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.