Effects of natural and human disturbances on the dynamics and spatial structure of Nothofagus glauca in south-central Chile.

Aim The general model for Nothofagus regeneration dynamics states that the species of this genus tend to occur in frequent, coarse-scale disturbance sites and/or in harsh conditions. The aim of this study is to test this regeneration model in deciduous mediterranean Nothofagus glauca forests at the northern limit of range of this genus, which include some of the most diverse but least well understood forests in the region. The study also aimed to advance general understanding of the ecological role of N. glauca in these forests, and its response to disturbance, by examining the composition, structure and regeneration patterns of N. glauca forests in the mediterranean zone following three different disturbance histories: mature, post-logging, and post-landslide second growth. Location The study was conducted at the southern limit of the mediterranean climatic zone, in the foothills of the Andes (36°35′ S, 71°28′ W) in south-central Chile, near the south limit of range of N. glauca. Study sites were located at a range of altitudes between 660 and 850 m a.s.l. Methods Two fully mapped 0.12–0.14-ha plots were located at each of three sites in order to sample age, composition, structure, canopy density and spatial patterns of the stands. The spatial description of sites was estimated with point-pattern analysis using Ripley's L-function (univariate and bivariate versions). Shannon's index was used to account for stand structural diversity by height class. Leaf area index (LAI) and canopy density were assessed with fisheye digital photos. Results Nothofagus glauca was the dominant canopy tree species at all sites. In the mature site there was abundant regeneration of N. glauca saplings, leading to a complex open canopy and an all-aged structure with a total basal area (BA) of 21–33 m2 ha−1 (including all tree species), despite the presence of shade-tolerant tree species and the bamboo Chusquea cumingii. The canopy was complex (Shannon's index up to 1.7), but was relatively open (16%). There was no evidence of a shift in regeneration from shade-intolerant Nothofagus species towards more shade-tolerant species. In the post-logging secondary growth stand an evenly aged structure prevailed, with patchy, more open canopies (15–21%, total BA = 9–38 m2 ha−1), but with a less diverse canopy structure. The last site, located in the debris fan of a landslide, presented the lowest LAI and highest canopy opening (23–30%) where the site occupancy is not yet complete (total BA = 14–23 m2 ha−1). At the harshest sites, saplings had a positive spatial association with adult trees, which suggests facilitation from adult trees to saplings. Main conclusions Our study supports the general model for Nothofagus regeneration dynamics, except that N. glauca does not attain sufficient canopy density to require disturbance for regeneration, even in the best sites. Nothofagus glauca can be both a pioneer in harsh site conditions, as well as a gap strategist in mature forests. In these generally harsh sites, the most species, the densest canopies and the most diverse structures occur in the oldest forests. Low canopy density allowed for diverse understorey species at all sites. [ABSTRACT FROM AUTHOR]