fagus grandifolia subsp. mexicana - Tree Ring Research

TREE-RING RESEARCH, Vol. 74(1), 2018, pp. 94–107 DOI: http://dx.doi.org/10.3959/1536-1098-74.1.94

TREE-RING RESEARCH OF MEXICAN BEECH (FAGUS GRANDIFOLIA SUBSP. MEXICANA) A RELICT TREE ENDEMIC TO EASTERN MEXICO ERNESTO CHANES RODRÍGUEZ-RAMÍREZ1 , ISOLDA LUNA-VEGA1 *, and VICENTE ROZAS2 1 Laboratorio

de Biogeografía y Sistemática, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, Mexico

2 Área

de Botánica, Departamento de Ciencias Agroforestales, Universidad de Valladolid, E. U. de Ingenierías Agrarias, Campus Duques de Soria, 42004 Soria, Spain

ABSTRACT Mexican beech (Fagus grandifolia subsp. mexicana) has been classified as an endangered species because of its restricted distribution. The current distribution of Mexican beech, which is considered a Miocene relict, is limited to Tropical Montane Cloud Forests (TMCF) in the mountains of the Sierra Madre Oriental in eastern Mexico. We used dendroclimatic techniques to evaluate the effects of climate variability on the growth of Mexican beech within three forest fragments. The independent chronologies developed for the three sites were 152–178 years long. Cross-sections helped to assess the quality of the crossdating and detect false rings. Over the last 180 years, Mexican beech trees have lower mean radial growth than rates exhibited by other Fagus species. Mexican beech growth appears to be influenced by growing-season temperatures, especially mean maximum temperature. The response appears to be positive at the beginning of the growing season but becomes negative later. These results suggest that the persistence of Fagus-dominated forests in Mexico is dependent on local-scale climatic conditions of the TMCF. Mexican beech forests are associated with micro-climatic conditions that will control the fate of these forests in the face of on-going climate change. Keywords: climatic change, endemism, growth rings, Mexican beech, Sierra Madre Oriental, tropical montane cloud forest.

INTRODUCTION Most dendrochronological studies in Mexico have been performed in arid, temperate and montane regions characterized by well-defined dry or cold seasons (Stahle et al. 2000b; González-Elizondo et al. 2005; Villanueva-Díaz et al. 2007, 2015; Franco-Ramos et al. 2016). These studies have mainly been conducted on gymnosperms in northeastern and central Mexico, such as Douglasfir (Pseudotsuga menziesii (Mirb.) Franco; Cleaveland et al. 2003; Villanueva-Díaz et al. 2003; Arreola-Ortiz et al. 2010), several species of pine [e.g. Mexican pinyon (Pinus lagunae [Rob.-Pass.] Passini; Constante 2007; Pompa-García and Hadad 2016); pinyon pine (P. pinceana Gordon & Glend.; Villanueva-Díaz et al. 2009), Bischicuri (Tarahumara *Corresponding author: [email protected] C 2018 by The Tree-Ring Society Copyright

language; P. cembroides Zucc.; Santillán-Hernández et al. 2010), white pine (P. pseudostrobus Lindl.; Carlón-Allende et al. 2016), bald cypress or ahuehuete (Taxodium mucronatum Ten.; Correa-Díaz et al. 2014) and oyamel (Abies religiosa (Kunth) Schltdl. & Cham; West 2011; Carlón-Allende et al. 2016). Other studies have tried to extend tree-ring analyses to species inhabiting tropical humid vegetation types, such as tropical rain forests (Ricker et al. 2007; Paredes-Villanueva et al. 2015; Williams et al. 2015). The radial growth of trees in Mexico has been extensively studied because growth-rate variation among trees appears to be strongly associated with differences in precipitation and large-scale climatic phenomena such as El Niño-Southern Oscillation (ENSO; Stahle et al. 2000a). Recent studies (e.g. Williams-Linera et al. 2003; Fang and Lechowicz 2006; Ponce-Reyes et al. 2012; Sánchez-Ramos and 94

Dendroclimatic Signal in Mexican Beech

Dirzo 2014; Cai et al. 2015) have documented a significant association between climatic variability and rates in tree radial growth. In this context, recent trends in climate change and their impact on the radial growth of individual trees can be evaluated through the study of tree rings in natural populations. Under appropriate climatic conditions, the analysis of tree rings can be used to make inferences on past climatic conditions beyond the period recorded by meteorological stations (Fritts 2001; Speer 2010). Furthermore, these analyses can also be used to predict the growth of trees under future climate scenarios (Speer 2010; D’Arrigo et al. 2014). Most of the tree species inhabiting the Tropical Montane Cloud Forest (TMCF) develop reliable and anatomically distinctive annual tree rings (García-Suárez et al. 2009; Williams-Linera et al. 2000; Rozas and Muñoz 2016). Fagus grandifolia subsp. mexicana (Martínez) A.E. Murray (Mexican beech) is considered a Miocene relict (23.03 Ma BP; Graham 1976) and an endangered species under Mexican law (Webster 1995; SEMARNAT 2010). Mexican beech is endemic to small fragments (1–42.5 ha) of TMCF of the Sierra Madre Oriental in eastern Mexico. This represents the lowest latitudinal distribution of Fagus species worldwide (Rowden et al. 2004; Téllez– Valdés et al. 2006). Mexican beech can be found in areas with a range of annual precipitation from ca. 800 to 2500 mm, and mean annual temperature of 14.5–24.4◦ C (Ehnis 1981; Peters 1992; Téllez-Valdés et al. 2006; Table 1). This species usually thrives on steep (>35◦ ) rocky slopes with low soil waterretention, which increases the probability of water stress in trees (Peters 1992; Williams-Linera et al. 2003; Téllez-Valdés et al. 2006). Williams-Linera et al. (2000) performed a dendrochronological study with Mexican beech in Veracruz, Mexico. They confirmed that Mexican beech trees show slow growth. However, no information was reported on the sensitivity of Mexican beech to climatic and phenological variability, or to disturbance events that influence forest dynamics (Fritts 1966; Speer 2010; Fang et al. 2012; Villanueva-Díaz et al. 2015). Chronologies for species inhabiting TMCF can provide unique and valuable insights into the paleoclimatic history of Mexico (Villanueva-Díaz et al. 2003; Fang and Lechowicz 2006). As proposed by Therrell et al. (2002), these chronolo-

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gies can also be valuable for investigating climatic variability. We studied the annual tree rings of individual trees in three populations of Mexican beech forests. To date, the effects of climate on the radial growthrates of Mexican beech have not been analyzed. Thus, the goals of the present study were to (1) determine whether Mexican beech represents a good model for dendrochronological research and (2) examine the relationship between tree-ring width and climatic fluctuations and determine the effects of local climatic factors on the ring-width index of individual trees.

MATERIALS AND METHODS Site Description The present study was carried out within three fragments of Fagus-dominated TMCF located in the Sierra Madre Oriental in the state of Hidalgo, Mexico: (1) San Bartolo Tutotepec (Medio Monte), (2) Tenango de Doria (El Gosco) and (3) Zacualtipán de Ángeles (La Mojonera; Table 1, Figure 1). On these three sites, Mexican beech grows on north-facing slopes within an altitudinal range of 1557 to 1950 m a.s.l. The dominant taxonomic classifications of soils at the three sites are Humic (Th) and Vitric Andosols (Tv; FAO-UNESCO 1988). The soil has a light sandy-clay loam texture with pH values between 4 and 6 (Peters 1995). The Mexican TMCF appears to develop under specific micro-climatic conditions (Fang and Lechowicz 2006). More specifically, the TMCF is characterized by sufficient levels of available water throughout the year contributed by the continuous presence of fog (horizontal precipitation) and a closed canopy that prevents water loss. This results in constant levels of environmental humidity and the absence of a well-defined dry season. These sites have a temperate Cb climate (sensu García 1988) characterized by mild temperatures, dry cool season from October to January, dry warm season from early February to May, a long cool summer (June to September), and humidity levels of 60–85% resulting from frequent fog and precipitation throughout the year (Peters 1995; Williams-Linera et al. 2000). Mexican beech is often found in association with other species that can also be found at higher

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RODRÍGUEZ-RAMÍREZ, LUNA-VEGA, and ROZAS

Table 1. Climate and geographic characteristics at the three sites of Fagus grandifolia subsp. mexicana (Mexican beech) in the mountains of the Sierra Madre Oriental of eastern Mexico.

Latitude (N) Longitude (W) Elevation (m a.s.l.) Slope (degrees/direction it faces) Max Min Temperature (◦ C) Max Avg Min Monthly Precipitation (mm) Max Avg Min *

La Mojonera

Medio Monte

El Gosco

20◦ 38 33 98◦ 36 51.8 1780–1950

20◦ 24 50 98◦ 14 24 1800–1944

20◦ 19 37.8 98◦ 14 57.1 1557–1864

37.8/N 1.8/N

21.5/N 0.45/N

43.8/N 16.1/N

18.74 13.62 (2.0* ) 8.49

20.65 14.65 (1.6* ) 9.74

22.48 17.08 (1.6* ) 11.67

298.01 274.3 (90.3* ) 23.71

308.1 275.3 (98.9* ) 30.54

333.37 279.74 (98.9* ) 53.63

Standard deviation.

altitudes (Miranda and Sharp 1950; Williams-Linera et al. 2003), such as ocotes (Pinus patula Schltdl. & Cham, P. teocote Schltdl. & Cham.) and several oak species (Quercus spp.; encinos, quebrachos). The mid-canopy (10–20 m) of the Mexican beech forest is mostly composed of tarflower (Befaria aestuans L.), Mexican clethra (Clethra macrophylla M. Martens & Galeotti), rarely zapotillo (Sideroxylon portoricense subsp. minutiflorum [Pittier] T.D. Penn.), sweetgum (Liquidambar styraciflua L.), magnolia (Magnolia schiedeana Schltdl.), eastern hop hornbeam (Ostrya virginiana [Mill.] K. Koch), and sabino (Podocarpus matudae Lundell). The Mexican beech forests exhibit different degrees of fragmentation and disturbance. The largest and least fragmented in the entire country is the forest in La Mojonera (42.5 ha) (Rodríguez-Ramírez et al. 2013). The results of Rodríguez-Ramírez et al. (2013, 2016) show that the Mexican beech forest in Medio Monte, San Bartolo Tutotepec municipality, which covers ca. 13.99 ha, is the least affected by anthropogenic activity. The beech forest at El Gosco (4.5 ha) shows a high degree of deterioration, mainly resulting from clandestine logging and grazing.

Sample Collection and Chronology Development At each site, 20 dominant Mexican beech individuals were selected following the parameters described by Peters (1992) and Hukusima et al. (2013),

who recommended sampling dominant trees of Fagus that have (1) a diameter at breast height (DBH) ≥40 cm (Ehnis 1981) and (2) a height >10 m. Individual trees were selected to meet these two criteria excluding those exhibiting scars or rot. Two cores were sampled at 1.3 m (breast height) with a Häglof borer from trees with DBH ≥40 cm. A total of 120 cores were obtained from the three sites. In each site, three complete cross-sections from fallen Mexican beech trees were collected. These crosssections were considered a random sample illustrative of radial growth of Fagus species in each locality. Cross-sections were smoothed with a power plane and then sanded until the growth rings were clearly visible. We recommend that when extracting cores from beech trees, the increment borings should be taken in the dormant season and that holes should be filled with treated cork plugs (e.g. treated with a mixture 80% ethanol or isopropanol and 20% purified water, or Na ethylmercurithiosalicylate “Merseptyl”), which are effective against a broad spectrum of bacteria, fungi, and viruses (Lorenz 1944; Toole and Gammage 1959; Polge and Thiercelin 1970; Thiercelin et al. 1972). The wood cores were dried at room temperature and then mounted and polished with two successive coarse-grit sandpapers (100 and 360) and four finer-grit sandpapers (400, 600, 800 and 1000) until the xylem cellular structure was clearly visible in the transverse plane under up to 100 ×


Figure 1. Geographical location at the three study sites of Mexican beech. A = La Mojonera; B = Medio Monte; C = El Gosco.

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Figure 2. Mexican beech cross-section.

magnification (Figure 2). The tree-ring series were dated by assigning calendar years to the rings through the identification of diagnostic ring sequences (Stokes and Smiley 1996; Rozas 2001, 2005). False tree rings were re-examined and compared by using wood cores and several crosssections from fallen trees to correct the chronologies from each site (Figure 2; Fritts 2001; Haghshenas et al. 2016). We measured ring widths using a stereoscopic microscope and a Velmex tree-ring measuring system with 0.001-mm accuracy using the software TSAP-Win v. 4.67c (Rinn 2003). Tree-ring series were crossdated using the Northern Hemisphere criteria, i.e. assigning to every ring the year in which growth started. This dating was verified with software COFECHA (Holmes 1983; Grissino-Mayer 2001). The computer software COFECHA allowed us to identify missing rings, false rings and crossdating errors. The most frequent causes of missing pith are incorrect alignment of the borer, very short length of the core relative to bole radius, and rot (Rozas 2001). To obtain average of detrended tree-ring width indices (RWI), we standardized raw ring-width series with autoregressive modeling to remove serial correlation using the ARSTAN computer program (Cook and Holmes 1996). Non-climatic trends were removed from each tree-ring series using a cubic spline with a 50% response of 10-year periods, which was flexible enough to maximize highfrequency climatic information and minimize the non-climatic variance related to ontogenetic trends and/or local disturbances (Dittmar and Elling 2007; Gareca et al. 2010). Autoregressive modelling was performed on each standardized series to remove temporal autocorrelation (Box and Jenkins 1976) in order to maximize the climatic signal. To produce a standardized

chronology, the resulting indexed series were averaged using a bi-weight mean to reduce the influence of outliers (Cook 1985). Temporal autocorrelation in chronologies is common because of the residual impact of growing conditions from previous years (Speer et al. 2017).

Climatic Records Temperature and moisture are the two most important parameters directly affecting the growth of Mexican beech (Fang and Lechowicz, 2006). We obtained data for mean maximum, average, minimum temperature (Tmax , Tavg„ Tmin ) and monthly precipitation (Prec ) directly from nearby weather stations (Zacualtipán, 20.6◦ N, −98.7◦ W; Tenango de Doria 20.3◦ N, −98.2◦ W; Table 1), with records dating back to 1942. Data from a single weather station (Tenango de Doria) were used for Medio Monte and El Gosco because of their close geographical proximity. More specifically, we used data for the period 1942– 2011 from the CLICOM database (http://clicommex.cicese.mx/), which were complemented with data for the period 2012–2015 from INIFAP weather stations (http://clima.inifap.gob.mx/; Figure 3). These data were corroborated using information provided by Climate-Data (http://es.climatedata.org/). We compared the data to recorded drought years in Mexico (Cardoza-Martínez et al. 2013). To explore the climate sensitivity of Mexican beech growth, we used Spearman’s correlation coefficients as a measure of similarity using SigmaStat v.4 (Jandel Scientific 2016). They were calculated between the tree-ring chronologies and monthly climate data for temperature and monthly precipitation of previous-year June until currentyear September.


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Figure 3. The climatic characteristics at meteorological stations: A = La Mojonera; B = Medio Monte and El Gosco (1941–2015). Average monthly temperature (gray bars), maximum temperature (light gray bars) and average total monthly precipitation (black lines) were based on CLICOM database (http://clicom-mex.cicese.mx/).

RESULTS The independent chronologies spanned up to 188 years for La Mojonera, 168 for Medio Monte and 152 for El Gosco (Figure 4). The average diameter at breast height of sampled trees across the three sites was considerably smaller (82.53 cm) than the average diameter at breast height reported for the species worldwide (134 cm).

The mean sensitivity of individual trees was similar across chronologies from the three sites (Table 2). When we compared the Mexican subspecies of Fagus with other beeches worldwide (e.g. F. sylvatica, F. grandifolia, F. orientalis and F. crenata), it is evident that the annual radial growth range (0.069–2.713 mm) has been slower for Mexican beech during the considered length of chronology (last 180 years).

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Figure 4. Ring-width chronologies at the three study sites of Mexican beech. A = La Mojonera; B = Medio Monte; C = El Gosco; D = all study sites. Black vertical lines represent the decade; black circles represent ENSO events.

Relationship Between Radial Growth and Climate Mean maximum temperature and precipitation were significantly associated with radial growth of the trees. More specifically, decreased radial growth was associated with several drought periods (e.g. 1828–1830, 1850–1866, 1869–1890, 1905, 1913, 1918, 1929–1930, 1940, 1963, 1970, 1972, 1976, 1983, 1991, 1997 and 2012; see Figure 4). Figure 5 shows the correlation between monthly climatic factors (Tmax and Prec ) and the tree-ring widths of the three Mexican beech forest sites. Correlations with Prec were positive in previous October for Medio Monte (P = 0.4) and negative in previous August.

For El Gosco, the Prec was negative and the Tmax was positive in previous June (Figure 5A and B). We found significant correlations between the growth of tree rings and the Tmax in April (P ≥ 0.4) for La Mojonera, Medio Monte, and El Gosco. Tmax showed only positive correlations for El Gosco in May and June (P = 0.4), and negative for La Mojonera. Finally, the Tmax was positive for Medio Monte in May but negative in June (Figure 5B).

DISCUSSION Mexican beech is very susceptible to seasonal and late frosts. Being deciduous, Mexican beech


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Table 2. Tree-ring characteristics for three Mexican beech sites. Statistics

La Mojonera

Medio Monte

El Gosco

Sampled trees Crossdated seriesa Diameter at breast height (cm) Max Min Height (m) Max Min Master series (years) Crossdated ringsa False rings Growth rate (mm) Max Min Avg Series intercorrelationa Mean sensitivitya Autocorrelationa Common interval Mean/median age (years)b Signal-to-noise ratioc

20 24

20 28

20 28

118 54

110 43

225 50

25 14 1828–2015 2198 7

28 15 1847–2015 3094 5

20 12 1863–2015 2700 10

2.507 0.326 0.987 (0.33* ) 0.679 0.395 0.504 1942–2015 93.29/89 27.10

1.571 0.486 0.983 (0.25* ) 0.689 0.334 0.569 1899–2015 1105/119.5 16.21

2.184 0.0691 0.949 (0.43* ) 0.716 0.363 0.518 1949–2015 96.5/92 21.72

*

Standard deviation. Values obtained with COFECHA (Holmes 1999). b Values statistically different using a Mann-Whitney U-test (P = 0.01). c Values obtained with ARSTAN (Cook 1985). a

Figure 5. Estimated correlation coefficients between ring-width chronologies for Mexican beech, (1) total monthly precipitation and (2) mean maximum monthly temperature. Horizontal dotted lines represent 95% confidence intervals, * = P< 0.05; A = La Mojonera; B = Medio Monte; and C = El Gosco.

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trees lose almost all of their leaves during the winter months (December to January) in response to colder conditions and frost events (Ern 1976; Lauer 1973). However, late frosts often occur in late winter and early spring (February and March), which can significantly affect the growth of trees and the production of leaves and flowers buds because of necrosis (Ehnis 1981). In this study, the annual tree rings of Mexican beech were associated with climatic variability for the first time. Our results show that Mexican beech develops annual rings that can be used to reliably estimate the age of the trees. Also, these studies are fundamental to assess the inter-tree variability related to climatic responses. Furthermore, the inferred sensitivity of Mexican beech to climatic fluctuations and the observed correlation between growth-ring series within sites indicate that Mexican beech is a useful species for dendrochronological research. We found that La Mojonera and El Gosco showed average annual growth greater than 2.0 mm compared to average growth of 1.57 mm at Medio Monte. Despite its high average growth rate, El Gosco had the narrowest ring width (0.069 mm) sampled at the three sites (Table 2). This variation in the growth rates may be caused by climatic variations at the study site and/or the effect of anthropogenic activities (e.g. deforestation and grazing) observed at the study site (Rodríguez-Ramírez et al. 2013). In this context, the present study provides relevant new data on the response of TMCF species to regional climatic fluctuations. However, the independent chronologies from the three sites indicated that tree growth responded differently to climatic fluctuations (Figure 4). The presence of individual trees with maximum ages of 152–188 years old allowed us to evaluate the effects of climate on tree growth by comparing tree rings to weather station data back to the year 1942. Our results confirm that Mexican beech, like many species, tolerates environmental stresses such as drought and hurricane effects, as well as anthropogenic disturbances. The TMCF is a relatively low-stress environment, which may explain the persistence of these populations of Mexican beech in north-facing slopes of the Sierra Madre Oriental in eastern Mexico (see Figure 5, Tables 1 and 2). The differences in tree growth across sites may be a con-

sequence of relatively more stable mean temperatures documented for El Gosco and Medio Monte than those recorded for La Mojonera (Figure 4). Compared to other species inhabiting similar environments (Heather et al. 2016; Hu and Riveros-Iregui 2016; Rozas and Muñoz 2016), we found a low association between radial growth and monthly precipitation (Prec ) in the previous year, which is probably random error. However, we found a positive response to the mean maximum temperature (Tmax ). We believe this to be the result of the high levels of precipitation recorded for Mexican beech forests (annual precipitation 824–2458 mm). Therrell et al. (2002) and West (2011) described the tree-ring chronologies in populations of Douglas-fir within our study region (e.g. El Malpaso, Veracruz; 20.404◦ N, 98.467◦ W). These authors observed an extremely weak correlation with regional summer precipitation (June–September), results that agree with our studies of Mexican beech (Figure 5A). The TMCF in the Sierra Madre Oriental has mean maximum temperature ranging from 14.5 to 24.4◦ C, which has a significant effect on the establishment of Mexican beech forests (Peters 1992; Téllez-Valdés et al. 2006). Accordingly, our results showed significant correlations between radial growth and mean maximum temperature. These observations support the suggestion of Fang and Lechowicz (2006) that regional temperature and moisture have a large effect on the growth of Fagus in Mexico. Furthermore, mean maximum temperature also appears to be a determinant factor affecting the growth of other tree species restricted to TMCF, such as Austrocedrus chilensis (D. Don) Florin & Boutelje, Magnolia spp., Nothofagus dombeyi (Mirb.) Oerst., and Pinus contorta Douglas ex Loudon (Price et al. 2011). Beech radial growth can depend on the temperature of the preceding year and/or the dormancy period. Our results showed that in May, lower maximum temperatures hindered the radial growth of trees from La Mojonera, and that higher maximum temperature, also in May, promoted the growth of trees from Medio Monte and El Gosco (Figure 5B). Assimilated carbon as well as nutrients (e.g. N, P and K) decrease during the warm and effective growing season, causing limited growth in the following year (Drobyshev et al. 2010).


Bennett (1985) showed that populations of Fagus grandifolia in the United States were adapted to micro-climatic conditions. Accordingly, climatic variables did not show similar influences on the growth of Fagus across all sites (Figure 5A), despite their geographical proximity (9–46 km). Overall, there was considerable radial growth of trees during years with high temperatures (Tmax ) preceding the rainy season (Figures 5B), but we did not detect that Tavg and Tmin were significant in the Mexican beech growth. The higher signal-to-noise ratio detected in the detrended series from La Mojonera suggests that individual trees in this site are more sensitive to annual climatic variation that those of the other two sites. This ratio is not as important for chronology statistics as other indices, but it provides secondary support to the dendroclimatological legitimacy (He et al. 2011). However, in the case of La Mojonera, mean maximum temperature had a pronounced positive effect on the radial growth of Mexican beech because of the high elevations where this forest develops (up to 2000 m), which makes trees more prone to limiting factors such as frost events. Micro-climatic conditions might be at least partially influencing the establishment and growth of trees within Mexican TMCF (Huntley et al. 1989; Rodríguez-Ramírez et al. 2013; Gual-Díaz and Rendón-Correa 2014). El Gosco, which is known to have had human and naturally induced disturbances in the recent past (Rodríguez-Ramírez et al. 2013), has the youngest trees and the highest DBH values among the three study sites (Table 1). Similar broad-scale climatic conditions facilitate the establishment of Mexican beech within TMCF (Godínez-Ibarra et al. 2007), but microclimatic conditions at each site appear to be affecting the tree-ring width of individual trees. Thus, we suggest that the patterns observed in the growth chronologies are dependent on local and regional climatic conditions. Accordingly, the three studied sites were affected by the same climatic fluctuations. In this context, compared to El Gosco, La Mojonera and Medio Monte have less anthropogenic influence and have the most favorable micro-climatic conditions for the establishment and growth of Mexican beech. Thus, these two localities have the oldest recorded trees and the biggest tree-ring widths.

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Our results suggest that Mexican beech forests are associated with a wide range of micro-climatic conditions, which should promote their resiliency to climate changes. Similar results were found (referring to different drought events) for Pinus cooperi C.E. Blanco in Mexico (Pompa-García et al. 2015) and Tectona grandis L.f. and Pinus caribaea Morelet in Brazil (Venegas-González et al. 2016). Mexican beech forests are characterized by particular micro-climatic conditions that produce differential responses in the width of trees (Rodríguez-Ramírez et al. 2016) and that alleviated the effects of regional environmental events, allowing the persistence of Mexican beech populations in the mountains of the Sierra Madre Oriental. Several studies have suggested that Mexican beech forests are greatly dependent on local-scale climatic conditions for their establishment and survival (Williams-Linera et al. 2000; Godínez-Ibarra et al. 2007). These conditions do not allow trees to have sufficient plasticity (Montiel-Oscura et al. 2013) and adaptability to maintain ecological interactions within TMCF under large-scale climatic fluctuations and anthropogenic influence. The results of this study can contribute to further research related to site-specific environmental factors in the Mexican beech growth process, and provide a better understanding of how temperature and other factors impact the growth of other species from the TMCF.

CONCLUSIONS 1. Mexican beech distribution may be limited to the TMCF because it is an environment that provides a constant supply of moisture and relatively moderate temperatures. Consequently, we can say that Mexican beech is not necessarily highly adaptable. 2. Mexican beech shows little or no response to variations in precipitation, especially in the growing season. 3. Mexican beech growth is possibly influenced by growing-season temperatures, especially mean maximum temperature. The response appears to be positive at first, becoming negative at some point during the growing season. The three sites show this response with some differences in timing between them.

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ACKNOWLEDGMENTS Lorenzo Vázquez-Selem and Osvaldo FrancoRamos helped us with the growth-ring measurements and lent some specialized equipment; Othón Alcántara-Ayala and Rodrigo Ortega García supported the fieldwork; Santiago Ramírez-Barahona and Carlos Solís contributed critical observations. We thank the constructive suggestions and comments of Matthew D. Therrell, Connie Woodhouse, Malcolm Kent Cleaveland, and Steven Leavitt. This research was financed by DGAPA PAPIIT IV201015 project. The first author (ECR-R) also thanks the financial support granted by the postdoctoral fellowship DGAPA-UNAM 2015– 2016.

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