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Leafminers are insects that develop and live
within the leaves of plants. Typically, the leaves are injured
by the insect feeding on the soft interior tissues so that only
the papery, thin covering of the exterior leaf surfaces remain.
Cottonwood blackmine blotch leafminer: This beetle larva
produces an unusually dark and distinctive blotch leafmine. The
yellow and black adult beetle can be found chewing and
skeletonizing small areas in the exterior leaf surface from late
June through July. They also lay eggs during this period. Later
the larvae initiate mines. The image above
shows a so-called serpentine leafmine.
Seed Production and Dissemination- Seed production starts when
the trees are 5 to 10 years old, increasing rapidly in amount as
the trees become older and larger. Estimates of annual seed
production of a single open-grown tree have been as high as 48
million seeds (3). Good seed crops are the rule. About 35 liters
(I bushel) of fresh fruit yields 1 kg (2.2 lb) of seeds, or
about 770,000 cleaned seeds (19). Seed dispersal follows
flowering by about 2 months in southern populations and a
somewhat shorter period in the North. It is characterized by
considerable variation among trees as well as a lengthy
dispersal period for some individual trees (9). Seed dispersal
occurs from May through mid-July in the South and June through
mid-July in the North (19). The dispersal pattern results in
abundant deposits of seeds along water courses as spring flood
waters recede. Seeds may be carried several hundred feet by the
wind, aided by the "cotton" attached to the seed. Seeds falling
in water may be carried a long distance from the parent tree
before being left on silt deposits. Seedling Development- Unless
floating on or immersed in water, cottonwood seeds must reach a
favorable seedbed and germinate very soon after falling.
Germination of fresh seeds may exceed 90 percent. Seedlings are
delicate for the first few weeks. Rains, very hot sunshine, and
damping-off fungi kill many of them. Very moist, exposed mineral
soils, such as fresh silt deposits, are required. Germination is
epigeal. Growth rate of the fragile seedlings is slow for the
first 3 weeks but may be very rapid after that. Full sunlight
for a substantial part of each day is required after the first
few weeks. Fully mature seeds that are dried promptly to 5 to 8
percent moisture and stored at temperatures just above freezing
maintain viability for several months. Storage at -20° C (-4° F)
may prolong viability for 5 or more years (20). It is best to
increase the moisture content gradually when attempting to
germinate very dry seed.
Eastern cottonwood is frequently planted to give quick shade
near homes. Male clones, which have none of the objectionable
"cotton" associated with seed, are preferred. Windbreaks are
occasionally established with cottonwood as a component.
Cottonwood is suitable for soil stabilization where soil and
moisture conditions are adequate, as along stream or ditch
banks. Deep planting permits reforesting of nonproductive fields
with sandy soils having available moisture beneath a dry surface
layer. There has been considerable interest in cottonwood for
energy biomass, because of its high yield potential and
coppicing ability. There has also been interest in growing it
for inclusion in cattle feed, since it is a good source of
cellulose relatively free of undesirable components, such as
tannins. The new growth is high in protein and minerals.
Some scientists recognize three subspecies of eastern cottonwood
(7). These include angulata, a southern strain, missouriensis, a
central or intermediate strain, and monilifera, a northern
strain. These divisions are based upon minor differences in
morphological traits. Plains cottonwood (Populus deltoides var.
occidentalis), discussed in the next paper, appears to be a
legitimate race or subspecies, growing at higher titleitudes under
more adverse conditions.
Eastern cottonwood hybridizes freely with plains cottonwood and
crosses with several other species either naturally or
artificially. It is most noted for its excellent hybrids with
Populus nigra. Hybrid swarms with P. balsamifera, P.
tremuloides, and P grandidentata are reported (18), as well as
natural hybrids with P trichocarpa (4). The following natural
interspecific hybrids are recognized (12):
Populus x acuminata Rydb. (P. angustifolia x deltoides)
Populus x bernardii Boivin (P. deltoides x tremuloides)
Populus x jackii Sarg. (P. balsamifera x deltoides)
Populus x polygonifolia Bernard (P. balsamifera x deltoides x
tremuloides)
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