Introduction

The xerophilous scrub constitutes one of the most important vegetal communities in Mexico. It occupies around 40 % of the national territory, and has a floristic richness estimated in about 6 000 species, including a high proportion of endemic species (^{Rzedowski, 1991}, ^{Semarnat, 2006}). This plant community is a source of ecological, genetic, social, economic, educational, cultural, recreational and aesthetic wealth (^{Hooper et al., 2005}; ^{Santos and Tellería, 2006}). Despite its importance, xerophytic scrub has changed in its structure and composition over time, so it has a loss of biological diversity that leads to a deficit in its functioning (^{Hooper et al., 2005}). The above has its origin in multiple human activities, which have caused constant alterations in their coverage and changes in land use.

Within the xerophytic scrubland formations, there is the submontane scrubland, which is characterized by the development of hills and mountainsides below 2 000 masl (^{Rzedowski, 1978}; ^{González-Medrano, 2012}). It thrives in shallow sedimentary or igneous soils and in semi-arid climates with precipitation values between 450 and 900 mm (^{Rojas-Mendoza, 1965}). The submontane scrub is located along the Sierra Madre Oriental in the states of Hidalgo, Querétaro, San Luis Potosí, Tamaulipas, Coahuila and Nuevo León (^{Alanís-Rodríguez et al., 2015}). In this last entity, 228 species of vascular plants have been documented, and it is in the central and southern areas where there is great plant richness (^{Estrada-Castillón et al., 2012}).

Although the submontane scrub of northeastern Mexico is generally considered to be in a good state of conservation, it is evident that anthropogenic pressure is increasing for these types of communities, especially in areas close to urban areas (^{Alanís-Rodríguez et al., 2015}; ^{Mora-Olivo et al., 2016}).

The conservation and sustainable management of plant communities requires knowledge of the functional and structural characteristics of the target vegetation (^{Montagnini and Jordan, 2005}). For this reason, several studies assess the structure and composition of the submontane scrub in the northeast of the country, among them those by ^{Foroughbakhch et al. (2003)}, ^{Canizales-Velázquez et al. (2009)}, ^{Estrada-Castillón et al. (2012)}, ^{Alanís-Rodríguez et al. (2015)} and ^{Uvalle-Sauceda et al. (2015)} for Nuevo León; in addition to that of ^{Mora-Olivo et al. (2016)} for Tamaulipas. However, so far these works are considered insufficient to attain a more accurate assessment of these scrublands, especially when they include in their composition species of high socioeconomic value, such as Helietta parvifolia (A. Gray) Benth. known as barreta.

Helietta parvifolia is one of the most important timber taxa in the submontane scrub of northeastern Mexico; its shaft is used by local people for the construction of fences due to its high quality and wood density. Derived from the above, research has been made to evaluate its timber production (^{Maginot et al., 2014}), wood durability (^{Carrillo et al., 2013}) and biomass (^{Návar et al., 2001}), as well as its growth in forest plantations (^{Foroughbakhch, 1992}). Therefore, the objective of this research was to analyze the structure and particularly the plant diversity of a mature submontane scrub where Helietta parvifolia is dominant in northwestern Mexico.

Materials and Methods

Study area

The study was carried out in the northwest of Linares municipality, Nuevo León, in a preserved area of the submontane scrub where Helietta parvifolia grows (Figure 1). The zone is located between 24°42'01.97'' N and 99°38'03.02'' W, at an altitude of 470 to 490 m, with a weighted average of 480 masl. According to data from ^{INEGI (2009)}, the average annual temperature in the area fluctuates between 16 °C and 24 °C; the climate is a semi-warm - sub-humid with summer rains, of average humidity, and an average annual rainfall of 500 to 1 100 mm, with an average of 800 mm per year (^{INEGI, 2009}). Based upon the information gathered, the vegetation in the study area is mature, without disturbances or previous clearing as a result of the forestry activities that dominate the region; the only use given to the plant community was the collection of edible fruits and woody material from the ground.

A) Map of Mexico with the state of Nuevo León in black, B) Map of the state of Nuevo León with the Linares municipality in black, C) Map of Linares with the study area marked as a black point.

Figure 1 Study area. 

Results

Five tree species and five shrub species belonging to nine genera and six different families were recorded. Fabaceae was the most representative, with five species (Acacia greggii A. Gray, Acacia rigidula Benth, Caesalpinia mexicana A. Gray, Ebenopsis ebano (Berland.) Barneby & J. W.Grimes, and Havardia pallens (Benth.) Britton & Rose); while the rest of the families only had one taxon (Table 1). Helietta parvifolia exhibited the highest values in abundance, dominance and frequency; it concentrated 42.66 % of the IVI, followed by Cordia boissieri A. DC. and Havardia pallens, with 17.31 and 15.83 %, respectively (Table 2).

The density of individuals per hectare showed a negative exponential trend line as the diameter increased; the 0-10 cm diameter class exhibited the highest density of individuals, with values above 1 800 N ha^-1 (Figure 2).

Figure 2 Density of individuals according to the diameter classes in the study area. 

Helietta parvifolia had the highest Importance Value Index (Table 2); in addition, the same negative exponential trend was observed in the density of individuals, and for the 0-10 cm diameter class the highest value registered, was above 800 N ha^-1 (Figure 3).

Figure 3 Density of individuals according to the diameter classes in the study area for Helietta parvifolia A. Gray. 

The total height of the individuals varied between 2 and 12 m. The highest abundance of individuals corresponded to the 0 - 6 m height class, with 1 440 N ha^-1; in the higher height classes, a negative trend in the density of trees was observed as the height increased (Figure 4). Individuals with a height below or equal to 9 m amounted to 95 % of the total measured.

Figure 4 Density of individuals according to height classes. 

In the plant community under study, Margalef index values of 1.45, Shannon index values of 1.47, and True Shannon Diversity index values of 4.34 were estimated. In relation to the Vertical Index of Species (A), a medium structural diversity was observed in the high strata, with a value of 2.16, an A _max of 3.40 and an A _rel of 63.72 %; in the third stratum, 62.8 % of the evaluated individuals were registered.

As for the analysis of vertical distribution between the high, medium and low strata, the high stratum was formed by Helietta parvifolia and Havardia pallens, with 40 and 10 N ha^-1, respectively, representing 2 % of the total. The middle stratum is made-up by Helietta parvifolia, Havardia pallens, Juglans regia L., Ebenopsis ebano, Acacia rigidula, Diospyros texana Scheele and Cordia boissieri, with 535, 170, 25, 10, 25, 5, and 90 N ha^-1, respectively, i.e., 35 % of the total in the study area (Table 3). Helietta parvifolia, Cordia boissieri, Havardia pallens, Juglans regia, Diospyros texana, Ebenopsis ebano, Caesalpinia mexicana, Acacia rigidula, and Celtis pallida Torr. were recor/ded in the low stratum, with 705, 245, 195, 190, 30, 100, 20, 10, 35, and 5 N ha^-1, respectively, amounting to 63 % of the counted individuals.

Discussion

The Fabaceae family was the best represented, a result that agrees with what has been observed in several researches carried out on both mature and regenerated submontane scrub plant communities (^{García-Hernández and Jurado, 2008}; ^{Canizales-Velázquez et al., 2009}; ^{Estrada-Castillón et al., 2012}; ^{Alanís-Rodríguez et al., 2015}; ^{Canizales-Velázquez et al., 2015}; ^{Mora-Olivo et al., 2016}). This family is diverse and well represented in the different plant communities of northern Mexico, given their climatic affinity and the presence of different topographic and soil conditions of the region (^{Estrada et al., 2005}). The dominance of Fabaceae over other groups is due to their ability to fix nitrogen in the soil. This is because, in arid and semi-arid environments, nitrogen is the most limiting element of all soil nutrients required for plant development and growth, as well as an essential element for microbial growth and for the degradation of organic matter (^{Ferrara and Alarcón, 2001}; ^{Celaya-Michel and Castellanos-Villegas, 2011}).

Helietta parvifolia exhibited the highest Importance Value Index (IVI), with 42.66 %. This species is also recorded by ^{García-Hernández and Jurado (2008)} with the highest IVI (25 %) in a shrub located in Linares, Nuevo León, Mexico; H. parvifolia, together with Cordia boissieri, Havardia pallens, Acacia rigidula, Diospyros texana, and Celtis pallida made up 79.41 % of the Value of Importance of that community. This pattern of composition and dominance strengthens the observation of ^{Foroughbakhch and Ngangyo (2017)}, who suggest that a community dominated by the above species is, from a structural point of view, a high thornscrub.

^{Estrada-Castillón et al. (2012)} indicate that, among the xerophilous scrubs, the submontane scrub exhibits different physiognomic characteristics that divide it into high, medium and low; the first includes the association dominated by H. parvifolia. ^{Meléndez-Jaramillo et al. (2019)} indicate that H. parvifolia is one of the species with the greatest presence in the submontane scrub of northeastern Mexico that is located at an altitudinal range of 500 to 800 m.

Most individuals of Helietta parvifolia were found in the smaller diameter categories; of these, 0-10 cm concentrated the largest number of individuals, and a minority of individuals exceeded 15 cm in diameter. In the height classes, the corresponding 0 - 6 m included the largest number of individuals, and the smallest number occurred in the class above 12 m. This agrees with the observations of the study in southeastern Nuevo León by ^{Foroughbakhch et al. (2003)}, who recorded individuals with a diameter equal to or below 15 cm and heights ranging between 3 and 12 m. These findings agree with the vertical characterization of the submontane scrub, where Helietta parvifolia reached a maximum height of 6 m, as reported by García-Hernández and Jurado (2008).

The presence in the community under study of a high number of individuals (1 800 N ha^-1) belonging to the diameter class of 0 - 10 cm suggests the existence of an active regenerative community, composed mainly of saplings and young trees. Of this regenerative community, 47.7 % of them belonged to H. parvifolia, which had the highest Importance Value Index, was dominant in the upper stratum, and contributed more than 83 % of the individuals in the diameter classes over 20 cm.

H. parvifolia is a slow-growing, high-density wood taxon (^{Foroughbakhch et al., 2011}), whose seeds have dormancy as well as independence from direct light for germination (^{Jurado et al., 2000}, ²⁰⁰¹). All these characteristics indicate that H. parvifolia is long-lived and has some tolerance to shade, which explains its dominance in the regenerative community under the canopy of this mature plant community. The observation is in line with that made by ^{García-Hernández and Jurado (2008)}, in the sense that H. parvifolia is part of a mature or undisturbed vegetation.

Furthermore, the dominance of H. parvifolia in this study also suggests a relationship with at least two other factors: the allelopathic characteristics of H. parvifolia (^{Graue, 1981}) ―a product of the alkaloid-type substances in its leaves (coumarins of the furan-quinoline type) that can inhibit the germination and growth of those plants that come into contact with them (^{Graue, 1981}; ^{Gómez-Calvario et al., 2019})― and the history of land use in the site, which has no record of disturbances, previous clearings due to forestry activities, or exploitation of timber species like H. parvifolia, and has only been used for the collection of edible fruits and woody material from the ground.

Both the lower importance value indexes of the taxa typical of the first years of bush recovery (pioneers and long-lived pioneers such as Celtis pallida, Acacia greggi, Acacia rigidula) and the higher importance values of species with characteristics of mature vegetation organisms (increased life spectrum and greater tolerance to shade, such as Cordia boissieri, H. parvifolia) coincide with the site land-use history information, which suggests a mature scrub, as well as with the observations made by ^{García-Hernández and Jurado (2008)} regarding the composition of the bushes under pristine conditions.

The evaluated area had Margalef index values of 1.45 and Shannon index values of 1.47. The former is similar to other scrublands in northeastern Mexico; for example, the Tamaulipan thornscrub (^{Pequeño-Ledezma et al., 2012}; ^{Jiménez et al., 2012}, ^{Mora et al., 2013a}), but lower than that cited by ^{Alanís-Rodríguez et al. (2015)}: D _Mg = 6.02; and that of ^{Canizales-Velázquez (2009)}: D _Mg = 6.02. The Shannon entropy index is low compared to the Tamaulipan thornscrub (^{Pequeño Ledezma et al., 2012}; ^{Jiménez et al., 2012}, ^{Mora et al. 2013b}); lower than that indicated by ^{Alanís-Rodríguez et al. (2015)}, D _Mg = 3.02 and ^{Canizales-Velázquez (2009)}, D _Mg = 3.00. These differences are attributable to the fact that ^{Alanís-Rodríguez et al. (2015)} and ^{Canizales-Velázquez (2009)} evaluated submontane scrublands at higher altitudes and recorded a high number of species typical of Quercus forests.

The Vertical Index of Species (A) was 2.16, with an A _max of 3.40 and an A _rel of 63.7 %, signifying an average structural diversity in the altitude strata. A _rel values close to 100 % indicate that all species are equally distributed in the three height strata (^{Pretzsch, 2009}). These results are similar to those obtained in researches developed in thornscrub communities, by ^{Jiménez et al. (2009)}, ^{Mora-Donjuán et al. (2014)}, and ^{Pequeño Ledezma et al. (2012)}, who obtained an A _max of 4.56, 3.08 and 2.2, and an A _rel of 67.1 %, 53.8 % and 63.7 %, respectively. The Vertical Index (A) has not been estimated for other submontane scrublands. Based on the estimated value of A, it may be said that the vegetal communities of the Tamaulipan thornscrub and the submontane scrubs show similarity in relation to the Vertical Index of Species. They both exhibit a well-defined vertical structure, where strata II and III (medium and low), which concentrate the largest amount of species and abundances (N ha^-1), are the most prominent.

Conclusions

The studied community exhibits similar values of species richness and diversity in comparison with other plant associations of arid and semi-arid climate in Northeast Mexico. Diameter and height class graphs for the plant community and, specifically, for Helietta parvifolia show an inverted jack, which indicates the existence of an active regenerative community, composed mainly of saplings and young trees. The most important family, due to its contribution to the community, is Fabaceae; and the most important species are Helietta parvifolia, Cordia boissieri, and Havardia pallens.