Thu, Aug 18, 2022: 9:15 AM-9:30 AM
513F
Background/Question/MethodsInteractions between overstory and understory vegetation are a commonly described, but poorly understood, component of ecological dynamics in closed-canopy forests. Overstory trees exert strong controls on understory shrub and herbaceous layer species by limiting available light and growing space, but it is unclear the extent to which overstory removal, through harvesting, affects competitive and/or facilitative relationships among understory plants during the subsequent transition from open-to closed-canopy forests. We used plant species cover data from 108 permanent sample plots to test the direct and indirect effects of the hierarchical vertical interactions between tall statured understory shrubs and lower vegetation layers (low shrubs, forbs, graminoids, bryophytes) in mature boreal mixedwood forests in northwestern Alberta, Canada. We explored how these interactions change with intensity of variable retention harvesting (clear-cut, 10%, 20%, 50%, and 75% retention, unharvest control) and time since harvest (3 – 17 years post-harvest).
Results/ConclusionsThe interactions between the understory layers across the harvesting treatments, including the reference, were characterized by a mix of direct and indirect positive and negative interactions, with the specifics varying with time since harvest. Across all treatments, many weak or non-significant positive interaction effects observed at 3- and 6-years post-harvest appeared to change to weak or strongly negative relationships by 11- and 17- years post-harvest in clearcut and lower retention treatments. This largely suggested a shift from facilitative interactions immediately after harvesting, due to increased availability of light resources and growing space, to competitive interactions as resources become limited and the stand transitions to closed-canopy conditions. Our results confirm the existence of a dynamic relationship between forest understory plants that is moderated by overstory tree removal. Thus, harvesting may disrupt ecological interaction networks in the understory beyond the intended consequences.
Results/ConclusionsThe interactions between the understory layers across the harvesting treatments, including the reference, were characterized by a mix of direct and indirect positive and negative interactions, with the specifics varying with time since harvest. Across all treatments, many weak or non-significant positive interaction effects observed at 3- and 6-years post-harvest appeared to change to weak or strongly negative relationships by 11- and 17- years post-harvest in clearcut and lower retention treatments. This largely suggested a shift from facilitative interactions immediately after harvesting, due to increased availability of light resources and growing space, to competitive interactions as resources become limited and the stand transitions to closed-canopy conditions. Our results confirm the existence of a dynamic relationship between forest understory plants that is moderated by overstory tree removal. Thus, harvesting may disrupt ecological interaction networks in the understory beyond the intended consequences.