Introduction

Trees are an important resource in urban areas for the environmental services they provide to society, mainly for their ornamental role or scenic beauty (^{Corona, 2021}). The foliage and fruit of the trees offer habitat, shelter and food for birds, insects, and other animal groups that have adapted to live in anthropized ecosystems (^{de Almeida y Cándido et al., 2017}). In addition, urban trees are a source of oxygen, an significant carbon sink and contribute to lower the temperature at the microclimate level (^{Livesley et al., 2016}).

Although the study of urban forests has its foundations about two centuries ago, interest in their study has increased notably in recent years, especially in Latin America (^{Devisscher et al., 2022}). In Mexico, in particular, the creation of new urban forests has been supported for centuries, among the first very notorious are the Alameda Central and Bosque de Chapultepec in Mexico City (^{Benavides and Fernández, 2012}). The latter was created by King Netzahualcayotl, who planted the first ahuehuetes (Taxodium spp.) in 1430 (^{Alcántara, 2019}). Currently, urban trees are characteristic in large and small cities of the national territory.

In the case of northeastern Mexico, research on urban forests has been concentrated in the state of Nuevo León and more intensely in the metropolitan area of Monterrey (^{Rocha et al., 1998}; ^{Alanís, 2005}, ²⁰¹¹). Recently, studies of urban trees have been carried out in smaller cities such as Linares (^{Leal et al., 2018}) and Montemorelos (^{Canizales et al., 2020}). However, there are still some municipalities in Nuevo León, such as Hualahuises, where knowledge about their urban trees is still scarce.

Hualahuises is not a large place, although its artisan tradition and its deep-rooted culture of indigenous origin favored its nomination as Magical Town in 2015. Despite not having very big areas with urban trees, in this municipality there are green areas in squares, gardens and along streets and avenues. Given that until now the current state of the trees in this place was unknown, the present study was carried out with the objective of evaluating the floristic composition, structure and diversity of its urban trees.

Materials and Methods

Results

38 species of vascular plants distributed in 35 genera and 22 families were recorded, 63.2 % (25 species) are introduced and 36.8 % (21 species) are native. The most representative family was Fabaceae with four species, followed by Arecaceae, Fagaceae and Rutaceae with three species each (Table 2).

According to the characterization, the abundance of individuals was 384, the total basal area (dominance) of the urban trees was 6.77 m², with 4 605.81 m² of crown area. Based on the vertical and three-dimensional attributes, the crown area was 8 473.92 m² and the crown volume was 2 575.01 m³. The four species with the highest values in all the variables were Fraxinus Americana L., Quercus virginiana Mill., Thuja occidentalis L. and Cordia boissieri A. DC. (Table 2).

The two species with the highest Importance Value Index were Fraxinus americana (introduced species) and Quercus virginiana (native species). In addition, Q. virginiana showed the highest abundance values (obtaining a value more than double that of F. americana), crown area and presumed crown area, while F. americana presented the highest values of basimetric area and crown volume, in both variables obtained values of almost double that of Q. virginiana (Table 2).

Figure 1 shows the graph of diameter classes. The curve describes a negative exponential trend, as the number of individuals decreases as the normal diameter increases. This revealed that a high number of individuals with small diameters and a low number of individuals with high diameters prevail. In total, 136 individuals with a normal diameter less than 5 cm and 34 >30 cm were counted.

Figure 1 Number of tree and shrubs samples in Hualahuises downtown based upon normal diameter (cm). 

Figure 2 illustrates height classes, in which it is noted that the greatest presence of trees was recorded in the 3 to 5.99 m category with 192 individuals, followed by the ≤2.99 m category with 100 samples. There are few individuals with a height greater than 12 m. The distribution of the specimens with respect to their height category represented a positive asymmetric distribution.

Figure 2 Number of tree and shrubs samples in Hualahuises downtown based upon total height (m). 

According to the richness and diversity indexes, the evaluated plant community recorded a D _Mg Margalef index of =6.21 and a Shannon entropy index of H'=2.94. Shannon's true diversity index was 18.93.

Discussion

38 species were recorded in Hualahuises downtown, a value higher than the 13 species reported by ^{Canizales et al. (2020)} in the green areas of Montemorelos, but similar to the 41 species by ^{Leal et al. (2018)} in the green areas of Linares, to the 39 species declared by ^{Zamudio (2001)} in Linares downtown and to the 39 inventoried by ^{Alanís et al. (2014)} in a university campus also in Linares municipality. Qualitative research carried out in the Monterrey Metropolitan Area (AMM), Nuevo León State, has recorded 115 species (^{Alanís, 2005}) and up to 137 (^{Rocha et al., 1998}).

The Fabaceae family was the most representative with four species. These findings are consistent with the results of ^{Alanís (2005)}, ^{Alanís et al. (2014)}, ^{Leal et al. (2018} and ^{Canizales et al. (2020)}, who conducted their studies in northeastern Mexico with similar data. The families that follow are Arecaceae, Fagaceae and Rutaceae with three species each. From them, all three species of Arecaceae and two species of Fagaceae and Rutaceae are introduced.

In the present study, 63.2 % (25 species) introduced and 36.8 % (21 species) native were recorded. These results are common in the region, as found in ^{Canizales et al (2020)} who determined 54 % of introduced species in Montemorelos, in the study by ^{Alanís (2005)} identified 53 % of introduced species in Monterrey Metropolitan Area and in ^{Martínez-Trinidad et al. (2021)} work, 61 % are introduced in three parks in Mexico City. This pattern of more introduced than native species is recognized in much of the world and is related to the increase in the production of ornamental trees in nurseries (^{Pagès i Clavaguera, 2005}).

Fraxinus americana reached the highest value of Urban Importance Value Index, with 16.7 %. Various studies have established this species as the most ecologically important in urban green areas in northeastern Mexico (^{Rocha et al., 1998}; ^{Alanís et al., 2014}; ^{Leal et al., 2018}; ^{Canizales et al., 2020}). ^{Alanís (2005)} explained that it was frequently includedd in urban reforestation in northeastern Mexico in the 1980’s, even earlier, but this species was no longer used in the early 1990’s, when natives began to be used.

The second species with the highest urban importance value was Quercus virginiana with 16.64 %. ^{Leal et al. (2018)} and ^{Canizales et al. (2020)} also recorded Q. virginiana as the second most important species in Linares and Montemorelos, Nuevo León, and ^{Alanís et al. (2014)} identified it as the most significant in a university campus in northeastern Mexico. Their high presence is related to the greater preference for incorporating them in the green areas of the cities of northeastern Mexico during the 1990’s (^{Alanís, 2005}).

Thuja occidentalis was the third species with a high urban importance value. It is exotic as it is native to eastern Canada and much of north central and northeastern United States (^{Housset et al., 2015}); it is an evergreen conifer of the Cupressaceae family. It was introduced to several cities in Mexico, such as Guadalajara (^{MacGregor-Fors, 2008}), Monterrey (^{Rocha et al., 1998}) and Linares (^{Leal et al., 2018}) among others, due to its beauty and resistance to frost (^{Alanís, 2011}).

The graph of diameter classes (Figure 1) showed a negative exponential trend, since the number of individuals decreases as the normal diameter increases; therefore, it is concluded that a large part of the arboreal individuals in Hualahuises are young samples. These results are similar to those provided by ^{Martínez-Trinidad et al. (2021)} in four parks in Mexico City, but opposed to the positive asymmetric distribution identified by ^{Leal et al. (2018)} and ^{Canizales et al. (2020)} in the green areas of cities in northeastern Mexico. The greater presence of juvenile arboreal individuals than adults may be associated with the fact that afforestation activities have intensified in cities in recent years (^{Alanís, 2005}).

The height class plot (Figure 2) depicts a positive skewed distribution. This behavior was similar to that reported by ^{Leal et al. (2018)} and ^{Canizales et al. (2020)} in urban green areas of Linares and Montemorelos, respectively. Most of the trees are between 2 and 5.99 m tall, similar to the results of ^{Canizales et al. (2020)} in Montemorelos, but opposite to those of the Linares trees (^{Leal et al., 2018}), which with higher height (from 6.40 to 9.60 m) already prevail. The size of the Hualahuises trees may be a response to their youth (^{Alanís, 2005}), or to the fact that they have been pruned, since their vertical development interferes with cable lines and public lighting (^{Macías-Sámano, 2010}).

The Margalef richness index (D _Mg =6.21) suggests a medium-high diversity, a value similar to the 7.62 recorded by ^{Alanís et al. (2014)} for the urban trees of a university campus in Linares, Nuevo León, and at 5.24 of ^{Leal et al. (2018)} for the one corresponding to the city of Linares. The Shannon-Wiener index value was 2.94. ^{Ortíz and Luna (2019)} took the value of 1.50 as the minimum criterion for the Shannon diversity index in an urban area, so, in reference to the same, the results indicate high diversity. This value of 2.94 is higher than the 1.17 reported by ^{Canizales et al. (2020)} in Montemorelos and at 1.99 recorded by ^{Leal et al. (2018)} in Linares, but lower than 3.89 registered by ^{Martínez-Trinidad et al. (2021)} in Mexico City.

Conclusions

Individuals with small diameters are the most abundant (d _1.30 <5 cm), as well as those of intermediate heights (h>3 m and <6 m), which indicates a young plant community. Species richness presents intermediate values and high diversity, compared to that of other urban green areas in northeastern Mexico. More than half (63 %) of the species are introduced, among which Fraxinus americana and Thuja occidentalis stand out for their Importance Value index. In this context, the native species with the highest value is Quercus virginiana.

This research could provide important information on which tree species are most successful in urban reforestation in the city of Hualahuises, Nuevo León State.