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1993 Proceedings of the Midwest Oak Savanna Conferences

1993 Proceedings of the Midwest Oak Savanna Conferences

Dennis Nyberg, Ph.D. 
Department of Biological Sciences (m/c 066)
842 West Taylor Street
University of Illinois at Chicago
Chicago, IL 60607-7060 
Tel: (312) 996-2643




In 1821, Palos township was 11% marsh, 36% prairie, 23% savanna, 8% woodland, and 20% forest. Except for the floodplain of the Des Plaines River, the trees of the savannas, woodlands & forests were almost entirely oaks (the white oak group and black oaks). Outside of the floodplain greater densities of trees are associated with steeper slopes. Flat areas, both low and high elevation, usually had low densities of trees, while slopes, regardless of aspect, had high densities. Fire interacting with prevailing winds predicts higher tree densities on North or East aspects than on South or West, while drainage effects should be independent of aspect. For this reason, it is believed that the drainage of the land was as important as fire suppression in the increase in the density of trees since 1821.


INTRODUCTION  back to top

In PALOS township (R12E T37N) outlets of Glacial Lake Chicago broke through the Valparaiso moraine, and scoured the till down to bedrock, on their way to the Mississippi River. As a result of two breaches, Palos has greater topographic diversity than most Cook County townships. Currently, almost half the township is owned by the Forest Preserve District. It was among the first townships surveyed in northeast Illinois. All these reasons make understanding pre-European settlement vegetation in Palos township especially interesting. The public land survey (PLS) divided the land of the United States into square parcels which were sold or granted to individuals and corporations. Most of the surveys preceded the settlement of the land for agricultural purposes. To mark the property boundaries every half mile the surveyors set a post (or raised mounds in treeless areas). To keep those posts from being moved, they blazed the nearest trees and recorded the species, diameter, bearing and distance in their notebook. Surveyors also recorded the positions of transitions between ecological communities they recognized as they measured survey lines, the position of trees that intersected the line, and the location of all streams and rivers. They rated the soil, made comments on the topography and the species of woody vegetation that grew along the survey line, as well as noted trails and villages of people. Because the information in these surveys is specific and quantitative, they are more accurate than the tales of travelers and colonists. The land survey records are a "snapshot" of the ecology of the USA after a couple of hundred years of contact between natives and explorers, but before post-European settlement agriculture.

The PLS has been widely used to evaluate the pre-European settlement vegetation for conservation planning purposes. This paper will add to the knowledge base in two ways. First, the quantitative information of the surveys will be used to calculate the density of trees and the basal area per hectare at the time of the survey. The density of stems per unit area (or the basal area per hectare) allows classification of each point a post was set as forest, woodland, savanna or treeless. Second, about half of PALOS township is owned by the Cook County Forest Preserve District. The large size of the protected area enhances the comparison of changes that have occurred between 1821 and the present. 

METHODS back to top

Two main issues need to be dealt with to quantitatively analyze the PLS. They are: 1] What are the characteristics of the population of trees sampled? 2] What is the best way to convert the distances measured at each post into a density (of the population sampled)? Because the surveys did not explicitly state the population sampled, and do not have clear rules for the distances measured after the closest distance, one must offer an interpretation of these issues. Since over 90% of the blazed trees were over 10 inches (with a mode of 15 inches) I interpreted the blazed trees to be a sample of the trees greater than 10 inches DBH (Diameter at Breast Height). While trees down to 3½ inches were blazed I believe trees <10 inches were avoided (unless the surveyor had to go really far to find a tree greater than 10"). 

The density of trees can be estimated from the distance from the post to the tree(s). Cottam and Curtis (1956) showed that for a homogeneous community and using the point quarter method (nearest tree in all 4 quadrants), the average distance (of the four trees) was equal to the square root of the reciprocal of the density. There are two problems with applying the point quarter method to the PLS. First, at the time PALOS was surveyed, surveyor John Walls was marking at most two trees per post. Second, the points surveyed are not samples of a homogeneous community. The latter point has inhibited analysis, but with a slight change of perspective we can turn the survey method to our advantage. That change in perspective looks at each post as yielding an estimate of the density of trees at that point, not for the community in which that point lies. Ultimately, each point can eventually be classified according to the density of trees in 1821. 

I have estimated density by generating an upper bound and a lower bound to the true density. First, I explain how to estimate the upper bound. Since Walls picked the nearest tree and the nearest in the opposite quadrant, using the average distance (at each post) clearly overestimates the density of trees. Thus, we generate the upper bound estimate as DENMAX=1/D2. Next, the lower bound estimate, DENMIN, is based on the maximum distance the surveyor went to mark a tree, Dmax. This distance defines an area, ãDmax2, which contains at least the number of trees recorded by the surveyor. Since there may be trees in this area (for example, in the same quadrant as the closest tree) this is a lower bound to the true density. The true density must be between the upper and lower bounds. I have averaged the two estimates to estimate the density of trees greater than 25 cm DBH.

There is one other piece in this puzzle. Because sample points can fall quite close to trees, some estimates of density are higher than anything that can exist in nature (in an area of 1 hectare). Thus, I truncated density estimates at 400 stems per hectare if the recorded distances gave an estimate >400 stems per hectare.

Though most classifications of community type are based on percent of cover, the number of stems per hectare (1 hectare =2.47 acres=a square with 100 meter sides) or the basal area per hectare is another way to classify communities. The issue of community classification using stems (>25 cm or 10") per hectare and/or basal area per hectare is addressed in the Oak Ecosystem Recovery Plan. I will repeat the conclusion here, but not the methods. Prairie has 0 to 2 trees per acre, savanna has 2 to 20 trees per acre, woodland has 20 to 50 trees per acre and forest has more than 50 trees >10 inches DBH per acre.

I evaluated the accuracy of Mr. Walls as a surveyor and the aspect and slope at each point he set a post or raised a mound using a 1928 USGS topographic map enlarged to a scale of 1:10000. Walls notes each time he enters a new community (most of the time). By putting together all the notes, we can reconstruct the ecological community [as named by the surveyor] for every mile walked. Only a few transitions that were not explicitly noted had to be inferred. These were associated with moving from/to timber or prairie from scattering timber. Several prairie swamp transitions were not explicitly noted.

RESULT back to top

John Walls recorded notes on 135 kilometers that he walked in Palos in 1821. The interior of the township was surveyed in August. Because of backtracking, the total length walked was probably about 150 miles. In the case of Palos Township, John Walls went as far as 435 feet from the post to blaze a tree. Of the 218 trees blazed, 155 (71%) were called "W. Oak", 34 (16%) were called "B. Oak", which I interpret as black oaks. Eight other species ("Ash", "B. Ash", "Elm", "Hickory", "Maple", "R. Oak", "B. Walnut" & "Willow") were represent among the other 29 trees blazed. Wall's "Maple" is silver maple. "Maple", "Ash", "B. Ash" and "Elm" were noted only in the Des Plaines bottomland. Shrubs were never mentioned in the notes. The diameters ranged from 3½ to 30 inches with a mode of 15 inches. Of the 199 trees 10 inches in diameter or more, only 22 were 20 or more inches DBH.

Of the 133 corners and quarter section posts in Palos, 88 had one or more trees blazed and 45 had none. Of the 45, 11 were wetlands, 2 would have been in the Des Plaines River, and 32 were prairie. For the 95 posts with trees (88 plus 7 posts set on the bank of the Des Plaines River), at 17 only a single tree was blazed, while two trees were blazed at 72 posts and four trees were blazed at 6 corners (each visited twice). The 88 Palos corners with trees were classified as follows: 4 wetland, 16 prairie, 31 savanna, 10 woodland, and 27 forest. Including the treeless corners and converting the 133 corners to percentages, in 1821, PALOS was 11% wetland, 36% prairie, 23% Savanna, 8% woodland, 20% forest, and 2% river.

Because the transitions between communities were noted by Walls, one can estimate the percentage of communities based on the lines rather than the corners. Of the communities, 2.57% were bottomland timber, 51.51% were timber, 4.54% were scattering timber, 27.84% were prairies, 13.08% were wetlands and 0.47% was river. The densities of trees in the communities described as "scattering timber" is so low that they would now be called prairies.

Frequently, the community transitions are associated with topographic changes. Specifically, the location of raises are associated with a transition from prairie to timber. There is regularly a band of prairie between the timber of the hills and the wetlands of the Sag valley. The central, flatter areas of Mount Forest Island have lower densities of trees than the highly dissected edges.

An interesting pattern emerges when looking at the order of the community transitions. There were far more timber prairie wetland transitions than would occur if the communities were distributed at random. 

DISCUSSION back to top

In 1821, Palos Township was diverse. None of the five major communities, wetland, prairie, savanna, woodland or timber, was less than 8% nor greater than 36% of the land area. What were the factors leading to this diversity? Can the proximate factors affecting the density of trees be identified?

Currently, the frequency of fire is seen as the primary factor moving communities along a prairie forest continuum. High frequencies of fire are associated with prairie, while mesic forests had no fire. Typically, these models associate fire movement with that of the prevailing wind, SW, to explain the presence of prairie along west banks and woodlands along east banks. This model does not help explain the distribution of prairie, savanna, woodland and forest in Palos. In Palos, northerly and easterly aspects are as likely to be prairie as forest. The most consistent correlate with community is slope. Flatter slopes have fewer trees.

The independence of community with aspect and the association with slope are consistent with the hypothesis than hydrology was an important factor in keeping areas treeless. Lower slopes have also been associated with greater fire movement. This may well be the case because fire moves downhill only in extremely flammable conditions. There are currently many treeless wetlands in Palos despite many years of fire suppression. The conditions of these wetlands apparently prevent the recruitment of any woody vegetation except buttonbush. This suggests that soil saturation may have played as much of a role as fire in keeping the density of trees low.

The drainage ditches of the settlers as well as the suppression of fire, tended to encourage the increased density of woody vegetation in presettlement prairies, savannas, and woodlands. Currently about 40% of the trees >10" DBH are >20" DBH. I conclude that pre-European settlement trees were considerably smaller than those of today. 


Cottam, G. and Curtis, J.T. 1956. The use of distance measures in phytosociological sampling. Ecology 37:451460.


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