Study area

The study was carried out in a gallery forest adjoining a submontane shrub and a Tamaulipan thornscrub in northeastern Mexico, specifically in the Allende municipality in Nuevo León State. The community is in a mature condition without apparent anthropic disturbance and is difficult to access. Its geographical coordinates are 25º13’45” N and 100º03’18” W. The climate is dry (BS_Ohw), according to Köppen´s classification modified by ^{García (1988)} for Mexico. The mean annual temperature ranges between 20 and 22 °C, and the warmest months are July and August, while the lowest temperatures occur in December and January, with a mean of 13 to 14 °C.

Vegetation analysis

A 4 Km long stretch with an altitude range of 450 to 490 masl was considered; 15 rectangular sampling sites measuring 50 m² (10 × 5 m) each were established at random for a total sampling surface area of 750 m². The sites were located at the shores of the river, parallel to the riverbed, which flows from south to north (^{Canizales et al., 2010}); all the individuals with tree size with a DBH (d _{1.30 m}) ≥ 8 cm were included. The dasometric variables evaluated were total height (h) and diameter breast height (d _{1.30 m}), which were measured with a Vertez III^™ hypsometer and a Forestry Supliers Inc^™ diametric tape, respectively.

Data analysis

The horizontal structure of each species was evaluated by determining its abundance according to the number of individuals; its dominance, in terms of its basimetric area, and its frequency, based on its presence in the sampling plots. The results were used to obtain a weighted value at taxon level known as Importance Value Index (IVI), which acquires percentage values on a scale of 0 to 100 (^{Müeller-Dombois and Ellenberg, 1974}). The relative abundance (AR _i , for its acronym in Spanish) per species was estimated using the following equation:

Where:

A _i = Absolute abundance

The relative dominance (DR _i , for its acronym in Spanish) was estimated using the formula:

Where:

D _i = Absolute dominance.

The relative frequency (FR _i , for its acronym in Spanish) was calculated using the following equation:

Where:

F _i = Absolute frequency = P _i /NS

P _i = Number of sites where the species occurs

iNS = Total number of simple sites

The horizontal characterization of the vegetal community was carried out using the Importance Value Index (IVI), which is the sum of the percentage values of abundance, dominance and frequency; the value determines the presence of each taxon within the community and is defined as (^{Whittaker, 1972}; ^{Moreno, 2001}):

Where:

AR _i = Relative abundance of species i with respect to the total abundance

DR _{i =} Relative dominance of species i with respect to total dominance

FR _i = Relative frequency of species i with respect to total frequency

The characterization of the vertical structure of the taxa was carried out by using the vertical species distribution index (A) (^{Pretzsch, 2009}), in which three altitude zones were defined: zone I, 80 %-100 % of the maximum altitude of the area; zone II, 50 %-80 %, and zone III, 0 to 50 %. This index (A) serves to determine the structural diversity in the vertical distribution of the species and is estimated by the following formula:

Where:

S = Number of species present

Z= Number of altitude layers

p _ij = Percentage of species in each zone, estimated with the following equation:

Where

n _i,j = Number of individuals of the same species (i) in the zone (j)

N = Total number of individuals

Pretzsch’s index needs to be standardized for purposes of comparison; this is done using the A _max value, estimated as follows:

Thus, the value of A can be standardized using this equation:

Richness and diversity were determined by Margalef’s index (D _Mg ), which is based on the quantification of the number of species present (specific richness), and the Shannon-Wiener index (H´), based on the proportional distribution of the abundance of each taxon (^{Moreno, 2001}). The values thus obtained were compared using the Student’s t-test of a single sample, which compared the mean value of the indexes estimated in this study with the reference value of other studies. Prior to the analysis, normality was verified using the Shapiro-Wilks test (^{Zar, 2010}).

The structure of the community was described in terms of the abundance of each species using rank-abundance curves (^{Magurran, 2004}). The Null, preemption, lognormal, Zipf and Mandelbrot models were tested. The parameters were estimated using the maximum likelihood method, and the selection of the best model was based on the Akaike Information Criterion (AIC) in terms of the lowest value. The analysis was carried out using the R software, version 3.2.1.2 (^{R Core Team, 2017}), with the Vegan package (^{Oksanen et al., 2016}) and with the support of the RStudio platform (^{RStudio Team, 2016}).

Horizontal structure

Nine taxa belonging to an equal number of genera and family were registered. The assessed community exhibited an abundance of 2 187 N ha^-1, with a basimetric area of 496.3 m² ha^-1. The species that exhibited the greatest abundance, dominance and frequency was Taxodium mucronatum Ten., with an IVI value of 52.4 %, followed by Platanus rzedowskii Nixon & J. M. Poole (IVI = 23.2 %) and Carya illinoinensis (Wangenh.) K. Koch (IVI = 8.8 %). These amounted to 84.4 % of the IVI of the community, while 15.6 % corresponded to the remaining six (Table 1).

Four genera (Carya, Platanus, Fraxinus, and Taxodium) are regarded as characteristic of this type of vegetation in Mexico (^{Enríquez-Peña and Suzán-Azpiri, 2011}). ^{Treviño et al. (2001)} recorded 21 and 16 species for the tree stratum of two rivers in central Nuevo León, while ^{Canizales et al. (2010)} cited seven taxa along a river with different degree of anthropogenic disturbance in the same state. In such studies, as in the present work, Taxodium mucronatum is considered one of the most abundant species. In the research documented herein, unlike the previous ones, Platanus rzedowskii was observed to be abundant, while P. occidentalis L. was not.

^{Estrada-Castillón et al. (2013)} registered both Platanus species at the Cumbres de Monterrey National Park and point out that these are usually associated with genera of boreal forests, including Quercus and Carya, as indicated in the present study. Likewise, ^{Nixon and Poole (2003)} mention that the two species occur very close to each other in certain riparian systems of northeastern Mexico and are the most conspicuous in this type of vegetal community in the temperate areas of the Eastern Sierra Madre.

Citrus sinensis - an exotic species widely used for the production of oranges in Allende- was calculated with a density of 13 N ha^-1; therefore, its seeds dispersed by birds (^{Verea et al., 2009}) have germinated in the study areas, despite their difficulty of access.

Total abundance was 2 187 N ha^-1; in this regard, ^{Canizales et al. (2010)} estimated a total abundance of 970 N ha^-1 for a gallery forest community with a low degree of anthropic disturbance; ^{Burton et al. (2005)} determined a value of 1 233 N ha^-1 and a lower level of abundance for a community closer to an urban center (950 N ha^-1) in southeastern United States. This indicates that the anthropogenic disturbance and the touristic activity registered by the authors affect the abundance of tree individuals, though to lesser degrees, as the gallery forest community analyzed in the present study was located in an area of difficult access exhibiting no apparent alteration.

According to the importance value index, Taxodium mucronatum registered 52 % and Platanus rzedowskii, 23.2 %. ^{Canizales et al. (2010)} mentioned T. mucronatum as having the highest ecological importance value index in three different anthropic disturbance conditions in the Ramos river, located in Allende, Nuevo León, but did not record P. rzedowskii. However, ^{Treviño et al. (2001)} cite P. occidentalis, followed by T. mucronatum, as the ecologically most important species for the same location, and T. mucronatum, with the highest IVI, followed by P. occidentalis and Populus wislizenii Sarg., in the Cabezones river.

Vertical structure

The maximum height for the trees in the present study was 15.5 m, based on which the stratification was performed in order to estimate the A-index. Stratum I (high) is formed by three species and was scarcely represented, by 9.15 % of the individuals and 12.08 % of the basimetric area. Stratum II (medium) was formed by six taxa and had the largest number of individuals (55.49 %) and basimetric area (68.76 %). Stratum III (low) consists of eight species, with 35.37 % of the individual and 19.16 % of the basal area (Table 2). The most abundant taxon in all the strata was Taxodium mucronatum, followed by Platanus rzedowskii; this indicates that the vegetal community exhibits the incorporation of individuals of the low stratum (III) that abound in the higher strata (I and II). The value of Pretzsch’s index was 2.03, with a A _max of 3.29 and a A _max of 61.87, indicative of an average structural diversity in the height strata, for the values close to 100 % imply that all the species are distributed equally between the three height strata (^{Mora-Donjuán et al., 2014}; ^{Rubio et al., 2014}). It is important to mention that there is little literature where this index is used in relation to gallery forests.

Richness and diversity indexes

The diversity in the area was lower (H´ = 0.58) than that estimated by ^{Canizales et al. (2010)} in a gallery forest classified as the best preserved one in their study and exhibited no significant differences (t = -1.433, df = 14, p-value = 0.173). On the other hand, for the community with a degree of anthropogenic disturbance classified as alarming, these authors registered a lower value, but a significant difference (t = 3.711 df = 14, p-value = 0.002) with respect to the present study (Table 3).

As for the richmess of species, compared to that of ^{Canizales et al. (2010)}, there are no significant differences in terms of the two conditions of the gallery forest analyzed by the authors (Table 4). According to ^{Canizales et al. (2010)}, anthropogenic activity has a negative impact on the diversity and abundance of species; therefore, the values were lower in vegetal communities with greater anthropic disturbance. In relation to the study by ^{Treviño et al. (2001)} on the Cabezones and Ramos rivers, the diversity was greater than in the research documented herein; the statistical comparison indicated that there are significant differences with respect to the value registered in Allende, Nuevo León (Table 3). A comparison with the researches on gallery forests by ^{Burton et al. (2005)}, in an urbanization gradient, and by ^{Santiago et al. (2014)}, in an altitude gradient, shows the existence of a statistical difference with regard to the diversity observed in this study; the former two recorded higher values (Table 3).

The Margalef’s index determined by the present research is three times higher than the one documented by ^{Canizales et al. (2010)}; yet, no significant differences resulted with respect to either the preserved (t = -1.422, df = 14, p-value = 0.176) or the disturbed (t = 0.897, df = 14, p-value = 0.384) vegetal community.

Species’ rank-abundance curve

Figure 1 shows the rank-abundance curve for all the species registered in the vegetation samples. This curve is graphically adjusted to the Mandelbrot (AIC=40.83), Preemption (AIC=41.80) and Lognormal (AIC=42.80) distributions, but not to the Null distribution (AIC=119.75). Therefore, the abundance of species in the area may be described by the Mandelbrot, Preemption and Lognormal models, as the distribution of species adjusts to these, both graphically and analytically. However, the best fit was for the Mandelbrot model. According to it, one or a few taxa are highly abundant and predominant in the vegetal community, as well as at the initial stages of the succession (^{Magurran, 2004}). This model has been regarded as one of the best for describing the distribution of the abundance of species (^{Fischer et al., 1943}).

Abundancia = Abundance; Rango de especies = Species´rank

Figure 1 Species’ rank-abundance curve for the gallery forest and functions adjusted to this curve. 

The Preemption model, also known as preferential niche or geometric series model, had a slightly smaller adjustment than the previous one; this implies that a few abundant species are present in a large proportion on one part of the hyperspace of the niche, and therefore, their occurrence varies with those situations in which the habitat is sequentially occupied (^{Aguirre et al., 2008}). ^{Alanís-Rodríguez et al. (2017)} observed that the geometric model was the one that best adjusted to a mesquite forest in central Nuevo León, in northeastern Mexico. The authors suggested that this is a typical structure for a severely disturbed environment. However, the studied community is not subjected to a severe disturbance; therefore, the Mandelbrot model, with a better fit, is the one that best describes the community, since the species analyzed in the gallery forest respond to specific factors that determine the ecology of the community, including environmental factors like geomorphology, the water flow or organic matter flow, or biological factors of its own, such as a rapid growth and being shade-intolerant.

^{Enríquez-Peña and Suzán-Azpiri (2011)} point out that the elements that affect the reproductive growth of Taxodium mucronatum Ten. include the availability of water, the fertility of the soil and pollution. Likewise, one of the characteristics of T. mucronatum is its rapid growth (^{Rolston, 2001}), which is important for its development at the initial stages of the natural succession; furthermore, the species is shade-tolerant during the first stages of its life (^{Conafor, 2018}). In the case of the Platanus genus, its intolerance to shade may cause mortality to increase at the germination stage under advanced successional conditions, since it has a high crown coverage (^{Conabio, 2018}), but germinates and develops adequately in areas with little coverage (^{Carranza, 1994}).