Potential distribution maps
According to the potential distribution map (Figure 2), 23 municipalities have records of the presence of the species, and 31 municipalities have areas of high suitability for its potential distribution (0.59-0.92); of these, 12 have no previous records. There are also suitable conditions for the development of the species along the Amajaque river and other localities; however, no records exist, and therefore it is crucial to carry out more fieldwork in order to verify the veracity of the modeling. The presence of T. huegelii has not been documented along the Beltrán, Blanco, and Claro rivers, consistently with the obtained results, as these areas offer no suitable environmental conditions for the species despite the presence of bodies of water (Figure 2).
The potential distribution of T. huegelii is within the limits of the Eastern Sierra Madre, (Zacatecan-Potosin) Southern Highlands, and Gulf of Mexico biogeographic provinces. The prevailing climate types are Warm humid, Dry warm semi-dry, Dry temperate, Semi-cold semi-dry, and Temperate sub-humid. It has previously been pointed out that T. huegelii is a typical tree of low and semi-warm places, being uncommon in very warm places, at altitudes below 300 m, and at temperatures equal to or above 25 °C (Martínez, 1963), consistently with the results of the present study.
To the north of its distribution in the state, it forms green strips along rivers in dry regions with an arid to semi-arid landscape (Farjon, 2017). In addition to the environmental conditions evaluated in this research, other biotic factors associated with the types of climates in which the species is absent may restrict its geographic distribution.
Variables independent contribution to the model
Three of the environmental variables that contributed the most to the prediction are related to precipitation (Bio_16, 24.2 %; Bio_19, 20.5 %, and Bio_13, 10.4 %). Likewise, the response curves indicate that the habitat suitability of T. huegelii in the state of Hidalgo is high in regions where the precipitation of the wettest four-month period (Bio_16), the precipitation of the wettest month (Bio_13), and the annual precipitation (Bio_12) are less than 250, 100 and 500 mm, respectively.
According to the results of this research, most of the documented records are located on the banks of rivers and localities associated with springs in the municipalities of Chapantongo in the El Tanque spa, Epazoyucan in Xolostitla de Morelos, Metztitlán in Zoquizoquipan, Santiago Tulantepec de Lugo Guerrero in Ventoquipa, and Tecozautla in the springs of Taxhido. This is evidence of the close relationship between the development of the species and water sources. The presence of few or only one individual far from water bodies is the result of planting for ornamental purposes.
Since pre-Hispanic times, T. huegelii has been introduced and planted in many locations far from water sources in Mexico and Guatemala; however, all of these places have (or had) water tables that are reached by the deep root system of the trees. Most specimens correspond to planted trees (Farjon, 2017).
Villanueva et al. (2010) point out that the ahuehuetes of Barranca de Amealco, in Querétaro, are very sensitive to the precipitation that occurs in summer and is related to the increase in flow rates that cause an increase in oxygenation and finally greater growth. In addition, seasonal changes of 80 to 90 cm in the water table depth result in temporary reductions in water potential and a maximum net photosynthetic rate of the foliage (Campos-Ángeles et al., 2005).
The distribution of T. huegelii along riverbanks is indicative of riparian vegetation or gallery forest, which is important for maintaining biodiversity (Naiman and Decamps, 1997). During floods, this species traps nutrients and pollutants, in addition to stabilizing sediments, whereby it prevents soil erosion and improves water quality, this is of crucial importance, considering that the Tula river is one of the most polluted rivers in the country (Arredondo et al., 2017). It also reduces the speed of water flow by keeping a sinuous course that favors the recharge of water in the groundwater table at a high level (Patten, 1998); however, when the flow increases, due to prolonged flooding, it can affect regeneration by seeds and shoots (Gecy and Wilson, 1990). Other key factors in the decline of riparian vegetation include soil compaction (Kozlowski, 1985), lack of water, soil infertility, contamination, water scarcity (Kozlowski and Pallardy, 1997), the construction of river terraces (Enríquez-Peña and Suzán-Azpiri, 2011), and climate change.
The suitability of the habitat of T. huegelii in Hidalgo is high in regions where the minimum temperature of the coldest month (Bio_6) is not below 3 °C, consistently with what is cited in the literature in the sense that it is a frost-sensitive species, unlike T. distichum (L.) Rich. which withstands extremely low temperatures (Farjon, 2017), its optimum development interval is between 16 and 22 °C (Conafor and Conabio, 2019). In northern and central Mexico, high temperatures at the end of winter favor its growth during spring (Villanueva et al., 2013). The climatic variables of precipitation and temperature are closely related to the ecophysiology of T. huegelii (Fritts, 1976; Correa et al., 2014).
Altitude (Elev) contributes 4.5 % to prediction. In this research, the altitudinal range for T. huegelii in the state of Hidalgo was determined to be 112 to 2 472 m, consistently with the previously cited records (Martínez, 1963; Eckenwalder, 2009; Villanueva et al., 2010; Farjon, 2017, Martínez-Sifuentes et al., 2021).
Martínez-Sifuentes et al. (2021) predict a reduction in the distribution of T. huegelii in Mexico for future climate change scenarios (2050, 2070) under the riparian ecosystem criterion. In Hidalgo, there is no protected natural area that guarantees the conservation of genetic diversity for the species; therefore, the creation of at least one Ecological Conservation Area for the entity should be considered based on future studies of genotypic and phenotypic diversity.
As a riparian species, ahuehuete has a close relationship with bodies of water, which implies that its dispersal maybe much more difficult under climate change scenarios, compared to tree species whose survival does not strictly depend on the presence of bodies of water, therefore, in situ conservation is particularly important for this species.
The location of long-lived organisms is paramount and supports the continuation of efforts for the protection, restoration, formulation of ecotourism projects and conservation of the large extension of unique riparian ecosystems in Mexico in which T. huegelii is distributed (Villanueva et al., 2010; Villanueva et al., 2012). In Mexico City, it is used as an indicator of surface aquifers (Martínez and Chacalo, 1994). In the case of ahuehuetes, historical records allow to determine the seasonal variation of precipitation based on dendrochronology (Cortés et al., 2010; Correa-Díaz et al., 2014); such is the case of the climatic conditions prevailing during the establishment, flourishing, and decline of pre-Hispanic civilizations (Villanueva et al., 2010).
The ages of ahuehuete trees in gallery forests in Hidalgo have been little studied (Correa-Díaz et al., 2014) and remain virtually unknown. Among the states in which such data for T. huegelii individuals are available, the following can be cited: Aguascalientes, Chiapas, Coahuila, Guanajuato, Jalisco, Michoacán, Nuevo León, Oaxaca, Querétaro, Tamaulipas and Zacatecas (Suzán-Azpiri et al., 2007; Villanueva et al., 2010).