Fire, climate and vegetation linkages in the Bolivian Chiquitano Seasonally Dry Tropical Forest

Abstract

South American seasonally dry tropical forests (SDTFs) are critically endangered, with only a small proportion of their original distribution remaining. This paper presents a 12 000 year reconstruction of climate change, fire and vegetation dynamics in the Bolivian Chiquitano SDTF, based upon pollen and charcoal analysis, to examine the resilience of this ecosystem to drought and fire. Our analysis demonstrates a complex relationship between climate, fire and floristic composition over multi-millennial time scales, and reveals that moisture variability is the dominant control upon community turnover in this ecosystem. Maximum drought during the Early Holocene, consistent with regional drought reconstructions, correlates with a period of significant fire activity between 8000 and 7000 cal yr BP which resulted in a decrease in SDTF diversity. As fire activity declined but severe regional droughts persisted through the Middle Holocene, SDTFs, including Anadenanthera and Astronium , became firmly established in the Bolivian lowlands. The trend of decreasing fire activity during the last two millennia promotes the idea among forest ecologists that SDTFs are threatened by fire. Our analysis shows that the Chiquitano seasonally dry biome has been more resilient to Holocene changes in climate and fire regime than previously assumed, but raises questions over whether this resilience will continue in the future under increased temperatures and drought coupled with a higher frequency anthropogenic fire regime.
This article is part of the themed issue ‘The interaction of fire and mankind’.

Full text

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Research
Cite this article: Power MJ, Whitney BS,
Mayle FE, Neves DM, de Boer EJ, Maclean KS.
2016 Fire, climate and vegetation linkages
in the Bolivian Chiquitano seasonally
dry tropical forest. Phil. Trans. R. Soc. B 371:
20150165.
http://dx.doi.org/10.1098/rstb.2015.0165
Accepted: 17 March 2016
One contribution of 24 to a discussion meeting
issue ‘The interaction of fire and mankind’.
Subject Areas:
ecology
Keywords:
fire, Chiquitano, seasonally dry tropical forest,
climate, drought, Holocene
Author for correspondence:
M. J. Power
e-mail: mitchell.power@geog.utah.edu
Fire, climate and vegetation linkages
in the Bolivian Chiquitano seasonally
dry tropical forest
M. J. Power1, B. S. Whitney2, F. E. Mayle3, D. M. Neves4, E. J. de Boer5
and K. S. Maclean6
1
Natural History Museum of Utah, Department of Geography, University of Utah, UT, USA
2
Department of Geography, Northumbria University Newcastle, Newcastle-Upon-Tyne, UK
3
Centre for Past Climate Change, Department of Geography and Environmental Science,
University of Reading, Reading, UK
4
Royal Botanic Gardens, Kew, Richmond, UK
5
Faculty of Geosciences, Department of Geography, Utrecht University, NL, USA
6
2C Forthbridge Road, London, UK
South American seasonally dry tropical forests (SDTFs) are critically endan-
gered, with only a small proportion of their original distribution remaining.
This paper presents a 12 000 year reconstruction of climate change, fire and
vegetation dynamics in the Bolivian Chiquitano SDTF, based upon pollen
and charcoal analysis, to examine the resilience of this ecosystem to drought
and fire. Our analysis demonstrates a complex relationship between climate,
fire and floristic composition over multi-millennial time scales, and reveals
that moisture variability is the dominant control upon community turnover
in this ecosystem. Maximum drought during the Early Holocene, consistent
with regional drought reconstructions, correlates with a period of significant
fire activity between 8000 and 7000 cal yr BP which resulted in a decrease
in SDTF diversity. As fire activity declined but severe regional droughts
persisted through the Middle Holocene, SDTFs, including Anadenanthera
and Astronium, became firmly established in the Bolivian lowlands. The
trend of decreasing fire activity during the last two millennia promotes the
idea among forest ecologists that SDTFs are threatened by fire. Our analysis
shows that the Chiquitano seasonally dry biome has been more resilient to
Holocene changes in climate and fire regime than previously assumed, but
raises questions over whether this resilience will continue in the future
under increased temperatures and drought coupled with a higher frequency
anthropogenic fire regime.
This article is part of the themed issue ‘The interaction of fire and mankind’.
1. Introduction
Seasonally dry tropical forests (SDTFs) comprise over 40% of the world’s tropical
forested ecosystems, but they are afforded little protection, and have been exten-
sively deforested over the past several centuries. Of the remaining SDTFs, 16%
occurs in South and Southeast Asia and 50% in Latin America [1]. SDTFs are lim-
ited to regions of the tropics that experience seasonal drought, occurring where
annual rainfall is less than 1600 mm with a five- to six-month dry season [2].
SDTFs require fertile soils, hence they have been heavily cleared for agriculture
over the past several centuries. Occurring under a similar climate, cerrado
(upland) savannas differ from SDTFs in that they are structurally open, containing
abundant fire-tolerant woody species and a xerophilic herbaceous ground cover.
Generally, cerrado (sensu stricto) savannas occur on less fertile soils compared
with SDTFs, but the boundary dynamics between SDTFs and cerrado savannas
are notoriously complex and have inspired many ecological studies todisentangle
the key drivers of forest– savanna ecotones (e.g. [3,4]).
Fire, whether natural or anthropogenic in origin, has long been recognized as a
key control upon fine-scale SDTF–savanna boundary dynamics over short-term
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ecological time frames, but the relationship between fire and
SDTF– savanna dynamics under high-amplitude climate
change predicted over the coming century is poorly understood
and a key concern for the conservation community, given the
endangered status of both these ecosystems [5]. The lack of
obvious plant–fire adaptations in a number of SDTF species
has led many ecologists to infer that this is not a fire-adapted
biome, and that it is therefore highly vulnerable to burning
[6], a cause for concern in the future, as climate change is
predicted to lead to increasing temperatures and drought [7],
which would no doubt lead to increased risk of fire.
(a) Fire and the Bolivian Chiquitano seasonally dry
tropical forest
Our study area is the Bolivian Chiquitano SDTF—the largest
intact, old-growth block of SDTF in South America [1]. This
semi-deciduous to deciduous forest covers most of the calcar-
eous rolling hills of eastern Bolivia (Chiquitanı
´a), interspersed
with patches of cerrado savanna where soils are either too nutri-
ent-poor or too thin to support forest [8]. These upland SDTF
and cerrado savannas form an abrupt boundary at the Boli-
via– Brazil border with the seasonally flooded savanna
wetlands of the Pantanal basin, most of which lies in Brazil,
to the east (figure 1). Fires occurring in the SDTF-cerrado
savanna mosaics of Chiquitanı
´a today are primarily caused
by humans clearing land for agricultural purposes. The season-
ally wet and dry climate, with a June– October dry season,
coupled with edaphic and hydrologic controls on vegetation
composition and structure, support large wildfires during the
driest years, when there is abundant dry biomass [9].
However, a serious cause for concern among tropical
ecologists and conservation biologists is that anthropogenic
fires are becoming increasingly common, not only burning
in savannas, but spreading into SDTF [10]. This concern is
predicated on the assumption by most ecologists that,
unlike savanna, SDTF is not a fire-adapted ecosystem due
to the lack of fire adaptation/tolerant features of many con-
stituent arboreal species, and, in particular, the prevalence
of fire-sensitive columnar cacti (Cereus spp.) in this ecosystem
[6]. However, the fact that some arboreal taxa (e.g. Astronium
(Myracrodruon) urundeuva and Aspidosperma quirandy)have
fire-resistant bark, raises the possibility of some degree of fire
resilience within this ecosystem. The likelihood of local con-
trolled fires escaping to become uncontrolled SDTF wildfires
will only increase in the coming decades as forest flammability
increases under a future warmer and drier climate [7], poten-
tially posing a serious threat to the long-term viability of the
SDTF ecosystem.
(b) Tropical climate and fire linkages
The climate of the study area—the Chiquitanı
´a region of east-
ern Bolivia and the adjacent Brazilian Pantanal—experiences
a highly seasonal climate whereby most of the annual precipi-
tation falls during the austral summer (December– February;
figure 1). This precipitation comes from the South American
summer monsoon (SASM) and is delivered to the study
area via the South American low-level jet (SALLJ) [11]. The
study area has a six-month dry season and average annual
precipitation between 1700 and 1000 mm, which decreases
from north to south [12]. Mean annual temperature is
approximately 258C, but large changes in seasonal
temperature have been recorded in the twentieth century
with highs greater than 408C during the austral summer
and low temperatures below 108C during the austral winter
[10].
Modern climate–fire linkages in eastern Bolivia are influ-
enced by both tropical Atlantic and Pacific sea surface
temperatures, and moisture transport. Trade winds from the
Atlantic contribute to evapotranspiration as air masses move
across the Amazon basin [13]. The development of strong
convection (subsidence) over Amazonia provides a link to
the tropical Pacific via the east– west Walker circulation
[14,15], which results in large-scale redistribution of seasonal
moisture flux. The influence of climate variability on the
Chiquitano SDTF is probably linked to changes in tropical
Atlantic sea surface temperatures that may have a strong influ-
ence on moisture variability and fire regimes in this area [16].
Considering a more Atlantic origin for drought and fire
occurrence in Chiquitanı
´a– Pantanal may help explain changes
in fire activity. Periods in the past decade when above-average
convective precipitation occurs over a warm tropical North
Atlantic Ocean result in subsidence to the south, over the
Amazon and South Atlantic. The direct influence is a displace-
ment of Hadley circulation and northward migration of the
inter-tropical convergence zone (ITCZ), resulting in reduced
precipitation across the equatorial Atlantic and, subsequently,
a reduction in moisture in western and southern Amazonia
[17], causing an intensified fire season [16]. For example,
during the extreme drought of 2010, Chiquitanı
´a– Pantanal
experienced anomalously low precipitation and extreme fire
conditions that have been linked to reduced advection of Atlan-
tic moisture into the Amazon basin and the consequent failure
of the SALLJ to deliver moisture to the study area [16]
(figure 1). These recent episodes of severe drought and fire
could potentially cause rapid alterations in the composition
and structure of vegetation communities within the SDTF.
(c) Aims and approach
Here, we use a palaeoecological approach, based upon the
analysis of pollen and charcoal from radiocarbon-dated lake
sediments, to determine the long-term relationship between
fire, climate and the Chiquitano SDTF over the Holocene
period (the last 12 000 years). This multi-millennial time series
spans intervals of time when the climate was significantly
drier than present (centred on the Middle Holocene—
6000 cal yr BP), thus serving as a potential analogue (albeit
imperfect) of vegetation– fire– climate linkages under future
increased drought predicted by most global climate models.
This palaeoecological approach therefore provides the necess-
ary long-term perspective to assess the ecological significance
of SDTF responses to drought and fires observed by ecologists
over recent decades.
The key aim of our study is to assess the response of the
Chiquitano STDF to long-term fire activity and increased
drought, and thereby test the validity of the widely held assump-
tion that the SDTF is inherently susceptible to fire. If this
hypothesis is correct, one would predict that the Bolivian
Chiquitano STDF has a long-term history of low fire activity.
2. Study site and methods
The study site, Laguna La Gaiba (LLG) (217.788,257.728), is
a large (.55 km
2
), shallow (4– 5 m deep) ‘overflow’ lake
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located along the course of the Paraguay River (figure 1) on
the border between Bolivia and Brazil. It lies along a fault
line that defines the boundary between the upland Chiqui-
tano SDTF to the west and the seasonally flooded Pantanal
wetlands to the east, which includes both savanna and
treeless grasslands [18]. Seasonal rainfall over the Pantanal
basin and its river catchment causes widespread floods that
drain into the Paraguay River and its associated lakes. LLG
is therefore closely linked hydrologically to the Pantanal
basin.
This lake has yielded long-term palaeoclimate records,
inferred from reconstructed lake-level changes [19–21] and
pollen-based vegetation reconstructions [22,23]. These records
indicate that eastern Chiquitanı
´a and the Pantanal have
SDTF
SDTF
SDTF
Pantanal
Paraguay River
Paraguay River
Pantanal
Laguna
La Gaiba
Cerrado savanna
15°0¢0¢¢ S
60°0¢0¢¢ W
20°0¢0¢¢ S
Pantanal
Dry Chaco
0 100 200 400
km
I
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a
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e
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r
a
n
t
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o
r
e
s
t
Palm Swamp
5m
4m
3m
<2 m
250
30
temperature (°C)
28
26
24
22
20
Cuiaba, Brazil (151 m.a.s.l.)
200
150
100
precipitation (mm)
50
0
01
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
I
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o
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23456789101112
2010 CRU TS 3.22
Figure 1. Regional map of tropical South America, lower-right inset shows the location of Laguna La Gaiba (LLG) and vegetation communities around the research
site, including the SDTF, cerrado savanna, Dry Chaco, inundated tolerant forest, palm swamps and the seasonally flooded Pantanal. The coring site location is marked
with an ‘x’ and occurs in the deepest portion (more than 5 m) of the lake. Inset climograph from Cuiaba, Brazil, approximately 250 km northeast of Laguna La
Gaiba, showing the seasonal timing and length of dry season, May to September, based on 1981– 2000 climatology (grey). Red histograms and line plot show
precipitation and temperature values during 2010 at LLG from gridded CRU TS 3.22 climate data.
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undergone significant changes in hydrology, climate and
biome turnover since the Last Glacial Maximum. Laguna La
Gaiba is surrounded by a mosaic of plant communities, the dis-
tribution of which is dependent on local topography, edaphic
conditions and hydrology. The lowest elevation areas, such
as those adjacent to the northeastern boundary of LLG along
the Paraguay River, support extensive flooded wetland
savanna. Seasonally inundated savannas occur to the east of
the main trunk of the river and support mosaics of SDTF on
local areas of higher terrain, such as ancient river levees. In
the immediate vicinity of LLG, the steep slopes of the
Amolar Hills, rising up to 600 m above the influence of seaso-
nal flooding, form the eastern margin of the Chiquitano SDTF
[10]. Deciduousness of the SDTF increases with elevation and
open-canopy scrub vegetation and cerrado savanna occur on
the summit of the hills, where soils are too thin to support
forest [8].
Previously published pollen abundance and richness data,
together with limnological data, from LLG [21– 23] are com-
pared with a high-resolution charcoal record from the same
core to enable the relationship between the Chiquitano SDTF,
climate and fire, over the entire Holocene, to be explored for
the first time. The LLG record extends beyond 40 000 years
BP and demonstrates large-scale landscape rearrangement
and catchment change in the western Pantanal at the end of
the Pleistocene [21,22]. Our analysis focuses on the last 12 000
years; a period defined by landscape stability, the establish-
ment of modern lake conditions and the spread of SDTF in
eastern Chiquitanı
´a [22,23].
The fire history reconstruction from LLG was accom-
plished by analysing macroscopic (more than 125 mm)
sedimentary charcoal at contiguous 0.5 and 1.0 cm intervals
throughout the entire sediment core [24] using the chronology
of Whitney et al. [22]. Charcoal analysis for each sediment
sample was completed by placing the sample in a 15 ml tube
in a hot-water bath of 10% potassium hydroxide solution for
15 min and then gently washing it through a 125 mm screen.
All remaining residues were examined at 36magnification
and all charcoal particles greater than 125 mm were tallied.
Charcoal counts were then converted to concentration (par-
ticles cm
23
) and charcoal accumulation rates (CHAR,
particles cm
22
yr
21
).
The CHAR record was then decomposed into two
components: a low-frequency background trend and a high-
frequency peaks component, referred to as fire episodes.
Background charcoal (BCHAR) captures the slowly varying
trends in CHAR through time as vegetation composition and
structure changes, while the peaks component aids in the
identification of one or more fire events [25]. At LLG,
BCHAR was summarized using a locally weighted scatterplot
smoothing (LOWESS) that is robust to outliers with a 300-year
window width. Peak detection in the charcoal record was
tested for significance using a Gaussian distribution, and
only those peak values exceeding the 95th percentile were con-
sidered significant [26]. Once fire events were identified, all
charcoal peaks were further screened to eliminate those
peaks resulting from statistically insignificant variations or
noise in CHAR [27]. Three metrics of fire activity are con-
sidered from the LLG charcoal record: CHAR or changing
influx of charcoal through time; fire episode frequency,
expressed as number of fires episodes or ‘peaks’ in charcoal
production per 1000 years; and peak magnitude, a measure
of the total amount of charcoal associated with each peak,
which is probably related to the fire size, intensity and charcoal
delivery to the lake.
To explore possible fire– climate–vegetation drivers at LLG
through time, we obtained the relative contribution of fire and
precipitation proxies in explaining temporal variation in com-
munity composition of SDTF (16 taxa), savannas (12 taxa)
and riparian forests (21 taxa) by following methods similar to
those proposed by Legendre et al. [28]. Because these three
plant communities (SDTF, savannas and riparian forests) are
distributed across distinct edaphic conditions (eutrophic, dys-
trophic and seasonally flooded soils, respectively), with little
or no overlap in species composition, the variation partitioning
analyses were performed separately for each of them. We
tested the overall significance of the fire fraction (controlled
for variation in moisture) and the moisture fraction (controlled
for variation in fire) by applying a permutation test (999 per-
mutations) for redundancy analysis. All proxy datasets used
for the variation partitioning analyses were pre-smoothed to
have the same temporal resolution, and comprise (i) macro-
scopic charcoal influx data (this paper), (ii) algal community
change, interpreted to be controlled by precipitation-driven
lake level change (first axis of principal components analysis
in reference [19]), and (iii) relative pollen per cent abundance
data, which have been assigned to categories reflecting each
of the three main vegetation community categories around
LLG [22,23] (table 1). Even though the temporal resolution
applied here makes it challenging to address the role of fire
and flooding on short-term ecological processes driving com-
munity turnover, recent studies have shown woody
vegetation is shaped by regularly occurring floods and fires
[9,29]. All variation partitioning analyses were conducted
using the vegan [30] package in the R statistical environment
[31].
3. Results and interpretation
Partitioning the variation explained by the environmental
predictors revealed that precipitation (Pediastrum algae) and
fire-related (charcoal) proxies explain 27% and 2%, respect-
ively, of the turnover in community composition in savanna
ecosystems. In addition, 2% of the turnover in savannas is a
shared component between precipitation- and fire-related
proxies. In the riparian forests, precipitation- and fire-related
proxies explain 18% and 2%, respectively, of the turnover in
community composition, while the precipitation-related
proxy is the sole factor explaining turnover in SDTF compo-
sition (27%). Furthermore, 69%, 80% and 73% of the turnover
in savannas, riparian forests and SDTFs, respectively, remains
unexplained (figure 2).
Most of the turnover in community composition that we are
able to explain is assigned to the precipitation-related proxy.
However, the fire-related proxy or CHAR showed a weak, but
still important, signature in the turnover of savannas. The
clear fire adaptations of savanna plants—such as thick, corky
bark; the ability to root-sprout from substantial rhizomes and
protected buds—demonstrate that fire has been a key ecological
force in savanna [34– 36]. Because these adaptations appear to
be either lacking or have received minimal research attention
in present-day SDTF, ecologists have hypothesized that fire
was not a strong ecological force in this biome [6]. Furthermore,
even though the precipitation-related proxy explains most of
the turnover in riparian communities, the fire-related proxy
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also showed a signature in the turnover of these communities.
The ecological role of fire in SDTF, growing primarily to the
west of LLG today, in the Chiquitanı
´a region of eastern
Bolivia, remains uncertain due to the paucity of suitable
palaeo archives (lake and bog sediments) in this landscape.
North and east of LLG are the Pantanal tropical wetlands,
which are a mosaic landscape dominated by seasonally flooded
savannas and riparian gallery forests, and shaped by regularly
occurring floods and occasional wildfires [9,18,37].
The relative contribution of charcoal from different neigh-
bouring ecosystems—savanna, riparian communities and
SDTF—to the sediments of LLG is difficult to establish.
However, because the preponderance of pollen deposited in
LLG originates from upland Chiquitano SDTF to the west of
LLG [22,23], it is likely that most of the charcoal entering this
lake similarly originates from the Chiquitano SDTF, rather
than the Pantanal wetlands to the east. Furthermore, the lack
of correlation between sediment particle size and charcoal
abundance supports our inference that most of the charcoal
entering LLG is aeolian in origin, rather than entering the lake
via erosion from the surrounding slopes or pulses of fluvial
input from the Pantanal wetlands. It seems likely that, under
extreme drought conditions of the Middle Holocene, fires
occurring in the savanna and riparian communities of the Pan-
tanal would have penetratedinto the eastern Chiquitano SDTF.
While the environmental component ( precipitation and
fire) is relatively straightforward to interpret, there remains
a large fraction of turnover in community composition that
is unexplained (69%, 80% and 73% of the turnover in savan-
nas, riparian forests and SDTFs, respectively) in the LLG
record. There are many factors which are potentially impor-
tant to determining the community composition of
assemblages that we have not adequately accounted for,
such as: (i) ecological drift (cf. [38]) driving stochastic
rearrangements of species distribution ranges through time;
(ii) biotic processes that were not measurable (e.g. compe-
tition, natural enemies); and (iii) environmental factors that
were not measured and uncertainties inherent to the proxy
measurements (table 1). Despite these factors, such a high
proportion of unexplained variation, ranging from about
33 to 75% (e.g. [39–41]), is a common outcome in studies of
floristic turnover.
The partitioning analysis shows that drought influences
SDTF community composition and our pollen data show that
closed-canopy forest remained throughout the Holocene with
no evidence of increased savanna-type vegetation until
recently (figure 3), which would be expected if the forest
was replaced by an open-structure vegetation. The period of
Table 1. Summary of the environmental proxy data used for the variation partitioning analysis, the interpretation of the proxy measurements, and sources of
uncertainty in the environmental reconstructions.
environmental
variable proxy measurement proxy interpretation sources of uncertainty
data
references
fire activity macroscopic charcoal
(more than
125 mm) influx
past local fire activity is proportional to
charcoal influx
errors in age model of sediment
accumulation
this paper
vegetation relative per cent
pollen abundance
variations in pollen types reflect
community change in surrounding
vegetation
not all pollen types can be
assigned to a single vegetation
category
spatial and temporal variations in
relative influence of wind-
versus flood-transported pollen
Whitney et al.
[22,24]
precipitation Pediastrum community
change
variations in proportion of algal types
restricted to shallow water are
controlled by lake level change; driven
by regional precipitation
variation in influences of
evaporation, local hydrology
and precipitation on lake levels
organisms respond to multiple
interacting environmental
variables
Whitney &
Mayle [19]
100
90
80
70
60
50
% variation
40
30
20
10
0
savannas riparian SDTF
unexplained
fire
precipitation + fire
precipitation
Figure 2. Variation partitioning by redundancy analysis to determine how
much of the temporal variation in floristic composition in LLG was accounted
for by the environmental variables measured (charcoal, pollen and Pediastrum
proxies). Black fraction represents the precipitation component (controlled for
variation in fire); light grey fraction represents the overlap between precipi-
tation and fire components; and dark grey fraction represents the fire
component (controlled for variation in precipitation). Variation partitioning
among other palaeoclimate proxies (e.g.
d
18
O speleothem records from
[32,33]) and LLG charcoal and pollen data was explored, but results were
insignificant and are not shown.
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12 000 10 000 8000 6000 4000 2000 0
0
0.5
1.0
1.5
charcoal influx (particles cm–2 yr–1)
000
1
0
000
800
0
6000
400
0
200
0
0
4
8
12
16
20
% riparian forest pollen
0
2
4
6
8
10
% savanna pollen
0
10
20
30
40
% SDTF pollen
2
4
6
8
fire frequency
(no. fire episodes per 1000 years)
0
10
20
30
40
50
12 000 10 000 8000 6000 4000 2000 0
a
g
e (cal
y
r BP)
0
20
40
60
80
100
particle size
0
10
20
30
40
fire episode magnitude
(
p
articles cm–2
y
r–1)
0
20
40
60
80
100
% other pollen types
SDTF-type
savanna-type
riparian
forest-type
P. argentiniense
CHAR
fire
frequency
% sand
(125–1250 mm)
% clay
(4–62 mm)
fire
episodes
all other
pollen types
fire magnitude
%Pediastrum abundance
Figure 3. LLG charcoal influx (CHAR or particles cm
22
yr
21
) with LOWESS showing BCHAR (black line), fire episodes (top red vertical bars) as identified by a 300
year background window and local threshold [26]. Pollen-based biome summaries [22] for SDTF-type (green), savanna-type (yellow) (which may include aquatic
grasses) and riparian forest-type (blue) are presented as in Metcalfe et al. [21]. The percentage of all other pollen types not contributing to these three community
types including Poaceae (averaging 46%) and Cyperaceae (averaging 16%) for the Holocene are shown in grey. Particle size analysis shows the clay (4–62 mm) and
sand (125 – 1250 mm) fractions, Pediastrum argentiniense is shown as an indicator of shallow-water conditions [19] and fire frequency (red) is expressed as the
number of fire episodes per 1000 years. Lastly, fire episode magnitude (shown at bottom as a red histogram) represents the amount of CHAR exceeding average
background CHAR with each fire episode.
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Holocene drought, identified from high abundance of shallow
water indicators (Pediastrum argentiniense algae) from 10 000 to
4400 cal yr BP [19], corresponds with compositional change
shown by increases in the key dry forest taxon Astronium and
a reduction in Moraceae (figure 4d,b), which is more prevalent
in pollen assemblages from wetter climates [43,44]. Fire
activity, or CHAR, however, does not correlate with these
changes in forest composition (figure 3). Fire frequency is high-
est during the Holocene transition, 12 000 cal yr BP, but then
low CHAR values during the Early Holocene (10 000 cal yr BP)
(comparable to those of today) increase to maximum values by
7500 cal yr BP. Extreme fire activity by 7500 cal yr BP lags
behind the beginning of Early Holocene drought at Botuvera
Cave [32] and changes in SDTF composition at La Gaiba by
approximately two millennia (figure 5b), supporting the
results of our partitioning analysis that shows drought, not
fire, to be the key influence on SDTF vegetation turnover.
Although increasing CHAR values by 7500 cal yr BP do not
significantly change the abundance of the key SDTF taxa,
thus not leaving a signature in the SDTF community turnover,
the increased fire pressure causes a decline in vegetation rich-
ness captured by LLG pollen richness index (figure 4a).
Therefore, the combined effects of drought and fire negatively
impacted plant diversity of the eastern Chiquitano SDTF as
more drought-tolerant taxa (Astronium and Anadenanthera)
increased in relative abundance.
The floristic composition of the SDTF subsequently altered
towards a more moisture-dependent forest community in the
Late Holocene (e.g. expansion of Moraceae) as precipitation
increased once more, resembling the modern forest
composition by 3000 cal yr BP. This compositional change
was driven by rising precipitation, beginning 4400 cal yr BP
at La Gaiba [19], and corroborated by a number of lake records
across the region [46,47]. Floristic changes lagged behind the
reduction in fire activity by several millennia. There were no
discernible variations in the pollen record concomitant with
lower fire activity after 6000 cal yr BP when CHAR sharply
declined (figures 3 and 4c). Again, this temporal mismatch
between peak fire activity and precipitation-driven compo-
sitional changes confirms our partitioning analysis, which
indicates fire is important, but not the key driver of SDTF
community turnover (figure 2).
4. Discussion
(a) Influences of past fire and climate on seasonally dry
tropical forest
The charcoal record from La Gaiba indicates that firewas a per-
sistent feature in eastern Chiquitano SDTF, with periods of
intensive fire activity during extended droughts. Despite the
clear role of drought and fire in the development of the Chiqui-
tano SDTF over the Holocene, there is a complex relationship at
play among precipitation, floristic composition and fire at the
boundary with the Pantanal. Lower precipitation, and hence
reduced flooding compared with present, in the Early to
Middle Holocene would have resulted in greater coverage of
ignitable, dry savanna areas in the Pantanal, because the seaso-
nal inundation of the basin and length of flood season is highly
12 000 10 000 8000 6000 4000 2000 0
age (cal yr BP)
0
0.5
1.0
1.5
charcoal influx (
p
articles cm–2
y
r–1)
12 000 10 000 8000 6000 4000 2000 0
age (cal yr BP)
0
4
8
12
16
0
0.4
0.8
1.2
1.6
% Moracaea and Cactaceae
0
10
20
30
40
p
ollen richness index
Astronium
Anadenanthera
veg richness
moraceae
cactaceae
charcoal
cactaceae
% Astronium and Anadenanthera
(a)
(c)
(d)
(b)
Figure 4. (a) Vegetation richness, determined with rarefaction analysis of pollen data [42], (b) per cent Moraceae and Cactaceae pollen, (c) charcoal influx, and
(d) per cent Anadenanthera and Astronium pollen. All pollen percentage data are based on total terrestrial pollen sum [22].
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dependent on the strength of the SASM delivered to the region
through the SALLJ. Palaeo-precipitation reconstructions from
speleothem
d
18
O records approximately 1500 km southeast of
LLG (downstream in the path of the SALLJ) capture the centen-
nial-to-millennial scale droughts during the Early to Middle
Holocene (figure 5a,b) that have been linked to changes in
the strength of the SASM [32,33]. Modern precipitation-hydro-
logical studies have demonstrated a strong link between
annual rainfall over the Pantanal and the extent and length of
flooding over the last century [12]. Furthermore, historical
periods of extended drought across the Pantanal reduced sea-
sonally inundated landscapes and vegetation types [12].
Drying across the Pantanal basin in the Early to Middle
Holocene is demonstrated by the proxy data from La Gaiba
[19,20], but also corroborated by several records influenced
by the path of the SALLJ. These include Lake Titicaca, which
progressively decreased in lake level beginning approximately
7500 cal yr BP [46], and the Lapa Grande Cave record [33],
which reports several large-amplitude dry-climate ano-
malies, persisting for several centuries, centred at 7800 and
7400 cal yr BP (figure 5a). Similar century-long droughts
occur at Botuvera Cave, located at 278S, near the exit of the
SALLJ, demonstrating the regional impact of drought con-
ditions during the Early Holocene (figure 5b). Although
stable oxygen isotope profiles differ at times in the Holocene
from the LLG CHAR record, there are potential linkages
among these proxies at multi-centennial and millennial
scales. For example, centuries of reduced flooding in the
Pantanal during extreme drought centred at 7800 cal yr BP
would have limited inundation in savannas, creating optimal
conditions for frequent fire ignition in these highly flamma-
ble grass-dominated systems. Our partitioning analysis
supports this interpretation, as fire and drought are both
shown to contribute to a considerable fraction of the variation
in savanna community (figure 2).
This higher fire activity raging across the Pantanal basin
during the Middle Holocene period of intense drought prob-
ably penetrated the SDTF of the eastern Chiquitano region,
spreading through the undergrowth, as pollen representative
of key SDTF understorey taxa, such as Clavija and Sapium,are
removed from the rich diversity of taxa [23]. Interestingly,
pollen of Cereus columnar cactus, one of the key SDTF taxa
used to argue for the fire intolerance of SDTF [5], is present
during periods of higher fire activity around 8500 and
12 000 11 000 10 000 9000 7000 5000 4000 3000 2000 1000 0
age (cal yr BP)
–4.0
–3.5
–3.0
–2.5
–2.0
–1.5
–1.0
Botuvera Cave d18 O (ppm, VPDB)
–8
–7
–6
–5
Lapa Grande d18 O (ppm, VPDB)
12 000 11 000 10 000 9000 8000 7000 6000 5000 4000 3000 2000 1000 0
a
g
e (cal
y
r BP)
0.5
0.6
0.7
0.8
0.9
1.0
0
10
20
30
peak magnitude
0.5
0.6
0.7
0.8
0.9
1.0
LLG transformed charcoal influxLLG transformed charcoal influx LLG transformed charcoal influx
3720
3740
3760
3780
3800
3820
Titicaca Lake level (m.a.s.l.)
0.5
0.6
0.7
0.8
0.9
1.0
(a)
(c)
(b)
8000 6000
Figure 5. LLG transformed charcoal influx using the box-cox transformation [45] is compared with three palaeo-precipitation proxies. (a) The
d
18
O speleothem
record from Lapa Grande Cave, Brazil (– 14.42º lat, – 44.36º lon) [33], (b) the
d
18
O speleothem record from Botuvera Cave, Brazil (– 27.22º lat, – 49.16º lon) [32],
and (c) lake level reconstruction from two sediment cores: C90 m and C150 m, from Lake Titicaca.
d
13
C records were used from Lake Titicaca sediments to develop
transfer functions to infer past changes in lake level [46]. The charcoal influx curve is plotted in grey.
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again at 1500 cal yr BP (figure 4b,c). Its presence may reflect a
high degree of spatial variability and/or decreased fire fre-
quency in the eastern Chiquitano SDTF during the Early
Holocene and again in the Late Holocene. Its survival in
fire-influenced SDTF may reflect its dispersal capability
across the catchment, possibly favouring rocky areas with
little ignitable ground cover, allowing small populations to
persist (today Cereus spp. are common components of the for-
ests around La Gaiba). Despite the inferred changes to
the plant diversity and potential impacts of fire on the under-
storey composition as well as on the low-abundance taxa of
the eastern Chiquitano SDTF during the Middle Holocene
(e.g. temporary exclusion of Tabebuia and Phyllostylon), the
dominant tree taxa of the dry forest (e.g. Astronium and
Anadenanthera) were not replaced throughout the period of
intensive fire activity (figure 4d), implying that SDTF is at
least partially resistant to the fires originating in the Pantanal.
(b) Fire and conservation in seasonally dry
tropical forest
Fire has been a persistent feature in the eastern Chiquitano
SDTF, thus refuting our hypothesis that fire has played no
role in shaping the diversity patterns in the Chiquitano
SDTF (e.g. [6]). The recent decline in fire activity during the
last two millennia might have fuelled the perception that
fires have had a limited role in the evolution of SDTF. Critically,
the floristic composition and diversity observed today in the
eastern Chiquitano SDTF is a direct result of the palaeoclimate
history, which in turn, had a strong influence on its history of
varying fire activity. As suggested by Colinvaux [48],
the high species richness across Amazonia and adjacent
regions and is probably driven by the extensive land area, a
wide range of habitats, as well as intermediate levels of natural
disturbance, including both floods and fires.
Recent intense drought events have contributed to
reductions in rainfall and soil moisture and increasing air
temperatures, such as during the widespread droughts and
fires of 2005 and 2010 [16,49]. Future management of fire–
vegetation dynamics in the SDTF requires knowledge of
natural variability on short and long time scales and consider-
ation of how those processes link climate change to impacts on
biological diversity (e.g. [50]). The maintenance of closed-
canopy SDTF during periods of increased fire activity during
the Early Holocene may serve as an ecological analogue for
future conditions, as drought intensity and fire frequency are
expected to continue increasing [7]. The Chiquitano SDTF,
and neighbouring dry-forest corridor [51], however, may not
be representative of all SDTF systems, as they have a very
different flora and history compared to the endemic-rich, iso-
lated SDTFs of the inter-Andean valleys. These unique and
isolated SDTFs of the inter-Andean valleys have experienced
an evolutionary history separate to that of lowland SDTFs,
having been separated for ca 10 Myr [52]. Andean SDTFs
may very probably show a sensitivity to fire that is not demon-
strated by the lower-diversity Chiquitano forest biome, which
have shown resilience and adaptability in occupying new
regions through rapid post-glacial migration [23,53]. Regard-
less of whether dry forests of inter-Andean valleys would be
able to maintain a closed-canopy structure in the event of
high fire activity, they contain some of the highest concen-
trations of endemics in the world [54], and the negative
impact of fires on plant richness could have a devastating
effect on this biodiversity hotspot.
Our palaeoecological data show that the eastern Chiqui-
tano SDTF withstood periods of intense droughts combined
with increased fire activity during the Holocene (challenging
the widely held view of susceptibility of SDTFs to fire), but
the combined effects of these pressures will be exacerbated
in the future, with more frequent fires of anthropogenic
origin, in addition to the projected increase in drought fre-
quency/severity and warming in the region [7]. Not only is
the Chiquitano SDTF under increasing agricultural pressure,
but the neighbouring Pantanal savannas have been experien-
cing intensifying cattle ranching activities, which include
heavy use of fire and invasion of fire-tolerant grass species
[10]. The Chiquitano SDTF could very probably get caught
in a ‘squeeze’ from escaped fires from both systems, which
alongside increasing fragmentation and drought, could
cause irreversible changes to the vegetation, opening the
canopy and allowing for elevated fire frequency to persist
in the system. The future of SDTF ecosystems depends
partly on their inherent ability to survive fire impacts.
Authors’ contributions. M.J.P. coordinated the research and drafted the
original manuscript; M.J.P. and D.M.N. performed the data analyses;
M.J.P., B.S.W., F.E.M. and D.M.N. co-wrote an improved version of
the manuscript and contributed equally to the editing and revisions.
F.E.M., B.S.W. and E.J.d.B. contributed to fieldwork. E.J.d.B. contrib-
uted to editing and revisions; K.S.M. contributed to laboratory
analysis. All authors gave final approval for publication.
Competing interests. We declare we have no competing interests.
Funding. Fieldwork to core Laguna La Gaiba was funded by National
Geographic and The Royal Society (F.M.) and the University of
Amsterdam (E.J.dB.); charcoal sample preparation and counting
was funded by The University of Edinburgh (K.S.M.) and the
Erasmus Programme (E.J.dB.).
Acknowledgements. This paper was developed through presentations and
discussions emerging from the PAGES-sponsored 2013-14 La ACER
workshops and the 2015 Royal Society meeting ‘The Interaction of
Fire and Humankind’. We want to thank Francisco Cruz for sharing
data and knowledge and two anonymous reviewers who provided
many thoughtful and helpful comments greatly improving this
manuscript.
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