Introduction

Cities are home to a growing proportion of the world's population, and in the face of urban sprawl, green areas play a key role in maintaining social, economic and ecological well-being (^{McDonnell and Hahs, 2009}; ^{Acharya, 2011}; ^{Ramalho and Hobbs, 2012}). Urban parks are green areas managed mainly for the purpose of coexistence and recreation, and unlike forests or protected parks, they are the most accessible green spaces for the population residing in urbanized areas (^{Oleyar et al., 2008}; ^{Nagendra and Gopal, 2011}; ^{Niemelä et al., 2011}). Urban parks also provide multiple environmental services: mitigate air pollution, reduce noise, provide scenic beauty, recreation, opportunities for contact with nature (^{Dobbs et al., 2011}; ^{Escobedo et al., 2011}; ^{Zhang and Jim, 2014}); and they favor the conservation of biological diversity by giving habitat and food to the local fauna (^{Chace and Walsh, 2006}; ^{Carbó-Ramírez and Zuria, 2011}).

There is considerable information describing the composition and diversity of trees in urban parks in various parts of the world, and in the United States, Europe and Asia, where they have developed most (^{Lososová et al., 2011}; ^{Nagendra and Gopal, 2011}; ^{Jim and Zhang, 2013}; ^{Pesola et al., 2017}; ^{Riley et al., 2017}). In Mexico, there is not much information about the green areas within the urban areas of several cities. The best assessed areas are large metropolis such as Mexico City, where general studies have been conducted covering the entire urban area (^{Benavides, 1992}; ^{Checa-Artasu, 2016}) or on a particular area such as Chapultepec Forest (^{Benavides and Young, 2012}).

In the northeastern part of Mexico, there are few records on the subject of green areas and urban forests. ^{Alanís (2005)} and ^{Jiménez et al. (2013)} evaluated the green areas of the Monterrey municipality, Nuevo León, through geographic information systems and in situ forest inventories. In Tamaulipas, ^{Mora and Martínez (2012)} elaborated a work on the wild plants of an urban forest located in Ciudad Victoria. In the case of the municipality of Linares, Nuevo León, information is scarce and limited to studies carried out on the main square of the city on the trees on the sidewalks (^{Zamudio, 2001}) and on a student campus (^{Alanís et al., 2014}). Based on the above, the objective of this study was to determine the structure, composition and diversity of urban trees in the different squares, parks and ridges of the urban area of Linares, Nuevo León.

Materials and Methods

Study area

The Linares municipality (Figure 1) is located southeast of the state of Nuevo León. Geographically it limits towards the north with the Hualahuises and Montemorelos municipalities; both to the south and to the east with the state of Tamaulipas and to the west with Galeana and Iturbide municipalities. Its geographic coordinates correspond to 24°51' N and 99°24' W (^{Municipios Mx, 2018}).

Upper left image = NE of Mexico and SE United States of America; Lower left image = Linares municipality, Nuevo León; Right image = Linares urban area with its green areas; COA = Coahuila; NL = Nuevo León; ZAC = Zacatecas; SLP = San Luis Potosí; TAM = Tamaulipas.

Figure 1 Location of the study area. 

Results

There were 41 species belonging to 34 genera and 25 families of vascular plants (Table 1). The most representative family was Fabaceae with six species, Fagaceae with four species, later Moraceae and Arecaceae with three each, and finally, Bignoniaceae, Oleaceae, Pinaceae and Rutaceae with two. Of the 41 species recorded in the study, 14 were native and 27 exotic (Table 2).

A total abundance of 75.43 N ha^-1 was recorded. The most outstanding species was Fraxinus americana L. with 25.67 N ha^-1, which represents 34.03 % of the total. Quercus virginiana Mill. with 28.61 % and Washingtonia robusta var. gracilis (Parish) Parish ex Becc. with 15.15 % follow; the remaining species together sum 22.21 %.

The crown coverage of the urban green areas (parks, squares and ridges) was 2 611.31 m² ha^-1. Fraxinus americana registered greater dominance with 1 225.38 m² ha^-1, followed by Quercus virginiana with 552.04 m² ha^-1 and Carya illinoinensis (Wangenh.) K.Koch with 255.63 m² ha^-1; the remaining species represented 22.14 %.

Quercus virginiana stands out as the main species in urban green areas in Linares, as it was recognized 33 of the 41 green areas analyzed (17.65 %). Washingtonia robusta var. gracilis was in 25 of them, which is equivalent to 13.37 %, followed by Fraxinus americana in 22 (11.76 %).

The highest calculated value for the Importance Value Index was for Fraxinus americana with 30.91 %, followed by Quercus virginiana with 22.46 % and for Washingtonia robusta with 11.56 %, which together add up to 64.93 % (Table 3). In contrast, Citrus sinensis (L.) Osbeck, Punica granatum L., Prunus persica (L.) Batsch and Tabebuia rosea (Bertol.) DC.recorded 0.20 % each, which is the lowest value.

The density of individuals per hectare according to the diameter classes registered in the study showed that most of the individuals evaluated are concentrated in category II (14.5 - 29 cm) with a total of 1 011 N ha^-1 (Figure 2).

Densidad = Density; Clases diaméticas = Diametric classes.

Figure 2 Density of individuals according to diametric classes in the study areas. 

The mensuration variable of total height was calculated based on the density of individuals by height category. Category III (6.4 - 9.6 m) was the one that predominated with a total of 998 individuals per hectare, which corresponds to 48.3 % of all registered individuals, while category VI (16 - 19.2 m) was the lowest number of individuals with only 19 (0.91 %) (Figure 3).

Densidad = Density; Altura = Height

Figure 3 Density of individuals by height in the study area. 

The evaluated area showed values of Margalef's Index of D_Mg = 5.24 and of Shannon-Wiener of H´ = 1.99, and the val H'_max = 3.17.

Discussion

In the inventory of urban trees 2 066 specimens were registered, belonging to 24 families and 41 species. This number of species is close to the data of ^{Zamudio (2001)} in the center of Linares, who identified the existence of 39 species, with 19 families in 1995 and 49 species with 27 families in 1999. It is low compared to the study conducted by ^{Alanís (2005)}, who in an investigation in the metropolitan area of Monterrey, detected 115 species, grouped in 37 families, which surely responds to the difference in the sampled surface.

On the other hand, ^{Alanís et al. (2014)} observed a similar number of species (39) to the one registered in this study, by restricting the area to the university campus of the Faculty of Forestry Sciences of the UANL.

The Fabaceae family was the best represented with six species (three native and three exotic), followed by Fagaceae with four species (three exotic), Moraceae with three species (two exotic), and Arecaceae with three species (all exotic). These data coincide with those of ^{Alanís et al. (2014)}, who highlighted Fabaceae with 10 species (nine native). It also coincides with the information of ^{Alanís (2005)}, in which Fagaceae and Moraceae are the most important in the Metropolitan Area of Monterrey.

The species with the greatest ecological weight for this study was Fraxinus americana, which concentrates 30.91 % of the Importance Value Index, followed by Quercus virginiana, Carya illinoinensis, Whasingtonia robusta and Ligustrum japonicum Thunb. This percentage is higher than that calculated by ^{Alanís et al. (2014)} who granted Fraxinus americana 18.21 % of IVI. ^{Zamudio (2001)} defined the latter within the four species with the greatest ecological weight (13.40 % of IVI).

In the study described here, dominant were Fraxinus americana (46.93 %), Quercus virginiana (21.14 %), Carya illinoinensis (9.79 %) and Washingtonia robusta (6.17 %). For ^{Carabias and Herrera (1986)}, these percentages are not satisfactory, since they establish that no species should exceed 5 % of the total population in the public trees of a city.

Only 13 species are native (36.6 %) and 27 are exotic (63.4 %) (Table 2). The opposite is revealed by ^{Alanís et al. (2014)}, since of its total number (39), 12 are exotic and 27 are native; this corresponds to a higher percentage of species of the second origin in the urban trees of the campus of the forest school. For the Metropolitan Area of Monterrey, ^{Alanís (2005)} accounted for 61 introduced species (53 %) and 54 native species (47 %).

The evaluated plant community has a species richness of D_Mg = 5.24 and a diversity of H ́ = 1.99. These values are lower than those of ^{Alanís et al. (2014)}, D_Mg = 7.62 and H ́ = 3.05. When comparing the results observed in this research with those of ^{Zamudio (2001)} for diversity, for the year 1995 the value obtained was H ́ = 2.54 and for the year 1999 it was H ́ = 2.27.

If the results of diversity and richness determined in the present investigation are contrasted only with the native species consigned by ^{Alanís et al. (2014)}, it can be observed that these authors estimated high values of D_Mg = 5.80 and H ́ = 2.82, with respect to those here calculated of D_Mg = 2.12 and H ́ = 0.8127, which are lower.

Conclusions

The urban trees of the of Linares include a high number of exotic species; Fraxinus americana, Quercus virginiana, Carya illinoinensis and Washingtonia robusta var. Gracilis are dominant, the first one of which is the most representative with a total of 703 individuals, which are equivalent to more than 45 % of its green areas.

The actual research study revealed data about the number and surface areas of the squares, ridges and parks in the city of interest, as well as of the species and families of all the trees of the urban forest. This information is useful in the detection of priority areas for reforestation, as this is a reliable inventory that contributes to the management of public trees.