- 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
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
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.