Structural dynamics of riparian forests along a black cottonwood successional gradient

Black cottonwood Populus balsamifera L. subsp. trichocarpa (Brayshaw) dominated riparian forests are important habitats for organisms in both terrestrial and aquatic ecosystems in the Pacific Northwest. Using a chronosequence approach and fixed area plots, successional processes were quantified in 28 stands along 145 km of the Willamette River in Western Oregon. Cluster analysis yielded five successional stages; stand initiation, stem exclusion, early, mid, and late seral. Ordination, using non-metric multidimensional scaling, revealed stand variables were strongly associated with these groupings, e.g., stand age, cottonwood dbh, tree biomass, basal area, structural diversity. Tree densities decreased through time from a high of 96,200 to 410 trees/ha in late seral stands. Structural diversity increased through time as understory plant and late-successional tree species established in early seral stands, creating multiple vegetative layers. Aboveground standing tree biomass increased through time from <1 Mg/ha in stand initiation sites to as high as 594 Mg/ha in late seral stands. Grouping using early-successional tree species importance values revealed lower tree biomass in stands dominated by late-successional tree species when compared to late seral stands still dominated by cottonwood. Abundance of invasive plant species, e.g., reed canary grass Phalaris arundinacea L., appeared to negatively impact structural diversity and biomass accumulation by inhibiting understory re-initiation. A significant reduction in peak flow events in the last century may help explain the lack of riparian forest regeneration we noted, as germination and establishment of young cottonwoods was limited to low areas subject to annual flood inundation and scouring. This study reveals Willamette River black cottonwood-dominated forests as dynamic habitats consisting of a mosaic of diverse stands in varying successional stages. It is vital that we find ways to manage for regeneration of these forests as well as implementing control practices for invasive plant species.