Tree-rings record drought history in North Dakota
What can tree rings tell us about drought patterns? Rodney Stroh, specialist for the North Dakota Rural Water Systems Association, shares how rings develop within trees and his research on many tree specimens.
While looking through the binocular scope, I observed the closely spaced tree-rings from the 1930s on the cores of wood extracted from trees in the Sheyenne National Grassland. Grandparents and parents have often shared stories about the drought years known as the “Dirty Thirties.” Their faces would wrinkle as they reminisced of infamous dust storms, topsoil blowing into homes, crop failures, the drying-up of shallow wells, the lack of livestock water supplies, cutting weeds for hay, tumbleweeds rolling into fencerows, financial problems and many other difficulties of farming during a multi-year drought. “The droughts of the 1930s and 1950s are well-known for their severity and had disastrous social and economic impacts in the Great Plains,” say C.A. Woodhouse and P.M. Brown in their 2001 article, Tree-Ring Evidence for Great Plains Drought.
A tree-ring or annual ring is the new wood produced by a tree in one growing season. The concentric rings seen on a stump are tree-rings. If you scrape a small amount of bark off of a tree trunk, or a branch, and see a green area, this is the cambium. The thin cambium layer produces new vascular tissue every year, thus increasing the diameter of a tree. The vascular phloem (floem) is the cell pipeline just outside the cambium that carries photosynthetic sugars from the leaves to the rest of the tree. Some phloem cells die each year and become part of the bark. The cell conduit immediately inside the cambium is the xylem (zilem). This layer of sapwood, as it is also identified, moves water and nutrients from the roots to the leaves. Old xylem cells become the sturdy heartwood of a tree.
Most of a tree’s diameter growth and annual variation in growth is exhibited in the xylem. Early in the spring, as a rule, a tree will grow substantially. The xylem cells are large and pale looking to an observer. This is known as earlywood. Tree growth usually slows later in the year. The cells compress in size and express a darker color. This is identified as latewood. One year’s representation of earlywood and latewood shape a tree-ring.
The width of a ring is influenced by any biological and environmental factors which can limit tree growth. Biological examples could be competition between species or insect infestation. Environmental examples could be precipitation or air temperature. Wide rings reveal favorable growing conditions, while narrow rings record a reduction in the growth of the tree due to some limiting factor.
The possibility of ring anomalies and the difficulties involved with their identification are taken into consideration during the examination of rings. False rings can be the result of an abrupt change in the weather in one growing season. Missing rings can be caused by stress on the tree. A missing ring in oak trees is rare. The only known occurrence was in 1816, “the year without a summer.” To verify the validity of tree-ring data, comparisons can sometimes be made with core sediment profiles from local lakes, looking at fluctuations of salinity levels for example. The study of tree growth rings and their applications for society is known as dendrochronolgy.
My experience with treerings was for a research project completed in 2002 titled, Native Woodlands of the Sheyenne National Grassland, North Dakota. Age and growth rates were examined. Cores were extracted from trees at diameter breast height (dbh) using an increment borer. Tree wounds were sealed with antiseptic petroleum jelly. A binocular dissecting microscope was used for counting annual xylem rings. The cores of some tree species were soaked in a dye solution for rings not clearly distinguishable. No extrapolation of actual age from dbh age was attempted because of seedling growth variability. The oldest, average overstory age at dbh was 100 years for basswood (Tilia americana). The fastest growth rate was 0.54 inch increase in diameter per year for cottonwood (Populus deltoides), growing with competition. Bur oak (Quercus macrocarpa) grows the slowest at 0.17 inch diameter increase per year.
In the 1940s, a project was completed titled, Tree Ring Studies in North Dakota, by George F. Will, Sr., who found a living bur oak along the Missouri River that was 373 years old. The rings in this key specimen (master oak) were cross referenced with precipitation records of the U.S. Department of Commerce’s Weather Bureau. The State Historical Society of North Dakota had some bur oak and Rocky Mountain juniper (Juniperus scopulorum) archaeological samples in storage from abandoned Native American villages in the Missouri RiverValley. Most of the villages in this study were home to the agrarian Mandan. This collection of samples were cut into cross sections, smoothed and dressed in such a way that the rings could be examined.Will then took these cross sections from the village ruins and progressively overlapped rings and crossdated them with known dates of the master oak.
Nature’s Records of Drought
The tree-rings recorded an estimation of precipitation going back 535 years. The study related the water available to tree roots to local crops having adequate moisture for growth in the Missouri River Valley from 1406 through 1940. During this timeframe, there were nearly equal wet and dry years. Tree-rings recorded years with average precipitation and subsequently average corn crop production. A wet period produces an above-average corn crop and a dry period produces a below-average corn crop. For periods lasting five years or more, it was found that wet periods occurred nearly equally as often as dry periods, at about 20 for each period type. However, for periods lasting five years or less, wet periods would show a doubling, 14 wet periods compared to seven dry periods. The longest wet period lasted 39 years (1663-1702) and the longest dry period or drought was 16 years (1633-1649).
It would be beneficial for the inhabitants of North Dakota if we could plan for the complexities of a drought by determining that for a certain yearly duration, a drought would cycle into our area every so many years. This study, which looked back five centuries, revealed that there are no cyclic patterns apparent as to how often a drought will occur and the length of the droughty period.
Tree-rings can record historical droughts, but at the present time, the interpretation of ring chronologies cannot predict future droughts. Woodhouse and Brown in their aforementioned publication remark, “The dendroclimatic record of drought indicates that drought has been a regular feature of this region, and will undoubtedly continue to be so.”
The agrarian tribes in the Missouri River Valley maintained a successful occupation of the area over the centuries despite the droughts. It was smallpox that decimated the population.
An interesting note is George F. Will, Sr., owned and operated Will’s Pioneer Seed House in Bismarck. This horticultural business was formerly owned by his father, Oscar H. Will. Both of these individuals collected, crossbred and sold hardy seed cultivars from the Arikara, Hidatsa and Mandan tribes.