CORK OAK-——-RYAN AND COOKE 
A study of the temperature regions 
of the United States shows that no 
cork trees are growing beyond the 
established line of the Poleward limit 
of growth and that both old and young 
cork oaks are growing between the 
Mediterranean limits of 50° and 70° 
F. It is also shown that the United 
States contains two regions which are 
hotter than the Mediterranean maxi- 
mum of 70° F. Although there are 
no large old cork oaks in either of these 
regions, numerous young cork trees, 
planted during the past 7 years, are 
exhibiting. healthy and vigorous 
growth in the 70°-75° F. region of 
Texas and Florida and in the 75°-80° 
F, region of Texas. It is too soon to 
tell whether or not these young trees 
are growing more rapidly than others 
of the same age in cooler regions and, 
until this fact is determined by later 
observations, it will not be known 
whether the optimum cardinal growth 
temperature has been reached or ex- 
ceeded. The only known fact seems 
to be that cork will grow at temper- 
atures between the Mediterranean 
maximum of 70° F. and the United 
States maximum of 80° F., thereby 
indicating that the maximum car- 
dinal growth temperature is above 
80° F. 
The altitude at which the cork oak 
grows depends entirely upon the sur- 
rounding temperature. It will grow 
at any elevation where the tempera- 
ture is above the established minimum 
growth temperature. The highest 
cork oak tree on record for the Medi- 
terranean region is the one, shown in 
plate 7, which is situated at Teniet-el- 
Had in Algeria and growing at an 
altitude of 5,280 feet. The highest 
on record for the United States is 
the one, shown in plate 8, which is 
growing at an elevation of 4,520 feet 
and is situated on the Craig ranch in 
Superior, Ariz. 
When the effect of temperature on 
the rate of cork growth under constant 
rainfall in the same soil was studied, 
the results were as follows: 
365 
Mean annual Rate o 
temperature (°F.) growt 
BS Or ercctveponioie rove aitcis kore ken ess 0.618 
BD are Pretec Tclomrectetels ones .749 
he aay Re ere mc.c TOE COreG -908 
GQEDe. orarece eile!) teioveiena aueveTs leneveeehe re 1.000 
CEB ote emt olao cons Gosoe 1.101 
OP Re retevacsiaue sieve share berouateieretaccloagts 12335 
UD Vere hare crete iota ciettaysren neve apovellovcve eta 1.619 
BO ius che sovons, cue eyo wit cree cuajeversve hovers 1.962 
Rainfall (a general term including 
rain, snow, hail, and sleet) is almost 
the coequal of temperature in deter- 
mining the limit of growth of a par- 
ticular species or variety of plant. 
Within a given temperature region, 
the distribution of native plants and 
agricultural crops depends more upon 
soil moisture than any other factor. 
The amount of available water is 
usually measured in terms of the depth 
of rainfall over a_ specified time. 
Therefore, the moisture requirements 
of a given plant can be determined by 
a study of its distribution in relation to 
its annual rainfall in inches. In some 
plants, the season, or time of year, in 
which the bulk of rainfall occurs is 
equally as important as the annual 
quantity, but with trees the seasonal 
amount appears to be of no partic- 
ular consequence; the annual depth 
is the issue of paramount importance. 
On the annual basis, therefore, a 
study was made of the rainfall dis- 
tribution of the Mediterranean cork 
areas and the result is presented in 
table 3, which shows that natural cork 
oak is growing between the mean 
annual rainfall limits of 10 inches and 
60 inches and that the greatest amount 
(73.5 percent) of cork is growing in 
the 20- to 40-inch region. 
A study of the rainfall regions of the 
United States shows that there is one 
rainfall region below the Mediterra- 
nean 10-inch minimum and another 
above the 60-inch maximum. Large 
old cork oaks are growing in both 
regions. Cork growth in the region 
having less than 10 inches of rainfall 
is chiefly in Arizona. Although much 
of it is under irrigation, there is a 
sufficient quantity in the drier parts 
