Nathan Turk, Jose Lopez, Tom Ryan
FOR 332
Lab #6: Disturbance in Urban Forests
Patch Type Influences Soil Characteristics
Abstract
Soil characteristics can indicate distinctions between types of urban patches. Urban
vegetation patches that pre-date urbanization are associated with greater organic matter.
Patches that have arisen during or as a result of urbanization have less diverse biomass in
terms of structure and species. We expected to find a thicker buildup of soil surface organic
matter and evidence of better drainage at the remnant patch we studied than at the emergent
patch. We found that the remnant patch's O layer was bigger, but the smaller subsequent A
layer may result from lesser decomposition rates than at the emergent. Our results
regarding drainage were inconclusive. Relationships between patch type and
characteristics of drainage and soil layers have important implications for land use
management, including urban planting.
Introduction
Urban patches, patches of vegetation resulting from urbanization, are homogenous
in nature and distinct from their surroundings, and come in three categories: planted,
remnant and emergent (Bradley, 1995). We hypothesized that a remnant urban patch or
landscape that existed before urbanization, in Oakwood Cemetery would have a thicker
organic (“O”) layer. The O layer, which includes fresh surface litter and decomposed
matter (Cedergren, 1967), should be thicker in remnant patches than in planted or emergent
patches. Remnant patches are usually older (Zipperer, 1997), and therefore experience
more buildup of decomposed materials. We expected that an emergent patch, which arises
as a product of development or natural catastrophes (Bradley, 1995), off Genesee Street
would have thinner O layers. Additionally, we studied evidence of drainage at a remnant
and emergent patch with the hypothesis that soil of remnant patches would exhibit better
drainage. Remnant patches have greater amounts of biomass at more stratums (Bradley,
1995) than emergent patches, and therefore greater uptake of soil moisture.
Methods
The soil pits referred to as 1 and 2 in the graphs (figs. 1 and 2) were dug at an
emergent urban patch off Genesee Street. Soil pit #3 was dug at a remnant patch in
Oakwood Cemetery. We assumed 18” of soil free of mottles (or oxidized interspersed
particles) was the minimum depth for a classification of moderately well-drained. The
point where mottling begins is where the water table has rested long enough to deplete soil
of oxygen (Cedergren, 1967), and usually indicates the drainage depth. Though we didn’t
reach mottling, we compared our depths to those of correct drainage classes as outlined by
Ward and Elliot (1995). Drainage was really related to soil depth as follows:
Well drained: more than 24” of mottle-free soil
Moderately well drained: 16” to 24” of mottle-free soil
Somewhat poorly drained: 8” to 16” of mottle-free soil
Poorly drained: less than 8” of mottle-free soil
Very poorly drained: less than 4” of soil without mottles
In addition to drainage class, we also measured the thicknesses of the soil layers that our
digging penetrated.
Results
All soil pits revealed dark, heavily organic soils which ranged from brown to dark
brown in color. The drainage class of all soils was determined to be at least moderately
well drained. The thickness of organic layers of the 3 soil pits is recorded in Figure 1
(labeled O Layer Thickness.) The depth to the soil horizon between A and B layers of the
three pits is recorded in figure 2 (entitled Soil Horizon Depth.)
The remnant patch at Oakwood cemetery has a much thicker and less differentiated
organic layer compared to the emergent patch. The A layers of the emergent patch soil pits
(#1, 2) are much thicker than that of the remnant patch (#3).
O Layer Thickness
6
Thickness (in)
5
4
3
O Layer
2
1
0
1
2
Site Number
Figure 1
3
Soil Horizon Depth
0
1
2
3
Depth (in)
-5
-10
B Layer
-15
A Layer
-20
-25
Site Number
Figure 2
Discussion
We misunderstood how deep mottle-free soil had to be measured to qualify as
well-drained. In this lab we assumed 18” to be the threshold of well-drained soil, and
therefore stopped digging once this depth was reached at two of the three occasions.
However, reasons that soils of both patch types were at least moderately well-drained may
owe to both sites being at upper slope positions. Typically, the higher the slope position,
the greater the depth to mottling (Ward, 1995), so soil pits at different points along the
sites’ gradients may have shown different drainage trends. Soil series could not be
definitively determined because of the lack of collection of certain necessary information
about the soils of each pit.
One reason the A layer wasn’t as thick at the remnant patch as at the emergent patch
may be due to different nitrogen contents. Decomposed organic matter, which along with
mineral particles comprises the A layer (Cedergren, 1967), accumulates less slowly when
litter has lower N concentrations (Singer, 1987). Differing decomposition rates could also
be due to differences in populations of earthworms, which accelerate organic matter
decomposition, and detritivores, scavengers that feed on dead matter (Singer, 1987). The
presence of earthworms in the remnant forest’s soil suggests that decomposition is not a
problem there. (Earthworms were not observed at the emergent patch.)
The remnant patch had a thicker organic layer, as we hypothesized, but we were
surprised that the patch was not as crowded with vegetation as the emergent. In the
emergent patch we had trouble digging because of the persistence near the soil surface of
snarled sapling roots. However, crowdedness is not always a good indicator of biomass
because denser stands might simply have more biomass focused in larger trees in more
vertical layers
Conclusions
The results of this experiment suggest that greater amounts of organic buildup can
be found in remnant patches than urban patches. Our initial hypothesis regarding the O
horizon was supported. However, our results did support our hypothesis regarding
drainage. Accurate drainage classifications would require pits to be dug deeper to the point
that mottling was actually reached. Simply digging to verify an arbitrary minimum in terms
of drainage classification does not identify the true depth to mottling.
Digging a second pit in the remnant patch might have improved the likelihood that
the horizon depths we measured were represented of all of the stand’s soil. The relationship
of drainage class to urban patch type is important in landscape planning. For example,
well-drained urban patches can help with storm water retention in areas susceptible to
flooding such as lowland golf courses. Therefore, leaving existing remnant or emergent
patches or planting an urban patch can be beneficial. Additionally, knowing the thickness
of soil layers is useful in predicting what species will grow best in urban plantings. Species
that root into the B horizon and deeper layers might benefit from thick O and A horizons,
since their thickness might indicate more nutrients available for the B horizon to leach.
References
Bradley, Gordon A. 1995. Urban Forest Landscapes: Integrating Multidisciplinary
Approaches. University of Washington Press. Seattle: 137-150.
Cedergren, Harry R. 1967. Seepage, Drainage and Flow Nets. John Wiley & Sons, Inc.
New York, pp. 340-342.
Singer, Michael J. 1987. Soils, an Introduction. MacMillan Publishers. New York: 1987.
Ward, Andy D, William J Elliot. 1995. Environmental Hydrology. Lewis Publishers. Boca
Raton: 439-445.
Zipperer, W. C., Sisinni, S. M., Pouyat, R. V., and Foresman, T. W. 1997. Urban Tree
Cover: An Ecological Perspective. Chapman & Hall. London. Urban Ecosystems: 1,
229-246.
Hutton, Frank Z. Jr. and Rice, C. Erwin. 1977. Soil Survey of Onondaga County, New
York. United States Department of Agriculture Soil Conservation Service: 29.