CAMBIAL ACTIVITY IN ALASKAN WHITE SPRUCE

Cambial activity in white spruce stems in Alaska was observed during a 2-year period in 50-60year-old natural stands. Mitotic index was used as a measure of the rate of periclinal division of fusiform cells in the cambial zone. Anticlinal divisions were relatively rare, averaging only one per 278 periclinal divisions in most stems. Mitotic index, at any given time, appeared uniform throughout the cambial zone of an internode, among internodes of the same tree, and even among trees growing at markedly different rates. Diurnal variation in mitotic index was observed. There were three distinct growing season periods: early, grand, and late. Early period activity was characterized by reactivation of periclinal division, erratic mitotic indices, and an approximate doubling of the number of cambial zone cells per radial file (NCZ). Production of the first new xylem and phloem elements marked the beginning of the grand period. Rate of cell production in the cambial zone remained about equal to derivative production for the next 45-50 days, when about 80 % of annual xylem and phloem increment occurred. There was a drop in NCZ at the beginning of the late period coincident with a decline in mitotic index, and NCZ soon dropped to the dormant level. Complete termination of cambial activity was gradual, extending through late August and perhaps into September. A RECORD OF natural cytological events and developing anatomical features associated with secondary growth of tree stems is an essential step preceding a search for mechanisms promoting them. This has been the objective of a 2-year study of cambial activity in stems of Alaskan white spruce, Picea glauca (Moench) Voss, reported here. Alaskan white spruce is ideal for studies such as this. As in most gymnosperms, the secondary vascular tissues are comparatively simple. There are few cell types, and cambial derivatives in both xylem and phloem are arranged in radial files which can easily be traced to their point of origin in the cambial zone. The growing season in Alaska is relatively short, and seasonal patterns are accelerated and seem more sharply defined than in more temperate regions (Gregory and Wilson, 1968). Cambial zone cells are inactive throughout the long winter. Vernal reactivation occurs within a short and predictable period, usually before all the winter snow has melted. Soil moisture, a troublesome extraneous variable in many field growth studies, is therefore unlikely to be limiting, at least through June, the month of most vigorous growth. Overall, the native trees of interior Alaska seem adapted to ' Received for publication 16 July 1970. This study is part of a dissertation presented to the Graduate School of Oregon State University in partial f ulfillment of the requirements for the degree of Ph.D. 2 Present address: Forest Physiology Laboratory, Plant Industry Station, Forest Service, U.D.S.A., Beltsville, Maryland 20705. growing appreciably in a short season and to surviving in an inclement environment during the remainder of the year, making them well suited for field studies of growth and survival. MATERIALS AND METHODS-Sample materials were collected from two areas near Fairbanks, Alaska (640 51' N, 1470 44' W), each supporting 50-60-year-old mixed stands of white spruce and quaking aspen (Populus tremuloides Michx.). One area, located about 30 miles west of Fairbanks at about a 900-ft elevation, was sampled from April 25, 1964, into January 1965. The other area, 7 miles west of Fairbanks at about a 500-ft elevation, was sampled from l\lay 6 through August 1965. All crown classes of white sprucedominant, intermediate, and suppressed-existed in these stands. Diameters at breast height (bh) ranged from 2-14 in. and total stem heights from 15-70 ft. Over 300 sample blocks, consisting of bark, cambial zone, and usually all of the previous year's xylem, were removed by razor blades and wood chisel at various stem levels from over 100 trees in the two stands. The size of the tangential face (bark surface) of the blocks was 1 by 3 cm. Beginning April 25, 1964, one sample was taken at bh level from each of two dominant, intermediate, and suppressed trees at 6-day intervals through July 14, 1964. Then one sample was taken at bh level from each of two dominant trees on July 22, July 31, August 14, and August 31, 1964. Different tiees were sampled each time. Numerous dormant season samples were taken in November 1964 and January 1965.