Elsevier

Landscape and Urban Planning

Volume 146, February 2016, Pages 20-28
Landscape and Urban Planning

Green leaf colours in a suburban Australian hotspot: Colour differences exist between exotic trees from far afield compared with local species

https://doi.org/10.1016/j.landurbplan.2015.10.003Get rights and content

Highlights

  • Leaf colour of endemics and exotics were compared in a global hotspot.

  • Australian endemics were greyer, less chromatic, and darker than exotics.

  • Colour changes might occur with planting exotics from far afield.

  • Due to the genetic isolation of Australian plants, colour impacts may be greater.

  • Colour is a component of homogenisation with urbanisation.

Abstract

Endemic species are often replaced by plantings from non-local areas in new suburbs in the developed world. Does this lead to colour changes? This paper compares colour in the leaves of exotic trees planted in suburbs to that of endemics in the Southwest Australian Floristic Region. Colour in plant parts was assessed by the Natural Colour System of Sweden, which enabled quantitative comparison between species. Hue, chromaticness, percentage yellow, blackness, whiteness, luminescence, and visual lightness were determined. The leaves of Australian trees were less chromatic and darker than exotic trees, suggesting that colour changes are occurring with suburbanisation in this region.

Introduction

The colour green dominates tree-scapes of many urban and non-urban areas. Yet what colour is that green? Do particular greens reflect particular environments? The study described here grew out of concerns about the increased number of exotic trees planted in Australia in urban areas. Australia is the only continent almost defined by one plant genus, Eucalyptus, which is found in almost every biome and local region. Suburban development in Australia has led to endemic tree species such as eucalypts being removed and their replacement with exotic trees, here defined as those whose natural range is outside of Australia. This study addressed the question as to whether colour changes have occurred with the introduction of large numbers of exotic trees. While a concern about colour change due to exotics has been raised verbally by botanists, there has been little comment in either science or urban design (Fig. 1), although Grose (2012) asked if colour is a neglected visual aspect of conservation.

Urban expansion in much of Australia and in the world, such as the US (Alig, Kline, & Lichtenstein, 2004) and the mega-cities of Asia (Murakami, Medrial Sain, Takeuchi, Tsunekawa, & Yokota, 2005), is typically into agricultural areas; in contrast, urbanisation in south-western Australia is commonly into bushland of the Southwest Australian Floristic Region (SWAFR), one of the world's global biodiversity hot spots (Myers, Mittermeier, Mittermeier, da Fonseca, & Kent, 2000), and one of only two hotspots in the megadiverse country of Australia (Mittermeier, Myers, & Mittermeier, 1997). Hopper and Gioia (2004) suggest that at least 8000 endemic floral species will be confirmed for the region when current long-term taxonomic surveys are completed. In other global hotspots of Mediterranean-climate type, such as the Californian Floristic Province and the Mediterranean Basin, urban expansion and tourism have long been considered threats to biodiversity (Konstant and Mittermeier, 1999, Myers and Cowling, 1999). Likewise, the biodiversity of the coastal strip of the SWAFR is threatened by suburban development equal to the highest rate in Australia.Lozano et al. (2013) noted that in the Mediterranean hotspots of the SWAFR, California, and Spain, threats to biodiversity originating from human activities represented more than 80% of all threat types. The question about colour changes with suburbanisation has arisen because in the last thirty years suburban development in Western Australia has featured the processes of clear-felling, elimination of topography, removal of topsoil, and terracing of land prior to the establishment of new house blocks (Grose, 2010, Kullmann, 2014). This process of suburbanisation has led to a great loss of endemic trees and considerable ecological change in the region (Grose, 2011), although new design practices are challenging this process (Grose, 2009, Grose, 2010). Suburban expansion in the SWAFR has thus produced a highly constructed landscape.

Canfield and Runkle (1999), working in the USA, considered that there is now an eclectic mix of endemic and introduced tree species in urban landscapes but did not suggest a percentage breakdown. Also in the USA, McKinney (2005) and La Sorte, McKinney, and Pyšek (2007) noted the increasing number of non-native species among urban floras and across continents, with non-native species originating from outside the USA or Europe playing a secondary role as a ‘homogenizing source’ on urban floras, with the primary role of homogenisation coming from American species. In the SWAFR the situation appears to be quite different. While non-local species are now dominant in most urban areas (Powell & Keighery, 2003), a study of newer suburbs in the region confirmed that exotic species (those from outside of Australia) comprised more than 83% of new plantings (Farrelly, pers. comm., 2003). Concerns have been raised about the loss of endemic species per se and about biotic homogenisation with genetic changes (McKinney and Lockwood, 1999, McKinney, 2005, La Sorte et al., 2007) but subtle issues of visual change, such as colour, texture, form, density, and height of the top-storey remain unexamined (Grose, 2012). The lack of research into colour changes in suburbs is paralleled by the general lack of research in the whole arena of non-reproductive colour in plants (Lev-Yadun, Inbar, Izhaki, Nèman, & Dafni, 2002) beyond experimental and microscopic work on chlorophyll and the nature of autumnal colour changes. Recent work on plant structure has been slowly changing this situation, and thus it is timely to ask: what green is that?

In McKinney's (2005) study, he distinguished species as those outside of the US and those from internal, less distant sources, and suggested that introductions from nearby sources were more frequent than species from more distant sources. However, in Australia this pattern is generally the reverse, with introduced trees being overwhelmingly from distant sources—from Europe, North America, and north-eastern Asia; many of these are deciduous and placed into the overwhelmingly evergreen continent of Australia. In addressing the question as to whether such a suburban shift from endemic Australian trees to exotics from distant sources brings a visual colour change in green leaf colour (beyond the obvious one of ‘autumn’ foliage) the study reported here compared the green leaf colours of major endemic trees and those exotic trees which are commonly planted as street-trees and in public parks in the SWAFR.

Gage (1993) makes the point that the study of colour par se is a vastly daunting subject, and Lancaster (1996) considers colour one of the most problematic of interpretations relating to the visual world. Colour observation has difficulties noted for centuries. Monge (1789, cited in) commented: “So the judgements that we hold about the colours of objects seem not to depend uniquely on the absolute nature of the rays of light that paint the picture of objects on the retina; our judgements can be changed by the surroundings.” Measuring colour by eye is subjective, ephemeral, prone to changes with weather, time of day and age of the plant, thickness of vegetation, light and sun-angles, “gleam and glitter” (Fridell Anter, 1996), haze, adjacent colours, and the distance of the viewer from the subject. Colour is further influenced by art history and theory, general history, lexicography, the science of optics, and ‘the eye of the beholder’.

Importantly for the current study which compares floras, the colour differences which we see can occur due to structural differences in the plant itself (Glover, 2009, Whitney et al., 2011), with very small differences in structure producing changes in the wavelength of light reflected. Structural colour is also independent of pigment colour, and can overlay it (Glover, 2009). Since plant anatomy is very much influenced by the light conditions in which the plant has evolved (Vignolini, Moyroud, Glover, & Steiner, 2015), it must be noted that suburbanisation in the SWAFR is likely placing plants from very different light environments into more intense light environments in the SWAFR, where plants have evolved for greater intensity. Thus the colour which we see is a product of three things: of external conditions, our eyesight, and the coloured object itself, which in this study is a leaf. These three components determine what we can see when leaves appear a different colour green to the human eye.

Plant leaf colour is very little recorded, perhaps due to a prevailing belief that leaves have little colour variation in comparison to flower colour, as considered by Glover (2009). However, scale or degree of difference is the key to this issue, and there are subtle differences in the hue and chroma of leaves even when falling under the general description of ‘green’, which covers an enormous range of hues. In a study of colour descriptions of plants in the SWAFR from the eighteenth century to the present Grose (2007) showed that botanical descriptions of leaf colour remains qualitative, with descriptions such as ‘mid-green’, ‘dark green’, ‘glossy green’. Such descriptions remain typical globally.

A key challenge of assessing colour differences by eye in the field is replicable technique. Colour methods exist for comparative field examination of soils by eye. The Munsell Soil Color Charts (1905), with 322 different standard colour chips in common use, were conceived for the purpose of elucidating the colour of soils, and the Munsell Color Charts For Plant Tissue Analysis (1905) for the elucidation of colour changes due to plant disease or nutritional deficiencies. The colour of soil is revealing of soil chemical composition, possible fertility, and nutritional potential or problems for agricultural production. With the Munsell systems, soils or plants can be compared by eye in the field, wherever their locations. This is an important motif regarding the comparison of colours of plants in the field.

Colour systems exist for comparing colours visually using colour swatches. In graphic art and interior design, the Pantone series of colour is used for testing, clarifying, and standardising colours, and is now used as a test of colour printing in graphic design. The Royal Horticultural Society Colour Charts are widely used in horticulture. The Natural Colour System (NCS), developed in Sweden in 1979, is the most extensive colour range suitable for external, natural surface colour and is also widely used in interior architecture and general architecture: the NCS is based on how colours are perceived; black and white are taken as basic ‘achromatic’ colours, with yellow, red, blue, and green as ‘elementary’ colours. However, while plant colour can be measured using a number of existing colour systems, this has not led to precise and comparative analyses between floras, largely due to problems of representation and analysis. Such comparison could answer the question of whether urban and suburban development is changing part of the fundamental visual appearance or ‘colourscape’ in some regions (Lenclos & Lenclos, 1982). Green leaves make up a major portion of colourscape, in environments with and without seasonal leaf-fall.

Determination of leaf colours using the NCS has been limited to a small number of unspecified species in Sweden (Fridell Anter, 1996) and more detailed plant leaf and trunk colours in the Australia (Grose, 2014). The red colours of autumn leaves have been examined in the UK (Archetti et al., 2009, Döring et al., 2009, Lev-Yadun and Holopainen, 2009), and Glover (2009) noted the physiological basis of colour and colour differences. However, no purposeful colour work exists for establishing general patterns of differences between major families or bioregions for plants.

Colour has also been used as a parameter to establish, measure, and discuss features of global change biology, such as changes in satellite colour data to classify ocean environments for monitoring inter-annual changes in the oceans (Esaias et al., 2000, Siegel, 2005) and plant cover changes (Malmer et al., 2005, Shugart et al., 2001). Colour in animals has been examined more keenly. As examples, fish colour in Hawaii (Marshall, Jennings, McFarland, Loew, & Losey, 2003), and the coral colour in which fish live (Matz, Marshall, & Vorobyev, 2006) has been examined, and the colours of fossil beetles (McNamara et al., 2012, McNamara et al., 2014), ancient horses (Pruvost et al., 2011), and dinosaur skin colours (Xu et al., 2004, Xu et al., 2012, Zhang et al., 2010, Li et al., 2010) have gained world interest. Coral bleaching and bleaching prediction data have been used as an indicator of coral health (Donner et al., 2005, Bellwood et al., 2006, Todd et al., 2013).

Any study of surfaces, whether plant or inanimate, needs to consider the distance that colour is viewed. Fridell Anter (1996), working with building surfaces, found distant colour (which she termed perceived colour) most variable and responsive to time of year, growth stage of the plant, light and weather, angle of the sun and incident radiation, and distance of the viewer from the object. Colour observation at a close distance allows an understanding of colour ranges in nature (Fridell Anter, 1996). Seasonal changes can also occur in plants, even in Australian evergreens (George, 2002). A further complication is that individual trees contain leaves of various ages in evergreen species; for example, Jacobs (1955) found that most species of eucalypts retain their leaves for at least eighteen months and generally have two or three leaf-age cohorts present in the crown, and the proportion to which these are present may vary with seasons and the success of the season. Despite these issues, a colour comparison can be made through careful observation at known distances, as in the Munsell systems for soils and plant nutrition.

This current paper details field assessments by eye of leaves, to determine if measurable differences in colour existed between endemic trees and species of exotic trees commonly planted as street trees in urban areas of the SWAFR. Although colour differences are generated by physiology (Lee, 1991, Lee, 2007, Lee et al., 2000, Parker, 2000, Glover and Whitney, 2010, Whitney et al., 2011), this study was not a physiological study of plants but a visual study in response to a visual question of the built environment: do introduced exotic trees in the SWAFR appear a different colour green to endemic trees?

Section snippets

The plants examined

Plants were examined in the suburbanising landscape of the Swan Coastal Plain of Perth, Western Australia, which lies within the SWAFR. The study was confined to larger endemic species, with the focus on those vegetation areas where the greatest amount of urban development is occurring around the city of Perth, namely the immediate coastal dunes, the southern corridor of seasonally water-logged plains and paperbark flats, and the north-east corridor of Banksia woodlands. The most important,

Results

The NCS allowed simple evaluation and comparisons of leaf colour between species in the field. Some leaf surfaces were concolorous (when leaf surfaces are the same colour on both sides), while others were discolorous (leaf surfaces differ in colour); some discolorous leaves were markedly different and some subtly.

The main finding was that there were differences in colour between endemic and exotic leaves. Endemics had a lower chromaticness (brightness) and a higher blackness (% close to black)

Discussion

In A.D. Hope's poem Australia (http://www.poetrylibrary.edu.au/poets/hope-a-d/australia-0146006), the colour of Australian trees is described:

A nation of trees, drab green and desolate grey

In the field uniform of modern wars

The notion that Australia has a substantially grey green tree leaf colour was supported in this study in the SWAFR. The greater the chromaticism, the greater the visual distance from grey; the lower the chromaticism, the lesser the distance from grey. Trees endemic to the

Acknowledgements

I thank Robert Powell, of Conservation and Land Management, for assisting with site locations for several of the species in the SWAFR and for noting colour changes with suburbanisation. Karin Fridell Anter of Sweden gave invaluable advice with colour using the NCS, as did Paul Green-Armytage of the Colour Society of Australia, and Margaret Pope in Sydney; Sue Finch and Sandy Clarke of the University of Melbourne assisted with statistics. I also thank Simcha Lev-Yadun and an anonymous reviewer,

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