Browsing by Author "Segura, AM"

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    Limiting mechanisms in the regeneration of the Chilean matorral - Experiments on seedling establishment in burned and cleared mesic sites
    (2000) Holmgren, M; Segura, AM; Fuentes, ER
    The Chilean matorral is characterized by multispecific shrub clumps in dry areas but has a continuous canopy in wetter sites. It has been hypothesized that this difference is due to easier recolonization of open patches by shrub seedlings under more mesic conditions. Within the mesic range of the matorral we designed a field experiment to compare shrub seedling emergence, growth, and survival under the closed canopy of a secondary forest versus three types of open patches: burned, cleared of shrubs but with a herbaceous layer present, and clear without a herbaceous layer. After the first summer, survival of Quillaja saponaria seedlings was 75% in the burned site, 30% in the cleared patch without herbaceous vegetation, and 15% with herbaceous vegetation present, whereas there was 0% survival under the secondary forest canopy. After eight years, the percentages had dropped to: 22%, 12% and 3%, respectively. These results contrast strongly with the seedling establishment patterns in drier areas of the matorral where early seedling survival is higher under the shade of large shrubs. In the experimental mesic sites, seedlings did best on the burned site, not only in terms of survival, but also in terms of growth. After one year, seedling mean height was 10.2 cm in the burned site, whereas 3.8 cm and 5.3 cm in the cleared patches without and with herbaceous respectively. After eight years, mean height differences between treatments had increased further: 147.7 cm in the burned site, 40.3 cm in the cleared patch without herbaceous cover and 13 cm in the cleared patch with herbs. Our results indicate that the facilitative effect of nurse shrubs on seedling establishment found in dry ranges of the matorral is less important in more mesic sites. This difference may explain the continuous shrub cover in relatively mesic areas as opposed to the characteristic patchy structure of the matorral in its drier range.
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    The significance of fire intensity in creating local patchiness in the Chilean matorral
    (1998) Segura, AM; Holmgren, M; Anabalón, JJ; Fuentes, ER
    The hypothesis that high and low intensity human-made fires can produce ecologically different effects in the Chilean matorral is examined. We compared the abundance of naturally established seedlings under shrubs burned by low and high intensity tires on five north and five south facing slopes. On south facing slopes, we found 54 shrubs burned by law intensity fires and only 4 shrubs burned by high intensity fires. In contrast, north facing slopes had approximately the same number of shrubs burned by low and high fire intensity fires (24 versus 19, respectively). We only found seedlings under shrubs burned by low intensity fires and most of them were of A Muehlenbeckia hastulata and Trevoa trinervis. Also viable seeds were only found under shrubs burned by low intensity fires. Results indicate that fire intensity can be an important factor determining species distribution patterns in the matorral.
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    Why have European herbs so successfully invaded the Chilean matorral?: Effects of herbivory,soil nutrients, and fire
    (2000) Holmgren, M; Avilés, R; Sierralta, L; Segura, AM; Fuentes, ER
    We show field and experimental evidence that introduced herbivores and soil degradation strongly influence the distribution of introduced herbs in the Chilean matorral. In the field, the relative abundance of introduced species was higher on sites grazed by livestock and on very poor soils. Two factorial experiments assessed the effects of rabbit grazing, nutrient addition, and fire. Native grasses were more sensitive than introduced grasses to grazing. With nutrient addition, native grasses increased while introduced grasses tended to decline. Fire had no significant effects on our experimental plots. We discuss several factors that might explain why alien herbs are more successful in the Chilean matorral than in the Californian chaparral. (C) 2000 Academic Press.