TREE GROWTH

Waipa Post, Volume 52, Issue 3705, 10 January 1936, Page 4

TREE GROWTH

IS IT INFLUENCED BY SUNSPOTS ? Considerable interest has been aroused recently in the possibility of forecasting drought periods. It has been shown that there is a decided periodicity in the Adelaide rainfall which passes through well-marked cycles of about eleven years. It is not without interest, therefore (writes J.G.W. in the Melbourne Age), to discuss the investigations of Professor Douglass, of Arizona, on the estimation of rainfall of historic and prehistoric times from the measurements of the annual growth rings of trees. Most people now that transverse sections of the stems of trees and shrubs which live for many years show well-marked rings in the wood. W|ood is constantly being produced by a tissue in the living stem called the cambium. The cambium, however, shows a well-mared periodicity in its metabolism depending chiefly on the amount of water absorbed by the plant. This results in differences in the size, and also in the thickness of the walls of the wood vessels produced by the cambium. Generally speaking, the vessels are larger and thin-walled in the wet periods of the year, and smaller and thicker walled in the dry periods; consequently a cross section of a stem shows a sharp demarcation between the small, thick walled vessels of the wood and the large, thin-walled cells of the preceding wet season. Dry years leave abnormally narrow annual growth rings, and this is particularly evident in coniferous plants. ANCIENT TREES. Douglass in his investigations used the gigantic redwood Sequoia gigantea, the tallest trees in the world, which grow on the Pacific coast of North America. He was struck by the dependence of these trees on rainfall to the virtual exclusion of other factors. The problem of annual ring measurement is complicated by the fact that there is variation of growth rate with age, and also fires and insect pests may alter the growth rate. Their disturbing effects can, however, be allowed for. The trees live to a great age. One section in the Natural History Museum at South Kensington dates back to the Roman occupation of Great Britain, but this is youthful to several which extend to 1000 B.C.—that is, the trees were about 3000 years old. The annual rings occur in wellmarked cycles —. that is, groups of rings in which the size of the vessels gradually increase to a maximum size and thickness, and thereafter decrease to a minimum.

The growth records have been worked out especially for the whole period from 1300 A.D. to the present day. Statistical analysis of the data shows well-marked cycles of 11.30 years, lasting for more than 600 years, but with an interruption from 1630 A.D. to 1850 A.D. About the former year and well established after 1700 A.D., there is a group of 7, 14, and 21-year cycles. The last cycle has dominated tree growth in Arizona for the past 200 years. The very dry years of the Arizona trees analyse best on 14 and 21-year cycles, with major droughts at about 150year intervals and minor droughts at about 40 to 50-year intervals. RING CYCLES OF AUSTRALIAN TREES. I have recently been investigating in a preliminary way the ring cycles of arid Australian trees, the investigation primarily being to determine the age of trees like ntulga, sandalwood, etc., which are not regenerating from seed, and which are rarely younger than from 60 to 70 years. It was noticed that the thick-walled xylem vessels during the past 60 years group themselves approximately into 10-year cycles—that is, into alternating periods of wet and dry seasons.

A marked feature of Douglass’ work is the close correlation between growth rate and sunspot activity. It has been suggested that there is a close connection between exceptional rainfall and drought and sunspot activity. Sunspots appear to be cyclonic disturbances within the mass of the sun, with gases radiating outward at the upper end, and these vortices are in opposite directions in the north and south hemispheres of the sun. They were first discovered about two years after the invention of the telescope by Galileo in 1608. They vary greatly in size, and sometimes occur in groups. In 1843 a European observer showed that there was a periodicity in their activity over a 20year period which he studied. Shortly afterwards an examination of all records extending back to about 1610 established this without a doubt. On the average the sunspots increase in numbers for 4.62 years, and decrease in numbers for 6.51 years. The full period therefore is 11.13 years. UNCHANGING CYCLE. These are average figures, and the full period ranges from extremes of 7.3 to 17.1 years. The period from maximum to minimum is always greater than the increasing period, and also the more intense the outbreak of spots in any period the shorter the time for maximuni devel-

opment and also for subsequent decay. But underlying these periods there is an unchanging cycle, of which the 11.13 years period is the most important. Douglass has shown a very marked correlation of wood activity and sunspots. His 11.3-year cycle on the average compares well with the observed 11.13-year sunspot cycle. In the late 1700’s the rings show a 9.4 year cycle when the sunspot cycle was of that length. Generally he found growth maxima occur at observed sunspot minima. It m'ay be that this is a coincidence—an enterprising statistician has correlated numbers of suicides with sunspot cycles—maybe A not. But, in any case, the careful analysis of annual ring data in definite regions may give valuable data which will enable us to predict the onset of drought years. At present it is difficult to do this for trees in the dry areas, since information is lacking which tells us whether wood growth takes place continuously in these areas or occurs only after good rains. This information, however, may readily be obtained.