Why biodiversity increases after variable retention harvesting: A meta-analysis for southern Patagonian forests

• We used Nothofagus pumilio forests as model ecosystem and applied meta-analyses for synthesis of evidence.
• By eight years after harvest, forest specialist of plants, insects and birds recovered to original levels.
• Native species of associated habitats and alien species significantly increased post-harvest.
• Dispersed retention supported more alien and non-specialist species than aggregates.
• Increase in alien species is a conservation significant outcome of VR forestry and should be monitored.

Effects of forest harvesting on biodiversity can be varied and complex to understand. We provide a meta-analysis of 553 studies plants, insects and birds to identify the general responses to Variable Retention harvesting (VR) 1–8 years post-harvest in Nothofagus pumilio forests of southern Patagonia. The analysis is focused on: (i) richness and abundance, (ii) origin and habitat (native forest specialist species, native species of other habitats, alien species), and (iii) temporal trends after harvesting. Our objective was to evaluate why biodiversity increases after variable retention harvesting, by assessing the effects on (i) species richness and abundance in general, (ii) native forest specialists, native generalist species and alien species, and (iii) the recovery of biodiversity toward original conditions. Forests managed with VR supported higher overall richness and abundance of plants, insects and birds in aggregate and dispersed retention than unmanaged stands, but with similar values each other. However, origin and habitat of species affected responses to VR. Aggregates support higher native forest specialist plant and lower plants of habitats other than dispersed retention. However, both retention treatments increased alien plants, although its richness and abundance was higher in dispersed retention. Native forest specialist insects were reduced in comparison to unmanaged forest, while insects of other habitats showed a positive response to both aggregate and dispersed retention as well as did for bird species richness and abundance compared to unmanaged forests. We found evidence for recovery of original conditions for native forest specialist plants and insects, and plants of other habitats. In contrast, alien plants and native insects of other habitats increased continuously in the studies included through those representing 8 years post-harvest. Major differences among both retention patterns included significantly higher richness and abundance of alien plants and native insects of other habitats in dispersed retention. Our synthesis shows recovery toward original conditions for some taxa, but demonstrates long-term establishment of alien plants as well as insect species not associated with native N. pumilio forests. These have emerged as a main potential threat to conservation of forests under VR prescription, at least at the stand level. Retention forestry could play a fundamental role for conservation in productive temperate forests, but the influence of retention pattern and aggregate size are still unclear.