Limiting mechanisms in the regeneration of the Chilean matorral - Experiments on seedling establishment in burned and cleared mesic sites

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 sh rub seedlings under more mesic conditions. Within the mesic range of the ma torral we designed a field experiment to compare shrub seedling emergence, growth, and survival under the closed canopy of a secondary forest versus t hree types of open patches: burned, cleared of shrubs but with a herbaceous layer present, and clear without a herbaceous layer. After the first summe r, survival of Quillaja saponaria seedlings was 75% in the burned site, 30% in the cleared patch without herbaceous vegetation, and 15% with herbaceou s vegetation present, whereas there was 0% survival under the secondary for est canopy. After eight years, the percentages had dropped to: 22%, 12% and 3%, respectively. These results contrast strongly with the seedling establ ishment patterns in drier areas of the matorral where early seedling surviv al is higher under the shade of large shrubs. In the experimental mesic sit es, seedlings did best on the burned site, not only in terms of survival, b ut also in terms of growth. After one year, seedling mean height was 10.2 c m in the burned site, whereas 3.8 cm and 5.3 cm in the cleared patches with out and with herbaceous respectively. After eight years, mean height differ ences 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 cl eared patch with herbs. Our results indicate that the facilitative effect o f nurse shrubs on seedling establishment found in dry ranges of the matorra l is less important in more mesic sites. This difference may explain the co ntinuous shrub cover in relatively mesic areas as opposed to the characteri stic patchy structure of the matorral in its drier range.