Post Oak ,
Iron Oak,
Cross Oak
Quercus stellata
Fagaceae (white oak group)
-- Research by John J. Stransky
Post oak (Quercus
stellata), sometimes called Iron Oak, or the Cross Oak, is a
medium-sized tree abundant throughout the Southeastern and South Central
United States where it forms pure stands in the prairie transition area.
This slow-growing oak typically occupies rocky or sandy ridges and dry
woodlands with a variety of soils and is considered drought resistant.
The wood is very durable in contact with soil and used widely for
fenceposts, hence, the name. Due to varying leaf shapes and acorn sizes,
several varieties of post oak have been recognized-sand post oak (Q.
stellata var. margaretta (Ashe) Sarg.), and Delta post oak (Quercus
stellata var. paludosa Sarg.) are included here.
Post Oaks occur in
all areas of Texas except the High Plains and Trans-Pecos. A shrub or
tree ranging from 20 to 75 feet tall with stout limbs and a dense
rounded canopy, it grows in dry, gravelly, sandy soils and rocky ridges.
It often grows along with Blackjack Oak, and like it has been considered
an indicator tree of poor soils when seen in native conditions. It is
extremely sensitive to root disturbance and lack of oxygen in the root
zone, so construction activities that compact the soil, pave over the
roots, or change the soil grade can kill existing trees, as can
overwatering, such as when a lawn has been planted around an existing
tree. Post Oak leaves have a distinctive cross shape and are usually 4
to 5 inches long and they are thick and somewhat leathery. They are dark
green and shiny on the upper surface and lighter green and rough hairy
beneath. The bark of the Post Oak is thick with platelike scales, and
is similar to that of the White Oak, but somewhat darker and often
fissured into scaly ridges
The wood of post
oak, commercially called white oak, is classified as moderately to very
resistant to decay. The wood is hard and strong, and is very durable when
in contact with soil. Its use is mainly for posts, railroad ties, mine
props, and sometimes as fuel. In fact, the Post Oak sometimes is called
the Box White Oak. Acorns are 1/2 to 2/3 inches long and ovoid in shape.
The acorns mature in one growing season and drop soon after
ripening, from September through November. In common with many other oaks,
post oak begins to bear acorns when it is about 25 years old. Good acorn
crops are produced at 2- to 3-year intervals.
The post oak
is anti-social or intolerant of competition and is classed as intolerant
of shade. Because of its slow height growth it often is overtopped by
other trees, including most other oaks. On poor sites, however, post oak
tends to persist and become dominant because it is more drought resistant
than many of its associates.
The Post Oak sets their roots much deeper that most
other oaks in the sandy soils for protection from drought. The roots of
most oaks are within the top 12" of the soil surface. In years of heavy
rains of 40-50 inches, and then followed by a year of so or extreme
drought, these deep sandy soils that normally protect them from drought,
may hold excessive amounts of moisture, depleting the soil oxygen and
causing root rot. This can be devastating to groves with poor drainage and
mortality may show up two to three years following years of heavy
excessive rains.
Post oak is
susceptible to most insects, diseases, and pollutants that present a
threat to other oaks. Regeneration efforts are hampered by acorns being
destroyed by weevils. Insect defoliators, leafrollers, tent caterpillars,
Gypsy moth, sawfly, leaf miners, and skeletonizers may cause growth
losses, and when repeated, may cause mortality. The foliage also is
susceptible to attacks by aphids, lace bugs, various scales, gall wasps,
and mites. The trunk, twigs, and roots may be damaged by carpenterworms,
borers, beetles, twig pruners, white grubs, and cicadas (locusts). Some of
these cause defects that render the wood unfit for many commercial
purposes.
Oak wilt, a
vascular disease caused by the fungus Ceratocystis fagacearum, is
potentially the most destructive disease of both the red and white oaks.
It is widely distributed throughout the Central States. White oak is less
susceptible to oak wilt than the red oak species, as in the white oak each
cell is compartmentalizes from to the adjacant cell, thus waterproofing
each individual cell. Oak wilt infection is more localized, and the
tree may lose only a limb at a time, or may sustain infection by the
pathogen without ever showing symptoms or take years for the tree to secum
to oak wilt death. In the live oaks and red oak species there is osmosis
(water movement) between each cell and the disease moves very quickly
throughout the entire tree.
Chestnut
blight fungus (Cryphonectria parasitica) causes many defects as well as
mortality to post oak throughout its range. The tree also is subject to
oak wilt (Ceratocystis fagacearum), a vascular disease prevalent mostly
north of the 35th parallel, but not to the same degree as on red oaks.
Soil-inhabiting fungi may cause heavy seedling mortality by damping off.
Powdery mildews stunt and deform nursery seedlings.
Most oak trees are bad
parents and they do not want to compete with their offspring. If willife
does not consume or carry off the acorns, the tree will predispose a
fungus, that will destroy germination of the acorn. Oaks trees depend of
wildlife to carry the acorns away from the parent tree, as this helps
create forest diversity. Weevils, also play an important part to help
destroy the acorns. Post oak is a valuable contributor to wildlife food
and cover. Acorns provide high energy food during fall and winter and are
considered important in the diet of wild turkey, white-tailed deer,
squirrels, and many other rodents. When acorns are available animals
fatten quickly, go through the winter in good condition, and are most
likely to produce healthy young. Leaves are used for nest building by
birds, squirrels, and raccoons. Cavities provide nests and dens for
various birds and mammals.
THE ROLE OF PHYTOPHTHORAS
IN TREE YELLOWING AND DEATH IN TEXAS
The major symptom
of Phytophthora spp. infection was a generalized slight yellowing
of leaves, persisting for months or years. Later the color change
accelerated, the leaves become orange, and then turned brown and remained
attached. Two to four weeks later the tree died. Since 1934, symptoms of
foliage discoloration ("yellowing"), retarded growth, and premature death
of post oak, live oak, winged elm, and hackberry have been reported in
central Texas. These symptoms are distinctly different from those caused
by the oak wilt (Ceratocystis fagacearum) and oak decline (Cephalosporium)
fungi. Of the 34 trees sampled, 24 exhibited "yellowing" decline symptoms.
All of the symptomatic trees yielded Phytophthora in fruit culture
and 83% of these yielded Phytophthora upon culture on media. 30
percent of the symptomless trees yielded cultures of Phytophthora.
P. cinnamomi was observed in many of the isolates.
Symptomatic trees were prevalent in places with
frequent standing water, or water flows across the roots where there was
exposure to slightly saline or alkali well water. Gas leaks and cattle
trampling also increased susceptibility. Soil injections in
the root zone with ethazol and metalaxyl provided symptom remission
(greener leaves). French drains to carry saline water past low sites also
resulted in symptom remission. Phytophthora played an important
role in tree "yellowing" and subsequent death in Texas. These symptoms
could be minimized with control.
Research is
continuing to investigate the significance to the control of P.
cinnamomi using phosphite. Potassium Phosphite is increasingly being
used as a means of control for dieback caused by Phytophthora cinnamomi.
It is a selective, systemic fungicide/nutrient energizer with a high level
of environmental safety and very low non-target toxicity. This injection
immediately enters the tree and begins restoring the trees ability to
function again and serves to stabilize the tree. One injection protected
trees for at least 4 years.
Phosphorous is an
essential element for trees and critical for root production. Not only
does phosphite help roots, but is actually beneficial to the regeneration
of mycorrhizae on the roots of oaks and other trees. The Phosphite is
highly mobile in trees and moves bi-directional in the pholem and upward
to the leaves in the vascular systems. Because Phosphite has one less
oxygen molecule than phosphate, a higher degree of solubility and
mobility, within the plant is achieved. This unique characteristic permits
phosphites to be rapidly absorbed or taken up across the membranes of
plant foliage and/or roots, in both their nutritive and plant protective
roles, with immediate activity on contact.
The US Constitution's haul is 26 inches thick, it
is a laminated sandwich of 7" of Post Oak, 12" of Live Oak, and 7"
of Post Oak. The Post Oak is the water proof factor on the outer and
inner sides of the haul and with this combination the two woods provides a
super strong haul. Cannon balls bounced off the haul... thus the name :
"Old Ironside"
