Establishing low cost, biodiverse species assemblages in public lawns

Abstract

Urban grasslands are widespread, provide important ecosystem services, and serve as a major component of the greenspace encountered by human populations. They are often perceived as underperforming in their contribution to sustainability because of their low biodiversity and high financial and environmental maintenance costs. In my thesis, I explored whether opportunities exist to create biodiverse, low maintenance urban grasslands. Waikumete Cemetery is recognized for its diverse plant species assemblage including natives and wildflower populations all maintained with a single annual mow. To determine why these desirable characteristics exist here and explore the site as a possible model, plant species assemblages and environmental characteristics were surveyed to indicate the relationships between species and environmental variation. Waikumete Cemetery has highly nutrient poor soils and the probable drivers of vegetation variation here were identified as different components of soil fertility, and particularly soil P. Functional traits are an important way to link organismal features to competition and ecosystem function. In addition to a taxonomically based survey of Waikumete Cemetery, an analysis of the distribution of plants by functional traits was conducted. Species present were primarily perennial, evergreen, and clonal with graminoids and geophyte forbs as important components. Trait groups varied along the dominant environmental gradients. Many exotic weed species compete best under fertile conditions; therefore an experiment was conducted to analyse potential changes to lawn composition in Auckland by reducing nutrient levels. Carbon and nitrogen treatments were applied to alter nitrogen content and comparisons in species composition were made. Despite the comparatively short study duration, there was some indication that soil nutrient levels had been affected by the treatments; under high carbon addition, the abundance of a low fertility grass species (Agrostis capillaris) increased. Disturbance is an important mechanism shaping grasslands, and mowing fills this role in urban environments though costly to budgets and the environment. An assortment of mowing regimes was employed in an existing public lawn in Auckland and composition over time was compared to determine how species assemblage was affected. Cessation of mowing had the greatest effect with an increase in Lolium perenne and decrease in legumes and Agrostis capillaris. The surveys and experiments conducted suggest that plant composition of urban grasslands can be influenced by controlling soil nutrient conditions and disturbance regime.