Contrasting Impacts of Cultivated Exotics on the Functional Diversity of Domestic Gardens in Three Regions with Different Aridity

Josep Padullés Cubino,Sonja Knapp,Jeannine Cavender-Bares,Carlo Ricotta,Christopher Swan,Alienor Jeliazkov,Stefan Siebert,Daniel Sol,Dorothy Borowy,Zdeňka Lososová

doi:10.1007/s10021-020-00556-x

Abstract

Cultivated exotic plants are often introduced for their aesthetic value and today comprise a substantial fraction of the flora of urban domestic gardens. Yet, their relative contribution to the functional diversity of domestic gardens and how it changes across different climate zones is insufficiently understood. Here, we investigated whether the effects of cultivated exotics on functional diversity of three plant traits related to plant aesthetics (that is, plant showiness, plant height, and leaf area) varied in suburban domestic gardens in three regions (Minnesota, USA; Alt Empordà, Spain; and central South Africa) that differ in aridity. For each garden, we calculated the mean and variance of each plant trait considering all co-occurring species and also splitting them into co-occurring cultivated exotics and natives. Our results revealed that mean plant showiness increased linearly with the proportion of cultivated exotics both across and within studied regions. Moreover, co-occurring cultivated exotics were, on average, showier than natives in all regions, but differences in their trait variances were context-dependent. The interaction between cultivated exotics and aridity explained variation in mean plant height and leaf area better than either predictor alone, with the effect of cultivated exotics being stronger in more arid regions. Accordingly, co-occurring cultivated exotics were taller and had larger leaves than natives in warmer and drier regions, while the opposite was true in cooler and wetter regions. Our study highlights the need to consider the combined effects of exotic species and climate in future studies of urban ecology.

Highlights

As the world becomes more urbanized (UN DESA 2018), urban plant communities are becoming increasingly important for maintaining biodiversity (Aronson and others 2014, 2016) and providing ecosystem services (Tratalos and others 2007)
The scientific literature indicates that ecosystem-service based traits associated with visual aesthetics tend to be of high priority for the configuration of the floras in domestic gardens, we have a relatively poor understanding of the contribution of cultivated exotics to the diversity of these traits, and whether trait diversity of cultivated exotics and natives differs across climatic zones
We address three questions: (1) How does the proportion of cultivated exotic plant species relate to the functional trait diversity of domestic gardens? (2) Do cultivated exotics influence functional trait diversity of gardens differently across a global aridity gradient? (3) Do cultivated exotics differ from co-occurring natives in functional trait diversity in regions with different levels of aridity? Because homeowners often select for cultivated species with different colors and aesthetic characteristics (Goodness and others 2016), we hypothesized that higher proportions of cultivated exotics in domestic gardens would be positively associated with functional diversity of plant showiness

Summary

Introduction

As the world becomes more urbanized (UN DESA 2018), urban plant communities are becoming increasingly important for maintaining biodiversity (Aronson and others 2014, 2016) and providing ecosystem services (Tratalos and others 2007). Cultivated species in gardens are largely selected from the horticultural pool (Cavender-Bares and others 2020) based on homeowners’ preferences for certain traits associated with these species, including low water requirements, a specific size at maturity, and showy flower displays (for example, Kendal and others 2012a; Goodness 2018; Pearse and others 2018) These traits have often been referred to as ‘ecosystem-service based traits’ (Pataki and others 2013), as opposed to the suite of traits commonly used in ecological studies (Cornelissen and others 2003; Perez-Harguindeguy and others 2013). The scientific literature indicates that ecosystem-service based traits associated with visual aesthetics tend to be of high priority for the configuration of the floras in domestic gardens (reviewed in Goodness and others 2016), we have a relatively poor understanding of the contribution of cultivated exotics to the diversity of these traits, and whether trait diversity of cultivated exotics and natives differs across climatic zones

Methods

Results

Conclusion