Size-density relationships in Drimys winteri secondary forests of the Chiloe Island, Chile: Effects of physiography and species composition.

摘要

Size-density equations have become a relevant stand analysis tool for foresters and ecologists. Second-growth forests dominated by Drimys winteri (>60% basal area of this species) in the north-central part of the Chiloe Island, Chile, were analysed for variation in size-density relationships according to physiographical conditions that influence site quality and species composition. Site differences in this region are driven by drainage. It is poor in flat terrain and improves on slopes. Data from 62 sampled plots showed that density-dependent tree mortality commences once the mean stand diameter reaches 10 cm. It also showed that inclusion of plots in steep slopes (>30%) decreased the goodness of fit of the size-density relationships. Thus, size-density relationships were evaluated for secondary forests ranging from 10 to 20 cm in mean diameter in relatively gentle slopes and flat terrain. D. winteri averaged from 72 to 80% of the total number of trees, and from 80 to 85% of total basal area. The most important accompanying species in flat terrain were those of the Myrtaceae family, and Nothofagus nitida was almost exclusively found in these conditions; on inclined terrain, those characteristic of better sites, e.g. Laureliopsis philippiana and Eucryphia cordifolia, in addition to Proteaceae species, were the principal associated species. Inspite of these variations, site, as reflected by slope (0-30%) and aspect, did not significantly affect size-density relationships. Species composition, as evaluated by percentage basal area, did not have a significant effect in the size-density relationships for all plots and south-aspect plots, and was significant but nominal in north-aspect plots. In flat terrain L. philippiana and E. cordifolia increased the r2 of the size-density model by 14%. Here, these species became relatively important only in stands with a mean diameter >15 cm, likely due to improved drainage conditions, suggesting that a split of the data in plots under and over this diameter could render different results in terms of the effects of site in size-density relationships. The slopes of the equations for all plots or by terrain condition were greater (-1.78 to -1.86) than the "universal" -1.6 slope. Maximum size-density reference lines are provided for average physiographical conditions in the north-central part of the island, and for each aspect condition. Findings from the research expand our understanding of population ecology in this region, provide an important planning tool for silviculturists to use in controlling stand density, and give ecologists a surrogate of microsite environmental conditions..