Tree cavity density is a limiting factor for a secondary cavity nester in second-growth Andean temperate rainforests

AbstractCavity-nesting bird populations are most frequently limited by the number of tree cavities available in second-growth forests. However, this possible limitation of a key resource is less clear in old-growth forests. We compared forest attributes (i.e., basal area, density of larger trees, density of dead trees, and tree cavity density) in second-growth and old-growth stands in Andean temperate rainforests in southern Chile. To examine the role of nest-site availability in limiting the populations of Thorn-Tailed Rayaditos (Aphrastura spinicauda), a secondary cavity-nesting bird species, we conducted an experiment in which nest-boxes were added and removed in old-growth and second-growth forests during a five-year period (2008-2013). In old-growth forests, as compared to second-growth forests, we found a more than double basal area (99.6 vs. 43.7 m2/ha), a three times higher density of larger trees (88.2 vs. 36.4 trees/ha), and a 1.5 times higher number of small cavities (25.9 vs. 10.3 cavities/ha). The density of cavities also strongly increased with tree diameter and basal area. In second-growth forests, Thorn-Tailed Rayaditos showed a strong response to the addition and removal of nest-boxes, with population abundance increasing by 13% and then decreasing by 50%, respectively. In contrast, we found no impact in old-growth stands. Our experiment emphasizes the importance of maintaining large and dead trees in second-growth, disturbed, and managed forests. These trees provide suitable cavities for Thorn-Tailed Rayaditos, and likely many other secondary cavity nesters, increasing their abundances in a Globally significant Biodiversity Hotspot in southern South America.
MethodsStudy area We studied a small bodied secondary cavity-nesting bird species in Andean temperate rainforests of southern Chile (Thorn-Tailed Rayadito, 11.74 g; Altamirano et al. 2020), in the La Araucanía Region (39º16’ S, 71º48’ W) (Fig. 1). The South American temperate rainforests are one out of only seven of these temperate rainforest ecosystems in the world (Alaback, 1991). These forests cover more than 40,000 km2 along the south-western edge of the continent, mainly in Chile and a small adjacent area in Argentina (CONAF et al., 1999; Donoso, 1993), stretching in latitude south between 35° and 55° (Armesto et al., 1998). They are characterised by cool summers, with precipitation occurring throughout the year. Furthermore, they have been classified as among the world’s 200 biologically most valuable and critically endangered ecoregions (Olson and Dinerstein, 1998) and are considered a Global Biodiversity Hotspot (Myers et al., 2000). In Andean areas, native forest has been reduced by 44% over the last two decades (Altamirano and Lara, 2010), and in coastal areas, 67% of native forests have been replaced by other types of land cover since 1975 (Echeverria et al., 2006). We quantified forest attributes, cavity density, and the abundance of a secondary cavity nester (SCN) in 10 forest stands with different degrees of disturbance in the Andean Cordillera (Fig. 1). The stands ranged from recently burned areas with very low and occasional selective logging, representing second-growth forests (< 80 years), to non-burned old-growth forests (> 200 years old; Caviedes and Ibarra, 2017). Six of the 10 stands corresponded to second-growth forests, mainly with a predominance of broadleaf species such as Nothofagus obliqua, Nothofagus dombeyi and Laurelia sempervirens. The other four corresponded to old-growth stands of conifer-broadleaf mixed forest, principally with Saxegothaea conspicua, Laureliopsis philippiana and N. dombeyi. Forest stands were considered as old-growth when they had a core area with a minimal edge effect and maintained a complex vertical structure and the mostly unmodified species composition (Armesto et al., 2009). In both second-growth and old-growth forest stands, understory composition was dominated by bamboo species (Chusquea spp.), Rhaphithamnus spinosus, and a variety of species of Azara and tree saplings. As the home range of the focal bird species is unknown, we assumed a range of 3-4 ha for the Thorn-Tailed Rayadito, given the observed home-range for its close relative, Aphrastura masafuerae (Hahn et al., 2010). Our study stands were separated by a minimum linear distance of 1.6 km, 3.3 to 3.8 times the median natal dispersal reported by Botero-Delgadillo et al. (2017; 490 m females and 420 m males) to ensure that populations in one stand would not affect birds inhabiting adjacent study stands (Wiebe, 2011). Forest attributes and cavity density  To quantify forest structural attributes and cavity density in the stands, we established five vegetation plots per stand. These plots had their centre at each point where the abundance of rayaditos was surveyed (details below), with a survey area of 0.04 ha (radius = 11.2 m). Within these vegetation plots, we quantified the following stand attributes: density of...