Vegetation responses to past
volcanic disturbances at the Araucaria araucana forest-steppe
ecotone in northern Patagonia.
Running title
Volcanic disturbance in Araucaria
forest
Abstract
Aims
Volcanic eruptions play an important role in vegetation dynamics and its
historical range of variability. However, large events are infrequent
and eruptions with significant imprint in today vegetation occurred far
in the past, limiting our understanding of ecological process. Volcanoes
in southern Andes have been active during the last 10 ka, and support
unique ecosystems such as the Araucaria-Nothofagus forest as part
of the Valdivian Temperate Rainforest Hotspot. Araucaria is an
endangered species, strongly fragmented and well adapted to
disturbances. Yet it was suggested that volcanism might have increased
the fragmentation of its populations. To provide an insight into the
vegetation responses to past volcanic disturbances, a paleoecological
study was conducted to assess the role of volcanic disturbance on the
vegetation dynamics and if the current fragmentation has been caused by
volcanism.
Location
Araucaria forest-steppe ecotone in northern Patagonia.
Methods
Pollen and tephra analysis from a sedimentary record.
Results
During the last 9 kyr, 39 tephrafall buried the vegetation around Lake
Relem, more frequently between 4-2 ka. The vegetation was sensitive to
small tephrafall but seldom caused significant changes. However, the
large eruption of Sollipulli-Alpehue (~3 ka) might
change the environmental conditions affecting severely the forest and
grassland, as suggested by the pollen record. Ephedra dominated
early successional stage, perhaps facilitating Nothofagusregeneration recovering original condition after ~500
years. Slight increase of pollen percentage from Araucaria andNothofagus obliqua -type could be indicative of sparse biological
legacies distributed in the landscape. The analysis showed that
vegetation resisted without permanent changes, recovering relatively
fast after the large eruption.
Conclusion
The relative stability of Araucaria pollen in the study area
after several tephrafall suggests no change in its past geographical
distribution at the current forest-steppe ecotone, thus I found no
evidence that volcanic eruptions would have affected its current
conservation status.
Keywords : disturbance regime, vegetation resilience, long-term
vegetation dynamics, volcanic ecology, Valdivian Temperate Rainforest
hotspot
Introduction
Volcanic eruptions are important disturbance agents on Earth, and the
impacts can trigger sudden and large environmental changes, particularly
on the vegetation
(Crisafulli & Dale,
2018). Large volcanic eruptions are infrequent, episodic and stochastic
events in the history of an ecosystem
(Turner et al.,
1998). Volcanism creates complex patterns and dynamic processes on the
vegetation depending on the disturbance mechanism and the magnitude
(e.g., Crisafulli &
Dale, 2018; Foster, Knight, & Franklin, 1998). In example, the
vegetation growth rapidly after being buried by tephrafall (Zobel &
Antos, 2018), while the type and arrange of biological legacies in
disturbed areas drove regeneration patterns and the rate of vegetation
development after disturbance (del Moral & Grishin, 1999).
Understanding the lasting effects of process and patterns of vegetation
responses like the resistance, recovery or the develop of diversity
patterns may help in designing nature management
(Dale et al., 2005;
Franklin et al., 2002) especially in highly diverse biogeographical
areas influenced by volcanism. However, the study of the vegetation
dynamic is challenging since active volcanoes are located in remote
areas, the long-term monitoring requires many resources, the responses
last too long to be monitored in human-life span, or had occurred far in
the past (Swanson &
Crisafulli, 2018). Well-designed paleoecological studies, through the
study of tephra layers and pollen sub-fossil in sedimentary records,
could help to understand past volcanic disturbances and vegetation
responses.
Paleoecological studies have attempted to illustrate past impact of
eruption and aimed to contribute to disturbance ecology. For example,
for large-eruptions of Icelandic volcanoes that could affect past
climate conditions, palynological studies showed contradictory or weak
evidences that volcanoes disturbed the vegetation in western Europe
(Paine et al., 2013). Also in the Mt. Mazana, western North America, an
experiment was conducted in a small lake to infer the impacts on local
and regional vegetation (Egan et al., 2016). The authors concluded that
eruptions did not trigger significant changes in terrestrial pollen taxa
in a distant area, but the aquatic taxa changed due to enrichment of
nutrients. Unlike those areas, after Taupo eruption
(~1850 yrs BP), New Zealand, pollen evidences indicate
that this eruption destroyed the forest nearby, and up to 170 km east of
the vent the vegetation suffered a variable degree of disturbance
(Wilmshurst & McGlone, 1996). In many areas the forest could not
recover to its original conditions, and sites far from the crater
covered by thin tephra layer were strongly impacted. Despite the
expected role of volcanoes in several global biodiversity hotspots
little is known about the mechanism that volcanic eruption drove past
vegetation dynamics and their influence in the diversity patterns needs
to be further explored.
Active volcanoes during the Holocene are dispersed in the Earth, but one
of the most active areas is located in the south-eastern part of the
Pacific (Stern, 2004). The subduction of the Nazca-plate underneath the
south American-plate has triggered several large-eruptions in the last
10 kyr in the Andes region (Fontijn et al., 2014). The southernmost
section of the Andes supports unique forest ecosystems, featured by a
high endemism, which also have been influenced by these volcanoes
affecting structure and functions in this Andean forest and surrounding
vegetation (e.g., Veblen et al., 2016). As result of the eruptions, the
tree-line location can be depressed
(Daniels & Veblen,
2004; Veblen et al., 1977), or sustain uneven age forest in the
landscape
(Kitzberger, 2012).
Moreover, volcanism would be responsible to keep pioneer species such asNothofagus species and Araucaria araucana as dominant
(Veblen and Ashton 1978; Burns, 1991). The evidences show after the
eruption of the Puyehue-Cordon del Caulle Volcanic Complex (PCC) in 2011Nothofagus pumilio was the principal species resprouting in zones
buried by ~50 cm tephra, and several cohorts would
correspond to past eruptions (Montiel et al., 2016). Eastward from PCC
in 2011, the steppe vegetation was buried by <5 cm tephra and
rhizomatous geophytes species such as Poa spp. and Rumexacetosella increased while therophytes disappear
(Ghermandi et al.,
2015). Another case is the 2008-eruption of Chaitén volcano. Swanson et
al. (2013) described the early responses of the different types of
disturbance as similar to those happened after Mount St. Helens, where
each disturbance type impacted specific areas that created different
patches in the landscape. Furthermore, Moreno-Gonzalez et al. (2019)
pointed out that in the direct blast-zone of the disturbance gradient
the early vegetation establishment is associated with elevation
gradient, and that the regeneration depend on life-traits strategy and
the types of biological legacies remaining in the area.
Despite the volcanic characteristics and the unique vegetation, little
is known about the role of volcanic disturbance on the vegetation
dynamics in northern Patagonia, the rate of recovery or the capability
to resist disturbance, particularly in the Araucaria araucanaforest. It was hypothesized that past volcanic eruptions might
influenced the genetic variability of Araucaria , but also
affected negatively the population distribution and regeneration
dynamics (e.g., Bekessy et al., 2002; Veblen et al., 2016). Yet the
hypothesis is contradictory with the recognized morphological adaptation
to resist volcanic disturbance (Burns, 1991) that might helped to
persist in an angiosperm dominated ecosystems (Kershaw & Wagstaff,
2001). Despite Araucaria’ s socio-ecological importance and its
current conservation status, few long-term vegetation reconstructions
have been conducted so far in the Araucaria forest-steppe
ecotone. To contribute to the knowledge of vegetation dynamics and the
volcanic disturbance regime, this work aims 1) to reconstruct the
volcanic disturbance history in the Araucaria forest-steppe
ecotone, 2) to assess the vegetation responses to past volcanic
disturbance and 3) to evaluate if Araucaria distribution was
influenced by volcanism at the current forest-steppe ecotone affecting
its conservation status. To this end, I made use of a published pollen
record (Moreno-Gonzalez et al., 2021) and re-analysed to have a separate
insight into the volcanic influence on the Araucaria forest
dynamics. Due to the distance to the volcanic source of the
forest-steppe ecotone eastward of the Andes, it is affected mostly by
tephrafall that buried the vegetation. Tephrafall therefore is the main
volcanic disturbance type around the study area. Following this
assumption, I predict that the magnitude of vegetation responses should
be related with the tephra thickness where vegetation would be more
resistant and/or recover faster to thin tephrafall than to thick
tephrafall. Then, forest fragmentation should increase after thick
tephrafall.
Methods
The study area is located in the current Araucaria forest-steppe
ecotone around 39° S, in northern Patagonia (Figure 1). In particular,
vegetation history was reconstructed from a sediment core obtained from
Lake Relem (38°58’39” S; 71°4’51” W; 1268 m a.s.l.), Lake Relem is a
shallow and small lake (~2.5 m depth, ~1
ha, respectively), without river inflow or outflow. The climate in the
region is temperate with oceanic influence. The Pacific air masses
arriving with westerly winds create a sharp rain-shadow effect
discharging most of the precipitation on the western section of the
Andes (Mundo et
al., 2013). Climatic conditions when interacting with local topography
and natural disturbances generate complex vegetation patterns and plants
associations (Kitzberger, 2012; Roig, 1998). Araucariaforest occurs mainly above 1000 m elevation, normally reaching the
tree-line at 1700 m a.s.l.
(Gonzalez et al.,
2006) showing a strong fragmentation (Figure 1). Further details on
climate and vegetation description, as well as a full description of the
sediment analysis, pollen analysis and the chronology can be found in
Moreno-Gonzalez (2020) and Moreno-Gonzalez, Giesecke, and Fontana
(2021).
Volcanic setting and eruptive
history
The study area lies in the Southern Volcanic Zone of the Andes (SVZ).
The SVZ is a result of the subduction of the Nazca plate beneath the
continental South American plate, and is extended between 33° S to 46° S
(Gilbert et al.,
2014). In a radius of 100 km around Lake Relem there are 7 volcanoes
that have been active during the Holocene (Figure 1). Fontijn et al.
(2014) list all eruptions recorded so far to the SVZ, which are
summarized in Table 1. The Lonquimay volcano has had more regular
eruptions of considerable magnitude (VEI ≥3) while other volcanoes have
erupted every 10 years (e.g., Villarrica volcano) or have erupted at
least once during the last 10 k years (Fontijn et al., 2014). The
Sollipulli-Alpehue eruption (hereafter the So-A eruption) was one of the
largest in the recent past in northern Patagonia, it is dated back to
2951 cal yr BP
(Naranjo et al.,
1993). In relation to the So-A eruption, Lake Relem is located within
the isopach of 2 m (Fontijn et al., 2016).
Data analysis
Here we used only pollen data from terrestrial taxa per sample and
expressed as were summed to obtain the relative abundance, expressed as
pollen percentage. Later, we conducted a stratigraphically constrained
cluster analysis (CONISS) for the pollen samples based in Euclidean
distances (Bennett 1996). To assess pollen diversity changes, individual
rarefaction analysis is a powerful tool
(Birks & Line,
1992). Using rarefaction analysis, we estimated the palynological
richness at the minimum terrestrial pollen count of 400 pollen grains
(E(T400)), the pollen diversity estimated at
E(T10), and pollen evenness was calculated as the ratio
E(T10)/ E(T400)
(Matthias et al.,
2015). Compositional trend of the terrestrial taxa was explored through
principal component analysis (PCA). The percentage was square root
transformed and centralized. Furthermore, we fitted a Principal Curve
(PC) to the compositional data. The starting point was based on the age
of the samples, and the curve was fitted through a smooth-spline method
with complexity of 5. To estimate the Rate-of-Change (RoC), we
interpolated the pollen samples at a regular time interval of 50 years
with smooth-spline, and then with the Euclidean distances as a
dissimilarity coefficient
(Bennett &
Humphry, 1995).
To reconstruct the volcanic disturbance regime, we made use of every
tephra layer >0.5 cm thick. Here we considered each of
these tephra layers as a single and independent disturbance event -since
tephrafall is limited to a short time- and the tephra thickness is
considered as a measure of the magnitude of the impact. Furthermore,
each tephra was coded as a quantitative explanatory variable and
modelled as a simple exponential decay process (Lotter and Birks 1993).
This model is a simple but robust equation
(x-αt ), where x is the value for the ash
(arbitrarily set to 100 by the authors), α is the decay coefficient
equal to -0.5, and t is the sample depth (=time). Also, the
authors arbitrarily assigned a value before tephra deposition of 0. In
this manuscript, we preferred to describe the magnitude of the event of
each tephra by giving the ash-value corresponding to the tephra
thickness in centimetres. In doing so, we aim to describe the magnitude
of the eruptions. Unlike the record of Lotter and Birks (1993), in
sedimentary records from Patagonian the occurrence of multiple tephra is
quiet normal, therefore the value below a tephra layer corresponded to 0
only in some cases. The frequency of volcanic events was calculated as
the sum of events over 1000 years, and then was modelled with
smooth-spline method (spar=0.7). Both variables, frequency and
magnitude, were later used as an explanatory variable to constrain the
pollen samples in a multivariate analysis (Redundant Analysis) assessing
vegetation responses to volcanic disturbance regime. All statistical
analyses were conducted through RStudio 3.3.1
(RStudio Team,
2016), vegan-package 2.4-2
(Oksanen et al.,
2017) and Rioja 0.9-15
(Juggins, 2015).
Results
Reconstruction of the volcanic
regime
The volcanic history and its disturbance regime were reconstructed from
the tephra layer content in the sediment of Lake Relem (Figure 2). In
total, the sediment core registers 39 tephra layers well-defined
>0.5 cm thick. Most of them are between 0.5-7.5 cm thick
(Figure 2a) but only 18 are >1.5 cm thick, what could be
considered as significant peaks (crosses in Figure 2a). The tephra layer
corresponding to the So-A eruption is the biggest one, which in the core
recorded 216 cm. The tephra-value, modelled as an exponential decay,
shows similar patterns regards the tephra thickness, but indicates a
lowest effect regarding to the eruption magnitude while missing some
peaks above the threshold (Figure 2b). Considering all the tephra
layers, volcanic disturbance regime is relatively frequent (Figure 2c).
The disturbance frequency indicates that around 7 eruptions had occurred
every 1000 years between 4-1.8 ka. Before 4 ka volcanic frequency was
lower than 6 events/1000 years, with periods without disturbance around
7 and 5 ka. For the last 1.5 ka up to the present, few tephrafall are
recorded, but none deposited more than 1.5 cm of tephra into the lake.
Vegetation responses to volcanic
events
A full description of the vegetation history was published in
Moreno-Gonzalez (2020). It encompasses the last 9 kyr and it is
characterized by an overall vegetation transition from steppe grassland
(Poaceae) to forest (Nothofagus dombeyi -type) that occurred
~4.5ka (Appendix 1), likely as result of a change in
precipitation regime rather than fire (Moreno-Gonzalez et al., 2021).
Likewise, in the unconstrained ordination diagram (Figure 3a) the
compositional trends represent the gradual vegetation change. The first
component explains 78% of the total variance, and would be interpreted
as the long-term shift from steppe taxa dominance (e.g., Poaceae) to a
forest with the dominance of N. dombeyi -type. Poaceae,Mulinum and Cyperaceae, among others, are more abundant in the
zone 1, located on the left side of the ordination diagram. The second
component explains 11% of the variance and split the overall trend
where Ephedra is dominant. The abundance of Ephedra in the
pollen record rose after the So-A eruption (Figure 3, Appendix 1),
therefore the second axis is mostly related to the response of So-A
(Figure 3a). The volcanic eruptions had a significant influence on
vegetation composition (Figure 3b) explaining 20% of the data. The
explanatory variables, volcanic frequency and magnitude, had a
significant influence on the vegetation too. However, further analysis
excluding samples after the So-A eruption indicate that the other small
tephra-layers had no significant influence in the pollen composition. In
the multivariate analysis, Araucaria shows a weak negative
relation with volcanic frequency, while somewhat related with volcanic
magnitude suggesting that Araucaria increased its abundance
following the eruptions.
The figure 4 shows several indicators of the vegetation with the So-A
eruption (vertical red line) and other unknown tephra <10 cm
thick (vertical grey lines). Tephrafall that marked a disturbance event,
normally caused a decrease (increase) in Poaceae (N.
dombeyi -type) pollen percentage (Figure 4a). The decrease is not
proportional to tephra thickness, but it is more notorious before 4.5
ka, when vegetation was likely dominated by Poaceae and other grasses.
Only So-A eruption depressed the abundance of both dominant taxa, while
the few events after So-A do not seem to have affected any of both
pollen taxa. Along the record, PAR of all taxa together is variable
(Figure 4b), where PAR could be interpreted as an indicator of
vegetation biomass. Volcanic events normally caused small decrease in
PAR between 100-500 grains cm-2yr-1, while after So-A eruption PAR decayed more than
2000 grains cm-2 yr-1. Other drops
in PAR can be related to sedimentary processes or other disturbances.
The Principal Curve fitted after 21 iterations. The variation in the
distance gradient units indicates some sensitivity of the vegetation
composition to volcanic disturbances, notwithstanding only few
significant changes (Figure 4c). The most relevant changes occurred at
the time of So-A, but the curve also suggests that pollen composition
was relatively stable along the time. On the contrary, peaks depicted by
the rate of change were not sensitive to volcanic events, except for
So-A (Figure 4d). The result indicates that the small changes after
small tephra deposition did not have long lasting effects on the
vegetation and could recover rapidly after the events. Changes in
palynological richness are variable and not always related to volcanic
disturbance (Figure 4e). In responses to volcanic disturbance,
palynological richness can increase or decrease. In particular, before
the So-A eruption, palynological richness reached the smaller values
coinciding with the expansion of N. dombeyi -type in the area; but
interestingly after So-A eruption palynological richness increased.
Perhaps vegetation was not completely destroyed, with some biological
legacies and survival individual remained in sheltered areas. As the
dominant taxa was depressed, palynological evenness increased (Figure
4e), a pattern that normally occurred after other small volcanic
eruptions in this study.
Discussion
After volcanic disturbance episodes, the pollen richness and evenness,
as well as PAR, do not indicate a unique pattern of decreasing
biodiversity and biomass. The traditional successional model suggests an
overall decrease in plant biomass and diversity caused by volcanism that
initiate a new ecological succession, likely dominated by pioneer and
shadow-intolerant species. Contrary to this expected model, the findings
presented here showed a more complex dynamic in the Araucariaforest-steppe ecotone, what may be controlled by many interacting
factors. Recent advances in volcanic ecology (Crisafulli et al., 2015)
point that the vegetation resistance and the rate of recovery is a
result of factors such as climatic conditions, type of the impact and
topography (del Moral and Grishin, 1999), plants traits (Antos & Zobel,
1986), and biological legacies (e.g., Dale et al., 2005). Moreover,
those operating factors can vary in time and spatial extension from one
eruptive event to another. In the following paragraph, I attempt to
contextualize those patterns occurring around Lake Relem.
Ephedra was the unexpected dominant taxa during the early
successional stage after the So-A eruption since Nothofagusspecies and Araucaria are considered pioneer tree in the region
(Figure 5). Little is known about its ecology and its palaeoecological
significance in Patagonia. Ephedra might have a nursery effect onNothofagus and Austrocedrus after fire disturbance (e.g.,
Raffaele & Veblen, 1998). In that study, Ephedra is not the most
abundant species after disturbances and it has been studied around
~200 km southward from Lake Relem. In modern
pollen-samples it averages 2.7%, with maxima of 32.4%, but close to
Lake Relem is founded up to 20% (Paez et al., 2001). Ephedra’ s
pollen grains could disperse several kilometres far from the source area
(Maher, 1964). The pollen abundance of Ephedra before So-A
eruption was always low, but such increase in pollen percentage and PAR
should only be indicative of a very local presence –perhaps suppressed
or co-existing with other species- and the following increase as result
of the So-A eruption that facilitate its expansion under certain
conditions. For instance, Mallín Paso del Arco (Heusser et al., 1988), a
record within the 2 m isopach zone of the So-A, depict a slight increase
on Ephedra around 3 ka, above a thick tephra layer at the core
bottom. Unlike, other sites do not show an increase in Ephedra(Dickson et al., 2020; Fontana & Giesecke, 2017) and, to my knowledge,
none of the palynological records in northern Patagonia show a
significant increase of Ephedra after an eruption nor have been
reported during recent eruptions.
Climatic condition may play a role in the environmental responses by
determining the vegetation composition and structure, which in turn
influence potential colonizer species and the rate of revegetation after
disturbance. For example, before 4.5 ka when precipitation was low
(Jenny et al., 2003; Lamy et al., 2001) the vegetation was dominated by
grasses (Figure 4). Small tephrafall left some impact to the vegetation
dominated by grasses (Figure 4b), but recovering fast enough to not
register a peak in the rate-of-change analysis (Figure 4c). After So-A,
revegetation started short-after with Ephedra whilst forest
developed later. This eruption evidenced strong imprint in vegetation
composition, but due to relatively stable climatic conditions in a
millennial-scale it returned close to its original state after
approximately 500 years (Figures 4 and 5). In comparison, after Taupo
eruption in New Zealand, the vegetation returned to its original
condition after about 120-250 years (Wilmshurst & McGlone, 1996).
Later, when precipitation augmented in Patagonia, it promoted the
development of Nothofagus species in the area, the vegetation was
less sensitive to small tephra deposition or did not disturb
significantly.
Plant traits play also an important role to resist tephra deposition and
may determine the species performance. For example, Antos & Zobel
(1986) found that herbs could not resist being buried by
~15 cm, while the early establishment was dominated by
the shadow tolerant trees and shrubs under the undisturbed canopy.
Regarding to the So-A eruption, the early establishment ofEphedra was likely due to special adaptation to poor soil,
sprouting shoots, and the capacity to resist arid and cold weather
conditions (Luebert & Pliscoff, 2006). Roots growing seem to be a
common mechanism of plants in response to buried disturbance by
tephrafall. After the Taupo eruption (~1850 BP), the
spore of Pteridium, a fern species with rhizome root system, were
found increasing in several records in a wide area buried by tephrafall,
thus suggesting the species spread after resisting the impacts of the
eruption (Wilmshurst & McGlone 1996). The same occurred with the fernLophosoria quadripinnata after the 2008-eruption of Chaitén
volcano. The species, also with rhizome root system, that normally grows
under the forest canopy in the low elevation of the temperate forest.
Close to the blast-zone Lophosoria was one of the first species
regrowing one year after the eruption in low and mid-elevation zones
(Moreno-Gonzalez et al., 2019). Remarkably, after repeated past
eruptions in the areas close to Chaitén Volcano, Lophosoriaresponded positively short after the eruptions (Henríquez et al., 2015).
Furthermore, the following expansion of Ephedra in the area
(Figure 5, Phase II) can be related to its fleshy-fruit and dispersal
mechanism, principally through birds and rodents (Loera et al., 2015).
Probably Ephedra was the main food supply for birds and rodents
in a wide area devastated after So-A eruption. A similar mechanism of
plant dispersal was suggested after Taupo eruption, where the pollen of
some fleshy-fruits taxa were found (Wilmshurst and McGlone 1996); and in
the last eruption of the Chaitén volcano, the species with fleshy-fruits
were also found playing an important role in the early vegetation
establishment (Moreno-Gonzalez et al., 2019).
Trees are expected to be less affected by small tephrafall, particularlyAraucaria and Nothofagus species that are considered
pioneer species after volcanic disturbances (Veblen, 1982). After the
recent eruption of Puyehue, Nothofagus pumilio species were found
regrowing in areas buried by tephra (Montiel et al., 2017). However, in
the blast-zone of the last eruption from Chaitén Volcano noneNothofagus seedling were found one year after the eruption
(Moreno-Gonzalez et al., 2019). In palynological records the abundance
of N. dombeyi -type is always variable, and few studies have been
assessed directly the responses to volcanic events. Álvarez-Barra et al.
(2020) demonstrated that well developed forest was not affected in the
long-term by tephrafall smaller than 20 cm. However, after the
large-eruption of So-A, the pollen abundance of N. dombeyi- type
was strongly affected (this study and Dickson et al. 2020). Indeed, the
data presented here shows that Nothofagus species did not behave as
pioneer and expanded into the area after 500 years in advanced
successional stages following the decay of Ephedra (Figure 5,
phase III).
Multivariate analyses indicated that Araucaria was not affected
by any of the small tephrafall. It is largely documented that the thick
bark, flexible branches and smooth leafs allow the species to resist
moderate disturbance (Burns, 1991; Veblen et al., 1995). Hence, it is
possible that small tephrafall deposited in the trees crown did not
cause significant physical damage to the whole population. If chemical
change to soil occurred after tephra deposition, physiological harms
such as decreasing tree-ring growth might not last for a long time
(Tognetti et al., 2012), therefore unlikely to found significant change
in pollen abundance from sedimentary records. Since Araucaria did
not indicate significant change in pollen percentage and PAR after the
So-A eruption (the largest eruption in the area for the last 9 ka), it
is unlikely that volcanism affected negatively the distribution of the
populations increasing the fragmentation distant from volcanic source as
it suggested by Bekessy et al. (2002). A recent pollen record from Lake
Cilantro (Dickson et al., 2020) shows similar patterns as those observed
in Lake Relem, neither changes in pollen relative abundance after small
tephra layers nor changes in PAR after So-A eruption. Although not a
generalised pattern, Dickson et al. (2020), Moreno-Gonzalez et al.
(2021) and Nanavati et al., (2020) showed that pollen abundance in areas
nearby the current ecotone have been slightly increasing for the last
~4 ka despite of the influence of several volcanic
eruptions. Further palaeoecological studies need to be done to
understand the role of past volcanic eruptions in determining current
diversity patterns in Araucaria forest and that can be used in the
conservation of certain threatened species in the Valdivian Temperate
Rainforest hotspot.
Data accessibility
statement
Moreno-Gonzalez, Ricardo; Giesecke, Thomas; Fontana, Sonia L (2020):
Pollen and macro-charcoal analysis of Lake Relem sediment core, northern
Patagonia. PANGAEA, https://doi.org/10.1594/PANGAEA.923741
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List of figures and tables
Figure 1 a) Figure showing active volcanoes (red triangles) during the
Holocene in southern South America (Global Volcanism Program, 2013). b)
Map of the study area representing the position of Lake Relem, the
location of volcanoes closes to Lake Relem, and the distribution ofAraucaria araucana (red polygons). Base map source, Google (2020)
Figure 2. Volcanic eruption regime. a) Indicates the thick of tephra
layers deposited into the Lake Relem. Grey horizontal line shows
arbitrary threshold of >1.5 cm to determine relevant
eruptions (red crosses). b) Tephra thickness modelled as the exponential
decay after eruption. c) Eruption frequency every 1000 years. Grey
points indicate the sum of frequency every 1000 years. Trend of
frequency was obtained by smooth spline function (blue line) with spar
=0.7.
Figure 3. Ordination diagrams of pollen composition for Lake Relem, a)
unconstrained and b) constrained.
Figure 4. Vegetation responses to volcanic eruptions registered in the
sediment record from Lake Relem. a) All pollen taxa
Pollen-Accumulation-Rate used as a proxy for vegetation biomass changes.
b) Principal Curve and c) the rate of change showing significant
compositional changes along the time. d) Palynological richness
E(T400)) and e) palynological evenness
E(T10)/E(T400) comparing diversity
changes after volcanic disturbances. Red vertical line indicates the
Sollipulli-Alpehue eruption; grey vertical lines indicate other tephra
deposited >1.5cm thick into the lake.
Figure 5. Successional patterns of the main taxa caused by the
Sollipulli-Alpehue eruption. in pollen relative abundance (dotted curve)
and Pollen Accumulation Rate (continuous curve) of selected taxa. Small
boxes at the top indicate successional phases. Note that periods of time
are not exact and are used only with schematic purpose. where
pre-eruption is represented in green. Red area indicates the
“collapse” of the vegetation
Figure 1 a) Figure showing active volcanoes (red triangles) during the
Holocene in southern South America (Global Volcanism Program, 2013). b)
Map of the study area representing the position of Lake Relem, the
location of volcanoes closes to Lake Relem, and the distribution ofAraucaria araucana (red polygons). Base map source, Google (2020)