« ΠροηγούμενηΣυνέχεια »
been different. It may also be that if the leaves had been kept from forming for a short time the result would have been different. In general it seems fair to presume that the number of rings found represent with a fair degree of certainty the age of that part of the tree. To get the full age of the tree it should be remembered that the count should be made at a point low enough to get the sapling produced from the seed in the first year of growth.
ON THE INTERNAL TEMPERATURE OF TREE-TRUNKS.
R. A. EMERSON.
Observations on the internal temperature of trees were begun by the writer in the summer of 1894. The object of the work was to determine if possible whether the temperature of trunks and limbs exposed to the direct rays of the sun does not at times become injuriously high. Observations were made on several apple trees, a maple, and a cotton wood. Some of the apple limbs were shaded by their foliage, some by boards, and some were in direct sunlight. Half inch holes were bored in the limbs, some on the north side, some on the south, and some on the west. Each hole was bored so that a radius of the circle formed by a cross section of the limb was cut at right angles near its peripheral end. Each hole extended a little over half-way through the tree and left approximately one-half inch of new wood between it and the bark. For taking internal temperatures an accurate thermometer was used. Its stem was fitted in a cork which fitted snugly the hole in the limb, so that, when the thermometer was in place the hole was closed tightly. At each reading the thermometer was left in the hole two or three minutes and so indicated fairly accurately the temperature of the wood. Between readings the hole was kept closed with a cork. Readings were taken at the same times every day. In some cases they were taken in the morning, in some at noon, in others at night, in some both morning and noon, in others both morning and night. The temperature of the air was taken at the same times. For this cheap thermometers were used. They were first compared with the better thermometer and their scales corrected. They were hung on the limbs, one on the side in which the hole was bored, the other on the opposite side. Readings were taken continuously from July 4 to September 5, with but few interruptions.
Now as to results. In the first place the real object of the work, to determine whether the temperature of exposed trunks and limbs does not at times rise injuriously high, can hardly be said to have been accomplished. The highest temperature recorded was 119° F. Though this is probably above the optimum temperature for growth, it would be difficult to say whether it is particularly injurious or not. Of course the maximum temperature of the wood one-half inch in from the cambium layer may have been much less than that of the cambium itself. A few interesting points came out, however, that lead to a further study of tree temperatures. Some of the things shown by this first summer's work are: (1.) The temperature of the tree trunks follows closely that of the outside air. (2.) One side of a small limb may have a temperature much higher than that of the other side. (3.) The maximum daily temperature of a limb exposed to direct sunlight is often much higher than that of the outside air. (4.) The maximum daily temperature of the shaded limbs is below that of the air. (5.) Limbs exposed to direct sunlight show a greater daily variation in temperature than shaded limbs. As one illustration of the above points, a part of the readings taken from four apple trees on July 26, 1894, are given in the table below. Hole No. 3 was in a limb shaded by a board, No. 4 in a limb shaded by foliage, and Nos. 1, 2, and 5 in limbs exposed to the sun.
From this summer's work it became apparent that very little could be learned of tree-temperatures by making observations only once or twice daily. Therefore during a number of days in
the spring and summer of 1895, hourly observations were taken. This time a box-elder tree was used. Holes were bored about as before. A number of good thermometers were placed in the holes and remained there throughout the test, the holes being sealed by putting wax about the thermometer stems. The thermometers were arranged to study the following points: (1.) The temperature of the air, as indicated by a thermometer in the shade. (2.) The same, as shown by a thermometer in direct sunlight. (3.) The temperature of the northeast side of a live limb. (4.) That of the southwest side of the same limb exposed to direct sunlight. (5.) That of the southwest side of the same limb shaded from the sun. (6.) That of the southwest side of a dead limb exposed to direct sunlight.
In addition to the points brought out before, the following were noted: (1.) The temperature of tree-limbs rises and falls more slowly than that of the air. (2.) The temperature of a dead limb rises and falls more quickly than that of a live limb. (3.) The extreme daily variations of temperature are greater in a dead limb than in a live one.
In July the same thermometers were placed in limbs of an apple tree and the same points compared. The results were identical to those obtained in the box-elder tree.
In September the thermometers were moved to another apple tree. Results were the same again with one exception. The temperature of the live limb followed that of the air more rapidly than did the temperature of the dead limb, just the opposite of what had occurred in both the previous cases. The dead limbs used before had been alive the previous summer and their wood was sound, while the limb used in the last case had been dead longer and its wood was soft and slightly decayed. It would be difficult, however, to account for the difference observed in the two cases on this ground alone.
It was this difference in behavior that led to a continuation of the work another year. Up to this time no accurate measurements of the thickness of wood between the hole and the rk had been made. The limbs, having been left in their original
positions on the trees, received the sun's rays at somewhat different angles. This might have had something to do with the difference between the temperatures of the live and dead limbs.
In August of this year, 1896, the thermometers were again placed in limbs of an apple tree. The thermometers were the same ones used before. They were compared with a thermometer loaned for that purpose by the meteorological department of the university and were found to be sufficiently accurate. A live limb about 10 centimeters in diameter and with fairly smooth bark was chosen. It leaned slightly to the north. All limbs to the south of it were removed, so that the sun's rays might fall directly upon it through the greater part of the day. A dead limb about the size of the live one, with sound wood and fairly smooth bark, was then obtained and a section of it about a meter and a half long was hung up parallel to the live limb and about a half meter from it. The sawed ends of this limb were covered with wax to prevent, as far as possible, a loss or gain of water. Holes sixteen millimeters in diameter, just large enough to admit the thermometer bulbs, were bored in these limbs about two and one-half meters from the ground. They were so bored that the thermometer tubes placed in them were perpendicular to the sun's rays at about 1:30 P. M. One hole in the live limb and one in the dead one were bored as in all cases before. In both cases the wood between the hole and the bark was 10 m.m. thick. The bark on the live limb was 3.5 m.m. thick, on the dead limb 4 m.m. thick. In addition to these tangentially bored holes, another was bored radially in each limb about 30 c.m. below the first. These were bored as near the center of the limb as possible. Each was 40 m.m. from the outside of the bark on the south side of the limb. All the holes were carefully sealed with wax. A heavy cloth screen was made to shade the limbs or protect them from the wind as might be desired.
With these arrangements for accurate comparison between the dead and live limb, the results of the first two trials made in 1895 were confirmed. The temperature of the dead limb changed more rapidly than that of the live one. It was also noticed that,