Overstory and regeneration dynamics in riparian management zones of northern Minnesota forested watersheds

We quantified tree regeneration under different riparian management zone (RMZ) treatments along first-order streams in Minnesota, USA. A primary objective for long-term management of RMZs in the study region is to maintain some tree cover and promote establishment of later successional tree species and conifers. We also compared regeneration response to contrasting harvesting systems that differed in expected soil disturbance and impact on residual vegetation. Riparian treatments included: (1) full control (no cutting in RMZ (60m-wide in all treatments) or adjacent upland stand), (2) riparian control (RMZ uncut; adjacent upland stand clearcut); and partially-harvested RMZs (RMZ basal area reduced from 29 to 13m ²/ha, adjacent upland stand clearcut) and using (3) cut-to-length or (4) tree-length harvesting. Nine years after treatment, basal area of the full control had not changed appreciably, while basal area of the riparian control had declined by 28% and basal area of the two partial-harvest treatments had decreased by 54%; reductions were due to blowdown of residual trees. Total regeneration density was stable over time in the full control and riparian control, but increased substantially in the two RMZ treatments. Regeneration response was driven by early successional species, mostly Populus tremuloides and Betula papyrifera, and shrubs. Responses were similar between the two harvesting systems. Our results show that regeneration does increase with partial harvesting of RMZs, that the response is mostly due to early succession, shorter-lived deciduous species, and that harvest systems that differ in expected site and vegetation impact elicit similar results. A lack of significant increases in conifers and longer-lived trees in the RMZs following treatment, and the substantial increases in shrubs which may inhibit establishment of these species, suggests a need for more active approaches to establish these species, for example through under planting and competition control.