Effects of variable retention on the interaction between tree regeneration and understory plants at different regeneration heights over 15 years in Nothofagus pumilio forests

Abstract

Tree regeneration grows in forests mixed with understory plants, leading to complex interactions that can positively, neutrally, or negatively affect regeneration. This study analyzed the effect of variable retention harvesting on the interaction between understory plants and the tree regeneration at different regeneration heights over 15 years in harvested N. pumilio forests of Tierra del Fuego (Argentina). We evaluated accompanying species, co-occurrence frequency, interaction indices (intensity-INT and importance-IMP) and assemblage patterns of accompanying species in aggregated retention-AR, dispersed retention (influenced-DRI and not influenced-DR by aggregates) and unmanaged primary forest-PF, across 72 permanent plots over 2–18 years after harvesting. Eleven understory species were the most frequent interacting plants, including natives (Osmorhiza depauperata, Galium aparine) and exotics (Poa pratensis, Taraxacum officinale). Co-occurrence was observed in 73 % of samples, varying among treatments (DRI>AR≈DR>PF), with higher frequencies in harvested forests for 0–20 cm height category. INT differed significantly at the community level only for 0–20 cm height category (F=9.78; p < 0.001), with positive values in harvested forests (DRI>DR>AR) and negative in PF. Erect herbs and exotic species had the most negative INT in PF (-0.97, −0.84), while DRI showed positive INT for grasses and erect herbs (INT>0.24). IMP was mostly neutral across treatments. Our results suggested that interaction indices change with regeneration height from positive to negative in harvested areas. Additionally, the assemblage of accompanying species differed even among harvested treatments (PF≠AR≠DRI≈DR). Although light availability and seed production play dominant roles, interactions could be relevant for early regeneration establishment, in harvested forests.