Special modeling
The current and future distribution models developed in MaxEnt in the present study show a good performance in the AUC test (0.88) for the training data and (0.93) for validation, and an excellent adjustment in the partial ROC with results of 1.85 to 1.94 and significant values of Z (p < 0.01). Peterson et al. (2011) report AUC values between 0.7 and 0.9 and the performance of the model is considered good, and close values to two of partial ROC are adequate without random effects (Peterson and Nakazawa 2008; Garza-López et al. 2016).
The present study involved 250 records of the P. greggii species, where the quantity and quality of the records enhanced the model performance.
Stockwell and Peterson (2002) showed an increase in precision and suggested a minimum of 50 records to develop the species distribution analysis. Aceves-Rangel et al. (2018) developed a potential distribution analysis of Pinus species, but used only 33 records of P. greggii which resulted in an AUC of 0.95, although this AUC is greater than the one obtained in the present study, the high AUC may have been due to fewer records, the lack of debugging and calibration analysis, and a location in the state of Chiapas, being the P. greggii an endemic species of the Sierra Madre Oriental and the eastern section of the Neovolcanic Transversal Belt (Ramírez-Herrera et al. 2005).
A potential current area of 617,706.04 ha of P. greggii was identified in this study, in contrast to a study that was done by Aceves-Rangel et al. (2018) that determined a potential current area of only 550,300 ha. The difference between both estimations was 67,406.04 ha, although it should be noted that the previous study did not include edaphic variables, which are relevant in potential distribution studies (Cruz-Cárdenas et al. 2016; Manzanilla-Quiñones et al. 2019a).
Relevant Environmental Variables
The most important variable for this study was the BIO1 (mean annual temperature), coinciding with diverse studies related to the Pinus habitat, which have shown that this variable plays an important role for at least ten different species (Aceves-Rangel et al. 2018).
The high dependence shows the significance of the effect that temperature has in the establishment and growth of conifer species (Wang et al. 2016), which has been corroborated by the association of drought index and the rise of the temperature in arid zones (Vicente-Serrano et al. 2010; Ma et al. 2014).
One aspect to consider is that an increase in temperature generates an accelerated development of the species, but with reduction in their growth (Gennaretti et al. 2014). This behavior is attributed to elevated temperatures promoting an increase in evapotranspiration and, consequently, a metabolic alteration that impacts the assimilation of photosynthates (Girardin et al. 2012). Temperature is an important component in global climate change and has presented modulations in their variability (Medhaug et al. 2017), especially in northern Mexico where the climatic models forecast significant increases (IPCC 2014).
Studies like Gutiérrez and Trejo (2014); Martínez-Méndez (2016); Manzanilla-Quiñones (2019a) have found that an increase in temperature caused by climatic change will lead to a decrease in the area of Abies, Quercus, and Pinus genera.
In the study by Aceves-Rangel et al. (2018), the variable BIO11 (mean temperature of coldest quarter) was found to be important for P. arizonica, which is ecologically associated with P. greggii and found in similar climate conditions (López-Peralta and Sánchez-Cabrera 1996).
García-Aranda et al. (2018) found the variable BIO11 plays an important role in the distribution of P. nelsonii in northwestern Mexico, a species associated with P. greggii in the Sierra Madre Oriental and the Neovolcanic Transversal Belt. The mean value of the BIO11 variable was 4.3 °C in the present study and is very similar to the mean value found for P. nelsonii of 4.6 °C — an adequate temperature for the growth and development of P. greggii
.The SLO for Garcia-Aranda et al. (2018) found that this topographic variable presented great relevance in three Pinus species with distribution restricted to northeastern Mexico (P. cembroides, P. culminicola, and P. nelsonii), contributing to the model with 21.1% significance; close to that found in this study of P. greggii with 18.3% of contribution to the current distribution model.
The SLO is a significant variable for the establishment of tree species located on steep soils. Muñoz et al. (2012) mention that the P. greggii is present on slopes with up to a 5% grade; the mean slope value found in this study is 8%.
The BIO19 variable (precipitation of coldest quarter) was considered an important variable for at least seven Abies species in Mexico (Martínez-Méndez et al. 2016). According to the study of Aceves-Rangel et al. (2018) BIO19 was relevant for P. lumholtzii at a national level with 8.2% as the lowest value, in comparison to the 12.8% found for P. greggii. In this analysis, the mean BIO19 was 427 mm, however P. greggii is adapted to zones with low precipitation ranging from 293 to 747 mm (Ramírez-Herrera et al. 2005).
The annual oscillation of the temperature (BIO7) for the current distribution of P. greggii in the present study had a mean value of 25.4 °C, which, according to the study zone, is a suitable temperature in the ideal annual range for the species development. This variable is relevant in other studies as well: Hernández et al. (2018) found that BIO7 had an importance factor of 12% for distribution of Cedrela odorata in Mexico, slightly less than our 15.5% of the total in the present study of P. greggii; Martínez-Méndez et al. (2016) considered BIO7 a useful and relevant variable in the distribution of four species of Abies at a national level, which demonstrates BIO7 to be a relevant environmental characteristic for tree species in Mexico.
Future Scenarios
Diverse studies about climate change scenarios have been generated in Mexico for Pinaceae, the majority of which belong to temperate and cold climates. These studies agree with the theory of a significant reduction in the distribution areas of the Pinaceae for 2050 (Sáenz-Romero et al. 2012; 2015; Cruz-Cárdenas et al. 2016; Manzanilla-Quiñones et al. 2019a). However, this type of study has not been widely applied to Pinus species that grow in the arid and semiarid regions of the country.
According to the increases of temperature forecast by the CNRM-CM5 and HadGEM2-ES models with two radiative forcings (RCP 4.5 and 8.5) for 2041–2060, the ecological niche will decrease between 7.8% and 21.8% within the endemic zone of P. greggii, but with a tendency to modify their distribution with consideration for the climatic change scenarios between RCP 4.5 and RCP 8.5 as are mentioned by Gavilán (2008).
In the modeling study of pinyon pines under climate change scenarios in Mexico by Pérez et al. (2019), they found that P. culminicola, P. johannis, and P. pinceana will show a decrease in area in relation to the current area, with a greater area presented in RCP 8.5 relative to RCP 4.5. This situation is similar to that of P. greggii with respect to a scenario with constant trajectories of CO2 concentration and other with increases in concentrations on the atmosphere, because all four species are from similar environmental conditions located at the bottom of the mountains of arid and semiarid climates (Perry 1991; Farjon and Styles 1997).
In the analysis of future distribution based on the statistical model of scale reduction for P. arizonica and P. cembroides (species similar to P. greggii), Romero-Sánchez et al. (2017) found there would be an increase in area of 52.29% and 45.95% by 2050, respectively (in comparison to the current area in Sierra de Zapaliname, Coahuila). While the study of Romero-Sánchez et al. (2017) was regionalized, the tendency of increasing in area of forest species in arid climates foresees favorable effects on species such as P. greggii, P. arizonica and P. cembroides under climate change effects considering the area of a one forcing to other, suggesting that although effects of global climate change may be negative, the analysis at local level may be beneficial for some species.
Conservation Areas Of Niche
Studies conducted on conservation of ecological niche mention that species with the tendency to conserve their niche either adapt to climatic changes through time or move to colonize new geographical areas with characteristics similar to those of their original ecological niche of (Martínez-Meyer and Peterson 2006; Peterson 2011).
The study of niche conservation based exclusively in the ecological niche modeling provide the advantage of the comparison method of environmental values of the locations where the species can be found, with estimated values through an environmental probabilistic index for the species. This index provides detailed relevant information about the niche components because it applies the niche theory as a multidimensional hypervolume proposed by Huchitson (1957) and is complemented with the geographical analysis part by Soberón and Peterson (2005), in which some or most of its environmental components are preserved (McComarck et al. 2010) allowing one species to persist at a different temporal scales (Martínez-Meyer 2006; Peterson 2011). However, these types of studies are very scarce for the Mexican conifers and it is just beginning to be studied in the country.
Martínez-Méndez et al. (2016) mention (without testing the hypothesis) that the ecological niche (as determined for Mexico) of the genus Abies has remained stable over time. Manzanilla-Quiñones et al. (2019a) tested this hypothesis in Abies religiosa [Kuth] Schltdl & Cham, finding that the ecological niche of A. religiosa has remained stable or has been preserved since 6,000 years ago in the high and humid parts of the Neovolcanic Transversal Belt. The results obtained by our study through the conservation model HadGEM2-ES RCP 4.5 estimate 75.6% of the conservation niche. These results indicate a smaller conservation niche area in comparison to the study of Manzanilla-Quiñones et al. (2019a) for A. religiosa, which is due to a reduction in the amount of moisture and an increase in the temperature in the ecological niche of P. greggii.
According to the analysis of the interaction of the relevant variables in the current and future distribution, it was possible to delimit two conservation niche zones suitable for the generation of conservation and restoration activities of P. greggii inside their natural distribution in Mexico.
Aguirre and Duivenvoorden (2010), in their study about the potential modeling of 56 Pinus species in Mexico mentioned as a conservation proposal that one of most important zones to establish new protection areas for this genus is the Sierra Madre Sur where several currently modeled species exist, as well as few areas that are under regulatory protection. However, this conservation proposal was focused on Pinus species in a more temperate climate, which differed from the arid to semiarid of P. greggii.
Manzanilla et al. (2019b) proposed two zones of conservation and seed production for P. pseudostrobus and P. montezumae. Although they have different environmental requirements than P. greggii, the proposed zones of conservation and seed production are similar, with two delimited zones in both our study and Manzanilla et al. (2019b).
Most of the studies on this subject have been made on Pinaceae of temperate-cold climates, therefore, the findings of our study are relevant because it allows us to define and propose two important areas for the conservation and restoration of P. greggii in Mexico.
Finally, according the objectives of this study, it was possible to delimit and estimate the current natural distribution of P. greggii in Mexico and the most relevant environmental variables were also analyzed. From this data we were able to determine the presence of this important ecological and economic species in Mexico, and from the niche conservation analysis between the current and future distributions, it was possible to propose conservation areas, which do not compromise the development and growth of the species because they would be under similar current–future environmental conditions.