Composition, structure, and biomass of cottonwood-dominated gallery forests along a successional gradient, Willamette River, Oregon Public Deposited


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  • Recognizing the importance of native black cottonwood-dominated riparian forests is especially important to preserve, protect, and manage for biodiversity in the Willamette River Valley. Species composition, structure, and biomass along a successional gradient from stand initiation to late succession of black cottonwood (Populus balsamfera L. subsp. trichocarpa (T. & G.) Brayshaw) dominated gallery forests was investigated along 145 km of the Willamette River, Oregon. These forests were found to generally follow disturbance-initiated successional stages; stand initiation, stem exclusion, early succession/understory re-initiation, mid succession, and late succession. Young stands were dominated by black cottonwood and opportunistic herbaceous species. Understory shrub and late-successional tree species established 12 - 15 years after stand initiation. Oregon ash (Fraxinus latfolia Benth.) was the dominant late-successional tree species. Biotic habitat variables, in contrast to abiotic environmental variables, appeared to be the most important determinants of herbaceous and understory species presence and abundance. Relative stand age, as represented by average black cottonwood diameter at breast height (dbh), was the most important measured environmental variable based on non-metric multidimensional scaling (NMS) ordination using understory plant species composition and abundance in sites of all ages. Abundance of reed canarygrass (Phalaris arundinacea L.) was also an important variable across stand ages. Based on NMS ordination of stands >20 cm average dbh, this variable was the most highly correlated environmental variable with plant species composition and abundance. High abundance of reed canarygrass resulted in lower values of understory species diversity and total species richness by inhibiting establishment of understory tree, shrub, and herbaceous species as well as late-successional tree species. Total above ground tree biomass was calculated for all stands and ranged from <1 to 549 Mg/ha in stand initiation and late succession stages respectively. Black cottonwood biomass was 36 - 100% of total above ground tree biomass (<1- 427 Mg/ha). Biomass in these forests exceeded all other published above-ground biomass estimates for deciduous riparian forests. As cottonwood trees senesced and pioneer tree species dominance decreased, biomass also decreased. Annual biomass accumulation was lowest during stand initiation (0.01 Mg/halyr), peaked when stands were between 7 and 12 years old (23.7 Mg/halyr), and decreased thereafter as stands aged (2.9 to 7.3 Mg/ha/yr in stands >65 years old, n = 6). Structural diversity increased during early succession as understory trees, shrubs, and herbs established along with late-successional tree species, and created multiple vegetative layers. This understory re-initiation occurred around 12 to 15 years after stand initiation, when the cottonwood canopy opened up. Total tree densities were highest in early successional stands, 20,800 to 96,200 trees/ha, and decreased in older forest stands 443 to 3710 trees/ha. Large tree (>10 cm diameter at 1.3 meters in height) densities ranged from 180 to 1350 trees/ha. Standing dead tree density decreased with stand age from a peak in a 4 year old stand of 9,600 snags/ha. Standing dead tree biomass and downed wood biomass increased through time as stands aged from 0 to 12.7 Mglha and 0 to 2.1 Mg, respectively. Abundance of invasive plant species appeared to negatively impact structural diversity by inhibiting establishment of understory and late-successional tree species. This study indicated that Willaniette River cottonwood-dominated gallery forests are structurally diverse with high carbon storage potential, but as late successional species come to dominate, diversity and biomass decreased. Relative stand age was important across stands relative to species composition and abundance. However, considering only older stands, reed canarygrass was the most strongly correlated variable with species composition and abundance. Without intervention to control establishment and survival of reed canarygrass, and perhaps some other invasive species, such as Himalaya blackberry (Rubus arm eniacus L.) and English ivy (Hedera helix L.), it is likely that these species will become more influential and that plant diversity in Willamette River riparian forests will be negatively impacted. In addition, stands are small in area, few remain, and pioneer forest regeneration appears to be limited to areas where they are subject to scour and excessive inundation during high winter flow events. As cottonwood dominance continues to decline, late successional tree species and non-native understory vegetation are expected to increase in dominance at the riverscape level, decreasing overall biodiversity in the Willamette River Valley.
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