Despite the controversy about how disturbance influences diversity (Mackey and Currie 2001), there is some evidence that less disturbance leads to higher species diversity (Laurance et al. 2012). Forested tropical coastal freshwater wetlands in the southeastern Mexican Pacific are sensitive to disturbance. In this study, we observed that sites with medium to no disturbance had higher diversity and were richer with up to 12 species. However, the number of species was low compared to other forested tropical freshwater coastal wetlands in the Gulf of Mexico (Díaz et al. 2002, Zamora et al. 2008, Maldonado and Maldonado 2010, Maldonado et al. 2016). For instance, in Calakmul, Campeche, the number of species can be as high as 56 (Chiquini et al. 2017). It is possible that the lower diversity in El Castaño (Pacific) is related to the high dominance of P. aquatica (Rincón 2014). In this region, P. aquatica is a very adaptive species that grows in association with some mangrove species (Barrios-Calderón et al. 2018).
The dominance of P. aquatica was potentially associated with its germination requirements, which can have up to 18 practical salinity units (SPU) in the Pacific, in contrast to the Gulf of Mexico (Rincón 2014). The surface of TFFCW in El Castaño and the region has decreased in recent years due to anthropogenic activities, such as tree harvesting, agriculture, livestock, and villages (Romero et al. 2015, Barrios-Calderón 2019). This pressure on the ecosystem could lead to a successional stage, where the tree stratum cover decreases while the shrub and herb stratum may be more abundant (Kellogg et al. 2003). From this perspective, the characterization of the structure and composition of forest fuel beds is of great interest and should be studied in the long term. Precisely, an important field of application for studies related to the structural composition of forests is fire ecology, mainly in the TFFCW. These ecosystems have been affected by an irresponsible fire use regime (Chen 2006) and fire dynamics have also contributed to changes in their vegetation (Randerson et al. 2006, Parisien et al. 2010, Carmenta et al. 2011, Mistry et al. 2016), thus reducing their diversity.
In this study, the Shannon–Wiener (H’) indices in undisturbed forest areas were similar to those of other TFFCWs (Díaz et al. 2002, Pérez et al. 2005, Ramírez 2006), while the values obtained in high disturbance regions were low, as expected. In the Gulf of Mexico, the P. aquatica TFFCW can be low (4 m) and grow up to 30 m, as indicated by Moreno and Infante (2009). In contrast, in the Pacific, the same type of species grows as much as 17 m (Rincón 2014), values similar to those reported in this study. Furthermore, 59.1% of the canopy strata were 6 to 17 m, while smaller trees represented only 6%. However, this was not the case in high disturbance areas, where the most abundant height (76%) ranged from 2 to 7 m, thus verifying that disturbance had an effect on the vertical structure.
Medium disturbance in the Pacific TFFCW caused P. aquatica to grow higher than in undisturbed sites. This suggests that low fire frequency could be beneficial to the ecosystem (Keane and Karau 2010, Bowman et al. 2011), acting as a renewal mechanism. In areas with high disturbance, P. aquatica was more dominant but did not have a great height. Therefore, the level of disturbance influenced the vertical and horizontal structure, and in sites with a higher density of P. aquatica, the forest will not be as tall. The most common diameter class of live fuels on the horizontal plane ranged from 2.5 to 7.5 cm, independent of disturbance. Similar observations have been reported at other sites of the LEBRE, e.g., El Jicaro (Rincón 2014). Similarly, in TFFCW from the Gulf of Mexico in Veracruz, the most frequent class was from 3 to 13 cm (Infante et al. 2011a). This was especially observed in El Castaño in high disturbance sites. The density of trees made up of the live fuel stratum was higher in this study and had a similar range to that reported in sites in the Gulf of Mexico (Intante 2011). However, in other LEBRE sites with a higher state of conservation, such as Brisas del Hueyate, the density can go up to 3310 ind. ha−1 and the IVI of P. aquatica (285.28%) was higher (Barrios-Calderón 2015).
In the present study, the density of trees comprising the live fuel’s stratum was higher in the less disturbed sites, while woody and litter fuels showed the opposite pattern, having more accumulation in sites with greater disturbance. However, the average value of dead fuels (woody and litterfall) reveals that TFFCWs from the Pacific coast can have more fuel than other forested ecosystems in the tropics (Reyes and Coli 2009, Adame et al. 2015) and temperate regions (Amiro et al. 2001, Flores and Omi 2003, Martínez et al. 2018). In the study carried out by Barrios-Calderón et al. (2018) in the Brisas del Hueyate area within the LEBRE, loads were reported for the classes of 1 (2.75 ± 0.45 t ha−1), 10 (7.01 ± 1.65 t ha−1), and 100 h (18.58 ± 7.22 t ha−1) that are less than those obtained in this study. However, for the class of 1000 h rotten (20.67 ± 16.22 t ha−1) and firm (14.18 ± 9.33 t ha−1), these same authors reported loads similar to those obtained at no disturbance TFFCW sites in the present study. Thus, TFFCW with medium and high disturbance represent a greater potential to present fires under favorable temperature and weather conditions. Mainly high disturbance sites that would represent hot spots for fires with the worst implications for the ecosystem (Flores 2017).
Litterfall fuels, especially the surface litterfall (SL) from all studied sites, are higher than those reported by Rodríguez et al. (2011) in Quintana Roo, Campeche, and Yucatan, with loads up to 17.2 t ha−1. Litterfall productivity in TFFCW from the Gulf of Mexico (Veracruz) ranged from 9.3 ± 0.5 t ha−1 (Apompal) to 14.9 ± 1.0 t ha−1 (Chica) (Infante et al. 2011b), which is similar to that observed in this study. In both the Gulf of Mexico and the southeast Pacific (LEBRE, El Castaño), litterfall fuels are the result of tree and liana productivity. As suggested by Souza et al. (2019), high litterfall accumulation is determined by climatic factors that affect the vegetative phenology of tree species. Thus, the amount of litterfall fuels in the soil of the study area is twice that accumulated in Veracruz TFFCWs in the Gulf of México, thus, the accumulation rate is higher in the southeastern Pacific. In terms of fire potential, this also represents a greater threat for the El Castaño area, due to the high accumulation of these litterfall fuels (Westcott et al. 2014, Varner et al. 2015). However, the litterfall fuels in the fermented layer are lower compared to studies in the Yucatan Peninsula (Rodríguez et al. 2011), with maximum loads of 53.89 t ha−1. This layer is especially important because combustion is low, but energy is high, so underground fires are more severe than surface fires (Neri et al. 2009). Observations of this fuel layer in this study show no significant differences in the three conditions evaluated, so regardless of the disturbance level, TFFCW are vulnerable to underground fires in the LEBRE.
Some authors, such as Rodriguez (2014) point out that the amount of woody fuels and litterfall decreases with frequent fires. However, despite the fact that, in the last 10 years, two fires of low intensity have been recorded in the study area (CONANP 2018), a limited decrease in dead fuels has been observed in sites with high or medium disturbance. This is due to disturbances caused by timber extraction and the opening of new roads, according to information provided by forest rangers.
Regarding the average load of dead fuels (woody and litterfall), Barrios-Calderón et al. (2018) observed an average load of 225.06 t ha−1 at other study sites in the TFFCW of El Castaño. These loadings are very similar to those of high disturbance sites, which represent the highest average accumulation of dead fuels. In another study carried out at the TFFCW in Calakmul, Campeche, Contreras et al. (2006) reported total loads ranging from 43.15 to 154.5 t ha−1, which were within the range obtained for no and medium disturbance sites. However, the total amount of dead fuels was higher than other TFFCW dominated by P. aquatica in the Yucatan Peninsula, Mexico (16.46 t ha−1), as shown by Reyes and Coli (2009), as they are the lowest loads that could be considered in relation to those obtained in this work.
Recently, Flores et al. (2018) determined that the load or biomass of these dead fuels contains between 47.5% C for litter and 67% C for 1000 h firm fuels. Consequently, the total amount of C released into the atmosphere when the TFFCW is burned contributes to the amount of greenhouse gases. Thus, the conservation and management of these coastal ecosystems is important for the C cycle at the local and regional scales. Therefore, Mahdizadeh and Russell (2021) pointed out the need not only to estimate degraded areas but also to quantify the amount of C lost in the most disturbed areas in a given year, which can vary greatly.
Irrational logging causes alteration in the vegetation structure, which has led to a greater increase and continuity of forest fuels, resulting in a greater possibility of fires (Gao et al. 2020). Fuel beds in highly disturbed sites have a higher ignition potential and C emissions through fine fuels (1 and 10 h) and litterfall, while the medium fuels (100 h) spread the fire, coarse fuels (1000 h) are related to their intensity. Furthermore, dead fuel available at any level of forest disturbance is related to the potential for fire spread and movement on stairway fuels (Chávez et al. 2016). The latter is the result of the vertical continuity of fuels arranged in these ecosystems, ranging from herbs, grasses, shrubs, lianas, and trees connecting one stratum with the next. In general, sites with higher disturbances will have higher fire potential, these sites will require the implementation of strategies to prevent and mitigate fire. Furthermore, if there is a wide dominance of P. aquatica, a type of softwood that grows in tropical regions, conditions increase the decomposition rates of this type of woody material (Hararuk et al. 2015). Finally, the probabilities of underground fires are similar in the three TFFCW conditions, as there are no differences in the depth of the litterfall layer or in the amount of organic material.
The information generated in this study related to the characterization of fuel beds is a starting point for further studies to predict the ignition, spread, and impact of fires in these ecosystems. In general, the potential for fires in the TFFCW conditions evaluated is more evident in areas with high disturbance that require the implementation of management and conservation strategies.