Variable retention effects on vascular plants and beetles along a regional gradient in Nothofagus pumilio forests

LENCINAS, MARÍA VANESSA; SOLA, FRANCISCO JAVIER; MARTÍNEZ PASTUR, GUILLERMO JOSÉ

FOREST ECOLOGY AND MANAGEMENT

ELSEVIER SCIENCE BV

Año: 2017 vol. 406 p. 251 - 265

0378-1127

Variable retention has been proposed to mitigate harmful effects of traditional practices on biodiversity of forest ecosystems, preserving habitats for species affiliated with closed forests, while also providing habitats for early-seral species. In Nothofagus pumilio forests of Argentinean South Patagonian variable retention has been implemented and its effects over biodiversity of several taxa have been actively monitored in short and medium term studies. However, biotic responses to variable retention have rarely been investigated further than six years since harvesting, seldom considering multiple taxonomic groups in the same research. Furthermore, there is a lack of information about responses along the regional gradient of a forest type distribution. In this work, we evaluated the effect of variable retention silvicultural approach on vascular plant and beetle assemblages, seven to eleven years after harvesting, in three locations along a regional gradient of the natural distribution of N. pumilio forest in Tierra del Fuego (Argentina).We surveyed three silvicultural managements (aggregated retention-AR, dispersed retention-DR, old growth forests-OGF) at three localities (San Justo-SJ, Los Cerros-LC, Río Irigoyen-RI). We characterized the understory vascular plant community to species level and estimated ground cover. The beetle community was sampled by means of pitfall traps, which were collected after a week during mid-summer. We evaluated species richness, abundance, Shannon-Wiener diversity and Pielou evenness indices of plant and beetle samples, as well as community structure, using univariate and multivariate statistical methods. We found 58 vascular plant species belonging to 22 families, and 1437 individuals of 45 species/morphospecies of beetles. Despite forest type being the same, plant and beetle assemblages of old growth forests showed differences among the three different locations along the studied regional gradient, with only 25% vascular plant species and one species of beetle shared among them. Understory plant species distribution may be driven by differences in microclimatic and geographical conditions, while availability of food resources or habitat structural complexity could explain the occurrence of beetle species. Likewise, harvesting by variable retention modified original plant and beetle assemblages with greater effects in DR than in AR. However, this trend was not uniform for each taxa or locality (e.g., in SJ for plants and in LC for beetles), and seems to be related with the composition of original assemblages and the influx of species from surrounding environments. The quantity of specialist vs. generalists species in the original assemblage could influence the resistance/resilience of the community. In this sense, old growth assemblages with greater proportion of generalist and/or non-sensitive species could maintain more similarity between aggregated retention areas and old growth forest. This was observed in plants from LC and RI (approximately 60% specialist vs. 40% generalists), while beetles from SJ and RI (approximately 90% detector species vs. 10% non-sensitive species) evidenced the counterpart. The influx of species from surrounding environments (mainly generalists or exploiters) occurred mainly in the more impacted harvested areas, generating higher dissimilarities between DR and OGF assemblages. Although responses were similar between plants and beetles for one locality (RI), the correlation between these taxa was not so clear for the others (LC and SJ), which demonstrate effects of retentions cannot be generalized among taxa and localities. Finally, the utility of a species identified as potential bioindicators for a region in a particular locality, and vice versa, must be carefully evaluated. Our results highlight the importance of particularities in the biotic assemblages of different taxa in a regional gradient for management and conservation planning, and support variable retention as a useful strategy to combine conservation and productive objectives in a silvicultural managed landscape.