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Revista Chapingo serie ciencias forestales y del ambiente

versión On-line ISSN 2007-4018versión impresa ISSN 2007-3828

Rev. Chapingo ser. cienc. for. ambient vol.28 no.3 Chapingo sep./dic. 2022  Epub 08-Mar-2024 

Scientific articles

Growth ring components of Pinus hartwegii Lindl. at the altitudinal distribution limits in east-central Mexico

Yareli Soto-Carrasco1 

J. Jesús Vargas-Hernández1  * 

Philippe Rozenberg2 

Armando Gómez-Guerrero1 

1Colegio de Postgraduados, Campus Montecillo. km 36.5 Carretera México-Texcoco, Montecillo. C. P. 56230. Texcoco, Estado de México. México.

2INRAE, UMR 0588 BIOFOR. 2163 Avenue de la Pomme de Pin, CS 40001, 45075 Ardon Cedex 2. Orléans, France.



Climate change will have a differential impact on mountain forest growth linked to elevation.


To evaluate the interrelationships of growth ring components at the altitudinal gradient limits of Pinus hartwegii Lindl. in three mountains of east-central Mexico.

Materials and methods:

We analyzed 295 tree samples from Cofre de Perote, Pico de Orizaba and Monte Tláloc corresponding to the period 1960-2017 with a total of 17 700 observations per variable (ring width and density [TRW, TRD], earlywood [EWW, EWD], latewood [LWW, LWD] and minimum and maximum density [MID, MAD]).

Results and discussion:

Growth parameters were higher at the lower limit (≈3 500 m). The correlation of TRW with EWW (r ≥ 0.95) and LWW (r ≥ 0.78) was significant (P < 0.05). TRD correlated with EWD (r ≥ 0.83) and MID (r ≥ 0.72), as well as EWD with MID (r ≥ 0.92) and LWD with MAD (r ≥ 0.92). At the upper limit (≈4 000 m), correlations of TRW, EWW and LWW with TRD, EWD and MID were negative (-0.3 ≥ r ≥ -0.8). This may be explained by lower temperatures and growth periods reducing the relative importance of latewood in ring width.


The information provided contributes to understand the dynamics of P. hartwegii xylogenesis in response to climate and possible implications for radial growth facing climate change.

Keywords: Pico de Orizaba; Cofre de Perote; Monte Tláloc; radial growth; altitudinal gradient



El cambio climático repercutirá diferencialmente en el crecimiento de los bosques de montaña con respecto a la elevación.


Evaluar las interrelaciones de los componentes de los anillos de crecimiento en los extremos del gradiente altitudinal de Pinus hartwegii Lindl. en tres montañas del centro-oriente de México.

Materiales y métodos:

Se analizaron 295 muestras de árboles del Cofre de Perote, Pico de Orizaba y Monte Tláloc correspondientes al periodo 1960-2017 con un total de 17 700 observaciones por variable (anchura y densidad del anillo [ATA, DTA], de madera temprana [AEW, DEW], tardía [ALW, DLW] y densidad mínima y máxima [DMI, DMA]).

Resultados y discusión:

Los parámetros de crecimiento fueron mayores en el extremo inferior (≈3 500 m). La correlación de ATA con AEW (r ≥ 0.95) y ALW (r ≥ 0.78) fue significativa (P < 0.05). La DTA se correlacionó con DEW (r ≥ 0.83) y DMI (r ≥ 0.72), así como DEW con DMI (r ≥ 0.92) y DLW con DMA (r ≥ 0.92). En el extremo superior (≈4 000 m), las correlaciones de ATA, AEW y ALW con DTA, DEW y DMI fueron negativas (-0.3 ≥ r ≥ -0.8). Esto puede explicarse por menores temperaturas y periodos de crecimiento que reducen la importancia relativa de la madera tardía en la amplitud del anillo.


La información generada contribuye a entender la dinámica de la xilogénesis de P. hartwegii en respuesta al clima y sus posibles implicaciones en el crecimiento radial ante el cambio climático.

Palabras clave: Pico de Orizaba; Cofre de Perote; Monte Tláloc; crecimiento radial; gradiente altitudinal


  • P. hartwegii had higher density and radial growth at the lower limit (≈3 500 m).

  • The relationship of radial growth with ring components changed with elevation.

  • The altitudinal effect was different among Cofre de Perote, Pico de Orizaba and Monte Tlaloc.

  • Differences may be related to growing season length and temperature.


Wood formation is closely related to functional processes associated with water and nutrient transport, carbon storage and mechanical support, among others (Correa-Díaz, Gómez-Guerrero, Vargas-Hernández, Rozenberg, & Horwath, 2020). The characteristics of radial growth rings reflect the physiological conditions of the tree during wood formation, in terms of xylem cell size and cell wall thickness. This provides indirect information on xylem hydraulic function, carbon sequestration capacity and water economy during the life of the tree (Babst, Bouriaud, Alexander, Trouet, & Frank, 2014; Speer, 2010). Growth rings in woody species act as natural records of climate and xylem formation, reflecting tree functions (Mendivelso, Camarero, & Gutierrez, 2016; Rathgeber, 2017). Retrospective studies documenting this xylem-recorded information are essential for understanding the effects of climate on the response of woody species on temporal and spatial scales (Düthorn, Schneider, Günther, Gläser, & Esper, 2016), especially in mountain forest ecosystems where strong environmental gradients exist.

Growth ring density reflects the biomass present in a given volume of wood (Morgado-González et al., 2019). The density profile of each ring reveals the typical pattern of earlywood and latewood, corresponding to the main growth periods (Morgado-González et al., 2019). However, the combination of ring width and ring density allows a clearer understanding of biomass determinism and functional implications for the tree (Björklund et al., 2017).

High montane forests in Mexico are dominated by Pinus hartwegii Lindl. which defines the altitudinal limit of trees (Franco-Maass, Regil-García, & Ordóñez-Diaz, 2016). In its natural habitat it covers a range of 600 to 700 m in elevation (from 3 500 to 4 200 m) (Gómez-Guerrero et al., 2013; Silva, Gómez-Guerrero, Doane, & Horwath, 2015). The ecological conditions where it grows and its biological characteristics provide an ideal environment to study the behavior of trees under climate change (Gómez-Guerrero et al., 2013; Silva et al., 2015). Some studies report that maximum density in latewood and minimum density in earlywood are influenced by elevation and cambial age, also relating to processes of water transport, nutrients, and mechanical support of the tree; other studies also mention mountain aspect as an important variable (Morgado-González et al., 2019). This interaction suggests that differential responses in radial growth and its relationships with density and growth ring components in forest landscape should be expected according to site elevation. These differences might be more noticeable at the limits of the altitudinal distribution of species (Morgado-González et al., 2019).

The objectives of this study were to evaluate the change of radial growth and ring components at the limits of the altitudinal distribution of P. hartwegii in the Cofre de Perote, Pico de Orizaba and Monte Tláloc mountains of east-central Mexico, and to determine the effect of elevation on the interrelationships of radial growth and ring components. As a general hypothesis, it was proposed that, for the three mountains, radial growth is greater at the lower altitudinal limit and that interrelationships with growth ring components similarly change with elevation.

Materials and methods

Location of sampled populations

Natural populations of P. hartwegii were sampled in December 2018 at the limits of its altitudinal distribution, 3 500 m (lower) and 4 000 m (upper), in the Cofre de Perote, Pico de Orizaba, and Monte Tláloc mountains in the states of Veracruz, Puebla, and Mexico, respectively (Figure 1). Environmental conditions in the three mountains are similar regarding climate, soils, topography, and vegetation cover (Table 1). The climate of the study areas is type C(w) semi-cold humid with summer rainfall (Garcia, 2004). Soils correspond to humic Andosol, umbric Andosol and Leptosol units (IUSS Working Group WRB, 2015).

Pure natural stands were located on each mountain where mature and dominant trees of the species were selected. The specific elevation of each stand varied slightly, due to the natural spatial distribution of vegetation; for the purposes of the study, elevations are referred to as 'lower' and 'upper' (Table 1). Two sampling stands were selected at each site with a minimum spacing of 300 m in a straight line.

Figure 1 Geographic location of the three mountains of the Transmexican Neovolcanic System with natural populations of Pinus hartwegii sampled at the altitudinal limits (≈3 500-4 000 m) of their natural distribution (information taken and modified from Gómez-Guerrero et al., 2013). 

Table 1 Geographic characteristics and environmental conditions of Pinus hartwegii sampling sites. 

Mountain Altitudinal limit Site Latitude (N) Longitude (O) Elevation (m) Annual precipitation1 (mm) Mean annual temperature1 (°C) Samples2
Cofre de Perote lower 1 19° 31.130’ 97° 09.462’ 3 506 1 540 9.13 48
upper 2 19° 29.303’ 97° 09.050’ 4 076 2 062 6.79 49
Pico de Orizaba lower 1 19° 00.230’ 97° 20.117’ 3 517 1 427 9.16 49
upper 2 19° 00.117’ 97° 17.487’ 4 178 1 773 5.66 50
Monte Tláloc lower 1 19° 24.123’ 98° 44.326’ 3 533 1 022 9.84 50
upper 2 19° 24.427’ 98° 43.677’ 4 004 1 152 7.36 49

1Climate data extracted from the Climate NA V6.21 platform (Wang, Hamann, Spittlehouse, & Carroll, 2016). 2Usable samples after densitometric analysis.

Sample collection and analysis

We selected 25 mature, dominant, healthy, adult trees at each stand, with a vigorous crown, straight stem and without defects or physical damage (Villanueva-Diaz et al., 2007). From each tree a 5 mm diameter wood core was collected using an increment borer, at 1.30 m height, up to the center of the stem and in a perpendicular orientation to the soil slope to avoid compression wood. Wood cores were stored in open polycarbonate boxes to allow moisture loss, and dried at room temperature in the laboratory.

Samples were sawn about 2 mm thick, and resin was extracted with a pentane bath (C5H12) for 48 h. The samples were dried again and exposed to X-rays with the procedure described by Mothe, Duchanois, Zannier, and Leban (1998). X-ray images (negatives) were scanned at 4 000 dpi to acquire microdensity profiles using WINDENDRO® (Windendro 2008e Regent instruments Canada Inc.).

Microdensity profiles were visually verified, compared with the sawn samples, and dated to identify the year of formation of each growth ring. According to the procedures described by Rozenberg et al. (2012), for each ring, the following variables were obtained: total ring width (TRW) (mm), of earlywood (EWW), and latewood (LWW); and total ring density (TRD), of earlywood (EWD), latewood (LWD), minimum (MID), and maximum (MAD) density (g∙cm-3). Five profiles were excluded from the analysis because the ring boundaries were undefined due to an oblique grain angle, which slightly reduced the sample size at some sites (Table 1).

Analysis of densitometric data

Densitometric data were fitted with the residual method to remove the effect of cambial age, using independent models for each location (Rozenberg et al., 2020). Rings close to the pith were excluded in the analysis to avoid variation due to juvenile effect (Cerano-Paredes et al., 2016). Since the last ring (2018) had not been fully formed, it was also not included in the analysis. Based on these criteria and availability of instrumental climate data, it was determined that the most appropriate period to distinguish the effects of climatic factors was from 1960 to 2017.

With the average values of the growth ring components per tree, a two-way ANOVA was carried out with the MIXED procedure of SAS version 9.4 (SAS Institute, 2013), to estimate average values per location and evaluate the effect of elevation, mountain, and their interaction. Subsequently, the Pearson correlation coefficient was estimated separately for each location with the purpose of analyzing the interrelationships between annual radial growth and the components of the growth ring. The homogeneity of the structure of correlations between elevations and mountains was compared using the Chi-square test (Rathgeber, 2017).


Growth ring traits

The average value of growth ring components corresponding to the period 1960-2017 was different between mountains and altitudinal limits (P ≤ 0.05), except for EWD (Table 2). Table 3 shows that narrower growth rings (TRW and EWW), but with higher LWW and density (TRD, EWD, LWD, MID and MAD) were found on Pico de Orizaba than on the other mountains. The average value of ring traits in Cofre de Perote and Monte Tláloc were similar, except EWD and MID which are 6.7 % and 4.1 % lower in Monte Tláloc.

Table 2 Statistical significance (P-value) of the effect of mountain, elevation, and their interaction on mean values of growth ring components of Pinus hartwegii during 1960-2017. 

Mountain 2 0.0023 <0.0001 0.0108 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Elevation 1 <0.0001 <0.0001 <0.0001 <0.0001 0.6939 <0.0001 0.0229 <0.0001
Mountain*elevation 2 <0.0001 <0.0001 <0.0001 0.2318 0.2647 0.2329 0.0419 0.0617

TRW = total ring width, EWW = earlywood width, LWW = latewood width, TRD = ring density, EWD = earlywood density, LWD = latewood density, MID = minimum ring density and MAD = maximum ring density.

Table 3 Average value of growth ring components of Pinus hartwegii during 1960-2017 in three mountains of east-central Mexico. 

Mountain Width (mm) Density (g∙cm-3)
Cofre de Perote 2.80 a 2.12 a 0.70 b 0.488 b 0.444 b 0.626 b 0.384 b 0.684 b
-0.06 -0.04 -0.02 -0.005 -0.005 -0.006 -0.005 -0.006
Pico de Orizaba 2.53 b 1.76 b 0.79 a 0.523 a 0.465 a 0.660 a 0.407 a 0.719 a
-0.06 -0.04 -0.02 -0.005 -0.005 -0.006 -0.005 -0.006
Monte Tláloc 2.74 a 2.06 a 0.71 b 0.472 b 0.420 c 0.622 b 0.360 c 0.686 b
-0.06 -0.04 -0.02 -0.005 -0.005 -0.006 -0.005 -0.006

TRW = total ring width, EWW = earlywood width, LWW = latewood width, TRD = ring density, EWD = earlywood density, LWD = latewood density, MID = minimum ring density and MAD = maximum ring density. Different letters indicate statistical differences in growth ring components between mountains, based on Tukey's mean comparison test (P = 0.05). The standard error (±) of the mean (n = 97 to 99) is indicated in parentheses.

Regarding elevation, Table 4 shows higher average radial growth with wider growth rings at the lower altitudinal limit, due to greater earlywood and latewood formation than at the upper limit. Average ring density was also higher at the lower limit, mainly due to higher LWD and MAD, as EWD was unchanged and MID was slightly higher at the upper limit. The effect of elevation was similar in the three mountains, except for TRW, EWW, LWW and MID in Pico de Orizaba where elevation did not affect them (Figure 2).

Table 4 Mean value of growth ring components of Pinus hartwegii from 1960-2017 at altitudinal distribution limits in the mountains Pico de Orizaba, Cofre de Perote and Monte Tláloc.  

Altitudinal limit Width (mm) Density (g∙cm-3)
Lower (≈3 500 m) 3.25 a 2.37 a 0.92 a 0.507 a 0.444 a 0.662 a 0.377 b 0.731 a
-0.05 -0.04 -0.02 -0.004 -0.004 -0.005 -0.004 -0.005
Upper (≈4 000 m) 2.13 b 1.59 b 0.55 b 0.482 b 0.442 a 0.610 b 0.390 a 0.662 b
-0.05 -0.04 -0.02 -0.004 0.004) 0.005) 0.004) 0.005)

TRW = total ring width, EWW = earlywood width, LWW = latewood width, TRD = ring density, EWD = earlywood density, LWD = latewood density, MID = minimum ring density and MAD = maximum ring density. Different letters indicate statistical differences of growth ring components by elevation effect, based on Tukey's mean comparison test (P = 0.05). The standard error (±) of the mean is indicated in parentheses (n = 147 and 148).

Figure 2 Average value of width (a) and density (b) components of growth rings from 1960-2017 at the lower (≈3 500 m) and upper (≈4 000 m) limits of the altitudinal distribution of Pinus hartwegii in the Cofre de Perote (P), Pico de Orizaba (O) and Monte Tláloc (T) mountains. TRW = total ring width, EWW = earlywood width, LWW = latewood width, TRD = ring density, EWD = earlywood density, LWD = latewood density, MID = minimum ring density and MAD = maximum ring density 

Correlation structure between growth ring traits

Table 5 shows the correlation matrix between ring width and ring density traits; the Chi-square test indicated that the correlation structure was not homogeneous at the altitudinal distribution limits of P. hartwegii. The strong positive correlation of TRW with EWW (r ≥ 0.95) was similar at both altitudinal limits, but the correlation of these with LWW was higher at the lower limit.

Table 5 Matrix of significant correlations (P ≤ 0.05) between ring width and density components (1960-2017) at the lower (≈3 500 m, below the diagonal) and upper (≈4 000 m, above the diagonal) altitudinal distribution limits of Pinus hartwegii in the Cofre de Perote (P), Pico de Orizaba (O) and Monte Tláloc (T) mountains. 

TRW P 0.968 0.812 ----- -0.58 0.662 -0.629 0.691
O 0.957 0.779 ----- -0.362 0.356 -0.434 0.392
T 0.966 0.255 -0.521 -0.669 0.569 0.727 0.641
EWW P 0.974 0.643 -0.252 -0.633 0.675 -0.68 0.684
O 0.981 0.578 -0.262 -0.44 0.339 -0.498 0.392
T 0.975 ----- -0.639 -0.764 0.571 -0.815 0.653
LWW P 0.863 0.735 ----- -0.306 0.455 -0.343 0.511
O 0.924 0.84 ----- ----- 0.333 ----- 0.321
T 0.879 0.783 0.395 0.273 ----- ----- -----
TRD P --- -0.263 --- 0.827 0.372 0.731 0.382
O --- --- --- 0.939 0.642 0.872 0.554
T --- --- 0.355 0.942 --- 0.856 ---
EWD P --- -0.306 --- 0.86 --- 0.949 ---
O --- -0.289 --- 0.87 0.42 0.963 0.329
T --- --- --- 0.878 --- 0.938 -0.316
LWD P --- --- --- --- --- -0.252 0.966
O 0.525 0.502 0.543 0.626 0.414 0.287 0.97
T 0.582 0.544 0.599 0.522 --- -0.343 0.945
MID P --- -0.296 --- 0.716 0.938 -0.266 -0.253
O -0.307 -0.353 --- 0.754 0.923 --- ---
T --- --- --- 0.829 0.963 --- -0.494
MAD P --- --- --- --- --- 0.917 -0.343
O 0.629 0.607 0.625 0.576 0.307 0.939 ---
T 0.711 0.67 0.694 0.468 --- 0.926 ---

TRW = total ring width, EWW = earlywood width, LWW = latewood width, TRD = ring density, EWD = earlywood density, LWD = latewood density, MID = minimum ring density and MAD = maximum ring density. Values in ‘bold’ indicate homogeneous correlations between elevations and mountains. The absence of values indicates a non-significant correlation. Cells with gray background identify correlations between ring width and ring density components.

Correlations of TRD with EWD (r ≥ 0.82), EWD with MID (r ≥ 0.92) and LWD with MAD (r ≥ 0.91) were also homogeneous at the two altitudinal limits. At the upper limit, correlation of TRD with MID (r ≥ 0.72) was higher and with LWD (r ≥ 0.20) and MAD (r ≥ 0.07) was heterogeneous (Table 5). Correlations of width variables (TRW, EWW and LWW) with density variables (TRD, EWD, LWD, MID and MAD) showed a different structure at the two limits. At the lower limit, TRW, EWW and LWW had a weak, positive or negative correlation (-0.35 ≤ r ≤ 0.35) with TRD, EWD and MID, while, at the upper end, correlations with TRD and EWD were negative (-0.82 ≤ r ≤ -0.17); furthermore, correlations of LWD and MAD with TRW, EWW and LWW at the lower limit of Pico de Orizaba and with LWW at the upper limit of Monte Tláloc were not significant (P > 0.05). At the upper limit, correlations between growth and density were negative for earlywood components (EWD and MID), and positive for latewood components (LWD and MAD).

The structure of correlations was also heterogeneous between mountains; Cofre de Perote and Monte Tlaloc contributed more to this heterogeneity at the lower limit, and Pico de Orizaba and Monte Tlaloc at the upper limit. A more homogeneous structure of correlations between altitudinal limits was found in Pico de Orizaba (Table 5).


Differences in growth ring traits

The effect of elevation on radial growth and growth ring components of P. hartwegii was significant, with rings 50 % wider and 5 % denser at the lower limit (≈3 500 m) of the altitudinal gradient from 1960-2017. Morgado-González et al. (2019) describe similar results for this species, with smaller average ring width and density with increasing site elevation. For Larix decidua Mill., Rozenberg et al. (2020) also reported higher average ring width and density at the lower limit of the altitudinal gradient. However, in Fagus silvatica Lipsky a compensatory effect of elevation was found, reflecting higher average growth and lower wood density at the lower limit of the gradient (Topaloğlu, Ay, Altun, & Serdar, 2016).

Dissimilarities in radial growth between altitudinal limits may be due to differences in temperature and growing season. Data derived from the Climate NA model (Wang et al., 2016) show that, in the study mountains, at the lower altitudinal limit of P. hartwegii forests, mean temperature is 3 °C higher and the frost-free period in the year is 64 days longer than at the upper limit. Although high mountain species are adapted to low temperatures, mean annual temperature (6 °C) at the upper limit may not be enough to reach the optimal level of photosynthesis, which only occurs at times of maximum irradiance (Körner, 2012; Sanfuentes, Sierra-Almeida, & Cavieres, 2012). A suboptimal temperature, together with a shorter favorable growth period, is reflected in narrower growth rings and lower average density, as pointed out by Cerrato et al. (2019) in Pinus cembra L. The results show that there are better growth conditions at the lower limit of the gradient, which allow trees to modulate the xylem function more effectively, favoring hydraulic conductivity, growth, structural support, and carbon accumulation (Cerrato et al., 2019). In contrast, at the upper limit, the response emphasizes the hydraulic function, similar to that reported by Camarero, Rozas, and Olano (2014) in Juniperus thurifera L.

Studies in other conifers show that reduced soil moisture (due to lower precipitation, water freezing, or lower retention capacity) induces latewood formation, with smaller diameter tracheids and greater cell wall thickness, to maintain the xylem hydraulic function (Camarero et al., 2014; Moyes, Castanha, Germino, & Kueppers, 2013).

Differences in radial growth and density among mountains are also related to differences under environmental conditions. Narrower rings were found at Pico de Orizaba, but with a higher proportion of latewood and higher average density and component densities, including minimum and maximum ring density (Table 3), which could be related to an earlier transition to latewood formation and a shorter growth period, as suggested by the estimated climatic data. This mountain had a mean annual temperature (and during the frost-free period) 1 to 2 °C lower than the others; in addition, the frost-free period was 56 days shorter, starting 44 days later and ending 12 days earlier than in the other mountains, which may explain the higher relative proportion of latewood and higher ring density. The lower temperature at the top of Pico de Orizaba could explain the higher MAD (at the end of the growing season), to reduce cavitation risks at that time of the year, when the greatest temperature drop occurs (Arias, Bucci, Scholz, & Goldstein, 2015; Scholz, Bucci, Arias, Meinzer, & Goldstein, 2012).

Interrelationships between radial growth and ring components

In most cool-temperate conifers it is common to find a strong positive relationship between ring width and earlywood and latewood components, as reported by Rozenberg et al. (2020) for L. decidua (r ≥ 0.7). Bjorklund et al. (2017) also indicate positive relationship between average ring density and that of its components for 27 conifer species (r ≥ 0.5), in addition to a weak negative correlation between ring width and ring density and its components. Negative correlations between wood density and ring width are mentioned in P. hartwegii by Morgado-González et al. (2019); however, Correa-Díaz et al. (2020) found that maximum ring density had a positive, although weak (r = 0.3), correlation with ring width at Monte Tláloc.

In general, the pattern of correlations between growth ring traits in P. hartwegii is similar to that described for other conifers (Björklund et al., 2017; Dalla-Salda, Martinez-Meier, Cochard, & Rozenberg, 2011); however, in this species, the pattern is modified with elevation. At the higher limit, the correlation of TRW with LWW was weaker and the negative correlations of TRW, EWW and LWW with TRD, EWD and MID were stronger (Table 5). This effect is consistent with differences under climate conditions at the gradient limits; low temperatures and short length of the favorable growing season at the upper limit cause latewood to have less relative importance in growth ring width, as has been shown in other species (Correa-Díaz et al., 2020; Düthorn et al., 2016).

On the other hand, the lower photosynthetic capacity at the upper limit of the gradient, associated with lower average temperature, generates greater competition for resources, especially during earlywood formation when shoot growth also occurs (Antonucci et al., 2017; Li et al., 2017). As environmental conditions become harsher, processes associated with energy capture, such as photosynthesis, are reduced to a greater degree (Pompa-García & Venegas-González, 2016). In the upper part, lower temperature and lower soil water availability limit photosynthetic activity as noted by Pompa-García, Camarero, Colangelo, and Gallardo-Salazar (2021) in Pinus leiophylla Schl. et Cham. This could explain the stronger negative correlation of TRW and EWW with TRD and EWD at the upper limit of the gradient. This could also explain the difference in magnitude of the correlations of TRW and EWW with LWD and MAD at the upper limit of Pico de Orizaba compared to Cofre de Perote and Monte Tláloc. These correlations were stronger on Pico de Orizaba and Monte Tláloc, where the upper part is cooler. When conditions improve in this more unfavorable zone, earlywood density decreases and latewood density increases; that is, the tree can distribute its resources more efficiently between earlywood and latewood of the ring, to favor conduction functions with larger cells in earlywood, and xylem support functions, with thicker-walled cells in latewood. Under more limiting conditions of Pico de Orizaba, there are fewer possibilities than in Cofre de Perote and Monte Tláloc to distribute resources efficiently.

The lower radial growth of P. hartwegii at the upper limit, linked to lower wood density, reflects the effect of limiting factors and response mechanisms of the species to adjust to environmental conditions during wood formation, maintaining xylem functionality (Park & Spiecker, 2005; Rozenberg et al., 2020). These differences at the altitudinal gradient limits of the species help to understand the differential response of its populations to increased temperature and environmental fluctuation associated with climate change. However, more detailed studies regarding phenology of cambial activity are required to understand the process of xylogenesis and to identify possible compensatory mechanisms among the components of the growth ring. These mechanisms would be required to maintain the functions of mechanical support and xylem hydraulic conductivity (Gindl, Grabner, & Wimmer, 2000) in a scenario of greater climate fluctuation occurring at the altitudinal gradient limits of natural distribution of the species.


Radial growth of Pinus hartwegii differs between altitudinal gradient limits (≈3 500 to 4 000 m) and between the mountains Cofre de Perote, Monte Tláloc and Pico de Orizaba. The altitudinal effect on ring components coincides with differences in temperature and growing season length, reflecting the presence of adaptive mechanisms in the wood formation process. The results are useful for understanding the dynamics of the processes associated with wood formation and their impact on productivity, hydraulic functioning, and permanence of the species in a context of climate variability associated with the increase in temperature in recent decades. However, more detailed studies of cambium phenology and wood formation at altitudinal gradient limits are required to identify the eco-physiological mechanisms involved in the response of radial growth to fluctuating climate factors.


We are grateful for the support of the academic and scientific cooperation program CONACYT-ECOS Nord between Mexico and France through project number 0276777 ‘Impacto del cambio climático en la adaptación y plasticidad fenotípica del crecimiento en árboles forestales’-M16A01 'Impact du changement climatique sur la plasticité phénotypique de la microdensité du xylème'.


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Received: October 28, 2021; Accepted: July 18, 2022

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