Introduction
The cerrãdo savanna of central South America is the largest, richest, and likely
most threatened savanna in the world (DaSilva Meneses and
Bates, 2002) The cerrãdo is the second largest biome in South America, covering 1.86 ×106 km2 and is home to over 10 000 plant species
(Myers et al., 2000). The tropical forest–savanna
ecotones within the cerrãdo biome are of considerable interest to biologists
because of their high habitat heterogeneity (beta diversity), importance in
rainforest speciation (Russell-Smith et al., 1997) and
sensitivity to climate change (IPCC, 2014). According to current
estimates, however, only 20 % of the cerrãdo remains undisturbed and only 1.2 %
of the area is preserved in protected areas (Mittermeier et
al., 1999). Additionally, cerrãdo savannas have a significant role in the modern
global carbon cycle because of high CO2 loss associated with frequent
natural fire activity (Malhi et al., 2002). Currently savanna
fires are considered the largest source of natural pyrogenic emissions, with
the most fire activity of all major global land-cover types
(Pereira, 2003). In the last few decades, deforestation for
agriculture and increased drought have resulted in increased burning in
savannas, contributing to approximately 12 % of the annual increase in
atmospheric carbon (Van der Werf et al., 2010).
The cerrãdo biome comprises forest, savanna, and campestre (open-field) formations
(Abreu et al., 2012; Mistry, 1998). Cerrãdo sensu stricto is
characterized as a woody savanna formation composed of dense, thin, and
rocky outcrops with cerrãdo physiognomies that are distinguishable based on their
densities, heights, and scattered tree–shrub covers with roughly 50 %
trees and 50 % grass (Abreu et al., 2012). The principal
determinants of the growth and development of the cerrãdo vegetation types are
largely related to edaphic factors (Colgan et al., 2012). For
example, the distribution of major cerrãdo vegetation types is closely related to
the geomorphology of the Precambrian Brazilian Shield in South America
(Killeen, 1998a). The development of the variety of
cerrãdo vegetation communities is largely the result of the heterogeneous nature of the
edaphic features (Killeen, 1998a) including the depth of the
water table, drainage, the effective depth of the soil profile, the presence
of concretions (Haridasan, 2000), soil texture, and the percentage
of exposed rock (Junior and Haridasan, 2005).
In addition to edaphic constraints, climate also has a prominent role in
determining cerrãdo savanna vegetation structure and fire activity
(Ribeiro and Walter, 2008). The cerrãdo biome is dominated by a
warm, wet–dry climate associated with the seasonal migration of the
Intertropical Convergence Zone (ITCZ) (DaSilva Meneses and Bates,
2002; Latrubesse et al., 2012; Vuille et al., 2012). On synoptic
climatological timescales, temperature and precipitation are the most
important effects of climate on fire (e.g., months to seasons to years)
(Mistry, 1998). These factors govern net primary productivity
(NPP) and the abundance of available fuels (Brown and Power,
2013; Marlon et al., 2013). Warmer temperatures are typically associated
with increased burning through vegetation productivity and the occurrence of
fire-promoting climatic conditions. However, the role of temperature can be
mediated by precipitation (Brown and Power, 2013). Fire responds
differently to increases in precipitation depending on whether fuel is
initially abundant or limited in the ecosystem (Marlon et
al., 2013; Mistry, 1998). In arid and semi-arid environments, such as the
cerrãdo, increases in precipitation tend to increase fire, whereas increased
precipitation in humid environments can reduce fire
(Marlon et al., 2008, 2013).
The seasonality of the precipitation coupled with abundant wet-season
lightning ignitions (Ramos-Neto and Pivello, 2000) is linked to
high fire frequency in the cerrãdo (Miranda et al., 2009). Wet-season lightning fires
typically start in open vegetation (wet fields or grassy savannas), with
significantly higher incidence of fire in more open savanna vegetation
(Ramos-Neto and Pivello, 2000). High biomass production during the
wet season results in abundant dry fuels favoring frequent fires throughout
the year (Ramos-Neto and Pivello, 2000). Data show a positive
correlation with fine fuel build-up and both fire temperature and fire
intensity (energy output) (Fidelis et al., 2010). Thus,
increased wet-season fuel accumulation in the cerrãdo increases fire intensity.
Based on an ecosystems adaptation to fire it can be classified as
independent, fire-sensitive, and fire-dependent (Hardesty et
al., 2005). In fire-independent ecosystems such as tundra and deserts, fire
is rare, either because of unsuitable climate conditions or lack of biomass
to burn. Fire-sensitive ecosystems such as tropical rainforests are damaged
by fire, which disrupts ecological processes that have not evolved with fire
(Hardesty et al., 2005). Fire-dependent systems such as the
well-drained grasslands of the cerrãdo biome have evolved in the presence of
periodic or episodic fires and depend on fire to maintain their ecological
processes (Hardesty et al., 2005). Fire-dependent vegetation is
fire-adapted, flammable, and fire-maintained (Miranda et al.,
2009; Pivello, 2011).
The study of fire and vegetation change in the cerrãdo is increasingly important as
population, agricultural activity, and global warming create pressing
management challenges to preserve these biodiverse ecosystems
(Mistry, 1998). The long-term role of humans on vegetation and
fire regimes of the cerrãdo remains unclear. There is increasing evidence for
a late Holocene (3000 cal yr BP) increase in Mauritia flexuosa (M. flexuosa) and fire activity in
Bolivia, Colombia, Venezuela, and Brazil, which has been attributed to both
natural and anthropogenic drivers (Behling and Hooghiemstra, 1999;
Berrio et al., 2002a; DaSilva Meneses et al., 2013; Kahn and de Castro,
1985; Kahn, 1987, 1988; Montoya and Rull, 2011; Rull, 2009).
To investigate the drivers of vegetation and fire in the cerrãdo, a long-term
perspective is needed. The past few decades have experienced increased
global temperatures, increased atmospheric CO2, and unprecedented
levels of deforestation (Malhi et al., 2002). These recent
changes heavily influence modern ecological studies, thus limiting the
understanding of the role of natural variability in these systems. Long-term
paleoecological studies can provide baseline information on processes
shaping forest–savanna fire–vegetation dynamics from
centennial to millennial timescales (Mayle and Whitney, 2012). These
long-term studies can inform whether recent shifts in ecotones are the
result of a minor short-term oscillation around a relatively stable ecotone
or a longer-term (e.g., millennial scale) unidirectional ecotonal shift
forced by climate change (Mayle et al., 2000; Mayle and Whitney, 2012).
Additionally, long-term paleoecological records help form realistic
conservation goals and identify fire management strategies for the
maintenance or restoration of a desired biological state
(Willis et al., 2007).
In this study, the long-term paleoecological perspective provides a context
for understanding the role of centennial to millennial climate variability
in the evolution of fire and vegetation in cerrãdo savanna ecosystems. The purpose
of this research is to explore long-term environmental change of
cerrãdo savanna palm swamps in Bolivia from the Lateglacial (ca. 15 000 cal yr BP)
to present. Paleoecological proxies including lithology, magnetic
susceptibility, loss on ignition (LOI), charcoal, stable isotope, and
phytolith data are used to investigate long-term ecosystem processes in the
cerrãdo savanna. There are three primary hypotheses investigated in this study:
Edaphic conditions are the dominant control on the presence of savanna versus forest vegetation on the Huanchaca Mesetta.
Climate is the dominant control on savanna structure and floristic composition.
The late Holocene rise in M. flexuosa was driven by climate rather than a change in human land use.
Study site
Noel Kempff Mercado National Park (NKMNP), a 15 230 km2 biological
reserve in northeastern Bolivia, is located on the Precambrian Brazilian Shield near
the southwestern margin of the Amazon Basin, adjacent to the Brazilian
states of Rondônia and Mato Grosso (Burbridge et al.,
2004). It is a UNESCO World Heritage Site, in recognition of its globally
important biodiversity and largely undisturbed ecosystems, including terra firme (non-flooded)
evergreen rainforest, riparian and seasonally flooded humid evergreen
forest, seasonally flooded savanna, wetlands, upland cerrãdo savannas, and
semi-deciduous dry forests (Mayle et al., 2007).
NKMNP occupies an ecotone between Amazon rainforest to the north and dry
forests and savannas to the south, containing 22 plant communities (Fig. 1) (Burn et al., 2010). Huanchaca Mesetta palm swamp
(14∘32′10.66′′ S, 60∘43′55.92′′ W;
elevation: 1070 m a.s.l.) is located within NKMNP on the Huanchaca Mesetta
– an 800–900 m elevation table mountain. The palm swamp is approximately
200 by 50 m, comprised entirely of a monospecific stand of the palm
M. flexuosa.
Climate
The climate of NKMNP is characterized by a tropical wet and dry climate
(DaSilva Meneses and Bates, 2002). The mean annual
precipitation at NKMNP derived from nearby weather stations (Concepción,
Magdalena, San Ignacio) is ca. 1400–1500 mm per year, with mean annual
temperatures between 25 and 26 ∘C
(Hanagarth, 1993; Montes de Oca, 1982; Roche and
Rocha, 1985). There is a 3- to 5-month dry season during the Southern
Hemisphere winter (May to September–October), when the mean monthly
precipitation is less than 30 mm (Killeen, 1990). Precipitation
falls mainly during the austral summer (December to March), originating from
a combination of deep-cell convective activity in the Amazon Basin from the
South American summer monsoon (SASM) and the ITCZ
(Vuille et al., 2012). The SASM transports
Atlantic moisture into the basin and corresponds to the southern extension
of the ITCZ. The ITCZ is driven by seasonal variation in insolation; thus,
maximum Southern Hemisphere insolation and precipitation occur in the
austral summer (Bush and Silman, 2004;
Vuille et al., 2012). During winter (June, July, August), cold, dry polar
advections from Patagonia, locally known as surazos, can cause short-term cold
temperatures to frequently decrease down to 10 ∘C for
several days at a time (Latrubesse et al., 2012;
Mayle and Whitney, 2012). These abrupt decreases in temperature may
potentially influence the distribution of temperature-limited species on the
Huanchaca Mesetta.
Geomorphology
The Huanchaca Mesetta table mountain is near the western limit of the
Precambrian Brazilian Shield and dominates the eastern half of NKMNP. It is composed of
Precambrian sandstone and quartzite (Litherland and Power,
1989). The top of the mesetta is flat, with a gently rolling surface and at
elevations ranging from 500 to 900 m above sea level (a.s.l.)
(DaSilva Meneses and Bates, 2002). The substrate of the
mesetta is rocky, and soils are thin and low in organic material
(Litherland and Power, 1989). Continuity of the crystalline or
sedimentary blocks of the mesetta is broken by an extensive network of
peripheral or inter-mesetta depressions formed from a combination of
erosion, dolerite dike intrusions, and faulting on the mesetta
(DaSilva Meneses and Bates, 2002; Litherland and Power,
1989). These depressions act as catchments for sediment and water, resulting
in sediment accumulation, which supports more complex vegetation
communities. High species diversity exhibited on the Huanchaca Mesetta,
compared with other savanna regions of South America, is attributed to the
long history of isolation of this edaphically controlled table-mountain
savanna (Mayle et al., 2007).
Huanchaca Mesetta study site (a) vegetation map of Noel Kempff
Mercado National Park (NKMNP) modified from Killeen (1998b), (b) view
from atop Huanchaca Mesetta, (c) Huanchaca Mesetta palm swamp, and
(d)
monospecific stand of Mauritia flexuosa. Photos by F. Mayle.
Vegetation
The cerrãdo savanna on Huanchaca Mesetta is dominated by a continuous
grass cover with sparsely scattered small trees and shrubs that grows on the
thin, well-drained, nutrient-poor soils (Killeen, 1998b). Woody species
include Byrsonima coccolobifolia, Caryocar brasiliensis, Erythroxylum suberosum, Vochysia haenkeana, and Callisthene fasciculata. Trees
and shrubs include Qualea multiflora, Emmotum nitens, Myrcia amazonica, Pouteria ramiflora, Diptychandra aurantiaca, Kielmeyera coriacea,
Ouratea spectabilis, and Alibertia edulis. Small shrubs include
Eugenia punicifolia and Senna velutina, and herbaceous species include
Chamaecrista desvauxii and Borreria sp. Monocot families
include the Rapateaceae (C3) (Cephalostemon microglochin),
Orchidaceae (Cleistes paranaensis) (CAM,C3), Iridaceae
(Sisyrinchium spp.) (C4), Xyridaceae (Xyris spp.)
(C4), and Eriocaulaceae (Eriocaulon spp., Paepalanthus
spp., Syngonanthus spp.) (C4) (Killeen, 1998b). In the interfluvial depressions organic-rich soil is sufficiently deep to
support humid evergreen forest islands, which are typically dominated by
monospecific stands of M. flexuosa (DaSilva
Meneses and Bates, 2002; Mayle and Whitney, 2012). M. flexuosa is a
monocaulous, arborescent palm, averaging 20–30 m tall which is typically
associated with a low, dense understory (da Silva
and Bates, 2002; Furley and Ratter, 1988; Kahn, 1988). M. flexuosa
is confined to lower elevations (< ca. 1000 m elevation) in warm/wet
climates (Rull and Montoya, 2014).
M. flexuosa swamps favor interfluvial depressions that remain
flooded during the dry season, when the surrounding terrains dry out
(Huber, 1995a, b; Kahn and de Granville, 1992). The
abundance of M. flexuosa in permanently flooded, poorly drained
soils is the result of pneumatophores (aerial roots), which enable its growth
in anaerobic conditions (Kahn, 1988;
Rull and Montoya, 2014). Seasonal water deficits saturate the soil profile in
the wet season and desiccate soil during the dry season, resulting in a
dominance of herbaceous versus woody plants surrounding the interfluvial
depressions (Killeen, 1998b). The seasonal dryness leads to
drought, plant water stress, and frequent fire activity, resulting in the
development of xeromorphic and sclerophyllous plant characteristics on the
open mesetta (Killeen, 1998b). The spatial distribution of
evergreen forest versus drought-tolerant savanna vegetation is additionally
constrained by edaphic conditions, limiting the expansion of forest vegetation
because of the heavily weathered sandstone soils dominant outside the
interfluvial depressions (Killeen and Schulenberg,
1998). Limited soil development precludes rainforest from developing on the
large, rocky expanses of the mesetta (Killeen and
Schulenberg, 1998). The essentially treeless campo cerrãdo that grows
around Huanchaca Mesetta palm swamp is edaphically constrained and has likely
grown on this mesetta for millions of years (Mayle and Whitney,
2012). Thus, the vegetation of the Huanchaca Mesetta is influenced by both
climatic and non-climatic controls including seasonal hydrologic conditions,
edaphic soil constraints, and frequent fire activity
(Killeen and Schulenberg, 1998).
Materials and methods
Sediment core
A 5.48 m long sediment core from Huanchaca Mesetta palm swamp was collected
in 1995 using a Livingstone modified square-rod piston corer from the center
of the swamp. The uppermost 15 cm, containing a dense root mat, was
discarded because of the presence of fibrous roots and potential for
sediment mixing. Huanchaca Mesetta sediment cores were transported to the
Utah Museum of Natural History for analysis. They were photographed and
described using a Munsell soil color chart. Visual descriptions, including
sediment type, structure, texture, and organic content, were undertaken to
assist interpretation of the paleoenvironmental data.
AMS radiocarbon dates from Huanchaca Mesetta.
Lab number
Material
Depth
14C age
δ13C
IntCal13 2σ
(cm)
(yr BP)
ratio
(cal year BP)
UGAMS 15158
Macrofossil
17
190 ± 20
-28.8
0–289
UGAMS 17252
Bulk sediment
58
2310 ± 25
-18.8
2211–2356
UGAMS 15264
Bulk sediment
118
1360 ± 20
-22.9
1272–1305
UGAMS 12023
Bulk sediment
190
2480 ± 20
-22.62
2473–2715
UGAMS 17253
Bulk sediment
225
3365 ± 25
-20.7
3561–3689
UGAMS 17254
Bulk sediment
277
6545 ± 30
-22.6
7422–9622
UGAMS 15159
Bulk sediment
320
8600 ± 30
-22.8
9524–9622
UGAMS 17255
Bulk sediment
380
11 905 ± 35
-16.3
13 577–13 789
Chronology
The chronological framework for Huanchaca Mesetta was based on eight
accelerator mass spectrometry (AMS) radiocarbon dates from non-calcareous
bulk sediment and wood macrofossils analyzed at the University of Georgia
Center for Applied Isotope Studies (Table 1). The uncalibrated radiometric
ages are given in radiocarbon years before AD 1950 (years before “present”,
yr BP). Radiocarbon ages were calibrated using CALIB 7.0 and the IntCal13
calibration data set (Reimer et al., 2013). IntCal13 was
selected in place of the SHcal13 calibration curve because of the
latitudinal location (14∘ S) of Huanchaca Mesetta and the proximal
hydrologic connection with the origin of the South American monsoon in the
Northern Hemisphere. The seasonal migration of the ITCZ is thought to
introduce a Northern Hemisphere 14C signal to the low-latitude Southern
Hemisphere (McCormac et al., 2004). This study
area is located in the low latitudes (14∘ S) and within the range
of the ITCZ migration; thus, the IntCal13 calibration curve was selected for
the radiocarbon calibrations. Following calibration, the mean age value of
calibrated years before present (cal yr BP) of the largest probability at 2σ standard deviation was used to reflect both statistical and
experimental errors) (grey bars in Fig. 2). These mean ages were used to
create the smoothing spline age model using classical age–depth modeling in
the package CLAM (Blaauw, 2010) within the open-source
statistical software R.
CLAM age–depth model for Huanchaca Mesetta. Grey bars represent 2σ error.
Loss on ignition
The variability in the organic and carbonate content of sediments is used,
in conjunction with magnetic susceptibility, to identify periods of
variability in sediment composition and organic content throughout the
Holocene. Organic and carbonate sediment composition was determined by
loss on ignition (LOI), conducted at contiguous 1 cm increments throughout
the cores. For each sample, 1 cm3 of sediment was dried in an oven at
100 ∘C for 24 h. The samples underwent a series of 2 h
burns in a muffle furnace at 550 and 1000 ∘C to
determine the relative percentage of the sample composed of organics and
carbonates. Concentration was determined by weight following standard
methodology (Dean Jr., 1974).
Magnetic susceptibility
Magnetic susceptibility (MS) was measured to identify mineralogical
variation in the sediments (Nowaczyk, 2001). The MS of sediments
is reflective of the relative concentration of ferromagnetic (high positive
MS), paramagnetic (low positive MS), and diamagnetic (weak negative MS)
minerals or materials. Typically, sediment derived from freshly eroded rock
has a relatively high MS, whereas sediments that are dominated by organic
debris, evaporites, or sediments that have undergone significant diagenetic
alteration typically have a low or even negative MS (Reynolds
et al., 2001). Shifts in the magnetic signature of the sediment can be
diagnostic of a disturbance event (Gedye et al., 2000).
Sediment cores were scanned horizontally, end to end, through the ring
sensor.
MS was conducted at 1 cm intervals using a Barington ring sensor
with a 75 mm aperture.
Charcoal
Sediment samples were analyzed for charcoal pieces greater than 125 µm using a modified macroscopic sieving method (Whitlock and
Larsen, 2001) to reconstruct the history of local and extra-local fires.
Charcoal was analyzed in contiguous 0.5 cm intervals for the entire length
of the sediment core at 1 cm3 volume. Samples were treated with 5 %
potassium hydroxide in a hot water bath for 15 min. The residue was
gently sieved through a 125 µm sieve. Macroscopic charcoal (particles
> 125 µm in minimum diameter) was counted in a gridded Petri
dish at 40 × magnification on a dissecting microscope. Non-arboreal charcoal was
characterized by two morphotypes: (1) cellular “graminoid” (thin rectangular
pieces; one cell layer thick with pores and visible vessels and cell wall
separations) and (2) fibrous (collections or bundles of this filamentous
charcoal clumped together). Arboreal charcoal was characterized by three
morphotypes: (1) dark (opaque, thick, solid, geometric in shape, some
luster, and straight edges), (2) lattice (cross-hatched, forming rectangular
ladder-like structure with spaces between), and (3) branched (dendroidal,
generally cylindrical with successively smaller jutting arms)
(Jensen et al., 2007; Mueller et al.,
2014; Tweiten et al., 2009). Charcoal pieces were grouped into non-arboreal
and arboreal categories based on their morphology, which enabled the
characterization of fuel sources in the charcoal record
(Mueller et al., 2014).
Charcoal counts were converted to charcoal influx (number of charcoal
particles per cubic centimeter) and charcoal influx rates by dividing by the deposition
time (yr cm-1) using CharAnalysis statistical software
(Higuera et al., 2009). In CharAnalysis, charcoal data were decomposed to distinguish background charcoal from distinct charcoal peaks based on a standard methodology to calculate a set of threshold
criteria (Higuera et al., 2007). The background threshold was calculated
using a 700-year moving average. If the charcoal data exceed that background threshold, the charcoal peak is interpreted as a fire episode.
Stable isotopes
Stable carbon isotopes were analyzed as an additional proxy for changes in
vegetation structure and composition. Carbon isotopic composition of
terrestrial organic matter is determined primarily by the photosynthetic
pathway of vegetation (Malamud-Roam et al., 2006). Previous
research on δ13C values of the Huanchaca Mesetta have been used
to determine the relative proportions of C4 savanna grasses versus
C3 woody and herbaceous vegetation
(Killeen et al., 2003; Mayle et al., 2007).
Sediment δ15N integrates a variety of nutrient cycling
processes including the loss of inorganic N to the atmosphere through
denitrification (McLauchlan et al., 2013; Robinson, 1991).
Denitrification and the subsequent enrichment of δ15N requires
abundant available carbon, available nitrate, and anaerobic conditions
(Seitzinger et al., 2006). Thus, wet, anoxic soils tend to
have enriched values of δ15N. Environmental conditions that
alter from wet (anaerobic) to dry (aerobic) conditions also enrich δ15N values (Codron et al., 2005). During dry periods,
denitrification is shut off because of an increase in available oxygen in
sediments, and thus δ15N values decrease. If dry soils become
hydrated, there is a preferential loss of 14N, enriching δ15N values (Codron et al., 2005). Stable isotope analysis
was conducted at 3 cm resolution for total carbon (C) and nitrogen (N)
throughout the length of the sediment core. One cubic centimeter of bulk sediment
was dried, powdered, and treated with 0.5 M hydrochloric acid to remove
carbonates. A range of 1–25 mg of the dried carbonate-free sediment was
weighed into tin capsules depending on organic matter content. The samples
were analyzed on a Finnigan Delta dual-inlet elemental analyzer at the
Sirfer Lab at the University of Utah. 13C / 12C and
15N / 14N ratios are presented in delta (δ) notation, in ‰ (‰ relative to the PDB and N2 air standards)
(Codron et al., 2005).
Phytoliths
Phytoliths preserve well in sediment records and are especially useful in
areas with intermittent dry periods. Phytoliths were used as a proxy to
reconstruct past vegetation composition and are especially useful in the
lower taxonomic identification of grasses (Piperno and Pearsall,
1998). Grass phytoliths can provide important paleoecological information.
Tropical C4 grasses, adapted to open environments with high seasonality
of rainfall, typically expand at the expense of C3 grasses and other
tropical forest species during drier intervals
(Hartley and Slater, 1960; Hartley, 1958a, b;
Piperno, 1997). C4 Panicoideae grasses are generally adapted to warm
moist conditions, whereas C4 Chlorideae grasses are adapted to warm, dry
conditions (Hartley and Slater, 1960). C3 subfamilies,
including the Pooideae, are adapted to cool and moist conditions, and are
currently confined to temperate climates with lower temperatures
(Hartley, 1961, 1973; Iriarte, 2006). The presence
of C3 Pooideae grasses from phytolith data from southeastern
Pampas grasslands in Uruguay have been interpreted to indicate a shorter dry season
with overall conditions that were cooler than during the Holocene
(Iriarte, 2006). Phytolith samples were taken every 4 cm along
the sediment core. The extraction and slide preparation of phytoliths were
conducted at the University of Exeter, UK, following standard procedures
described by Piperno (2005). Slides were scanned and counted at the
University of Utah Power Paleoecology Lab using a Leica EMED compound light
microscope (400–1000 ×). The number of phytoliths counted varied from 101 to 320
per slide. The modern palm swamp is a monospecific stand of M. flexuosa that produces
globular echinate phytoliths but does not produce hat-shaped phytoliths
characteristic of other Arecaceae (Piperno, 2005). Although other
palms produce globular echinate phytoliths, the current monospecific stand
supports the identification of globular echinate phytoliths as belonging to
this palm.
Given the abundance of M. flexuosa during the middle and late Holocene, phytolith
percentages from globular echinate phytoliths were calculated separately.
Percentages of non-Mauritia phytoliths were calculated on the basis of the total sum
of phytoliths excluding M. flexuosa. Phytolith identification was made by comparison
with modern plant reference collections curated at the University of Exeter
Archaeobotany Lab. The classification of Poaceae implemented a three-partite
morphological classification related to grass taxonomy
(Panicoideae–Chloridoideae–Pooideae) (Twiss et al., 1969)
and further developed in both North America (Fredlund and Tieszen,
1994) and the Neotropics
(Bertoli de Pomar,
1971; Iriarte and Paz, 2009; Iriarte, 2003; Piperno and Pearsall, 1998;
Piperno, 2005; Sendulsky and Labouriau, 1966; Söndahl and Labouriau,
1970; Teixeira da Silva and Labouriau, 1970; Zucol, 1999, 2000, 1996, 1998).
The phytolith percentage diagrams were plotted using Tilia and Tilia*Graph software (Grimm, 1987). CONISS was used to calculate
phytolith zones (Grimm, 1987). CONISS is based on cluster
analysis, with the constrain that clusters are formed by hierarchical
agglomeration of stratigraphically adjacent samples to minimize dispersion
within the clusters (Bennett, 1996; Grimm, 1987). The divisions
were chosen using a broken-stick model to determine the number of
statistically significant zones at the lowest dispersion within the clusters
(Bennett, 1996).
Results
Five distinct zones were identified, i.e. zone 1, the Lateglacial
(14 500–11 800 cal yr BP);
zone 2, the early Holocene (11 800–9000 cal yr BP); zone 3, the middle Holocene (8000–3500 cal yr BP); and zone 4 and zone
5, the late Holocene (3500 cal yr BP to present).
Huanchaca Mesetta phytolith data separated by zones created by
constrained cluster analysis (CONISS). Grey bars indicate core breaks.
Huanchaca Mesetta lithology: (a) lithological description of the core
profile, (b) magnetic susceptibility, (c) loss on ignition (LOI), and (d) bulk
density. Zones derived from phytolith data. Grey bars represent core breaks.
Huanchaca Mesetta stable isotope data: (a) δ13C, (b) %
total carbon, (c) carbon to nitrogen ratio, (d) δ15N, and (e) % total N.
Zones derived from phytolith data. Grey bars indicate core breaks.
Zone 1: 14 500–11 800 cal yr BP: Lateglacial
The Lateglacial vegetation on Huanchaca Mesetta was dominated by arboreal
taxa, grasses, and Asteraceae (opaque perforated platelets) phytoliths
(Fig. 3). The phytolith assemblage likely contains both in situ vegetation
production and wind-blown vegetation from the surrounding rocky savanna.
Both C4 Panicoideae and C3 Pooideae grass phytoliths were present
during the Lateglacial. The presence of C3 Pooideae grasses is
interpreted as the result of cooler Lateglacial conditions compared to present. The
Lateglacial vegetation community at Huanchaca Mesetta lacks a modern
analogue plant community in NKMNP. The presence of both C3 Pooideae
and C4 Panicoideae grasses suggest some degree of landscape
heterogeneity. A consistent layer of very dark sandy silt dominated the
lithology of Huanchaca Mesetta during the Lateglacial. The magnetic
susceptibility and bulk density values were low and exhibit minimum
variability compared to the rest of the record (Fig. 4). Coupled with LOI
organic values below 10 %, the sediment lithology was summarized as a
low-energy depositional environment with relatively low nutrient input.
Organic matter deposited during the Lateglacial had δ13C values
of -16 ‰ (Fig. 5), indicating a contribution
of C4 grasses to organic matter composition. The proportion of
C3 to C4 grass contribution was calculated by using values of
C3 and C4 grasses and a simple two-pool mixing model
(Perdue and Koprivnjak, 2007) with end-member values of
-27 ‰ for C3 and -12 ‰ for
C4 plants. The contribution of C4 vegetation
was ca. 80 %, higher than any other time in the Huanchaca record. Modern
δ13C values in the basin range from -18 to
-22 ‰. The location of these C4 drought-adapted
grasses was likely the surrounding plateau. Organic carbon concentrations
gradually increased from 1 to 4 % during the Lateglacial, indicating
relatively low amounts of organic matter in the system compared to those of
today. The C : N ratio ranged from 20 to 30, indicating a terrestrial organic
matter source. N concentrations were low, from 0.1 to 0.2 %, and the δ15N values were ca. 5 ‰, indicating minimal
denitrification during the Lateglacial. The δ13C, % C4
contribution, and high C : N values coupled with the phytolith data dominated
by trees and grasses suggest a predominantly terrestrial signal
characterized by an open savanna grassland during the Lateglacial (Fig. 6). The δ15N values suggest that sediments within the swamp
were drier than present creating aerobic conditions and low denitrification
rates.
Charcoal influx levels were low during the Lateglacial (14 500–12 000 cal yr BP).
The fire return interval (FRI) was two fire episodes per 1000 years (Fig. 7). Based on the 0.5 cm sampling resolution of this record, fire
“episodes” were interpreted as periods of increased fire activity rather
than isolated fire “event”. The charcoal signature was consistent with
frequent, low intensity fires that likely occurred in the open,
grass-dominated mesetta surrounding the basin. Low charcoal influx levels
coupled with low-magnitude charcoal peaks suggest that the non-analogue
vegetation structure of C3 Pooideae, C4 Panicoideae, and arboreal
phytoliths likely created a fuel structure that lacked sufficient density or
fuel connectivity to produce abundant arboreal or grass charcoal. Low
charcoal influx coupled with low fire frequency suggest that the Lateglacial
environment was likely fire-sensitive within the basin.
Zone 2: 11 800–9000 cal yr BP: early Holocene
There were decreased C4 Panicoideae grasses, with consistent levels of
C3 Pooideae grasses, arboreal, and Asteraceae (opaque perforated
platelets) phytoliths. The presence of C3grasses, and the absence of
M. flexuosa, the dominant component of the modern basin vegetation, suggest temperatures
cooler than present. The lithology, magnetic susceptibility, bulk density,
and LOI values indicate minimal shift during the vegetation transition.
Organic geochemistry reflected a change in organic matter source, with
δ13C values becoming more negative, indicating an increase in
the contribution of C3 vegetation at ca. 11 000 cal yr BP. The δ13C contribution of C4 grasses decreased dramatically from 60
to 20 % during this period (Fig. 8). These data correspond to a decrease
in C4 Panicoideae grass phytoliths and an increase in arboreal
phytoliths. Low levels of terrestrial organic input into the system were
indicated by low carbon concentrations and C : N values ranging between 25 and
30. N cycling changed during the time of this zone, with δ15N values
exhibiting greater amplitude and higher frequency variability. The δ15N values ranged between 4 and 8 ‰, indicating
increased variability in denitrification rates associated with increasing
wet (anaerobic) to dry (aerobic) conditions. The N concentrations were low,
between 0.05 and 0.01 %, indicating minimal nitrogen availability in the
system.
C : N ratio to δ13C stable isotopes by zones determined from
phytolith data.
Charcoal influx at Huanchaca Mesetta increased at ca. 11 200 cal yr BP coupled
with an increase in the fire frequency to five episodes (periods of increased
burning) per 1000 years. The peak magnitude values indicated two substantial
fire episodes (periods of increased burning) at ca. 10 200 and
9100 cal yr BP.
The lack of significant change in the lithology suggests that taphonomic
conditions were consistent during this interval. The increase in grass
phytoliths during this period coupled with the increase in charcoal influx
and fire episodes suggests that the early Holocene vegetation community was
becoming increasingly more fire-dependent and vegetation was likely adapting
to the increase in fire frequency associated with the period.
Huanchaca Mesetta charcoal data (a) charcoal influx in grey, black
background; (b) charcoal influx log base 10 in grey, black
background;
(c)
peaks indicated by crosses; (d) peak magnitude; and (e) fire episodes per 1000
years. Zones derived from phytolith data. Grey bars indicate core breaks.
Huanchaca Mesetta summary: (a) charcoal influx in grey, black
background; (b) fire episodes per 1000 years; (c) peaks indicated by
crosses;
(d)
ratio of non-arboreal to total charcoal; (e) ratio of trees to trees and
palms; (f) ratio of C3 to total grasses; (g) ratio of palms to total
phytoliths; (h) % C4 contribution; (i) lake level of Titicaca in
m a.s.l.; and (j) insolation at 15∘ S. Zones derived from phytolith data.
Grey bars indicate core breaks.
Zone 3: 8000–3750 cal yr BP: middle Holocene
Significant vegetation changes occur through the middle Holocene. From 8000
to 5500 cal yr BP, C4 Panicoideae (warm/wet) grasses were at the
lowest values in the record. C3 Pooideae (cold/wet) grasses
diminished after ca. 7000 cal yr BP and remain absent for the remainder of
the record. Arboreal phytoliths reached the highest levels in the record at
8000 cal yr BP, followed by a slight decline to 3500 cal yr BP. δ13C values ranged between -24 and -22 ‰ from 7900 to 5100 cal yr BP. These values corresponded to a diminished
C4 contribution to organic matter (approximately 18 %). Decreased
C4 grass phytoliths from 8000 to 5000 cal yr BP was interpreted as a
decrease in vegetation density in the open mesetta surrounding the basin
caused by drying conditions on the mesetta. After 5000 cal yr BP, C4
Panicoideae grasses and C4 Chlorideae (warm/dry) grasses gradually
increased in the surrounding watershed, coupled increased δ13C
values to -19 ‰. M. flexuosa phytoliths first appeared at 5000 cal yr BP, and gradually increased to modern levels by 3750 cal yr BP. The δ13C values decreased, potentially associated with the development of
the C3 M. flexuosa community. A dark-brown clay–sand mixture from 8000 to 3750 cal yr BP dominated the lithology that transitioned to black detrital peat at ca.
3750 cal yr BP associated with the establishment of M. flexuosa. After
4000 cal yr BP,
LOI, magnetic susceptibility, and C : N values increased, indicating increased
organic material. Nitrogen cycling continued to fluctuate throughout this
period. δ15N values exhibited the greatest frequency and
amplitude of variability from 8000 to 3750 cal yr BP, ranging from 2 to
12 ‰, indicating repeated and extensive dry periods on the
mesetta.
Increased charcoal influx at ca. 8000 cal yr BP was followed by an abrupt
decrease to the lowest values during the record from ca. 7900 to ca. 3800 cal yr BP. Peak frequency reached the highest levels of six fire episodes
(periods of increased burning) per 1000 years during the middle Holocene. These
data corresponded to the highest levels of δ15N values,
indicating extended dry periods that likely promoted frequent fires on the
mesetta. The first evidence of grass charcoal appeared at ca.
6500 cal yr BP, suggesting a change in the fire ecology on the mesetta. From 5000 to 3750
cal year BP, grass charcoal increased. This is coincident with the
establishment of M. flexuosa palm swamp and increased C4 grasses in the surrounding
watershed. After 3900 cal yr BP, charcoal influx and fire frequency
increased. Significant increases in grass charcoal reflected a change in the
fuel composition in the watershed. Phytolith, isotope, and charcoal data
suggest that, after 3900 cal yr BP, the M. flexuosa within the basin became increasingly
fire-sensitive and the occurrence of a fire within the palm stand would have
had consequences for the vegetation not adapted to fire. The fire-adapted
C4 grass-dominated watershed continued to be fire-dependent.
Zone 4: 3750 to 2000 cal yr BP: late Holocene
There is a decrease in arboreal taxa coupled with increased values of M. flexuosa.
C4 Panicoideae (warm, wet) grasses continued to dominate the
surrounding watershed. The lithology consisted of black detrital peat at ca.
2450–2050 cal yr BP associated with high LOI values (ca. 22 % organics)
and magnetic susceptibility values (ca. 1000 × 10-5). After 2500 cal yr BP the %C, %N, and δ15N increased, suggesting moist,
anoxic conditions that enabled moderate denitrification from the swamp.
These lithologic and isotopic data represented the establishment of modern
palm swamp characterized by increased autochthonous organic accumulation.
The δ13C values reached modern levels by 2800 cal yr BP,
although values exhibit increased variability co-varying with
the C4 grass contribution.
Charcoal influx at Huanchaca Mesetta remained low 3750 to 2000 cal yr BP
with a FRI of five episodes (periods of increased burning) per 1000 years. Grass
charcoal reached the highest continuous levels at ca. 2800 to
2000 cal yr BP, corresponding to high levels of fire-adapted C4 grass phytoliths.
Increased grass charcoal coupled with low peak magnitude values and high
fire frequency indicated that the vegetation surrounding the palm swamp was
fire-dependent and fire-adapted. However, within the moist M. flexuosa palm stand, the
vegetation remained fire-sensitive.
Zone 5: 2000 cal yr BP to present: late Holocene
M. flexuosa reached the highest levels in the record at ca. 1800 cal yr BP, followed by
decreasing values towards the present. The presence of hat-shaped phytoliths at ca.
200 cal yr BP indicates very low concentrations of other palm species during
this time. There was a gradual decrease in M. flexuosa towards the present coupled with the
highest levels of C4 Panicoideae grasses at ca. 200 cal yr BP and a
decrease in C4 Chloridoideae (warm, dry) grasses in the surrounding
watershed. The lithology was dominated by dark-brown detrital peat. After
ca. 800 cal yr BP δ13C values were ca. -18 ‰ and the % C4 contribution was ca. 50 %. These data corresponded
to the highest levels of C4 Panicoideae grass phytoliths in the
record. The dark detrital peat lithology was interrupted by two coarse sand
layers at ca. 1550 and ca. 300–200 cal yr BP, followed by a shift
back to black detrital peat from ca. 200 cal yr BP to present. These sand layers
were characterized by a decrease in LOI from ca. 22 to 2 % organics, C : N
ratios from ca. 25 to 0, and δ15N from ca. 5 to
0 ‰ coupled with increased magnetic susceptibility and
bulk density values, suggesting clastic flood events associated with sandy
sediments low in organic material. After 300 cal yr BP, %C values increased
from ca. 1 % to > 20 %, reaching the highest values in the
record. The %N values increased from ca. 0.2 to the peak Holocene values
of 1.2 at present. The dramatic increases in both %C and %N were
likely the result of in situ carbon cycling and nitrogen fixation.
Charcoal influx increased after 2000 cal yr BP at ca. 1400 to 1200 cal yr BP, and reached peak Holocene values at ca. 500–400 cal yr BP. Increased
charcoal was coupled with the lowest FRI values in the record. Peak
magnitude increased significantly around 1200 cal yr BP and the largest peak
magnitude values at ca. 200 cal yr BP. These charcoal values were cropped for
plotting and visualization purposes. Raw counts exceed 1200; thus the values
are also provided log-transformed (Fig. 8). Peak frequency increased
after ca. 400 cal yr BP to ca. 4 fire episodes (periods of increased
burning) per 1000 years towards the present. There was a decrease in grass
charcoal,
indicating increased woody biomass burned. The increased charcoal influx
coupled with low FRI and more woody charcoal was interpreted as being the result of fire
episodes that infrequently penetrated the fire-sensitive palm stand and
burned the M. flexuosa woody biomass. The charcoal, phytolith, and isotope data
collectively suggest that the vegetation surrounding the palm swamp was fire-dependent and fire-adapted, while the vegetation within the palm swamp was
fire-sensitive.
Discussion
First-order control: edaphic constraints
Modern vegetation distribution of cerrãdo savannas are largely related to edaphic
factors (Colgan et al., 2012; Killeen, 1998a). Since the
Lateglacial, the vegetation, soil geochemistry, and fire history indicate
that edaphic constraints were the first order of control on vegetation on
Huanchaca Mesetta. Despite significant climate variability since the
Lateglacial (Baker et al., 2001;
Cruz et al., 2005), the open savanna surrounding the basin was continuously
dominated by fire-adapted C4 grasses. Within the basin, soil was
sufficiently thick to support more complex vegetation communities that
exhibited greater response to climate variability through time. On the
highly weathered quartzite plateau, however, vegetation was limited to
drought- and fire-tolerant C4 grasses, as indicated by the continued
presence of C4 Panicoideae grass phytoliths that co-varied with the
δ13C values.
The first hypothesis, that edaphic conditions are the dominant control of
vegetation on the plateau, was supported based on phytolith and isotope data. Irrespective of changes in
temperature, precipitation, and fire activity, savanna vegetation has been
present on the mesetta for the past 14 500 years. Edaphic conditions on the
open rocky plateau have limited species composition to C4 drought-adapted grasses. Arboreal and palm vegetation was limited to the topographic
depressions present on the plateau where soil was sufficiently deep to
support more complex vegetation communities.
Second-order control: climatological drivers
Lateglacial surazo winds and Mauritia flexuosa
Non-analogue Lateglacial vegetation communities are documented from low-elevation sites including Laguna Chaplin (14∘28′ S,
61∘04′ W; approximately 40 km west) and Laguna Bella Vista
(13∘, 37′ S, 61∘, 33∘ W; 140 km northwest of Huanchaca Mesetta).
The absence of Anadenanthera, a key indicator in present-day deciduous and semi-deciduous
dry forests, was interpreted as being the result of reduced precipitation (e.g., longer and/or
more severe dry season), increased aridity, and lowered atmospheric CO2
concentrations. These conditions favored C4 grasses, sedges, and drought-adapted savanna and dry forest arboreal species (Burbridge
et al., 2004). Similarly, the non-analogue Lateglacial vegetation community
at Huanchaca Mesetta is notable for the absence of M. flexuosa. M. flexuosa can tolerate a broad
precipitation gradient ranging from 1500 to 3500 mm annually in areas
with annual temperature averages above 21 ∘C, roughly
coinciding with the 1000 m a.s.l. contour line
(Rull and Montoya, 2014). M. flexuosa is dependent
on local hydrology, including water table depth and flooded conditions
(Kahn, 1987). The presence of M. flexuosa in the lowland records at Laguna
Chaplin and Laguna Bella Vista (ca. 200 m a.s.l.) during the Lateglacial
(Burbridge et al., 2004) indicates that conditions were
sufficiently warm with a locally wet habitat below the mesetta to support
the palms despite an estimated 20 % decrease in precipitation
(Mayle et al., 2004; Punyasena,
2008). Temperature was thus likely a limiting factor for the establishment
of M. flexuosa on the mesetta. However, temperature reconstructions of Lateglacial
conditions from Laguna La Gaiba (ca. 500 km SE of Huanchaca Mesetta)
indicate temperatures reached modern conditions (ca. 25 to 26.5 ∘C) around 19 500 cal yr BP and have remained relatively
stable to present (Whitney et al., 2011). However, previous
studies have suggested the increased frequency of surazo winds (Bush and
Silman, 2004). An ice cap located on the Patagonian Andes generated an
anomalously high pressure center in northwestern Patagonia, resulting in
increased surazo cold fronts blowing cold, dry, southerly winds
northward and penetrating the NKMNP region (Iriondo and Garcia, 1993;
Latrubesse and Ramonell, 1994). The surazos may have been no more intense than
those of present but likely occurred more often and lasted more of the year
(Bush and Silman, 2004). Increased frequency of surazos would have had
little effect on the absolute temperature minima, but the mean monthly and
annual temperature minima may have been ca. 5 ∘C lower
(Bush and Silman, 2004). Based on a lapse rate of 6.4 ∘C km-1 (Glickman, 2000), the 400 m difference
between the lowland sites (Laguna Chaplin and Laguna Bella Vista, ca. 250 m a.s.l.) and Huanchaca Mesetta (ca. 650–800 m a.s.l.) could have resulted in
up to ca. 2.6 ∘C difference in average annual temperatures.
Despite near-modern annual temperatures at ca. 19 500 cal yr BP, the
elevational lapse rate coupled with lower mean monthly and annual
temperature minima accompanying more frequent surazos likely resulted in climatic
conditions below the thermal optimum of 21 ∘C for M. flexuosa
(Rull and Montoya, 2014). Thus, during
the Lateglacial, increased frequency of surazos likely resulted in increased
biological stress on the vegetation community at Huanchaca Mesetta, resulting
in vegetation dominated by trees and grasses as opposed to M. flexuosa.
Interpreting CharAnalysis in paleofire reconstructions at Huanchaca Mesetta
The charcoal record from the Huanchaca Mesetta provides one of the first
subcentennial paleofire records from the cerrãdo savanna ecosystem. Previous
experimental studies on sedimentary charcoal from African savanna ecosystems
support the use of sedimentary charcoal to reconstruct past fire activity in
savanna systems (Aleman et al., 2013; Duffin et al.,
2008). The Huanchaca Mesetta charcoal record presents a novel approach,
combining charcoal influx data, CharAnalysis software (Higuera
et al., 2007), and arboreal/non-arboreal charcoal ratios in Neotropical
savanna ecosystems. Originally, CharAnalysis was designed as a
peak-detection tool for forest ecosystems with low FRI in the Northern
Hemisphere (Higuera et al., 2007). Paleoecological
investigations in fire-prone systems such as savannas can be challenging because of the annual to multi-annual FRI complicating the identification of fire peaks and isolated fire events.
To address the challenge of reconstructing cerrãdo paleofire activity, charcoal
influx was compared with the ratio of arboreal to non-arboreal grass
charcoal to infer the primary fuel source during periods of elevated fire
activity. Low charcoal influx values, coupled with low arboreal charcoal, were interpreted as periods of decreased burning.
Increased charcoal influx values and/or increased arboreal charcoal that
exceeded the background threshold were identified as fire episodes. Because
of the temporal resolution of the record, fire episodes were not interpreted
as isolated fires but rather as periods of time that experienced increased
fire activity (indicated by higher FRI values). Thus, an increase in the FRI
from 2 to 5 episodes per 1000 years, as seen from 8000 to 6000 cal yr BP,
represents more than a 50 % increase in the periods of burning over that
2000-year period. These data indicate a substantial shift in paleofire
activity during the middle Holocene, particularly as there were no
significant changes in the vegetation record on the Huanchaca Mesetta during
this time.
Holocene precipitation, fuel moisture, and fuel availability
During the middle Holocene in lowland Amazonia, the presence of dry forest
taxa and increased charcoal influx at Laguna Chaplin and Laguna Bella Vista
indicate a combination of seasonally flooded savannas and semi-deciduous dry
forests (Mayle et al., 2004). At
Laguna Orícore (13∘20′44.02′′ S, 63∘31′31.86′′ W; 335 km northwest), peaks in drought-tolerant arboreal taxa, coupled
with maximum charcoal concentrations, indicate drier and regionally more open
vegetation (Carson et al., 2014). Laguna Granja
(13∘15′44′′ S, 63∘, 42′37′′ W; 350 km northwest)
was also characterized by open savanna vegetation. These data suggest lower
mean annual precipitation (< 150 cm) and a longer dry season
(> 5 months with < 100 cm) during the middle Holocene
(Burbridge et al., 2004; Mayle et al., 2000).
Additionally, water levels at Lake Titicaca were ca. 100 m below present
(Fig. 8), attributed to precipitation levels ca. 40 % below present
(Baker et
al., 2001; Cross et al., 2000; D'Agostino et al., 2002).
The discrepancy in increased fire activity in the lowland sites and
decreased fire activity on the mesetta is attributed to fuel connectivity.
In the lowland sites of Laguna Bella Vista, Laguna Chapin, and Laguna
Orícore, dry forest–savanna vegetation provided sufficient fuel and
increased fire activity during the middle Holocene. At Huanchaca Mesetta,
decreased available moisture limited vegetation growth and fuel
availability, particularly in the edaphically constrained rocky mesetta
surrounding the basin. The lack of fine C4 grass connective fuels
resulted in decreased burning on the mesetta.
Lake Titicaca reached modern water levels between 3750 cal yr BP and the present (Rowe et al., 2003), indicating
wetter regional conditions with less severe dry seasons. The pollen
assemblages of Laguna Bella Vista, Laguna Chaplin, and Laguna Orícore
indicate an expansion of humid evergreen closed-canopy rainforest vegetation
coupled with significant decreases in charcoal concentrations
(Burbridge et al., 2004;
Burn et al., 2010; Carson et al., 2014). The rainforest–savanna ecotone is currently at its most southerly extent in the Amazon Basin in at least the last 50 000 years.
(Mayle et al., 2000; Mayle
and Whitney, 2012; Burbridge et al., 2004). The progressive
succession through the Holocene in the lowlands of NKMNP from
savanna/semi-deciduous forest to semi-deciduous/evergreen forest to
evergreen rainforest is part of a long-term unidirectional trend of
climate-driven rainforest expansion associated with the regional increase in
precipitation associated with a stronger SASM
(Mayle et al., 2004). The basin-wide
increase in mean annual precipitation and reduction in the length/severity
of the dry season is attributed to increasing summer insolation at
10–15∘ S driven by the Milankovitch precessional forcing
(Mayle and Whitney, 2012). The wet conditions of the late
Holocene created ideal waterlogged conditions for the establishment of the
M. flexuosa palm swamp in the drainage basin.
During the late Holocene, the asynchrony of charcoal records between the low-elevation sites and Huanchaca Mesetta is attributed to fuel flammability.
Increased precipitation led to different effects on fire frequency, with
decreases in the lowlands and increases in Huanchaca Mesetta. Increased
precipitation in the low-elevation closed-canopy rainforests decreased fuel
flammability along with fire activity. Whereas increased precipitation
resulted in the buildup of fire-adapted C4 grasses on the
surrounding plateau. Lightning-caused fire is common in cerrãdo savannas today and
highest in more open savanna ecosystems, such as the Huanchaca Mesetta
(Ramos-Neto and Pivello, 2000). Increased precipitation would have
been accompanied by increased incidence of lightning-caused fire, fueled by
the abundance of fire-adapted grass fuels in the surrounding watershed.
The second hypothesis, that climate was the dominant control on savanna
vegetation structure and floristic composition, was supported by the
vegetation and fire data. Since the Lateglacial, climate change has
coincided with both the vegetation composition and fire regimes on the
plateau. The asynchrony in response to regional climate forcing at Huanchaca
Mesetta and the low-elevation sites emphasizes the need to obtain more
paleorecords across an elevational gradient to determine the effects of
climate variability across heterogeneous ecosystems.
Human versus natural drivers on the evolution of Mauritia flexuosa
The development of M. flexuosa swamps and increases in charcoal influx have been seen
in numerous paleoecological records from savanna ecosystems in Colombia
(Behling and Hooghiemstra, 1998, 1999; Berrio et
al., 2002a, b), Venezuela
(Montoya et al., 2011b;
Rull and Montoya, 2014; Rull, 1999, 2009), and Brazil
(DaSilva Meneses et al., 2013). Previously two hypotheses
have been proposed to account for the late Holocene development of these M. flexuosa palm
swamps. The first hypothesis suggests that the increase in M. flexuosa and charcoal
influx is attributed to increased precipitation and wet-season lightning
fires driven by strengthened SASM activity (Kahn and de
Castro, 1985; Kahn and de Granville, 1992; Kahn, 1987). The second
hypothesis suggests that the simultaneous rise in M. flexuosa and charcoal was linked to
intentional planting or semi-domestication of M. flexuosa for human use
(Behling and Hooghiemstra,
1998, 1999; Montoya et al., 2011a; Rull and Montoya, 2014). Currently there
is insufficient archeological evidence from any of these savanna sites to
support a robust anthropogenic signal
(Rull and Montoya, 2014). Previous
paleoecological studies in the lowlands demonstrate humans were the dominant
driver of local-scale forest–savanna ecotonal change in those areas (e.g.,
Bolivian Llanos de Moxos) dominated by complex earth-moving pre-Columbian cultures
(Carson et al., 2014; Whitney et al., 2014).
These studies suggest that even in areas with extensive geometric
earthworks, inhabitants likely exploited naturally open savanna landscapes
that they maintained around their settlement, rather than practicing
labor-intensive deforestation of dense rainforest
(Carson et al., 2014). Evidence for human
occupation of the lowlands has been found with ceramics from soil pits in an
interfluve ca. 25 km northwest of Laguna Chaplin and abundant ceramics and
charcoal dating to ca. 470 cal yr BP recovered from Anthosols (terra preta)
throughout La Chonta ca. 150 km west of NKMNP (Burbridge et
al., 2004). Implementing a new methodology to concentrate and isolate
cultigen pollen (Whitney et al., 2012), the
re-analysis of pollen data from Laguna Bella Vista and Laguna Chaplin
revealed Zea mays pollen was present around 1000 to 400 cal yr BP; approximately
2000 years after the initial increase in M. flexuosa at these sites (B. Whitney, personal communication, 2014). Although humans were present in NKMNP, there
is no evidence that they drove regionally significant ecotonal changes in
forest–savanna boundaries. The patterns of forest–savanna shifts exhibited
at these sites are consistent with climate forcing
(Burbridge et al., 2004). The absence of archeological data
on Huanchaca Mesetta dominated by nutrient-poor, rocky soil, which would have
been infertile for the practice of agriculture, coupled with the limited
access to the mesetta would have made human habitation unlikely. Although
the M. flexuosa swamps may have been used for hunting and gathering purposes, these data
do not suggest humans were the driving mechanism behind the initial
establishment or proliferation of M. flexuosa in the interfluvial depressions of the
mesetta.
The comparison of the Huanchaca Mesetta record to previous studies coupled
with the absence of archeological remains on the mesetta support the third
hypothesis, that expansion of M. flexuosa at this site was largely controlled by
natural drivers (edaphic, climate, lightning caused fires) as opposed to
anthropogenic drivers. In contrast to the conclusions from other studies,
this record provides no evidence for an anthropogenically driven fire
regime, deforestation, soil erosion, or cultivation on the mesetta. These
data suggest that natural drivers control the continued presence of savanna
vegetation and fire activity on the Huanchaca Mesetta for the past 14 500
years.