Study area

The study was carried out in the municipality of Pesquería, NL (Northeast of Mexico), in areas surrounding the Pesquería power station located between coordinates 25° 45’ 17.78” North latitude and 99° 58’ 01.40” West longitude, with an altitude of 300 m (Figure 1).

Figure 1 Location of the study area. In the left image, Mexico can be seen, in the upper right image, the state of Nuevo Leon and in the lower right image, the Pesquería municipality indicating the study area with a red dot.  

Data collection

In this research the methodology applied by Dalmasso et al. (1997), which considers the taking of samples along and starting from the emitting source (clearing zone) two transects depending on the prevailing winds, which in this case were those from the Northeast and Southeast (INEGI, 1989). According to the contiguous surface of the emission source with native vegetation in each transect, three measurement points were established: sector 1, at 100 m from the clearing zone, sector 2, at 350 m and sector 3, at 700 m, considering that there should be a gradient of decrease in contaminant levels in relation to the increase in distance from the source (Figure 2).

Figure 2 Sampling point diagram.  

In each section, three species (Cordia boissieri, Acacia amentacea, Havardia pallens) representative of the Tamaulipas thorny scrub were evaluated, which present high value index values of importance (^{Pequeño et al. 2012}; ^{Donjuan et al., 2013}; ^{Alanís et al., 2016}). Individual samples of leaves were taken from the side of the cups oriented towards the source, at a variable height between 1.5 and 2.5 m, for the ease of access (Dalmasso et al., 1997). The sample size was 40 grams, obtaining a total of 18 samples, result of the 18 points to be evaluated in each of the stages of this study (^{Astorga et al., 2011}). The extracted material was conditioned in bags and taken to the laboratory for processing (Figure 3).

Figure 3 Sampling. a) selection of individuals; b) sample collection; and c) bagging.  

The extraction of the sedimentary particulate material (MPS) was carried out by washing, stirring periodically with distilled water and passing a small brush to favor the detachment of the particles. Glass beakers of 350 ml capacity were used. The extract obtained was filtered in a funnel through Whaltman filter paper No. 42 (previously tared), dried in an oven and the weight of the MPS was recorded. The leaves washed free of MPS, are being weighed, after drying in an oven at 75 °C until the constant weight was obtained (Figure 4). The data are expressed in grams of MPS per kg of dry matter of leaves (g kg^-1).

Figure 4 Removal of sedimentable particulate material. a) obtaining sample size; b) washing of plant material; c) filtering of plant material; d) drying of plant material; e) weighing plant material to a constant weight; and f) weighed filter paper after drying with sedimentable particulate matter.  

Analysis of the information

Once the statistical assumptions of normality, homoscedasticity and independence between the values were verified, an analysis of variance (Anova) of one factor (p≤ 0.05) was performed. As a post hoc test the Tukey HSD test was used. The statistical program used was the SPSS version 19.0 (SPSS Inc., Chicago, IL, USA).

To determine the effect of the time elapsed from the removal of vegetation in the concentration of particulate material on the surrounding vegetation, Figure 5 was generated, where it is observed that the amount of dust registering if it presented difference over time (gl= 2; F=4.307, p= 0.019).

Figure 5 Concentration of sedimentable particulate material according to the sampling months. Average values ± standard error. Different letters (a, b, c) indicate significant difference (p< 0.05).  

In Figure 5, a clear tendency is observed over time, decreasing the amount of sedimentary particulate material as time passes, which is directly related to the start and end of the vegetation removal work in the place. This result coincides with those recorded by ^{Röösli et al. (2001)}, ^{Celis et al. (2007)} and ^{Pachon and Vela (2014)} who in their pollution studies by particulate material also recorded a direct relationship between the decrease of the same and stages with low and no intensity of anthropogenic activity, finding the smallest amounts of sedimentary particulate material in stages close to the cessation of these activities.

It was also evaluated if there was an effect between the distance of the zone of emission of particulate material and the direction of the dominant wind in the concentration of particulate material of the vegetation. In the samplings of May 2014 (gl= 5; F= 0.426, P= 0.822) and February 2015 (gl= 5; F= 0.719, p= 0.621) there were no significant differences in the amount of dust at different distances. A possible explanation for this would be related to the particle size, since it is possible that larger particles could be present during the samplings of May 2014 and February 2015, as a result of the impact of the work of clearing and removal of vegetation, while that for the month of September 2015, the impact was smaller, generating smaller particles.

^{Márguez et al. (2011)} mention that the smaller the particles, the longer they remain in the air will be greater, due to their density and size, they also mention that this type of particles can be suspended for weeks, and can be transported over long distances. This information also coincides with the results of ^{Corleto and Cortéz (2012)}, who mention that larger particles are deposited more quickly and closer to the source of emission, while medium-sized particles move further away and are deposited at some distance from the source of emission and smaller particles are transported by winds at greater distances.

In the evaluation carried out in September 2015, if there was a significant difference in the amount of dust in the different distances (gl= 5; F= 4.576, p= 0.014), as can be seen in Figure 6. In the sampling carried out in September 2015 the highest value in concentration of sedimentary particulate material in SE direction was registered at 700 m, followed by SE at 350 meters with 2.93 and 1.47 g of MPS kg^-1 of MSF respectively. This is because there are prevailing winds coming from the northwest in that season of the year which disperse the smaller particles suspended in the environment transporting them long distances from the source.

Figure 6 Concentration of sedimentary particulate material (MPS) by heading and distance for the September 2015 sampling. NO= northeast, SE= southeast and the values 100, 350 and 700 are the distances of the emitting source. Average values ± standard error. Different letters (a, b, c) indicate significant difference (p< 0.05).  

These values are low compared to those recorded by ^{Dalamasso et al. (1997)}, who in a similar study in an area where some cement industries are installed evaluated the total suspended particles deposited on the foliage of the adjacent arboreal vegetation composed of Acacia caven, Celtis tala, Eucaliptus camaldulensis, E. viminalis, Geoffroea decorticans, Prosopis alba, Prosopis nigra, Aspidosperma quebracho blanco and Schinus fasciculatus. They recorded maximum average values of 36.22 g of MPS kg^-1 of MSF and minimum of 4.01 g of MPS kg^-1 of MSF. This difference may be due to the morphology of the species they used for their study. What coincides with ^{Beckett et al. (2000)} who mention that the structure of the tree and specific variants such as leaf size, shape and texture of the leaf are factors that determine the ability to capture particulate material in each of the species. ^{Granados and Mendoza (2002)} indicate that the dust spread in open plantations tends to be larger and its presence varies according to the distance and origin of the particles. In addition, ^{Codina et al. (2002)} indicate that some characteristics of plant material such as foliage density, roughness of the foliage, presence of epidermal hairiness, as well as the interaction between the leaf surface and the surrounding air can be considered as factors in the retention capacity of the species.

Conclusions

Based on the results, it is concluded that the concentration of particulate material deposited in the surrounding vegetation is inversely proportional to the elapsed time of the removal of vegetation, causing a decrease in sediments as the time elapsed is greater. It is also concluded that the direction of the wind affects the concentration of particulate material of the vegetation, observing a greater concentration of sediments in the vegetation in the direction of prevailing winds and between a greater sediment concentration according to greater distance in the same direction of the prevailing winds with respect to the area of emission of particulate material.