Subject development

Urban forests are important ecosystems that provide numerous environmental services to humans and wildlife (^{Martínez-Trinidad et al., 2021}). Due to its importance, interest has arisen in studying said woodland to learn about some of its ecological aspects. Traditionally, works have focused on the structure and floristic composition of these ecosystems (^{Martínez-Trinidad et al., 2021}), but recently, others have been directed towards the origin of the species (native or introduced), based on the argument that the native flora is more recommendable than the exotic one, because, among other reasons, it can become invasive (^{Alanís-Rodríguez et al., 2023}).

It is still evident that exotic trees are more popular in public parks as shown by studies worldwide (^{Saavedra-Romero et al., 2019}). Particularly, in northeastern Mexico, research has been carried out in urban areas indicating that there is a higher proportion of introduced species and individuals, which have more species richness, basimetric area and crown area than the native ones (^{Leal et al., 2018}; ^{Canizales et al., 2020}; ^{Alanís-Rodríguez et al., 2022}). However, there is not much information about rural areas in this regard, despite the fact that the use of trees of both origins is increasingly common.

Therefore, the objective of this work was to evaluate the diversity and structure of native and introduced trees of the six most populated rural localities of Hualahuises, Mexico. It was based on the hypothesis that the introduced species will record higher values of species richness, Margalef index, Shannon index, basimetric area and crown surface area compared to native species.

The study was carried out in the six rural localities with the largest number of inhabitants in the Hualahuises municipality, Nuevo León State (Northeast Mexico). The localities have a population between 92 and 253 inhabitants (^{Secretaría de Economía, 2020}). The area belongs to the North Gulf Coastal Plain province and to the San Fernando-Soto la Marina hydrological region. It has Vertisol soil and a semi-warm sub-humid climate with summer rains. The rainfall range goes from 700 to 900 mm (^{INEGI, 2010}). Table 1 shows the names of the towns with data on their latitude, longitude and altitude.

In April 2022, a census of urban trees was carried out based on the tree species that were located in public parks, squares and ridges in six rural towns in the Hualahuises municipality, Nuevo León, Mexico.

The normal diameter (d _1.30 ) was determined with a 283D/5m Forestry Suppliers Inc^® diameter tape, total height (h) and crown length (Lc) with the manual Haglöf Vertex^® III hypsometer, and the crown diameter (k) with a TP50ME 50 m Truper^® fiberglass tape. The correct nomenclature of species was based in Tropics (2022). For each species, its abundance, coverage and frequency were determined according to its presence in the sites of interest (^{Alanís et al., 2020}). To evaluate the crown, the surface area and its volume were calculated. With the aforementioned information, the Urban Importance Value Index (UIVI) was estimated according to ^{Saavedra-Romero et al. (2019)}.

To test the null hypothesis (H ₀ ) of no difference in species richness, basal area, and crown surface area of native and introduced trees from the six rural locations, the proportions test was used with a confidence level of 0.95 using the command prop.test in R (^{R Core Team, 2021}).

The total number of individuals was 252 of 32 species distributed in 29 genera and 19 families for the six rural locations that were evaluated. The Fabaceae family was the largest represented by seven species. Due to their origin, of the 32 taxa, 19 are native (59.37 %) and 13 introduced (40.63 %), 77.79 % of the individuals are native and 22.25 % introduced. Regarding their dimensions, the percentage differences are more pronounced: the native ones represent 91.69 % of the basimetric area, 92.36 % of crown area, 92.22 % of crown surface area and 96.78 % of crown volume, while the exotic recorded numbers lower than 9.00 %.

At the species level, the most outstanding in structural terms were Prosopis laevigata (Humb. & Bonpl. ex Willd.) M. C. Johnst., followed by Vachellia farnesiana (L.) Wight & Arn. and Ebenopsis ebano (Berland.) Barneby & J. W. Grimes. The three species are native belonging to the Fabaceae family and together they represent 44.05 % of the abundance and 63.71 % of the Urban Importance Value Index (Table 2). The most abundant introduced species and UIVI are Fraxinus americana L., Leucaena leucocephala (Lam.) de Wit and Cupressus sempervirens L., adding 18.22 % abundance and 8.93 % of UIVI.

Based on the analysis of proportions, it was determined that the native species registered higher values of species richness, Margalef index and Shannon index (p<0.001) than those introduced in four evaluated rural locations, while in the remaining two it was found equality in species richness. Likewise, it was determined that there is a difference between the basimetric area and the surface area of the crown of the native and introduced trees of the evaluated rural localities, since the native species reached higher figures (p<0.001).

In this study, 32 species were recorded, a value slightly lower than the 38 reported by ^{Alanís-Rodríguez et al. (2022)} in the center of Hualahuises municipality, 39 species in the center of Linares (^{Leal et al., 2018}) and 39 species on a university campus (Graduate School of Forest Sciences of the Autonomous University of Nuevo León) also in the Linares municipality (^{Alanís et al., 2014}). This slight decrease in the number of species can be explained by the fact that the other locations gather more exotic species and, therefore, increase the total species richness.

Fabaceae is the family with the largest number of species. This result is similar to those described by ^{Alanís et al. (2014)}, ^{Leal et al. (2018)}, ^{Canizales et al. (2020)} and ^{Alanís-Rodríguez et al. (2022)}, who conducted their studies in urban forests in northeastern Mexico. Legumes also have a wide presence in the submontane scrub (^{Mora-Olivo et al., 2016}) and in the Tamaulipan thorny scrub (^{Jiménez et al., 2013}), natural communities that grow in the studied municipality.

According to the comparison of native and introduced species, the results obtained differ from others that evaluate urban trees in the cities of northeastern Mexico; in these cases there is a higher proportion of introduced species and individuals, which have a greater basimetric area and crown surface area than the native ones and therefore, species richness (^{Alanís et al., 2014}; ^{Leal et al., 2018}; ^{Canizales et al., 2020}; ^{Alanís-Rodríguez et al., 2022}). In this study, the native species behaved in the opposite way. This could be explained by the knowledge and use given to species showers in rural locations. ^{Estrada-Castillón et al. (2022)} mention that in these places the elderly people pass on traditions, ideas and experience in the use of plant species from generation to generation.

At the species level, Prosopis laevigata had the highest UIVI. This species is considered an important resource for arid and semi-arid zones in Mexico, due to the different uses that it is given such as fodder for cattle, human food in the form of flour, fermented drinks and in pods (^{Estrada-Castillón et al., 2022}). The three species with the most presence and UIVI are the native ones, in addition to the previous one, Vachellia farnesiana and Ebenopsis ebano, which can be proposed for use in reforestation programs in other rural and urban environments.

The proposed hypothesis is rejected, because a good adoption of native tree species is observed in the rural localities of Hualahuises, Nuevo León, as they had outstanding values of species richness, Margalef index, Shannon index, basimetric area and crown surface area than the exotic species in the localities that were assessed.

The native species show an outstanding size compared to the exotic, since they surpass them in 90 % in all the variables (91.69 % of basimetric area, 92.36 % of crown area, 92.22 % of crown surface area and 96.78 % of crown volume).