations, however, were assigned in national and international contexts, and these species lack formal assessments of local conservation statuses. ...
Article
The North American beaver Castor canadensis is an invasive species in the Tierra del Fuego Archipelago. Due to this biological invasion, Argentina and Chile signed an agreement to restore affected ecosystems by eradicating beavers. In southern Patagonia, the beavers’ ecological impacts are well studied, but there is a relative lack of information on how their invasion (and potential removal) could affect bird communities. In the southern portion of Tierra del Fuego’s “big island” (Isla Grande), we conducted passive acoustic monitoring and avian point counts in intact riparian forests, beaver ponds, and beaver meadows (i.e., drained ponds) to assess spatial and seasonal differences in acoustic activity and avian abundance, species diversity, and functional diversity. During spring and summer, acoustic activity was significantly higher in meadows than in forests, with ponds exhibiting intermediate values. Abundance and species diversity exhibited similar patterns, driven largely by resident passerines, while functional diversity tended to be highest in ponds, largely due to ducks and raptors. Effects were weaker in fall and winter. Acoustic metrics exhibited moderate to strong correlations with all point‐count‐derived metrics. Synthesis and applications: At the patch‐level, the avian community was more abundant and diverse in beaver‐modified habitats than in intact riparian forests, though communities in modified patches may not differ substantially from those in analogous natural open and wetland habitats. Dam breaching and pond drainage did not yield a return to an intact forest bird community, indicating that active reforestation may be necessary to restore avian communities to pre‐beaver conditions in the short to medium term, as sought by the binational agreement. Given the immense challenges of eradication and restoration, its social‐ecological costs and benefits—including those related to avifauna—should be thoroughly considered in establishing goals or indicators of success.
... ). A partir de la propuesta teórica, se han identificado "Estados" MartínezPastur et al. 2006). Esto demuestra lo complejo que puede ser el repertorio de situaciones que es posible encontrar en los ñirantales. ...
Thesis
El uso ganadero de los bosques de Nothofagus antarctica de Tierra del Fuego (Argentina) ha sido históricamente extensivo y poco planificado, y usualmente basado en la reducción de cobertura arbórea mediante cortas para aumentar la producción de forraje bajo su dosel. El objetivo de esta tesis fue analizar estructura y rasgos funcionales de las comunidades de aves terrestres, diurnas y residentes estivales (principalmente paseriformes), en diferentes estados del bosque de N. antarctica evaluando el efecto de variables ambientales a escala local y de paisaje, así como el potencial uso de las aves como indicadoras de impactos para mejorar las propuestas de estrategias de manejo foresto-ganadero existentes. Este trabajo se realizó en 4 Estancias (Los Cerros, Rolito, Las Hijas y Cabo San Pablo) ubicadas en el centro y este de la Isla Grande de Tierra del Fuego (Argentina). Se estudiaron 4 tipos de bosque: un tipo con raleos (Abiertos) y tres tipos sin raleos (Muy Cerrados, Cerrados y Muy Abiertos). Las aves se estudiaron en puntos de observación fijos, donde se realizaron conteos durante los meses de enero y febrero del 2017 y 2018 en Los Cerros, 2018 y 2019 en Rolito, y 2019 y 2020 en Las Hijas y Cabo San Pablo, para caracterizar estructura (composición, riqueza, densidad, biomasa e índices de diversidad) y rasgos funcionales (grupos tróficos, uso de sustratos, tipo de nidificación y estatus migratorio). En Los Cerros y Rolito también se analizó la susceptibilidad a la depredación de nidos artificiales. Los efectos analizados fueron: a escala local, la estructura forestal (altura dominante, área basal, cobertura de copas, densidad de árboles y diámetro medio), la cobertura del suelo (sotobosque, residuos leñosos, plantas no-vasculares, renovales y suelo sin vegetación), y la oferta alimenticia, vegetal (riqueza y cobertura total, de gramíneas y de dicotiledóneas consumidas por aves) y de la artropofauna (abundancia total y proporción de los órdenes más importantes); y a escala de paisaje, área, perímetro y forma del parche; área, número de parches, índice del parche más grande y conectividad de bosques y áreas abiertas; y total de bordes y densidad de bordes. Las variables fueron evaluadas mediante modelos lineales generalizados (GLM) y generalizados mixtos (GLMM), y multivariados (PCA, MRPP, IndVal, y CCA). Se pudo identificar mayor riqueza y diversidad de especies de aves en bosques Abiertos (raleados) y Muy Abiertos, dadas por la presencia de especies de áreas abiertas o de bordes de bosque. Por otro lado, se identificaron menores densidades de omnívoros y granívoras en bosques Cerrados y Muy Cerrados, respectivamente, vinculadas a los sustratos de alimentación. El uso de los sustratos difirió entre tipos de bosque, estando el uso de sustratos bajos y copas relacionado con su disponibilidad. Mientras que el uso de ramas y fustes, y la densidad de individuos volando se asociaron a la estructura forestal (cobertura de copas y área basal, entre otras). Por otro lado, la densidad de especies que nidifican en huecos y copa abierta, así como la densidad de residentes y migradoras, también difirieron entre tipos de bosque y se asociaron a la altura dominante y a la cobertura de copas. Los bosques raleados, si bien presentaron algunas diferencias con los Cerrados y Muy Cerrados (mayor riqueza de especies, diversidad, uso de sustratos bajos, densidad de aves volando y de migradoras), fueron parecidos en otros rasgos (ej. densidad de aves que utilizan ramas y copas o que nidifican en huecos), observándose en ellos incluso especies especialistas de bosque (Pygarrhichas albogularis y Aphrastura spinicauda). Si bien no se encontraron diferencias significativas en la susceptibilidad a la depredación de nidos artificiales entre tipos de bosque, los nidos que se encontraban en bosques Muy Abiertos fueron depredados más rápido. Este estudio permitió identificar especies indicadoras de bosques Cerrados y Muy Cerrados (A. spinicauda, P. albogularis), Abiertos (Spinus barbatus) y Muy Abiertos (ej. Tachycineta leucopyga). Por lo tanto, estas especies se podrían utilizar para el monitoreo de los raleos en estos bosques. Al analizar variables de diferentes escalas espaciales (local y paisaje), se pudo observar que las mismas influyeron sobre diferentes grupos de especies. Por ejemplo, la estructura forestal (área basal, cobertura de copas) y el área del parche de bosque tuvieron mayor efecto sobre P. albogularis. Se concluye que ciertas características a nivel local (ej. mayor área basal, cobertura de copas y de renovales, y proporción de himenópteros), así como a nivel de paisaje (ej. parches de bosque grandes e irregulares, alternancia con áreas abiertas), favorecen a una mayor diversidad de aves en los bosques de N. antarctica de Tierra del Fuego. El desarrollo de propuestas de manejo forestales y silvopastoriles que preserven o favorezcan la presencia de paisajes y parches de bosque con estas características contribuiría a que el manejo de estos bosques sea más sustentable.
... In South America, 25 pairs of North American beavers from Manitoba (Canada) were introduced in 1946 to Tierra del Fuego by the Secretary of the Navy of Argentina to enrich the community and for fur trading (Pietrek and Fasola, 2014). Since then, they have vastly altered the ecosystem there (Anderson et al., 2009;Henn et al., 2016), caused the establishment of non-native plants (Martínez Pastur et al., 2006) and are beneficial for native but also non-native fish species (Malmierca et al., 2011). Beaver impacts include those on agriculture through flooding and erosion due to dam building and burrowing into flood banks, on forestry through tree damage, on fishery due to beaver dams being migration barriers and raising water temperatures, and on infrastructure due to dam failure (Butler and Malanson, 1994;Härkönen, 1999b;Collen and Gibson, 2000;Müller-Schwarze, 2011). ...
Chapter
a.Aim Provides an overview of selected taxonomic groups of invasive freshwater species: plants, bivalves, crayfish, fish and mammals. b.Main concepts and main methods covered For invasive species in each taxonomic group, major introduction pathways are outlined as well as main impacts and management actions. c.Conclusions Invasive freshwater species can be found in a variety of taxonomic groups, only a few of which can be covered here. Many freshwater invaders are still expanding their ranges, and have large effects on biodiversity and socio-economics. Their ecology and impacts need to be more thoroughly investigated in order to improve their management.
... Algunos ejemplos de esta acción sinérgica es la facilitación de invasión del castor norteamericano a varias especies de herbáceas invasoras (Martínez-Pastur et al., 2006;Wallem et al., 2007). Así mismo existe evidencia de que la modificación de hábitat del castor hace que este sea más utilizado por la rata almizclera, la cual a su vez, en ambientes alejados de la costa marina, pueden aportar hasta un 50% de la dieta de los visones (Crego et al., 2016). ...
Chapter
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Resumen. Este capítulo analiza la información disponible respecto de los principales motores de cambio global que operan en la Patagonia: cambio climático y su impac-to sobre la biodiversidad; la introducción de especies exóticas; cambio en el uso y cobertura del suelo, y algunos motores de cambio global emergentes tales como las floraciones algales nocivas (FAN) y el incremento en la conectividad de las poblaciones humanas asociado a la construcción y ampliación de la carretera austral, y el puente sobre el canal de Chacao en Chiloé. Ponemos énfasis en las complejidades asociadas al cambio global debido a las sinergias entre los distintos motores del cambio global en la Patagonia, tales como la introducción de especies exóticas; clima y aumento de probabilidad de incendios, y entre las FAN, clima, y aportes de nutrientes. Los modelos climáticos globales evaluados para la Patagonia chilena proyectan que al 2070, asumiendo un escenario de modificación en las concentraciones de gases de efecto invernadero moderado (RCP 4.5), la temperatura promedio aumentará entre 0,9°C hasta 1,4 °C. De manera similar, para la precipitación se proyecta una disminución que va entre 5,5 y 116 mm en promedio; la máxima disminución en precipitación corresponde a una reducción de 221 mm, con una moda de reducción de 21 mm. En algunas áreas se proyectan incrementos en la precipitación de hasta 77 mm. A pesar de que los ecosistemas patagónicos han sido resilientes y capaces de adaptarse a las modificaciones del clima holocénico, la evidencia sugiere grandes y abruptos cambios asociados a la colonización europea en el siglo XX que van de la mano con el aumento en la incidencia de incendios, pérdida de hábitat e invasión de especies exóticas. En particular, un incremento en las plantaciones de especies exóticas, junto a un clima más seco y cálido y un incremento en la abundancia de herbívoros exóticos que impacten la regeneración de las especies nativas, puede traer profundas consecuencias sobre el funcionamiento de los ecosistemas patagó- nicos. Este estudio concluye con una serie de recomendaciones que abordan tanto las brechas de conocimiento detectadas, como los impactos del cambio global en la zona. Entre estas están aquellas asociadas a la regulación de actividades productivas (turismo y acuicultura); la necesidad de llevar a cabo un diagnóstico periódico del estado de los ecosistemas patagónicos y los servicios que la naturaleza provee a las personas; mejorar nuestro conocimiento del funcionamiento de los ecosistemas y en particular del impacto del cambio climático y los efectos de la acción sinérgica de distintos motores de cambio global sobre su resiliencia.
Article
Beavers (Castor fiber, Castor canadensis) are one of the most influential mammalian ecosystem engineers, heavily modifying river corridor hydrology, geomorphology, nutrient cycling, and ecosystems. As an agent of disturbance, they achieve this first and foremost through dam construction, which impounds flow and increases the extent of open water, and from which all other landscape and ecosystem impacts follow. After a long period of local and regional eradication, beaver populations have been recovering and expanding throughout Europe and North America, as well as an introduced species in South America, prompting a need to comprehensively review the current state of knowledge on how beavers influence the structure and function of river corridors. Here, we synthesize the overall impacts on hydrology, geomorphology, biogeochemistry, and aquatic and terrestrial ecosystems. Our key findings are that a complex of beaver dams can increase surface and subsurface water storage, modify the reach scale partitioning of water budgets, allow site specific flood attenuation, alter low flow hydrology, increase evaporation, increase water and nutrient residence times, increase geomorphic heterogeneity, delay sediment transport, increase carbon, nutrient and sediment storage, expand the extent of anaerobic conditions and interfaces, increase the downstream export of dissolved organic carbon and ammonium, decrease the downstream export of nitrate, increase lotic to lentic habitat transitions and aquatic primary production, induce ‘reverse’ succession in riparian vegetation assemblages, and increase habitat complexity and biodiversity on reach scales. We then examine the key feedbacks and overlaps between these changes caused by beavers, where the decrease in longitudinal hydrologic connectivity create ponds and wetlands, transitions between lentic to lotic ecosystems, increase vertical hydraulic exchange gradients, and biogeochemical cycling per unit stream length, while increased lateral connectivity will determine the extent of open water area and wetland and littoral zone habitats, and induce changes in aquatic and terrestrial ecosystem assemblages. However, the extent of these impacts depends firstly on the hydro-geomorphic landscape context, which determines the extent of floodplain inundation, a key driver of subsequent changes to hydrologic, geomorphic, biogeochemical, and ecosystem dynamics. Secondly, it depends on the length of time beavers can sustain disturbance at a given site, which is constrained by top down (e.g. predation) and bottom up (e.g. competition) feedbacks, and ultimately determines the pathways of river corridor landscape and ecosystem succession following beaver abandonment. This outsized influence of beavers on river corridor processes and feedbacks is also fundamentally distinct from what occurs in their absence. Current river management and restoration practices are therefore open to re-examination in order to account for the impacts of beavers, both positive and negative, such that they can potentially accommodate and enhance the ecosystem engineering services they provide. It is hoped that our synthesis and holistic framework for evaluating beaver impacts can be used in this endeavor by river scientists and managers into the future as beaver populations continue to expand in both numbers and range.
Chapter
In the past few decades, natural scientists and natural resource managers have come to recognize that environmental problems have causes and consequences that are both ecological and social (Carpenter et al. 2009). For example, as an academic discipline, ecology increasingly acknowledges that ecosystems are not only composed of biotic and abiotic elements but also that humans form an integral part of what are ostensibly socio-ecological systems (Pickett and Ostfeld 1995; Anderson et al. 2015a). As a result, efforts are being made to expand the ways that issues, such as biological invasions, ecological restoration, or biodiversity conservation, are studied and managed to address them not only as ecological systems but also their social domain and human dimensions (Collins et al. 2011; Díaz et al. 2015; Pascual et al. 2017). Yet, significant work still remains to achieve this goal. For example, studies about invasive exotic species have a clear biological bias at the national level in Chile (Quiroz et al. 2009), at the regional scale in Patagonia (Anderson and Valenzuela 2014) and the Southern Cone (Ballari et al. 2016), across the Latin American continent (Pauchard et al. 2011) and even globally (Estévez et al. 2015; Vaz et al. 2017). Therefore, the relative dearth of interdisciplinary, applied, and social studies about biological invasions explain why they continue to increase and currently constitutes a barrier to addressing it as a socio-ecological problem.
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The geomorphology of a Fuegian Andes watershed colonized by the introduced species Castor canadensis has been analyzed. The main objective was to distinguish the physical conditions of the occupied streams. Geomorphological analysis and mapping techniques were applied; also, colonization monitoring surveys in the entire watershed was made through terrestrial and aerophotogrametrical methods. Were made aerial surveys during the autumn. Streams of first and second order and low gradient (0-6o) were those chosen by Castor canadensis for settlement. The higher-order streams were seasonally occupied, but restricted only to their secondary landforms. The northern part of the watershed is larger in size, drainage density and stream numbers; therefore, it offers more settlement possibilities. The rivers of the lateral hanging valleys were more densely occupied than those of the slope units. Several of the conditions of rivers for Castor canadensis settlement observed in Tierra del Fuego were similar to those observed in the Northern Hemisphere.
Article
Quantitative investigations of vegetational composition and structure in 15 beaver-impacted wetlands in Franklin and Hampshire Counties, Massachusetts, were conducted between 1980 and 1995. Four wetland types were identified based on species composition. Five distinctive physiographic zones, each with a characteristic assemblage of plant species, and three successional pathways were observed in the study sites. While soil moisture is clearly a strong influence on plant community structure and composition in beaver-impacted wetlands, hydrological dynamics including frequency, duration, and extent of flooding appear more influential than depth to water table.