Effects of Physical Soil Properties on the Coverage of Native and Non-native Plants in Urban Riparian Areas

Melanie M. Sharp

Master of Science in Environmental Science

Portland State University

Presented February 20, 2002.

Abstract

I hypothesized that differences in physical soil conditions may have effects on the coverage of native and non-native plant species in urban riparian areas. Twenty-three sites were randomly established in urban riparian areas in the Portland, Oregon, metropolitan area. Native and non-native plant species coverages were compared to soil physical properties, including soil bulk density, percent organic matter, soil texture and soil horizon depth. Patterns of non-native plant coverage were assessed at different scales and at different distances from streams.

Out of the 104 plant species found, 26 of them were non-native. Of the non-native species, 6 were invasive. Non-native plant species coverage overall was 38% at a five meter sampling interval. The percent of silt and the depth of the O, L, and A-horizons were negatively correlated with non-native vegetation. Percentage of sand, percentage of organic matter in the A-layer and soil bulk density had a positive correlation with non-native plant coverage (p<0.05).

Using a forward stepwise regression, the variables percentage shade, percentage sand, O-horizon depth, percentage organic matter of the A-layer and A-layer depth were significant at p<0.05 with an R²= 0.498. Although soil properties are typically highly variable, a sampling scale of 5m was most predictive.

Higher levels of sand in the soil would correlate to lower soil moisture holding ability, which, in the dry summers of the study area, is detrimental to most native plant survival. Lower percentage of soil organic matter also decreases soil moisture holding, in addition to lowering nutrients available to plants. A reduction in soil horizon depth also reduces available nutrients plants can utilize before their root reach the nutrient limited lower layers. It is likely that native plants in this study are adapted to soil conditions that existed in the study area before human disturbance approached its current levels. With increased compaction, decreased organic matter, decreased horizon depths and changes in soil texture, non-native plant species are able to invade and out-compete native species in many cases. Management of urban riparian areas should incorporate these findings when trying to prevent or restore natural riparian function.