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Forest canopy openings modify the natural environment, producing changes in light quality and intensity, precipitation and temperature. In turn, these changes promote the acclimation of understory species. However, little work has been done on underground responses to those environmental changes. The objective of this work was to determine how Osmorhiza depauperata, Phleum alpinum and Poa pratensis change its root length density and root colonization by mycorrhiza as a function of light availability in a Nothofagus pumilio (i.e. lenga) forest harvested following the variable retention prescription.We selected three microenvironments in an old growth forest harvested by the variable retention prescription: aggregated retention, dispersed retention with influence of aggregated retention and dispersed retention. A non-harvested primary forest (PF), similar to the harvested one, was used as a control. Every 2 months, from October 2008 to April 2009, we took soil cores from randomly selected plants. From these soil cores, root length density and colonization percentage (CP) by arbuscular mycorrhizae were estimated.Light availability changed significantly among the microenvironments. In general, root length density was significantly greater in P. pratensis than in P. alpinum and both species greater than in O. depauperata. Light availability increased root length density in all species, although the magnitude of these increases difference among species. Root length density was 187% greater in P. pratensis, 101% in P. alpinum and 94% in O. depauperata in the disperse retention system than in the PF. Mycorrhiza CP was higher in O. depauperata than in P. alpinum and P. pratensis. Also, it was lower in the PF than in the harvested microenvironments. CPs were very low.