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Some Basics About Cottonwood
Establishment and Survival
by Larry Larson and Michael Borman
A number of factors influence the establishment
and survival of plants within riparian corridors. The periodic occurrence
of flooding, erosion, deposition, and drought directly influences
plant composition. Knowledge of plant adaptations is important
to interpret the site potential of a riparian corridor. The purpose
of this article is to illustrate environments that favor black cottonwood
establishment and survival within the riparian corridor. Information
of this type is necessary for establishing appropriate instream
flow regimes to restore these riparian ecosystems.
Establishment
Cottonwood flowering and pollination
generally coincides in the spring with rising high water in riparian
systems and is followed by seed development and dispersal which
occurs as water levels recede. The timing of these events is critical
to cottonwood seedling establishment. Individual cottonwood seeds
are quite small and have a life expectancy of 1-2 weeks which is
further reduced to 2-3 days upon wetting. As a result, seed germination
and establishment has a narrow window of opportunity and requires
a specific environment.
Typical cottonwood establishment is associated with moderate
to slowly receding waters that expose freshly deposited mineral
substrate (fine sand or a fine sand/gravel mix). This yields an
environment free of competition, a mineral soil in which root penetration
can maintain contact with a zone of moist substrate as waters recede,
and an environment that is not subject to additional erosion, deposition,
or prolonged flooding during the first growing season. From a stream
classification (Rosgen) perspective we are, in general, describing
a “C” channel which provides colonization opportunities through
point bar formation and the deposition of substrate in remnant channels
that also carry flood water. The stream gradient in this scenario
will likely be less than 2 percent allowing fine sands or a sand/gravel
mix to form the surface layer of the exposed point bar with layers
of mixed and coarse material beneath. The mixed and coarse materials
are typically deposited during periods of higher stream velocity.
The stream gradient also suggests that floodwaters will tend to
pond within this reach of the stream and then recede at a slower
rate than would occur on steeper gradient streams.
This sequence of events may occur only once in ten years
or longer on many streams in eastern Oregon. This gives cottonwood
stands an even-aged appearance (similar height and size) because
a large number of seedlings tend to become established at the same
time and then thin as the colony matures. In addition, cottonwood
populations associated with point bars may give the appearance of
being formed in a series of lines or arcs of even-aged trees, reflecting
the periodic establishment of seedlings along a receding water line.
All of these factors are encompassed in the “Recruitment Box” model
proposed by Canadian scientists Stewart Rood and John Mahoney.
An application of the model is shown in Figure 1.
Figure 1.
Parameters of the cottonwood seedling recruitment box model applied
to the Bow River, Alberta (Mahoney and Rood, 1998; Wetlands (18):
634-645).
In addition to seedling establishment, cottonwoods can
also become established through the burial of broken or detached
branches and through the development of suckers that sprout from
shallow roots. Black cottonwoods shed branches (cladoptosis) throughout
the winter and early spring as part of a natural pruning process.
Winter winds and snows can also break branches from parent plants,
which fall at the water edge. These tree parts represent potential
sources for new tree establishment. In this case, high waters may
transport and bury or simply bury the branch in place on point bars
or other sites of substrate deposition. Then, as the high water
recedes, the branches sprout forming new plants. Reproduction via
root suckers is also common in black cottonwood. Suckering tends
to increase when the parent tree has crown and/or shallow root damage.
Survival
Cottonwoods are susceptible to both extended drought and
flooding conditions. Young plants are especially susceptible to
drought when moisture from the water table drops below their rooting
zone. This is a major cause of seedling death on over-steepened
point bars and on steeper stream gradients where water levels can
drop at a faster rate than root growth. Juvenile and mature trees,
while less susceptible to drought, can show signs of pruning, leaf-drop,
and yellowing due to cavitation (air bubble formation in water transporting
tissue). Extended periods of drought will result in stunted growth
and/or death in juvenile and mature trees.
Cottonwood has several adaptations that allow it to survive
flooding events, but it is not as well adapted to prolonged flooding
as a number of other riparian species. Cottonwood trees that occur
in these areas are often associated with soils that contain a layer
of coarse substrate. These soils drain more quickly than fine textured
soils and thereby effectively reduce the length of time that a root
system must survive in a flooded environment (little or no available
oxygen). Cottonwoods typically show signs of stress when flood conditions
extend beyond a few weeks. The roots on mature trees tend to survive
flooded conditions by utilizing anaerobic respiration (respiration
without oxygen) to continue essential metabolic functions. However
anaerobic respiration can not be continued indefinitely. It is roughly
20% as efficient as oxygen-based respiration and the by-products
from these chemical reactions accumulate within the plant tissue
where they become toxic. Reliance upon this adaptation requires
a slowdown or stoppage of plant growth and will be limited by the
amount of carbohydrate reserves stored within the roots and the
subsequent accumulation of toxic compounds. A second way that cottonwoods
overcome the lack of oxygen in flooded soils is through the presence
of lenticels along the stem and root crown area of the tree. Lenticels
are small cracks or pores that develop in the bark. Oxygen entering
the tree through these pores will migrate toward areas of low oxygen
concentration. In most cases, this oxygen is supplied to adventitious
roots. Both of these adaptations can occur within the plant at the
same time but in different portions of the root system.
Concluding
Remarks
The riparian corridor is a complex mosaic
of moisture and disturbance patterns. Plants that form communities
within those corridors survive on sites where their basic requirements
for establishment, growth, and reproduction are being satisfied.
It is obvious that restoration efforts in riparian areas require
an understanding of both the environmental mosaic and the life history/adaptations
of riparian species. Species-specific knowledge of this type is
extremely useful to determine instream flow regimes designed to
restore riparian vegetation ecosystems.
Michael
Borman, Extension Rangeland Resources Specialist, Oregon
State University, Corvallis, OR, (541) 737-1614, Michael.Borman@orst.edu
Larry Larson,
Professor,
Range Ecology, Oregon State University, Corvallis, OR,
Llarson@eou.edu
An original
version of this article appeared in October 2000, Issue No. 305,
of The Grazier, a newsletter published by the Department
of Rangland Resources http://www.orst.edu/dept/range/grazier/GRAZ305.htm.
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