Development and assessment of an approach for urban riparian restoration

Abstract

Urban waterways are highly degraded; recent research indicates that management by planting riverbanks can reduce water pollution and increase wildlife habitat. There are ecological, social, economic and legal reasons for restoring riparian zones (areas of land adjacent to waterways) in and near cities. Christchurch, New Zealand, was used as a case-study to develop an approach for selecting riparian plant species, and ascertaining their environmental ranges, to enhance planting success. The approach involved choosing relevant environmental variables, simple field sampling methods, and data analysis and presentation methods suitable for use by non-specialist restoration practitioners. Environmental data for self-established and planted individuals of several riparian species were collected from semi-natural riparian areas in the Low Plains Ecological District. The environmental variables measured in the field included soil moisture, elevation and distance from water, broadly defined riparian class (eg., river, backswamp), and specifically defined riparian class (eg., lower bank, mid-bank), slope and slope shape (as indications of drainage), aspect, frost, and soil pH and conductivity.

Between species t-tests were performed to ascertain the most important environmental variables for each species. Within species t-tests were done for species where both self-established and planted data was recorded. Ordination assisted in identifying duplicate environmental variables. Species ranges were compared in Tukey boxplots.

Comparison of self-established and planted data for several tree species and Phormium tenax indicated that planted individuals were found in significantly higher, drier and more frosty sites than their self-established counterparts (most p-values >0.05).

Soil moisture, riparian class, canopy and slope were the most distinguishing environmental variables. Species responded to different combinations of these variables and were within the expected ranges for the Christchurch area. With refinement, frost and aspect may also be useful variables. Variables not fully captured but considered important are water level fluctuation and soil texture. It was concluded that the methods were most applicable in areas where little information on species ranges exists, or for monitoring species in modified urban restoration projects.

Several variables could be summarised into a graph and table, however the development of a simple species database is suggested for inputting, analysing, and updating complex data. GIS is considered to be a useful tool for mapping, planning and monitoring riparian restoration projects in large scale or sensitive projects.

One future direction for research is the development of an interactive database, compiling information on species attributes and riparian function (eg., for erosion control), to support the riparian management decision process.

In conclusion, the approach developed will assist in selecting suitable species for the environmental gradients at a riparian restoration site, and should lead to a more rigorous and successful approach to riparian planting and monitoring.