It has been argued that if the cost of investment in cold resistance mechanisms detracts significantly from growth rates, then height growth tradeoffs should control species sorting on latitudinal temperature gradients. Applying the same logic to altitudinal temperature gradients, it can be predicted that species' upper altitudinal limits will be fixed by their degree of cold resistance, whereas the lower altitudinal limits of species growing higher up will be set by competition from faster-growing thermophilous species. We tested this prediction for three Nothofagus species on an altitudinal gradient (1,100 -1,700 m) near Termas de Chillán, on the lower western slopes of the Andean Cordillera. We used retrospective methods to measure height increment during the previous growing season, for juvenile trees growing under open conditions at 12 sites. As expected, height growth rates generally decreased with increasing altitude. However, relationships between altitude and height growth did not differ among species. As growth rates of cold-resistant N. pumilio were very similar to those of its more thermophilous competitors N. obliqua and N. dombeyi, our initial prediction was not upheld by the data, and we can not explain what determines the lower altitudinal limit of N. pumilio.KEYWORDS: altitude, cold resistence, competition, temperate forest, tradeoff RESUMEN Se ha postulado que, si la inversión de las especies arbóreas en mecanismos de resistencia al frío es a costa de la tasa de crecimiento, entonces la distribución de las especies a lo largo de los gradientes latitudinales de temperatura debería ser determinada por un compromiso entre el crecimiento en altura y la resistencia al frío. Aplicando la misma lógica a los gradientes altitudinales, se puede predecir que mientras los límites superiores de las especies serán fijados por su capacidad de resistir el frío, sus límites inferiores en el gradiente serán determinados por la competencia con especies de crecimiento más rá-pido. Pusimos a prueba esta idea con un sistema modelo de tres especies de Nothofagus, en un gradiente altitudinal (1100 -1700 m) cerca de Las Termas de Chillán, en las faldas occidentales de la Cordillera de los Andes. Utilizamos métodos retrospectivos para medir el crecimiento en altura durante la temporada anterior, en árboles juveniles en 12 sitios con condiciones abiertas. De acuerdo a nuestras expectativas, por lo general las tasas de crecimiento en altura disminuyeron con el aumento en altitud. Sin embargo, las relaciones entre altitud y tasa de crecimiento no difirieron estadísticamente entre especies. Dado que las tasas de crecimiento de N. obliqua, resistente al frío, fueron muy similares a las de sus congéne-res más termófilas N. obliqua y N. dombeyi, no se apoyó nuestra predicción inicial. Por ende, nuestros datos no permiten explicar las causas del límite altitudinal inferior de N. pumilio.PALABRAS CLAVES: altitud, bosque templado, competencia, tolerancia al frío Gayana Bot. 59(1): [21][22][23][24][25] 2002
Pentachlorophenol (PCP) is a widely used pesticide, particularly for the preservation of wood. Given its high toxicity and resistance to degradation it has become a dangerous environmental pollulant. Due to its high lipophilicity, PCP is able to partition into the lipid bilayer of cell membranes disrupting several vital functions. The present research was concerned with the effects that the chronic administration of PCP could produce in vivo to the sciatic nerve of rats. X-ray diffraction patterns obtained from freshly dissected and dried sciatic nerves of PCP treated rats did not show significant differences in their reflections with respect to those present in the patterns from untreated animals. However, morphological studies performed by optical and electron microscopy showed degenerative changes in about 10% of the A and B type of nerve fibers.
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