Plant traits link people's plant preferences to the composition of their gardens

Abstract

Gardens are ubiquitous in western cities, comprising up to a third of the total urban area and often containing a majority of the vegetation present. Gardens are the cumulative result of many individual plant choices, yet we know relatively little about the role of preference in these choices. We investigated peoples’ preference for different garden plants and reasons for plant choices using a postal questionnaire (n = 224) containing 30 colour photos of garden plants and questions about gardening behaviour. Preferences were compared with the plants growing in the gardens of 48 randomly selected respondents. Objectively measured plant traits were used to relate preferences to the plants growing in people's gardens. Significant relationships were found between survey responses and both the traits and taxonomy of plants growing in respondent gardens. The results also show that people's preferences are very diverse, and that these preferences were related both to aesthetic traits such as flower size, leaf width and foliage colour, and non-visual traits such as nativeness and drought tolerance. Together these findings provide evidence that garden floras have responded to their social environment, and suggests that the very high levels of diversity observed in gardens can in part be attributed to the heterogeneity observed in this social environment.

Introduction

As the world is becoming more urbanised (United Nations, 2010), residential gardens are becoming an increasingly important contributor to people's health and wellbeing (Dunnett & Qasim, 2000) and the ecological functioning of cities by maintaining biodiversity (Daniels and Kirkpatrick, 2006b, Smith et al., 2006) and through the provision of ecosystem services (Tratalos, Fuller, Warren, Davies, & Gaston, 2007). Gardens are the cumulative result of many individual decisions about plant choice over time that combine to determine the social and biophysical benefits provided. These benefits can vary greatly between gardens and depend on the characteristics (traits) of the individual plants in them. For example, gardens with many native plants may provide habitat that suits native birds more than exotic birds (e.g. Daniels & Kirkpatrick, 2006b) or increase ‘belonging’ for some people (Head & Muir, 2006) while those with large canopies may provide more cooling (McPherson et al., 1997) and increase health benefits (Mitchell & Popham, 2008). There is a body of research across disciplines identifying a range of factors influencing the occurrence of plants in residential gardens, including attributes of people such as household income (Martin, Warren, & Kinzig, 2004) and cultural background (Fraser & Kenney, 2000), attributes of plants such as flowering (Marco, Barthelemy, Dutoit, & Bertaudière-Montes, 2010) and fruiting (Acar, Acar, & Eroğlu, 2007) and physical attributes such as rainfall (Daniels & Kirkpatrick, 2006a). Preference (Behe and Nelson, 1995, Berghage and Wolnick, 2000, Townsley-Brascamp and Marr, 1994) and the related concept of aesthetics (Head and Muir, 2006, Marco et al., 2010) influence people's plant choices. However, there has been little research into the ways that people's preferences for plants actually shape the composition of the plants occurring in their gardens. This study strengthens our understanding of this relationship by examining people's preferences for plants and plants in gardens using measures that are more objective and more comparable than approaches typically used in landscape preference studies.

The few studies that do explore whether preferences influence the plants grown in gardens have been limited in several ways. Some are dependent on coarse, subjective measures of self-reported garden composition or practice that are difficult to generalise to other contexts. Kurz and Baudains (in press) found that people who self-reported choosing native plants had much higher preference for high-habitat gardens, and Larsen and Harlan (2006) found that people have self-reported garden styles that mostly matched their expressed preferences. There have been several studies at the garden landscape level that link preference to the garden style presented in photographic stimulus, but provide little insight into how people choose particular plants. For example, van den Berg and van Winsum-Westra (2010) show that people's preferences vary by the formality of the garden design, and Yabiku, Casagrande, and Farley-Metzger (2008) found that people's preferences for mesic and xeric garden styles differed in the desert city of Phoenix, Arizona. Other studies have related psychological traits other than preference to garden characteristics. Zagorski, Kirkpatrick, and Stratford (2004) found that garden species composition was related to attitude towards the effort of gardening and to the environment, while van den Berg and van Winsum-Westra (2010) found that the personality trait Personal Need for Structure was related to self-reported garden style.

Our study builds on this body of work by relating people's preferences to actual observations of plants in their gardens, and by characterising plants using objective measures of plant traits. The use of plant traits provides significant benefits for the understanding the relationship between plant preferences and plants occurring in gardens. Our focus on plant traits draws on approaches in ecology, where plant traits are commonly used to analyse the functional response of plants to their environment, and to allow global comparisons of vegetation communities that are taxonomically distinct (Cornelissen et al., 2003). The study of plant traits in ecology has focussed explicitly on how the functional traits of plants (such as leaf mass per area and canopy height) vary in relation to physical environmental drivers such as climate and nutrient levels (e.g. Wright, Westoby, & Reich, 2002). Garden plants exist in an environment that has both physical (e.g. temperature and water availability) and social drivers (e.g. people's preferences), and consequently may show trait responses to both. Loosely defined plant traits have been used in many landscape preference studies, typically by expert categorisation of stimulus photographs (e.g. small, medium or large foliage) (Williams, 2002) or in respondent self-assessment of garden style categories (e.g. wild, manicured or romantic) (van den Berg & van Winsum-Westra, 2010). However, the use of categories with loose definitions makes comparison between studies and generalisation to other contexts difficult. This study uses objective measures of plant traits to explore the relationship between preferences and the plants in people's gardens, an approach which will allow our results to be objectively compared with results from other places. Plant traits that have been related to people's preference in garden and non-garden contexts include leaf and flower colour and size (Kaufman and Lohr, 2004, Kaufman and Lohr, 2008, Kendal et al., 2008, Todorova et al., 2004, Townsley-Brascamp and Marr, 1994), leaf width (Williams, 2002), the provision of shade or fruit (Fraser & Kenney, 2000), tree canopy shape (Lohr and Pearson-Mims, 2006, Sommer and Summit, 1995, Williams, 2002) and nativeness (Williams, 2002). This study will explore the role of preference in people's garden plant choices by focussing on plant traits including flower size, leaf colour, nativeness and leaf width. We hypothesise that garden plants will have traits that show responses to both social and physical drivers, and that people's preference for aesthetic plant traits such as foliage colour and flower size will be reflected in the traits of the plants in their gardens.