4 DISCUSSION
With the understanding of biogeographic processes such as dispersal of
organisms, creation of barriers that promote vicariance, and generation
of new lineages and species (cladogenesis), it becomes clear that
climate change is central to these evolutionary processes, triggering
the origin and extinction of organisms within a given historical context
(Haffer, 2008). Some hypotheses (such as Pleistocene refuges and
Disturbance-vicariance) have been proposed to explain the great
contemporary Neotropical and Brazilian biodiversity (Haffer, 2008;
Aleixo et al., 2010). Heating and cooling cycles are thought to have
triggered adaptive processes in different biological groups in the
numerous paleogeographic phases. Episodes of retraction and expansion of
forest and open formations combined with isolation and speciation would
have affected Cerrado biodiversity (Silva & Bates, 2002; Haffer, 2008).
Regardless of the extent of space occupied by populations, the process
is a change that causes impacts that are not always ideal for the
environment.
It is therefore important to predict the effects not only on possible
new phytogeographic arrangements, but also on habitat functionality. The
results of this study demonstrate that, even in the most optimistic
climate change scenarios (ie RCP 4.5), climate requirements will
potentially have a strong impact on woody assemblages of savannas from
the central-north portion of Brazil in the near future (2050). The ENMs
estimated changes in the spatial pattern of environmental suitability
for the species and flora composition at macro-regional scale and showed
that 70% of the species will occupy new niches and 30% will have
smaller ideal ranges.
Due to its central and ecotonal location, greater turnover can be
expected in future savanna domains in response to climate change,
especially in areas of steep west-east moisture gradient (Davidson et
al., 2012), such as in central-north Brazil. Although the modeled
species are typical of Cerrado, they are eventually recorded in ecotonal
areas under the influence of neighboring domains (BFG, 2018) and, as
they are heliophilous plants, they will potentially respond by expanding
their habitats, possibly increasing competition within and between
floras inside the Biome and in adjacent sites. Thus the generated models
are inclusive for all species using all available records of occurrence
within the study area, regardless of whether the distribution is
exclusive to the Biome or ecotonal (typical or ecotonal species,
generalist or marginal species), focusing on potentially habitable sites
for these species (Siqueira & Durigan, 2007).
Generalist species demonstrated growth in the projected range. For
Beisner, Haydon, & Cuddington (2003) it is expected that resilient
species adapt to frequent environmental disturbances. The projection was
not much different for species typical of the northern marginal Cerrado.
However, P. reticulata , S. convallariodora and H.
articulatus may lose climate suitability and be restricted to
alternative stable states (Beisner et al., 2003), adopting new
environmental equilibrium ranges as refuge zones, as is currently the
case found on the edges of the Amazon domain and in the disjunct savanna
fragments between the Amazon and Caatinga.
Most species showed spatial range gain in the projections, providing
minimal survival conditions for heliophilous species. This corroborates
the assumption of expansion of xeric environments possibly initiated in
the Holocene and maintained in the present period (Simões et al., 2019).
The idiosyncratic response of some species (three species showed
decreased projected area) reflects the lack of a pattern in the
scenarios and indicates that climate change will have a variable
influence on the varied plant communities of Cerrado (“synareal
floras”, in personal communication by J. M. Costa-Coutinho et al. in
press). Moreover, considering only the climate factor, the Climate
Observatory/SEEG (http://www.observatoriodoclima.eco.br) forecasts a
72% increase over the average temperature and a decrease in
precipitation, with the most negative trends in the Cerrado and the
Amazon (Azevedo et al., 2018; Penereiro, Badinger, Maccheri, &
Meschiatti, 2018).
Impacts on biodiversity vary depending on the biome, and even within a
biome, responses may vary at different scales (eg Siqueira & Peterson,
2003; Siqueira & Durigan, 2007; Terribile et al., 2012). The difficulty
of native floras to adapt to climate change tends to aggravate the
natural forest degradation in a few decades. Even widely distributed
species can have population sizes reduced in some areas and suffer local
extinction in others, in long time series with seasonal tendency to
aridity (Simões et al., 2019). It can be extrapolated that the
structural simplification, increased mortality, and reduced average
plant density are indicators of changes caused by the
“desertification” of the northeastern savannas and semi-arid steppes,
the “savannization” of the Amazon, and the “erosion” of central
savannas, contributing in all cases to the contemporary reduction of
phylogenetic diversity and to new patterns of ecological niches (Costa,
Carnaval, et al., 2012; Terribile et al., 2012).
Recent studies have shown considerable niche resilience to drought, but
also interactions between deforestation, fire, and potential carbon
storage discharge and precipitation (Davidson et al., 2012). Although
the climate is a predominant driver of community changes, several
current factors operate in the disturbance of habitats, preventing the
recovery and/or natural growth of landscapes and interfering with the
real environmental suitability. Under the conditions analyzed in this
study, all species are expected to find adequate habitats in focal
areas, even in the worst climate scenario. However, it is noteworthy
that other environmental and anthropogenic complications were not
considered, neither the inherent biological relationships of
inter-population and intra-ecosystem interactions. Research has
indicated the importance of soils in the distribution of plant species
(Siqueira & Durigan, 2007), but edaphic variables were not included in
the projections of northern savanna, because previous analyses (see
summary of data available as a file in
https://doi.org/10.5061/dryad.9cnp5hqd4) pointed to the
topographic and climatic attributes as the main modelers of these
savannas, showing less relevance to edaphic aspects, which are
subordinated to climate variations, especially the high rainfall regime
of Amazonian savannas and drought and sharp thermal amplitude in the
northeastern savannas. Similarly, through a modeling study in the
Amazon-Cerrado transition, Dionizio et al. (2018) found that the
dynamics of environmental effects along the latitude-longitudinal
gradient are particularly due to climate, and then due to the frequency
of fires and phosphorus limitation in the soil.
The dynamics of environmental suitability are most likely to decrease in
the Cerrado-Amazon border, where deforestation is greatest and where
climate and plant diversity move between ecosystems (Davidson et al.,
2012). In line with these authors, lower precipitation in these ecotones
will make the conditions between forest (short drought) and savanna
(long drought) less limiting, promoting its expansion. However, it is
not possible to measure the direction and extent of displacement in
these borders because some of the adopted algorithms (MAXENT, for
example) may extrapolate the projections of occurrence in their
adjustments (Vale, Tarroso, & Brito, 2014). Siqueira and Durigan (2007)
showed that models generated from “pure” data, with elements from a
single biome, were more effective in terms of prediction than “mixed”
models, which had species with broader distribution, also occurring at
edges of fragments belonging to other plant formations.
For Vale et al. (2014), the analysis of species distribution in marginal
ranges (transition zones) for ecological model predictions requires
greater caution in selecting the extent, resolution and boundaries of
the studied area. Terribile et al. (2012) also recommend separating the
different sources of uncertainties in the modeling to assess the
reliability of the predictions. For this reason, three other algorithms
(GLM, ANN, RF ) in addition to MAXENT, in different circulation
models (AOGCMs) and scenarios, were included in the present analyses so
as to increase the percentage of hits of the resulting model when
compared to emission scenarios, and align the results with recent works.
The prediction of the real influence of environmental changes on the
different elements that act on plant diversity is based on conjectures
and are conditioned to fluctuations in direct (temperature,
CO2 emission, solar radiation, precipitation, sea ice
extent) and indirect (food availability, pests, soil moisture, sea
level) components that impact biodiversity (IPCC, 2018). According to
Costa, Carnaval, et al. (2012), the latest diversification cycles of
some biological groups were correlated with recent climate changes; for
other groups, though, such changes contributed little to today’s
richness and geographical distribution. Terribile et al. (2012) consider
appropriate to examine both the past and the future in order to map the
most likely areas of Cerrado in the future. In this context, assessing
the consequences of variations in phenological and reproductive
patterns, ecological interactions, length of annual seasons, and
responses linked to adaptability, plasticity, migration or extinction in
populations, for example, is to be a goal of current research (Azevedo
et al., 2018; IPCC, 2018).
ENMs in Cerrado plant lineages contributes to this discussion and gives
an idea of the susceptibility of this Biome to current trends in land
use and chronic use of natural resources, ensuring the information of
Silva and Oliveira (2018), which outlined potential areas of
“topoclimate” suitability shared by species across present and future
climate scenarios. The present study presented novel data showing that
the largest inclusive extent of the projected refuge will potentially be
concentrated in the center of the studied area (central-north or
mid-north Cerrado), coinciding with biome “subhotspots” considered by
Castro and Martins (1999) as the Cerrado’s peripheral (ecotonal)
boundaries of high peaks of diversity (see database available at
https://doi.org/10.5061/dryad.9cnp5hqd4). As discussed by J. M.
Costa-Coutinho et al. (in press ) in personal communication, lower
and plastic diversity is expected from peripheral floras in fragmented,
ecotonal habitats with high structural complexity acting as
environmental filters that cause the homogenization of diversity (Kortz
& Magurran, 2019). But as the models show, for the species analyzed,
many of these areas will provide the ideal conditions for some Cerrado
communities. These results reinforce the authors’ interest in this
ecotonal region of high scientific interest in the meeting of
supercenters of Cerrado biodiversity (Castro & Martins, 1999; Castro et
al., 2010).
Endorsing the work of Terribile et al. (2012), biodiversity
concentration reservoirs are indicated as priorities for conservation
actions of Brazilian savannas although the protocols, scales and focal
areas are different. However, current protected areas proved to be
poorly effective for relictual refuges in the modeled scenarios. The
projection of the climatically stable focal area is mostly connected,
but largely unprotected, overlapping degraded areas. Notably, less than
10% is housed by currently established CUs.
Disjoint refuge fragments to the northeast, south and east of the
projected area do not overlap with any of the CUs. They are small
remnants of natural vegetation, but without protection. Combined with
climate change, deforestation and fires are the factors with the biggest
impacts, stimulated by the growing importance of the northern savannas
as a cultivation lands for South America, due to a growing competitive
advantage. Agricultural expansion and urbanization, which are not
limited to a single phytogeographic region, impose an environmental
footprint that furthers the Cerrado’s vulnerability. Contrary to the
current environmental policy of the Brazilian government, the
identification of these environmental refuges and indication for
preservation represent the minimal palliative measures to stop anthropic
actions, yearning for greater future effectiveness of CUs, certainly as
buffer zones for these events in ecosystem functioning and conservation.
The predictive power of ENMs showed high reliability and the results
provided evidence that climate change will affect the distribution
performance of the ten investigated oreadic woody species and alter the
potential extent of their fundamental niches. The projected occurrence
range coincides with the extension of the Cerrado biome, but
interpenetrations in adjunct biomes are estimated to be greater,
especially in the Amazonian border. There was no pattern of displacement
of species towards higher elevations and towards climatically milder
areas. With the gradual atmospheric warming, plants in the central-north
Cerrado will be affected even in the most optimistic scenario. In both
distribution status, the pattern of climatic influence of the species
was not the same, which could have consequences on the ecological
relationships and functionality of the floras. The main impact for the
largest number of species, consistent in most scenarios, was the
expansion of potential areas of occurrence, leading to suppression or
cohabitation with species of other biomes. Therefore, northern Cerrado
vegetation tends to benefit from the expansion of thermal suitability,
although fragmentation and/or displacement of optimal environmental
suitability is expected as the scenarios advance, especially for
typically marginal plants. The confluence of the most suitable areas is
considered a refuge and the largest extension is foreseen in the
central-north area of the studied area, mainly involving parts of
Maranhão, Tocantins and Pará. Combining data from legal reserves and
vegetation suppression, the protected areas as a whole have the
potential to protect less than 5% of the identified stable climatic
areas. The models generated here show environmental refuges for species
of the central-north savannas as the most indicated areas to be focused
in conservationist measures. The findings also demonstrate the
insufficiency of the CUs for protecting present and future oreadic
floras, thus suggesting optimizations of maintenance strategies in the
Brazilian Cerrado.