Human-induced changes in atmospheric composition are expected to affect primary productivity across terrestrial biomes. Recent changes in productivity have been observed in many forest ecosystems, but low-latitude upper tree line forests remain to be investigated. Here, we use dendrochronological methods and isotopic analysis to examine changes in productivity, and their physiological basis, in Abies religiosa (Ar) and Pinus hartwegii (Ph) trees growing in high-elevation forests of central Mexico. Six sites were selected across a longitudinal transect (Transverse Volcanic Axis), from the Pacific Ocean toward the Gulf of Mexico, where mature dominant trees were sampled at altitudes ranging from 3200 to 4000 m asl. A total of 60 Ar and 84 Ph trees were analyzed to describe changes in growth (annual-resolution) and isotopic composition (decadal-resolution) since the early 1900s. Our results show an initial widespread increase in basal area increment (BAI) during the first half of the past century. However, BAI has decreased significantly since the 1950s with accentuated decline after the 1980s in both species and across sites. We found a consistent reduction in atmosphere to wood (13) C discrimination, resulting from increasing water use efficiency (20-60%), coinciding with rising atmospheric CO2 . Changes in (13) C discrimination were not followed, however, by shifts in tree ring δ(18) O, indicating site- and species-specific differences in water source or uptake strategy. Our results indicate that CO2 stimulation has not been enough to counteract warming-induced drought stress, but other stressors, such as progressive nutrient limitation, could also have contributed to growth decline. Future studies should explore the distinct role of resource limitation (water vs. nutrients) in modulating the response of high-elevation ecosystems to atmospheric change.

It is well known that tree growth is strongly affected by climate at high elevations, but it is still unclear how climate variability influences the distribution of montane forest ecosystems, particularly tree line species. To advance knowledge in this field, we combined temporal (tree ring measurements) and spatial data (remotely sensed variables), to quantify how forests responded to climatic variability across altitudinal gradients in Central Mexico. Normalized difference vegetation index (NDVI), tree‐ring chronologies, and site‐level climatic data were used in a vegetation trend analysis of greenness and browning and to reconstruct canopy vigor for the last century. Although a common ring width chronology was developed, we found significant site‐dependent forest growth response, where young trees (<100 years) exhibited heterogeneous growth trends, without an altitudinal pattern. However, mature trees (100–200 years) showed a common growth decline during the middle twentieth century, regardless of their altitude. Annual maximum NDVI anomalies did not show a general greening effect at high‐elevations. The forest showed both greening and browning zones denoting spatial variability in tree vigor. Furthermore, temperature from the previous year had a positive effect on both NDVI and ring width index but negative at beginning of the growing season. The significant relationship between winter‐spring NDVI (December to March) and ring width index (r = 0.64, p < 0.05), was useful to reconstruct the canopy vigor for the last 151 years. Thus, the greening effect derived from NDVI should be carefully interpreted as a direct forest growth increase. These results highlight the potential of integrating remotely sensed and dendrochronological methods to improve predictions of forest ecosystem responses.

The effect of thinning and cultural practices (multinutrient fertilization, pruning) on total aboveground biomass increment and growth efficiency was studied over three consecutive 2-year periods (1981–1987) in young Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) plantations. Net aboveground biomass increment over the 6-year period averaged 14.5, 7.8, and 5.5 Mg•ha−1•year−1 for the high-, medium-, and low-density plots, respectively. Growth efficiency, after dropping sharply between leaf area indexes of 1 and 6 m2/m2, remained relatively constant up to a leaf area index of 17, the highest measured. Consequently, aboveground biomass increment continued to increase at leaf area indexes well above that at which the Beer–Lambert law predicts maximum light should be absorbed. Foliage analyses indicate that thinning improved nitrogen, potassium, and magnesium nutrition and increased the translocation of potassium from 1-year-old foliage to support new growth. However, fertilization increased foliar nitrogen and phosphorus contents only when coupled with pruning, suggesting that trees favor total leaf area over individual needle nutrition. Indications of potassium and magnesium limitations in this study are supported by other recent studies in Douglas-fir. Further work on the role of multinutrient deficiencies in this species is warranted.

RESUMENCon el objetivo de determinar la composición, diversidad, similitud florística y valor de importancia de las especies arbóreas, se evaluó la regeneración de tres rodales en donde se aplicó el método de regeneración natural de árboles padre durante el periodo 1989-1995. Dentro de cada rodal, se definieron unidades relativamente homogéneas considerando características como pendiente, exposición, altitud y número de árboles padre; resultando once unidades. Contiguas a éstas se seleccionaron áreas sin intervención. En cada unidad se estableció un sitio circular de 314.16 m 2 (10 m de radio), el cual se dividió en cuatro subparcelas de 78.54 m 2 . Se evaluó la regeneración establecida de especies arbóreas (pinos y latifoliadas), en las áreas con tratamiento y las especies arbóreas en las áreas sin tratamiento. Se determinó la diversidad de especies y la densidad. Se calculó el valor de importancia (IVI). Se determinaron los índices de diversidad e índices de similitud. Los resultados indican que los rodales estudiados presentan una mezcla de especies de pino y latifoliadas, en donde dominan relativamente las primeras. Los índices de diversidad calculados señalan que a medida que pasa el tiempo, después del tratamiento de regeneración, los valores de diversidad y riqueza son menores, aunque éstos no cambian en aquellos rodales sin tratamiento. Al comparar los índices de diversidad de especies entre los rodales tratados, todos son estadísticamente diferentes. Sin embargo, al compararlos con los rodales sin tratamiento, sólo en un rodal y en su adyacente, no son estadísticamente diferentes, los cuales obtuvieron entre sí el valor más alto del índice de similitud. En general, la mezcla de especies en los rodales con tratamiento de regeneración y en las áreas adyacentes, se mantiene con pocos cambios, por lo que se concluye que la aplicación de la corta de regeneración no tuvo efectos negativos sobre la composición y diversidad de especies, mas aún propició un ligero incremento de éstas. ABSTRACTNatural regeneration of three stands harvested with the seed-tree method during the period 1989-1995 was evaluated in order to determine the composition, diversity, floristic similarity and importance value of the tree species. Within each stand, relatively homogenous units were identified taking into account characteristics such as slope, exposure, altitude, and number of seed trees, resulting in eleven units. Adjacent to these units, stands without silvicultural treatment were selected. Inside each unit a 314.16 m2 (10 m radius) circular site was established, each of which was subdivided into four 78.54 m2 subplots. Established regeneration of tree species (pines and hardwoods) was evaluated in the stands under management while in stands without management only the tree species were recorded. Species diversity and density were determined, along with the importance value index. Both diversity and similarity indices were also calculated. The results indicate that the stands under study have a mixture of pine and hardwood sp...

Se realizó un análisis comparativo de la diversidad de especies arbóreas y se cuantificaron diversas variables para caracterizar la estructura espacial de rodales de pino-encino en Ixtlán de Juárez. Para ello se aplicó un muestreo aleatorio estratificado y se registró información dasométrica en 91 sitios circulares de 500 m2 cada uno. A nivel de taxón se determinó el valor de importancia y de acuerdo con el dominante se definieron ocho comunidades fitosociológicas: Pipat, Pioax, Pipse, Pidou, Piaya, Piteo, Pirud y Pilei. La diversidad arbórea se estimó mediante el Índice Shannon (H’). Con información de las parcelas de muestreo se conformaron 2,612 grupos, cada uno constituido por cinco árboles vecinos, a partir de ellos se hizo la caracterización estructural. Se determinaron: la mezcla de especies (índice Mezcla de Especies, Mi), el patrón de distribución horizontal (índice de Uniformidad, Wi) y la diferenciación de tamaños de árboles (índice de Dominancia, Ui). De acuerdo con H’, las comunidades Pipat y Piaya presentaron la mayor diversidad de árboles (2.19 y 2.10, con 26 y 19 especies, respectivamente). Las comunidades Piaya, Pilei, Pioax y Pipat, tuvieron la mezcla de especies más alta (0.591< Mi < 0.687); en las ocho comunidades fitosociológicas el arreglo espacial fue aleatorio (0.530 < Wi < 0.569). El análisis del tamaño de los diámetros, por comunidad mostró que de 46-51% de los árboles fueron más delgados que los individuos de referencia. Se concluye que la diversidad de especies arbóreas y la estructura de los rodales son diferentes en función de la comunidad fitosociológica.