Note: Figures have been excluded from this article.
Department of Geology
The College of Wooster
As part of a continental-scale series of drought reconstructions, Cook et al (1999) conducted dendroclimatological research that included the development of a ring-width oak chronology in northeastern Ohio extending from the seventeenth century to 1985. The information contributed to the development of tree ring PDSI reconstructions that are used for interpretation of past climate variation, particularly drought. The purpose of the present study is to extend the chronology further back into time and also update it for the last fifteen years. Using Cook et al’s (1999) Johnson Woods data for comparison, a separate, independent pool of data was collected. Three visits to Johnson Woods were used for coring fallen logs and for collection of several sections of trees cut previously by a chainsaw. These samples not only bolstered the data from Cook et al (1999) study by adding substantial sample size, but also effectively extended the chronology, from 1605 through 1999.
Johnson Woods is a Ohio Department of Natural Resources Nature Preserve. Sampling was done in cooperation with the Ohio Department of Natural Resources Division of Natural Areas and Preserves. The tree cores and sections from this study and the data are archived at The College of Wooster Tree Ring Lab housed in the Department of Geology.
Tree rings have been very useful tools in many places around the world for the study of landscape erosion and for the dating of structures and other items that contain wood. Tree rings are also used extensively as indicators of past climate. The field of tree ring study that deals with climate is called dendroclimatology. First developed A.E. Douglass, Waldo S. Glock, and Edmund Schulman, the field uses variations in annual growth rings in trees in order to draw conclusions about climatic variation within a geographic area. In many areas around the United States, and indeed around the world, there exists modern meteorological observational records for at most, the last 100 years, that are used by dendroclimatologists in correlation to tree ring data. These two pieces of data, tree ring and meterological records, can be used together in order to make reconstructions of past climate that will show variations in temperature or moisture. This information is valuable not only for development of paleoclimatic models, but also for agricultural contingency planning (Cleaveland et al 1992).
Tree Growth and Ring Formation
In woody plants such as trees, there is a process of development that must occur in order for the undefined cambium to differentiate into new and deceased wood cells. The method by which this occurs is cell division. A plant cell is distinct from animal cells mainly bcause of the presence of a cell wall. Inside this wall, exists the protoplast, various organs such as golgi-vesicles, dityosomes, mitochondrion, an endoplasmic reticulum, and ribosomes. Dictyosomes are very helpful in the formation of a cell wall during cell division. During this process of cell wall formation, three fibrous layers are created to make up this final wall. Before the walls thicken fully, the cell expands so that the fibers become intertwined. The structure of the cell wall depends entirely on the function the cell performs for the plant.
From this single plant cell, differentiation occrs. At this point, the apical meristem develops a plant body whose cells on the tips of its roots and sprouts are the first to divide. Layers of cells eventually develop, and from these layers, the entire woody plant surrounding the stem begins to protect it like a coat. This new structure is called the cambium. Through continued development, xylem begins to form on the inside of this cambium, and phloem grows around the outside of it (Figure 1). The stem of the plant now begins to grow in two directions, anticlinal growth occurs to increase the size of the plant vertically, and periclinal growth occurs to increase the radius horizontally (Schweingruber 1996). In the xylem, two functions are performed, providing support for the tree, and conducting water through the plant.
Principles of Dendrochronology
There are nine basic principles that are accepted as being imperative to the proper study of tree rings. These principles have been formulated through repeated observations and experience in studies from around the world.
Oak Rings and Structure
During a year in a tree’s growth a single ring is produced. This ring contains two major divisions, referred to as early wood and late wood. Early wood is the first growth which a tree puts on during its growing season. In northeastern Ohio oaks, this begins in early spring, usually early April. This growth continues and advances into the late wood growth during the summer. In many trees, the difference between these two parts of a ring can be as subtle as a slight change in color. However, in hardwoods such as oak or elm, these two parts are quite distinct. The early wood in these trees contains very large vesicles, whereas the late wood does not (Figure 2).
Oak trees in northeastern Ohio are quite common, but old growth oak trees are very uncommon. These old growth oaks can live to be up to 400 years old. These are dying out however, and are being replaced by beach-maple forests in most of the region. One of these last remaining stands of old growth oak forest in northeastern Ohio is Johnson Woods, a 200 acre piece of land that lies just a few miles north of the city of Orrville, Ohio, Wayne County. One previous study in this wood was conducted by Cook et al (1999).
The purpose of the present study was to extend Cook et al’s (1999) data back into time, and to update it for the last fifteen years. To do this, thirty-one trees were sampled, producing forty-five usable ring width series. Some trees were sampled more than once in order to minimize variations in ring widths around the circumference. Samples were taken from trees that were mainly lying dead on the forest floor. Some of these were found to have been dead for up to sixty years. One sample, JW0100, was a section cut by a chainsaw recently that was found on the forest floor.
These samples were taken back to the tree ring laboratory at the College of Wooster, Ohio, for preparation and analysis. Cores were taken out of the straws, and mounted into poplar grooved mounting sticks with wood glue. These were then left to dry overnight. Sanding occurred next, and was begun with 50 grit paper, progressively sanding to a grit of 600, leaving a highly clear surface. Next, rings were counted under a dissecting microscope, and marked in the standard way with one pencil dot per decade, two per half-century, and three per century. Ring widths were measured using a digital devise with a precision of 0.001mm, and recorded as Edit II files. Crossdating was performed with the aid of COFECHA (Holmes, 1983) and raw data was processed into ring-width chronologies using the ARSTAN method (Cook, 1985).
The data that was analyzed for this study correlated well with the data collected for the previous study conducted by Cook et al (1999). Figure 3 presents the most interesting information. This graph depicts sample size in conjunction with the normalized ring width data and a weighted mean curve. This type of graph is very useful for viewing the entire picture of the northeastern Ohio climate over the last 400 years, and also provides insight into the quantity of data that was available for the following conclusions to be drawn. From AD 1700 on, a strong sample size exists, and thus the record is considered reliable. A weighted mean curve was added to the graph so that the oscillations in record can be better observed.
This variations in this graph (Figure 3) are primarily a record of growing season moisture based on previous studies. The period of 1850-1899 was a time of apparently high moisture. The twentieth century does not appear to have been the driest period of time for the area. The early nineteenth century appears to be the driest interval of the entire record. There appear to be no obvious patterns in this drought record over the last 300 years.
Graphs 4-6 show various different depictions of the same tree ring data, in order to isolate different attributes of growth over the last 400 years.
There is more that can be done with Johnson Woods. It would be very useful to attempt to increase the sample volume for the period of 1600-1700. A period of 400 years would allow patterns in the data, if they exist, to possibly come to light. This study of Johnson Woods could also lead to fruitful studies of some of the other remaining old growth forests in Ohio such as Hueston Woods near Cincinnati. Eventually, a useful drought record for the state of Ohio and nearby regions may be created.
- The Science of Tree Rings, Dr. Grissno-Mayer, The University of Tennessee
- NOAA – Historical Palmer Drought Indices
- Principles of Dendrochronology
- Tree Ring Laboratory – Lamont-Doherty Earth Observatory
- NOAA – Drought Reconstructions (PDSI)