Subject development

Temperate forests provide a variety of ecosystem services, and posess high biodiversity (^{Prieto-Amparán et al., 2019}). Forest degradation negatively affects their functional and structural characteristics (^{Vásquez-Grandón et al., 2018}), it refers to processes that result in long-term and drastic changes in the ecosystems (^{Thompson et al., 2013}) and is generally associated with a reduction in biomass, changes in the structure and composition of species, as well as in their natural regeneration (^{Chazdon et al., 2008}).

Forest fires are considered one of the main natural or anthropogenic factors causing the degradation of temperate forests (^{Pausas and Keeley, 2009}). In Mexico, the fire regime is characterized by its moderate intensity, occurring at intervals of two to ten years (^{Santini et al., 2019}). Some temperate forest (TF) species can benefit from low to moderate fires, because clearing the land stimulates seed release and promotes natural regeneration; however, when fires are severe, the seed viability declines (^{Rodríguez-Trejo and Fulé, 2003}).

Multiple areas of the Cumbres de Monterrey National Park (CMNP) have been degraded due to forest fires that have occurred over time (^{Cantú et al., 2013}). The level of species diversity and germination capacity after a fire of the El Tejocote Laguna de Sánchez community, located in the central zone of the CMNP, is unknown. For this reason, the following objectives were proposed: (1) To describe the ecological importance and diversity of tree and shrub species in a burned area, and (2) To determine the percentage of germination of tree and shrub species surviving after a fire.

The burned site has a surface area of 10 ha, and the geographical coordinates of the center of the area are 25°19'54" N and 100°15'09" W, at an average altitude of 1 905 masl. The average rainfall there is 640 mm, and the average temperature is 13 ºC (^{Cantú et al., 2013}).

A ten percent of the study area was sampled one year after the fire occurred. Ten circular sampling units (SU) of 1 000 m² were established, randomly located at a minimum distance of 20 m between sites (^{Silva-García et al., 2021}). Within each SU, all shrub and tree species were recorded in a census and identified; total height, crown diameter and basimetric diameter at 10 cm were measured for each species. The crown diameters were measured with a model TP20ME Truper^® longimeter, the basimetric area with a model 283d Forestry Suppliers^® diametric tape, and the total height, with a model B084G8TRG9 Nikon^® laser rangefinder.

In order to determine the relative ecological value of the species, the relative frequency (RF; Equation 1), relative abundance (RA; Equation 2), relative dominance (RD; Equation 3) and Importance Value Index were calculated (IVI; Equation 4). The IVI value was divided by three to standardize an interval of 0 to 100, as the maximum value is 300 when the RF, RA, and RD are added together (^{Manzanilla et al., 2020}). The equations for their calculation were as follows:

Where:

RF = Relative frequency of the species

n = Number of the SU where the species is present

N = Total number of SUs

Where:

RA = Relative abundance per species

n = Number of individuals of the species

N = Overall number of all the species

Where:

RD = Relative dominance of the species

i = Crown area of the species

N = Overall crown area of all the species

Where:

IVI = Importance Value Index

RA _i = Relative abundance per species

RD _i = Relative dominance of the species

RF _i = Relative frequency of the species

The indexes to measure species diversity were: (1) Margalef (D _Mg ; Equation 5), whose values above 5 indicate high diversity, while values below 2 suggest low diversity (^{Margalef, 1972}), (2) Simpson's complement (ʎ; Equation 6), the range of whose values fluctuates between 0 and 1, high diversity is associated with values close to 1, and a value close to 0 indicates a site with low diversity (^{Soler et al., 2012}; ^{Salmerón et al., 2017}), and (3) Shannon-Weiner (H’; Equation 7), whose values range from 0.5 to 5, values above 3 indicate high diversity, while values below 2 denote the opposite (^{Somarriba, 1999}). The equations used to calculate the diversity indexes were as follows (^{Moreno, 2001}):

Where:

D _Mg = Margalef diversity Index

S = Number of species present

nl = Natural logarithm

N = Total number of individual

Where:

ʎ = Simpson Index

pi = Abundance according to species i

Where:

H’ = Shannon-Wiener Index

Pi = Proportional abundance of the species

S = Total number of the species

nl = Natural logarithm

n _i = Number of individuals of species i

N = Total number of individuals

An analysis of the percentage of germinated seeds was carried out; the experiment consisted of the following: (1) Collecting seeds from living individuals after the fire, (2) Identifying seeds with no apparent damage, (3) Performing buoyancy tests to detect empty seeds, (4) Applying a pre-germinative process depending on the type of seed: (i) Pinus L. and Quercus L. were immersed in water for 24 hours, (ii) Juniperus L. seeds were placed in 100 % sulfuric acid for 15 minutes, while (iii) Agave L., Arbutus L. and Rhus L. underwent no treatment (^{Antonio et al., 2020}), and (5) Sowing 144 seeds of each of the seven identified species in a 288-cavity model ch288 GLL^® semi-rigid plastic container.

A completely randomized design distributed in four replications (36 seeds per replication) was used; the response variable was germination, and seven treatments were considered for each of the tree and shrub species. For the germination data, a Shapiro-Wilk normality test was performed (^{González-Estrada y Cosmes, 2019}), and no normality was found (p≤0.00097). Therefore, a Kruskal-Wallis test was performed to determine the presence of differences between treatments (^{McKight y Najab, 2010}); the comparison between the means of the different species was carried out by means of a Mann-Whitney U test (^{McKight y Najab, 2010}).

The taxa present in the study area were Pinus teocote Schltdl. & Cham. (twisted leaf or Aztec pine), Pinus pseudostrobus Lindl. (smooth-bark Mexican pine), Juniperus flaccida Schltdl. (Mexican juniper), Quercus laeta Liebm. (white oak), Rhus virens Lindh. ex A. Gray (evergreen sumac), Arbutus xalapensis Kunth (Texas madrone), and Agave scabra Ortega (rough agave or maguey).

The white oak was the most abundant species (Table 1), which is related to the behavior of other species of the same genus (Quercus) that survived after the severe fires in Chipinque Ecological Park (^{Alanís-Rodríguez et al., 2011}). The most dominant tree species was the Aztec pine (Table 2); this agrees with what has been reported in other temperate forests in the state of Nuevo León with fire damage, where Pinus is the most dominant tree over all other species (^{Alanís-Rodríguez et al., 2011}). The most frequent tree species were Pinus teocote and Quercus laeta (Table 1); this behavior is normal in this type of ecosystems because in other temperate forests of the Sierra Madre Occidental with different environmental conditions and floristic compositions, the frequency percentages for pines and oaks are similar to those obtained in the present research (^{Hernández-Salas et al., 2013}). Both species registered a higher IVI (Table 2); this relationship differs from the one reported in Chipinque Ecological Park after a fire, where Quercus had a higher IVI compared to the value of Pinus (^{Alanís-Rodríguez et al., 2011}). However, this coincides with what was reported by ^{Méndez et al. (2014)} in a fire-affected TF in the Sierra Madre de Guerrero, where the IVI of Pinus radiata D. Don was high, while that of Quercus glaucescens Bonpl. was low.

One year after the fire, species diversity indices indicate low values in El Tejocote (Table 2), consistently with the findings of ^{Méndez et al. (2014)} and ^{Méndez et al. (2018)}, who also cite low values for the Margalef and Shannon-Weiner indices in a burned temperate forest of the Sierra de Guerrero.

Significant differences were obtained between the germination of shrub and tree species (H=17.42, Hc=17.62, p=0.0015). Pine species had contrasting germination percentages (Table 3); similar values have been documented for Pinus pseudostrobus in Michoacán State (^{Sáenz et al., 2011}) and for P. teocote in Veracruz State (^{Ramírez, 2000}). The low germination of Quercus laeta (Table 3), can be associated with the fact that Quercus seeds are recalcitrant and lose their viability quickly due to fast desiccation (^{Wawrzyniak et al., 2022}).

Agave scabra reached a high germination percentage (Table 3), which has been recorded in several Agave species (average germination of 91 %) under different environmental conditions (^{Campos et al., 2020}; ^{Castillo et al., 2022}). The low germination of Rhus virens and the null germination of Juniperus flaccida and Arbutus xalapensis suggest a change in the scarification used: For J. flaccida it is proposed to increase the exposure time to sulfuric acid to 5 hours (^{Martínez-Pérez et al., 2006}), as well as cold stratification (^{Yucedag et al. 2021}). A similar treatment is recommended for R. virens and Arbutus xalapensis, but with sulfuric acid exposure times of 30 minutes (^{Olmez et al., 2007}).