Introduction

In recent years, urban areas have experienced an expansion in both extension and population, which in turn has increased demand and importance (^{CAF, 2018}). Since these spaces provide cities and their inhabitants with environmental services such as climate regulation, erosion control, recreation, tourism, pollination, aesthetics, habitat for wildlife, carbon capture, conservation of biodiversity, among others (^{Mora-Olivo and Martínez, 2012}; ^{Gómez-Baggethun et al., 2013}), it is relevant to know them as their structure, composition and diversity to strengthen the planning and management of urban green areas for present and future generations (^{Flores-Xolocotzi and González-Guillén, 2010}).

The literature that describes the diversity and composition of urban trees in different parts of the world is diverse (^{Nagendra and Gopal, 2010}; ^{Fahey et al., 2012}; ^{Ortíz and Luna, 2019}; ^{Moussa et al., 2020}). In Mexico it is not so abundant but most of it comes from Mexico City (^{Checa-Artasu, 2016}; ^{Maldonado-Bernabé et al., 2019}).

Research in northeastern Mexico that has assessed green areas and urban forests focus on the municipalities of the metropolitan area of Monterrey and Linares in the state of Nuevo León (^{Zamudio, 2001}; ^{Alanís-Flores, 2005}; ^{Alanís et al., 2014}; ^{Leal et al., 2018}), as well as in Ciudad Victoria, Tamaulipas (^{Mora-Olivo and Martínez, 2012}). The city of Montemorelos is the third most populated in Nuevo León, with 45 108 inhabitants (^{INEGI, 2010}) and studies in this regard have not yet been carried out.

Based on the above, the objective of this work was to know the structure (abundance, dominance, frequency and importance value index), floristic composition, diameter and height classes, relative abundance pattern, square meters of green area per inhabitant and the diversity of trees in the different parks and ridges in the urban area of the city of Montemorelos, Nuevo León.

Materials and Methods

Study area

The study was carried out in the city of Montemorelos, a municipality of the same name, in the state of Nuevo León, northeast of Mexico. It is located between 24°54'and 25°25' north and 99°36 'and 100°11' west, between 500 and 2 700 masl (Figure 1). The prevailing climate is semi-warm subhumid with rains in summer, with lower humidity. The average annual temperature is 16 to 24 °C, with a precipitation interval of 500 - 1 100 mm. The dominant soil types are Leptosol and Vertisol. The urban area is established on land previously occupied by agriculture and scrubland (^{INEGI, 2009}).

Figure 1 a). State of Nuevo León, b). Montemorelos municipality, c). Urban green areas assessed in Montemorelos city. 

Results

There are 21 ridges and 6 parks in Montemorelos city, in which all the tree species were registered, which together totaled 47 173 m². 918 individuals were registered, 497 in ridges and 421 in parks. 13 species distributed in 11 genera and nine families were identified. Seven are to exotic and six, native. The most representative families were Fagaceae and Fabaceae, with three species each (Table 2). The green area per inhabitant was calculated at 0.87 m².

Total density of trees was 194.6 N ha^-1 and absolute dominance of 7 425.23 m² ha^-1 (Tables 3 and 4). The sum of the area of the total urban trees was 54.80 m², with a crown area of 35 044.88 m² and a volume of 187.73 m³. The species with the highest average height were Washingtonia filifera (Linden ex André) H.Wendl., Quercus laurina Bonpl., Casuarina equisetifolia L. and Quercus rubra L.

The relative abundance of Fraxinus americana L. was the most outstanding (62.75 %), as well as the carrier of the largest crown area, with 71.31 % of relative dominance. The second most abundant species was Quercus virginiana Mill. (22.11 %) and with a relative dominance of 15.97 %. The most frequent species were Q. virginiana and F. americana with 26.03 % and 27.40 % respectively but in Table 4, they were ordered according to their Importance Value Index (IVI) value; thus, the highest were F. americana (53.82 %), Q. virginiana (21.37 %) and Ebenopsis ebano (Berland.) Barneby & J.W. Grimes (9.72 %).

In the diameter classes, the individuals followed a normal distribution with a slight positive asymmetry (Figure 2). The highest abundance was recorded in the diameters of 20.01 - 30 cm (74 N ha^-1), while the 10.01 - 20cm and 30.01 - 40 cm categories concentrated 41 and 40 N ha^-1, respectively. Under the assumption that the diameter could be indicative of the state of development of the trees, most of the urban trees of the city are made up of young specimens and few mature and long-lived individuals (Table 5).

Figure 2 Absolute abundance (N ha^-1) of the individuals in the urban green areas of Montemorelos city according to normal diameter (cm). 

The height class graph shows a scarce presence of short trees (category 2 - 4 m; 19 N ha^-1) and the highest in the category 4.01 to 6 m (91 N ha^-1). The distribution of individuals in height categories had a positive asymmetric distribution (Figure 3).

Figure 3 Absolute abundance (N ha^-1) of individuals in the urban green areas of Montemorelos city according to total height (m). 

The list of ridges and plazas in Montemorelos (Table 5) indicates the number of individuals, species richness, basal area (m²), crown area (m²) and volume (m³) of the identified taxa. In general, it is noted that the squares always had the highest values in all the variables compared to those of the ridges.

The range / abundance curve showed a negative exponential trend, in which F. americana exhibits a clear dominance in the tree community of the urban green areas of Montemorelos municipality (Figure 4). The evaluated area recorded a Margalef index of D _Mg = 1.9, while the Shannon entropy index was H'= 1.17. Shannon's true diversity index was 3.22.

Gray bars indicate native species, black bars indicate introduced species.

Figure 4 Range of species / abundance of species curve of urban green areas of Montemorelos city. 

Discussion

The number of taxa identified in the study (13 species, 11 genera and nine families), was lower than the results of similar studies in northeastern Mexico, such as those of ^{Zamudio (2001)} and ^{Leal et al. (2018)} both in Linares municipality, who recorded 41 and 39 species respectively, while ^{Alanís et al. (2014)} counted 39 species in 16 families for the green areas of a university campus also in that locality. In larger urban areas such as the Monterrey Metropolitan Area (AMM) Nuevo León, ^{Rocha et al. (1998)} documented 137 tree species belonging to 68 families. In the same metropolitan area, ^{Alanís-Flores (2005)} determined that the tree diversity was 115 species, grouped into 37 families. This difference in the number of taxa observed in the different urban green areas can be explained by the difference in surface area, since the city of Montemorelos makes up around 50 % of the surface of the AMM. However, several studies indicate that the difference in the number of taxa is not only explained by the different extensions, but also responds, to a large extent, to the management actions carried out in them, as it is the case with the high number of vascular species in urban parks (^{Cornelis and Hermy, 2004}).

In the present study, the Fabaceae and Fagaceae families were the most representative with a total of three species each one. ^{Leal et al. (2018)} considered the first as the most important with six species in green areas of the Linares municipality, which coincides with the results of ^{Alanís-Flores (2005)} and ^{Alanís et al. (2014)}.

In spite of the fact that native species have the advantage of their natural adaptation to the environment, and, therefore, better growth and lower maintenance costs (^{Alanís-Flores, 2005}), the introduced species make up a high percentage of the trees within urban areas, and this is considered a good practice, by easily and rapidly increasing diversity (^{McKinney, 2006}), which is also related to the correct application of management measures (^{Cornelis and Hermy, 2004}).

In the case of Montemorelos, seven (54 %) have this condition. Similar results have been described in different parts of the world. Thus, ^{Alanís-Flores (2005)} determined 53 % of species introduced for the AMM, while ^{Ortíz and Luna (2019)} for the city of Resistencia, Argentina, 69 %. For the Chennai district in India, ^{Muthulingam and Thang Pavel (2012)} recorded 33 % non-native species.

In the present study, the green area per inhabitant (0.87 m²) was lower than that observed by ^{Mejía Salazar and Gómez (2015)} for Tepic, Nayarit (1.2 m²) and lower than that calculated by ^{Alanís-Flores (2005)} for Monterrey (4.6 m²). All these values, including the one in this study, were lower than the provision of urban green areas suggested by various authors, which represents a deficit in these places, both in Montemorelos municipality as in other cities in Mexico (^{Palomo, 2003}; ^{Wang, 2009}; ^{Molinar, 2015}).

Regarding abundance, that registered in the area of interest (194.6 N ha^-1) was lower than that of ^{Alanís et al. (2014)} for the trees of a university campus in Linares (207 N ha^-1) and that of ^{Benavides and Fernández (2012)} for a section of Bosque de Chapultepec (295 N ha^-1) but greater than that of ^{Leal et al. (2018)} for the urban trees of Linares (75.43 N ha^-1). The abundance value in this study (194.6 N ha^-1) compared to the other investigations is considered acceptable.

Regarding dominance, both ^{Alanís et al. (2014)} as ^{Leal et al. (2018)} reported lower coverage values than those of the present study (7 359.99 m² ha^-1 and 2 611.31 m² ha^-1, respectively vs 7 425.23 m² ha^-1). The latter and those of ^{Alanís et al. (2014)} are very similar, with 74 % canopy coverage. Regarding the coverage of urban trees (35 044.88 m²), only 0.25 % of the urban area has it.

Fraxinus americana is the taxon with the highest ecological importance (IVI of 53.82 %), that is, it has the highest representativeness in the study area according to its abundance, dominance and frequency values. ^{Alanís et al. (2014)} and ^{Leal et al. (2018)} have consigned this species as the most ecologically relevant in the urban green areas of northeastern Mexico, as other authors agree in these areas of the country (^{Rocha Estrada et al., 1998};^{Zamudio, 2001}) and that it is extensive to spaces of this nature from Argentina, for example (^{Ortíz and Luna, 2019}).

On the other hand, ^{Alanís-Flores (2005)} refers that this species was frequently used in urban reforestations in northeastern Mexico in the 1980s; and, despite the fact that in some cities it was replaced by native oak species and by others of tall stature such as Ebenopsis ebano (Berland.) Barneby & J.W. Grimes and Ehretia anacua (Terán & Berland.) I.M. Johnst., among others, there are still localities such as Montemorelos in which F. americana is the most common in its green areas.

Quercus virginiana was the second species in abundance and in regard to the value index of ecological importance (21.37 %), which coincides with that confirmed by ^{Leal et al. (2018)} (IVI of 22.46 %) in Linares. This observation is associated with the fact that the Quercus genus has become relevant in the forestation of green areas in cities. In this regard, ^{Alanís-Flores (2005)} points out that in the San Pedro Garza García municipality in the 80s, the use of native species (mainly oak) was encouraged, and later, in the 90s, this practice became general in the AMM, with massive plantations.

On the other hand, under the objective of protection and conservation against exotic or native pests and diseases, ^{Santamour (1990)} proposed the 10-20-30 rule for urban forests (which includes parks, ridges and other metropolitan green spaces). The objective of the 10-20-30 rule is to achieve diversity in urban plantations and maintain a range of ages to plan the removal and replacement of dead specimens (^{Flores et al., 2018}). This strategy assumes that, in terms of the arboreal diversity of these spaces, it is correct that the presence of a tree species does not exceed 10 % to 20 % of the same genus, nor 30 % of a taxonomic family (^{Kendal et al., 2014}). For this case, Fraxinus americana and Quercus virginiana exceed 10 % of presence in regard to the species and Quercus, 20 % in terms of genus, but no family exceeded 30 % as recommended.

Similar results occur in other urban green areas evaluated as an urban forest in Mexico City, where a frequency greater than 10 % was determined for four species, one that exceeded 20 % per genus and no family greater than 30 % (^{Saavedra-Romero et al., 2019}). For the trees of a university campus in the Valley of Mexico, ^{Islas-Rodríguez et al. (2012)} reported that three of the most abundant species did not comply with the 10-20-30 rule. However, in the study area, the management and design practices have not been in charge of diversifying and maximizing the potential of the species by not complying with the also known rule of 10 % (^{Flores et al., 2018}).

According to the results obtained for density (N ha^-1) -Normal diameter (cm), it can be observed that the distribution is normal, with a high number of mature trees, since most of the individuals (74 N ha^-1) are grouped into the 30 cm and 40 cm category (40 N ha^-1). These data were different from those of ^{Alanís et al. (2014)}, who evaluated the green areas of a 30-year-old university campus where they have made constant plantings and have a high number of low diameter class trees and a reduced number of high class trees, which has resulted in a negative exponential trend line in the density of individuals as the diameter increases.

The range of species / abundance curve indicates that there is a dominance of F. americana (Figure 4). This may be due to the preference for this tree for ornamental use in the northeast region of the country (^{Alanís-Flores, 2005}; ^{Alanís et al., 2014}; ^{Leal et al., 2018}). However, this dominance is not recommended in communities established in urban environments (^{Santamour, 1990}), where, as a matter of pest control, a greater diversity of species is sought.

^{Ortíz and Luna (2019)} took as a minimum criterion in an urban area the value of 1.5 for the Shannon diversity index for Barcelona, so their results indicate low total diversity with an entropy index of H '= 1.17 and a diversity true of 3.22 for the area of interest. The values recorded in the present study are also lower than those of others of this type in northeastern Mexico; ^{Leal et al. (2018)} obtained an entropy index of H ́ = 1.99 for the urban trees of the city of Linares. For the year 1995 ^{Zamudio (2001)} calculated an entropy value of H ́ = 2.54 and for 1999, of H ́ = 2.27 for the urban environment of Linares, and ^{Alanís et al. (2014)} of 3.05 for the trees of the university campus in Linares. ^{Ortíz and Luna (2019)} reported a value of 3.60 for the Shannon diversity index, for the city of Resistencia, Argentina.

Finally, it is considered that the value obtained from the richness of species is low (1.9) in regard to that of other assessed areas such as that of the Linares university campus, which was 7.62 (^{Alanís et al., 2014}) and that is classified as high, and that registered by ^{Leal et al. (2014)} of 5.24. In the urban area of the city of Resistencia, Argentina,^{Ortíz and Luna (2019)} determined 10.55 for the Margalef index, which represents an outstanding species richness. The foregoing suggests that the management and design actions of the different cities have promoted a greater mix of species in urban green areas.

Another argument that could explain this contrast is that Montemorelos municipality has a smaller surface area of urban green areas compared to Linares and the Monterrey Metropolitan Area. Likewise, the diversity of species is a relevant factor for the permanence of the vegetation, since it controls the reproduction of pathogenic organisms and a low plant diversity leads to the reduction of green areas (^{Flores et al., 2018}).

Conclusions

The diameter class curve indicates that there is a higher proportion of individuals of intermediate size (DN = 30 cm). The dominance-diversity curve characterizes an arboreal community with high dominance of few species. The green area per inhabitant (0.87 m²) is lower than that recommended in previous studies. There are low values of richness and diversity of species compared to other urban green areas in northeast Mexico. More than half of the species are introduced, among which Fraxinus americana stands out for its importance value index. This research evaluates and quantifies important variables of the urban trees of Montemorelos, Nuevo León, which provides the necessary information for its proper management.