Despite the fact that there is controversy on how disturbance will influence diversity (Mackey and Currie, 2001), there is some evidence that less disturbance results in higher species diversity (Laurance et al. 2012). Tropical freshwater forested wetlands in the Mexican Southeast Pacific, are sensitive to disturbance. In this study we observed that sites with medium to none disturbance are higher in diversity and richer with up to 12 species. The amount of species however, is low in comparison to other tropical freshwater forested wetlands from 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 go as high as 56 (Chiquini et al. 2017). It’s possible that the lower diversity in El Castaño (Pacific) is related to the high dominance of P. aquatica (Rincón, 2014). Given that in this region, P. aquatica is a very adaptive species, so much so that it grows in association with some mangrove species (Barrios-Calderón et al. 2018).
The dominance of P. aquatica is potentially associated to its germination requirement, that can go up to 18 salinity practical units (SPU) in the Pacific in contrast with the Gulf of Mexico (Infante et al. 2011b). The TFFW of El Castaño and the region have reduced their surface in recent years, due to anthropogenic activities such as tree harvest, 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 further reduces its cover percentage, while shrubs and herbs may be more abundant (Kellogg et al. 2003). From this perspective, the characterization of the structure and composition of the forest fuels bed is of great interest, so they should be studied over time. Precisely, an important field of application of studies related to the structural composition of forests and jungles is the fire ecology, mainly in the TFFW. These ecosystems have been impacted by a regime of irresponsible use of fire (Chen, 2006) and fire dynamics have also contributed to changes in its vegetation (Randerson et al. 2006; Parisien et al. 2010; Carmenta et al. 2011; Mistry et al. 2016), thereby reducing its diversity.
In this study the Shannon-Wiener (H’) index in none disturbance forest areas are similar to other TFFW (Díaz et al. 2002; Pérez et al. 2005; Ramírez, 2006), while the values obtained in the zones high disturbance are low as expected. In the Gulf of Mexico, TFFW of P. aquatica can be so low (4 m) and grow up to 30 m, as indicated by Moreno and Infante (2009). In contrast, in the Pacific same type of species grows as much as 17 m (Rincón, 2014), similar to the values reported in this study. Furthermore, 59.1% of the canopy strata goes from 6 to 17 m, while smaller trees represent only 6%. However, this is not the case in high disturbance areas were the most abundant height (76%) ranges from 2 to 7 m, thereby verifying that the disturbance has an effect on the vertical structure, which is evident with the supression of the trees.
Medium disturbance in the TFFW of the Pacific, results in P. aquatica growing higher than in sites none disturbance. This suggests that low fire frequency could be beneficial for the ecosystem (Keane y Karau, 2010; Bowman et al. 2011), acting like a renewal mechanism. In high disturbance areas, P. aquatica is more dominant however does not have great high. Therefore, the level of disturbance will influence vertical and horizontal structure, and in sites with more density of P. aquatica the forest will not be as tall. The most common diameter class of live fuels on the horizontal plane goes from 2.5 to 7.5 cm, independent from disturbance. Similar observations have been reported in other sites of the LEBRE e.j. El Jicaro (Rincón, 2014). Similarly, in TFFW from the Gulf of Mexico in Veracruz, the most frequent class is from 3 to 13 cm (Infante et al. 2011a). This is specially observed in El Castaño in high disturbance sites. The density of trees in this study has a similar range to that reported in sites at the Gulf of Mexico (Infante, 2011). However, in other LEBRE sites with a higher state of conservation such as Brisas del Hueyate, the density can go up to 3,310 ind. ha− 1 and the IVI of P. aquatica (285.28%) is higher (Barrios-Calderón, 2015).
It is important to note that in the present study, the density of trees that make up the live fuels stratum is higher in less disturbed sites, while woody and litter fuels show an opposite pattern, having more accumulation in sites with greater disturbance. However, the average value of dead fuels (woody and litterfall) reveals that tropical freshwater forested wetlands from the Pacific coast can have more fuel than other forested ecosystems in the tropics (Adame et al. 2015; Reyes and Coli, 2009) 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 h (2.75 ± 0.45 t ha-1), 10 h (7.01 ± 1.65 t ha− 1), 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 report loads similar to those obtained at no disturbance TFFW sites in the present investigation. Thus, the TFFW 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 worst implication for the ecosystem (Flores, 2017).
Litterfall fuels, especially the superficial litterfall (SL) from all studied sites is higher than the ones reported by Rodríguez et al. (2011) in Quintana Roo, Campeche and Yucatan, with loads up to 17.2 t ha− 1. Litterfall productivity in TFFW from the Gulf of Mexico (Veracruz) goes from 9.3 ± 0.5 t ha-1 (Apompal) to 14.9 ± 1.0 t ha− 1 (Chica) (Infante et al. 2011b), which are similar to the observed in this study. In both sites Gulf of Mexico and the southeast Pacific (LEBRE, El Castaño), litterfall fuels is the result of the productivity of trees and lianas. As Souza et al (2019) point out, high litterfall accumulation is determined by climatic factors that affect the vegetative phenology of tree species. In this way, the amount of litterfall fuels in the soil of the study area doubles the amount accumulated in TFFW of Veracruz in the Gulf of México, therefore the accumulation rate is higher in the in the southeast Pacific. This in terms of fire potential also represents a greater danger 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 from the fermented layer are lower in comparison with sited in the Yucatan Peninsula (Rodríguez et al. 2011), with maximum loads of 53.89 t ha− 1. This layer is especially important because the combustion is low, but the energy is high, therefore underground fires are more severe than the surface fires (Neri et al. 2009). Observations of this fuel layer in this study shows no significant differences in the three conditions evaluated, therefore independent of disturbance level TFFW 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 las 10 years, two fires of low intensity have been recorded (CONANP, 2018), not much of a decrease of down dead fuels was observed in sites with high and medium disturbance. This is due to disturbance caused by wood extraction and 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) came to obtain an average load of 225.06 t ha− 1 at other study sites in the TFFW of El Castaño. These loads are very similar to obtained in the sites of disturbance high at the present investigation, which represent the highest average accumulation of dead fuels. In another study carried out at the TFFW in Calakmul, Campeche, Contreras et al. (2006) report total loads ranging from 43.15 to 154.5 t ha− 1, which are within the range obtained for none disturbance and medium disturbance sites. However, the total amount of fuels dead is higher than other TFFW dominated by P. aquatica dominated in the Yucatan peninsula, Mexico (16.46 t ha− 1) obtained by Reyes and Coli (2009), which 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% of C (for litter) to 67% of C (in fuels of 1000 h firm). In this way, the total amount of C released when the TFFW are burned contributes a greater amount of greenhouse gases, compared to other ecosystems that have lower amounts of dead fuels (necromass), since the flood forests are important C sinks. Thus, is important role of these coastal ecosystem to the C cycle at local and regional scale.
Fuel beds at high disturbance sites have higher ignition potential and C emissions through fine fuels (1 h y 10 h) and litterfall, while the medium fuels (100 h) propagate the fire and coarse fuels (1000 h) are related to its intensity. Furthermore, dead fuel available in any level of forest disturbance, will be related to propagation potential and movement of the fire on stairway fuels (Chávez et al. 2016). The latter is the result of the vertical continuity of the fuels arranged in these ecosystems, which goes from herbs, shrubs, lianas and trees connecting one stratum with the next. In general, sites with higher disturbance will have higher fire potential, these sites will require the implementation of strategies to prevent and mitigate fire. Even more so if you have a wide dominance of P. aquatica, a type of softwood that complies with the established by Haruk et al (2020) when developed in humid areas with high temperatures and annual precipitation greater than 1837 mm, and that favor the decomposition rates of this type of woody material. In this way the complex of woody fuels forms the necromass, that is, the dead wood that is available and susceptible to ignite. Finally, the probabilities of underground fire are similar in the three TFFW conditions, because there are no differences in the depth of the litterfall layer and amount of organic material.
The information generated in this study related to the characterization of fuel beds, is a starting point for further studies which will allow predict the ignition, propagation, and impact of fires in these ecosystems. In general, the potential for fires in the TFFW conditions evaluated is more evident in areas with high disturbance that require the implementation of preventive and efficient. This to counteract the possible presence of fires that could spread to adjacent ecosystems (mangroves, freshwater marshes and palmares). All information referring to the study of fuel beds and their composition, constitutes a starting point at the regional level to predict or diagnose the start, spread and impact of fires in these ecosystems, in the face of anthropogenic disturbances that have fragmented the connectivity and ecosystem functionality of these coastal wetlands.