Nathan Turk, Jose Lopez, Tom Ryan

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 2in 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).

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.