Variability in anatomical stem traits could be related with the environmental conditions where plants develop (e.g. low temperatures, drought, short growing season), and this variability refl ects the ability of a species to be established in habitats that differ in these conditions. Maihuenia poeppigii (Otto ex Pfeiff.) K. Schum is not only one of the Cactaceae species with the southernmost distribution of America, but also presents populations along a wide range of altitudes (approx. from 15 to 2,600 masl.), where the climate varies greatly. This study evaluates the variability in stem anatomical traits related to xylem conduction (density, diameter and relative hydraulic conductivity of vessels) and to water storage (degree of succulence of stem and leaves), in three populations of M. poeppigii that are established in an altitudinal gradient in the South-Central Chile. Additionally, the maximum height of the M. poeppigii cushions was determined as a measure of its exposure to the environment. Average density, diameter and relative hydraulic conductivity of vessels differed signifi cantly among populations from different altitudes. While vessel density and succulence of the stems increased in populations located at higher altitudes in the gradient, diameter of the vessels and plant height decreased. The succulence of the leaves did not differ among the three surveyed populations. The variation found in these anatomical traits refl ects a differential strategy in the use and storage of water between the populations established in the altitudinal gradient. This pattern of functional responses can explain the ability of this species to occupy sites that differ in altitude. | La variabilidad en atributos anatómicos de las plantas podría estar relacionada con las condiciones del medio donde ellas se desarrollan (bajas temperaturas, estrés hídrico, corta estación de crecimiento), y esta variabilidad refl ejaría la capacidad de una especie de establecerse en hábitats que difi eren en la disponibilidad de estas condiciones. Maihuenia poeppigii (Otto ex Pfeiff.) K. Schum. no sólo es una de las especies de la familia Cactaceae con distribución más austral de América, sino que además presenta poblaciones creciendo en un amplio rango altitudinal (aprox. desde 15 hasta 2.600 m s.n.m.), en donde el clima varía notablemente. Este estudio evalúa la variabilidad en atributos anatómicos del tallo, relacionados con la conducción del xilema (densidad, diámetro y conductividad hidráulica relativa de los vasos) y almacenamiento de agua (grado de suculencia del tallo y hojas), de tres poblaciones de M. poeppigii que se encuentran establecidas en un gradiente de altitud en la zona centro sur de Chile. Adicionalmente, se determinó la altura máxima de los cojines de forma de estimar el grado de exposición de estos al ambiente, considerando la variación en la altura de las plantas a mayores altitudes. El promedio de la densidad, diámetro y conductividad hidráulica relativa de los vasos varió signifi cativamente entre las poblaciones de distinta altitud. Con el incremento en la altitud aumentó la densidad de vasos y disminuyó el diámetro y el aporte relativo de los mismos a la conductividad hidráulica. Asimismo, con el aumento en la altitud disminuyó la altura de los cojines y aumentó la suculencia de los tallos. La suculencia de las hojas no difi rió entre las tres poblaciones. La variación encontrada en dichos atributos anatómicos refl eja una estrategia diferencial en la utilización y almacenamiento del agua entre las poblaciones establecidas en el gradiente de altitud. Este patrón de respuestas funcionales puede explicar la capacidad de esta especie de cactácea de ocupar sitios inhóspitos que difi eren en altitud.

Variability in anatomical stem traits could be related with the environmental conditions where plants develop (e.g. low temperatures, drought, short growing season), and this variability reflects the ability of a species to be established in habitats that differ in these conditions. Maihuenia poeppigii (Otto ex Pfeiff.) K. Schum is not only one of the Cactaceae species with the southernmost distribution of America, but also presents populations along a wide range of altitudes (approx. from 15 to 2,600 masl.), where the climate varies greatly. This study evaluates the variability in stem anatomical traits related to xylem conduction (density, diameter and relative hydraulic conductivity of vessels) and to water storage (degree of succulence of stem and leaves), in three populations of M. poeppigii that are established in an altitudinal gradient in the South-Central Chile. Additionally, the maximum height of the M. poeppigii cushions was determined as a measure of its exposure to the environment. Average density, diameter and relative hydraulic conductivity of vessels differed significantly among populations from different altitudes. While vessel density and succulence of the stems increased in populations located at higher altitudes in the gradient, diameter of the vessels and plant height decreased. The succulence of the leaves did not differ among the three surveyed populations. The variation found in these anatomical traits reflects a differential strategy in the use and storage of water between the populations established in the altitudinal gradient. This pattern of functional responses can explain the ability of this species to occupy sites that differ in altitude.