Cool Tree Architecture: A Descriptive Framework for a Tree Architecture Typology to Temper Urban Microclimates

Abstract

As the elementary unit of the urban forest, trees temper thermal extremes in urban microclimates through shading and evapotranspiration, and by altering the movement of air. Metrics on shade performances of different species, however, are currently limited, which can be remedied by the development of a method to describe the range of species and cultivars via a structured overview of physical characteristics impacting radiation reflectivity, absorptivity, and transmissivity. This paper proposes a descriptive framework based on the concept of "tree architecture, " which has developed into a recognized field of plant study from the perspective of their physiognomy, morphology, and morphogenesis. The framework describes various architectural sub-traits within the overall trait categories of Crown, Wood, and Foliage. The descriptive framework can be used to develop a "Cool Tree Architecture Typology" (C-TAT), in which trees can be organized into similar types based on common physical characteristics. Further elaboration of sub-traits using observations of trees in controlled field laboratories resulted in new derivative classes for use as key in classifications for the C-TAT. The C-TAT can be used to organize the many species and cultivars occurring in, for example, Cfb Atlantic climate zone cities, to a lesser number of architectural types. This allows for more rapid evaluation and cooling performance calculations of tree inventories and can also be of value in assisting tree managers to propose more accurate thermal performance standards for trees in urban projects. The elaboration of tree architecture from an urban microclimate perspective complements existing elaborations and approaches in the field of tree architecture.

● A descriptive framework to elaborate the architectural characteristics of tree species relevant to solar radiation reflectivity, absorptivity, and transmissivity

● Critical tree architecture classes for cooling including Crown Proportion, Wood Grain, Wood Density, Wood Zoning, Foliage Texture, and Foliage Luminance

● The descriptive framework can be used to develop a Cool Tree Architecture Typology to categorize trees based on common physical characteristics

● Tree architecture is a novel frame for developing metrics and standards of urban trees in relation to thermal microclimate amelioration

Graphical abstract

Keywords

Tree Architecture / Urban Heat Island / Climate Adaptation / Urban Microclimate Amelioration / Cool Tree Architecture Typology

1 Introduction

The consequences of climate change impact urban environments and communities in multiple ways, including rising temperatures and heat waves^[1]. Climate change exacerbates the existing phenomenon of urban heat island (UHI), which together are projected to have increasingly harmful effects on aspects such as human health and water availability^{[2] [3]}. Changes to a city's morphology, the design of its buildings, and the materialization of its unbuilt spaces are ways to make cities better adapted to deal with climate change and UHI^[4]. As a key part of urban space, urban green infrastructure (UGI) is seen as an effective way to adapt to climate change and associated heat stress^[5]~[7]. UGI refers to a broad range of green spaces such as urban woodlands, urban parks, street trees, green roofs, and green facades^[8]. The urban forest, understood as the mosaic of trees and associated vegetation growing in and around dense human settlements^[9], can be seen as the backbone of UGI^[10] and a key means to adapt cities to rising temperatures and heat stress^[11].

As the elementary unit of the urban forest, trees temper thermal extremes in urban microclimates through shading, evapotranspiration, and by altering the movement of air^[12]. Shading is the common term used for the characteristics of trees that affect the reflectivity, absorptivity, and transmissivity of solar radiation through the tree canopy, and ensuing radiant energy reaching objects and surfaces beneath them^{[13] [14]}. Shading performances of different species of street trees vary greatly, however, depending on their physical characteristics^{[15] [16]}. Insights into how these varying characteristics impact shading are currently limited, which can be remedied by the development of a method to describe and classify the various species and cultivars into groups via a structured overview of the physical characteristics impacting radiation reflection, absorption, and transmission. Such a descriptive framework may also then be used to develop a "Cool Tree Architecture Typology" (C-TAT), in which a large number of species and cultivars can be categorized into a lesser number of architectural types based on common physical characteristics. The C-TAT will allow for more rapid development of cooling performance metrics of many tree species and performance calculations of existing urban tree inventories, assisting tree managers to propose more accurate thermal performance standards for trees in urban development projects.

This paper focuses on trees in the temperate climate zone in Europe, or Cfb (temperate oceanic climate^†

Cfb, referring to temperate oceanic climate, is the humid temperate climate sub-type, generally featuring cool summers and mild winters with a relatively narrow annual temperature range and few extremes of temperature (source: Ref. [17])

) according to the Köppen–Geiger classification^[17], with The Netherlands as case study country within this climate zone.

2 Research Design

The development of a descriptive framework builds on the concept of "tree architecture, " which has its early beginnings in pre-Linnaean plant studies focused on plant form description. With the development of sexual classification in the Linnaean system, plant form was relegated to a minor role, but Johann Wolfgang von Goethe^[18] revived interest in the topic. Goethe's work informed later classifications such as by Alexander von Humboldt and Aimé Bonpland^[19], who elaborated for the first time the overall structure of a plant community based on vegetation types and physiognomic features. Following Humboldt and Bonpland's work, Christen Raunkiaer presented a system including seven life-form classes^[20]. Important contributions also came in the field of landscape architecture, such as from James Rose who studied the form of temperate trees and grouped them by their shared forms and the influence of these forms on human experience in (designed) environments^[21]. A complementary body of work appeared in tropical forestry study in the 1970s led by Francis Hallé, Roelof Oldeman, and Philip Tomlinson, who developed a woody-plant classification system based on morphological characteristics using the term "tree architecture"^[22].

The foundational structure for the descriptive framework builds on the work around tree architecture, augmented by insights from the field of tree physiognomy^{[23] [24]}, tree morphogenesis^[25], and tree morphology^[26]. Research on morphogenesis, morphology, and physiognomy is similar to tree architecture, which focuses on the general life form of plants and their inherent organizational characteristics. This review results in three basic trait categories of Crown, Wood, and Foliage with descriptive sub-categories including crown shape, trunk morphology, branch morphology, twig morphology, leaf size, leaf shape, leaf color, leaf thickness, leaf texture, and leaf orientation. To elaborate and develop these characteristics towards a preliminary set of critical sub-traits and C-TAT classes, a comparative visual analysis of benchmark botanical illustrations was carried out (Step One). This was followed by the validation and refinement of the framework aided by field observations in dedicated outdoor laboratories. The definitive framework is divided into a descriptive section on critical sub-traits using drawings, diagrams, and text descriptions, and a section elaborating C-TAT classes (Step Two). How to deal with growth dynamics of trees was also addressed in this step. The framework was trialled using Prunus avium as a sample tree of Cfb climate zone.

2.1 Step One: Comparative Analysis of Benchmark Botanical Illustrations

Compared with floral paintings which belong to the world of art and aesthetic appreciation, botanical illustrations are pictorial depictions of plant traits primarily for scientific purpose^[27]. Botanical illustrations have a number of key characteristics which make them relevant for this research. First, they isolate and portray definitive physical characteristics of a tree through drawings, paintings or engravings, allowing for a detailed portrayal of the whole tree attributes with a minimum of unnecessary visual information. Second, they depict a tree as a living entity in its adult or mature growth stage, and invariably show multiple phenological phases and other aspects that could not appear in a single tree specimen at any one time due to temporal or situational variables. Third, they are based on detailed studies and recording of actual individual specimens, thus revealing the interactions between endogenous factors (the fundamental growth patterns of a tree) and exogenous factors (the specific environmental conditions of a location). Botanical illustrations do limit the impact of exogenous factors, however, by avoiding the use of specimens which grow in environments with limitations, or specimens that are overly damaged or pruned. As such, botanical illustrations can be said to depict a tree in its most ideal or "natural" state.

These considerations form the starting point of the elaboration of sub-traits in Crown, Wood, and Foliage for a particular tree species. The goal of the analysis was to distinguish various traits emerging from benchmark botanic illustrations which enable a comparative analysis and elaboration of sub-traits. Leading out from these considerations, the collections of botanical illustrations for this study and comparison were chosen based on the following criteria: 1) illustrations by professional/trained botanical illustrators, artists or architects; 2) the use of appropriate representation techniques; 3) the use of accurate elevation projections; 4) illustrations to scale; 5) the whole tree depictions based on specimens in ideal situations; 6) trees shown both with and without foliage (where relevant); and 7) depictions of adult or mature trees.

Two benchmark sets of illustrations published in the last ten years were chosen which meet these criteria: The Architecture of Trees (2019)^[28] with illustrations by Cesare Leonardi and Franca Stagi, and Illustrated Trees of Britain & Europe (2013)^[29] with illustrations by David More. The botanical illustrations by Leonardi and Stagi were drawn with pen and ink on transparent film and consisted of scaled, elevation views (both active and dormant seasons for deciduous trees) and illustrations of leaf traits at the leaf axis unit scale (Fig.1). Over the years, Leonardi and Stagi expanded the number of illustrations based on specimen trees in Italy, England, France, Austria, Switzerland, Czechoslovakia, Mexico, and Guatemala. Their selection of trees includes less hardy species which may not always survive severe winters, but nevertheless grow in various parts of Europe^[28]. The work of More forms part of a long-term project to develop a durable visual record of the tree species, varieties, and cultivars growing in the British Isles, Ireland, and north-western Europe. The selection of almost 2,000 trees in this volume includes less hardy species which can be found in most parts of Europe, but may not always survive continental winters. Although the plates are not as systematically scaled as the work of Leonardi and Stagi, the specimens are generally accompanied by a scaled feature such as a person or animal. Illustrations of trees in the active phase are invariably full color oil paintings with dormant phase depictions in pen and ink (Fig.2). Illustrations of leaf, bud, flower, fruit, and bark traits are also included in many plates.

Fig.1 Botanic Illustrations of Acer platanoides: summer profile, winter profile, and foliage (source: Ref. [28]).

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Fig.2 Botanic Illustrations of Acer platanoides: summer profile, bark, and foliage (source: Ref. [29]).

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The 232 trees illustrated in The Architecture of Trees formed the basis for a first selection of illustrations to study and compare in this paper. A selection of 101 trees was made, leaving out species with evident negligible impact on urban microclimates, such as dwarf cultivars (Tab.1). Sample illustrations of these species from both volumes were printed to scale for visual analysis.

Tab.1 Selected tree species for analysis

Abies alba		Arbutus unedo		Cornus florida		Fraxinus ornus
Abies cephalonica		Areca catechu		Corylus avellana		Ginkgo biloba
Abies concolor		Betula pendula		Corylus avellana var. contorta		Gleditsia triacanthos
Abies nordmanniana		Betula pubescens		Crataegus oxyacantha		Gymnocladus dioicus
Abies pinsapo		Broussonetia papyrifera		Cryptomeria japonica		Hippophae rhamnoides
Acacia baileyana		Carica papaya		Cryptomeria japonica var. Globosa Nana		Hyphaene thebaica
Acer campestre		Carpinus betulus		Cupressus arizonica		Ilex aquifolium
Acer monspessulanum		Carya ovata		Cupressus cashmeriana		Jacaranda ovalifolia
Acer negundo		Castanea sativa		Cupressus macrocarpa		Jubaea spectabilis
Acer opalus		Catalpa bignonioides		Cupressus sempervirens		Juglans nigra
Acer palmatum		Cedrela sinensis		Daphniphyllum macropodum		Juglans regia
Acer palmatum var. atropurpereum		Cedrus atlantica		Davidia involucrata		Juniperus chinensis
Acer platanoides		Cedrus atlantica var. glauca		Delonix regia		Juniperus communis
Acer pseudoplatanus		Cedrus deodara		Diospyros kaki		Kigelia pinnata
Acer saccharinum		Cedrus libani		Draecaena draco		Koelreuteria paniculata
Adansonia digitata		Ceiba pentandra		Elaeagnus angustifolia		Laburnum anagyroides
Adenium namaquanum		Ceratonia siliqua		Erythea armata		Lagerstroemia speciosa
Aesculus carnea		Cercis siliquastrum		Eucalyptus camaldulensis		Larix decidua
Aesculus hippocastanum		Cereus giganteus		Euphorbia abissina		Laurus nobilis
Agave americana		Chamaecyparis lawsoniana		Fagus sylvatica		Libocedrus decurrens
Ailanthus altissima		Chamaecyparis lawsoniana var. nidiformis		Fagus sylvatica var. asplenifolia		Liquidambar styraciflua
Ailanthus altissima var. erythrocarpa		Chamaecyparis obtusa		Fagus sylvatica var. Atropurpurea Group		Liriodendron tulipifera
Albizzia julibrissin		Chamaerops humilis		Fagus sylvatica var. pendula		Maclura pomifera
Alnus glutinosa		Chorisia insignis		Ficus bengalensis		Magnolia grandiflora
Alnus incana		Cinnamomum camphora		Ficus benjamina		Magnolia purpurea
Alnus incana var. pendula		Citrus sinensis		Ficus carica		Mangifera indica
Aloe dichotama		Cladrastis lutea		Ficus elastica		Melaleuca diosmifolia
Araucaria araucana		Clerodendrum trichotomum		Ficus sycomorus		Melia azedarach
Araucaria bidwillii		Cocos nucifera		Fraxinus excelsior		Mespilus germanica
Araucaria brasiliensis		Cornus controversa		Fraxinus excelsior var. pendula		Morus alba
Morus alba var. pendula		Pinus pinea		Quercus macrolepis		Sciadopitys verticillata
Nolina longifolia		Pinus strobus		Quercus palustris		Sequoiadendron giganteum
Nyssa sylvatica		Pinus sylvestris		Quercus petraea		Sequoia sempervirens
Ochroma grandiflora		Pistacia terebinthus		Quercus pubescens		Styphnolobium japonicum
Olea europaea		Pistacia vera		Quercus robur		Styphnolobium japonicum var. pendula
Olea europaea var. oleaster		Platanus occidentalis		Quercus robur var. pyramidalis		Sorbus aucuparia
Opuntia ficus-indica		Platanus orientalis		Quercus rubra		Sorbus domestica
Osmanthus fragrans		Populus alba		Quercus suber		Sorbus hybrida
Ostrya carpinifolia		Populus nigra		Quercus trojana		Sorbus hybrida var. pendula
Pandanus utilis		Populus nigra var. italica		Ravenala madagascariensis		Stelitzia augusta
Parrotia persica		Populus tremula		Rhizophera mangle		Tamarindus indica
Paulownia imperialis		Prunus amygdalus		Rhus typhina		Tamarix gallica
Phillyrea latifolia		Prunus avium		Robinia pseudoacacia		Taxodium distichum
Phoenix canariensis		Prunus cerisifera		Robinia pseudoacacia var. bessoniana		Taxus baccata
Phoenix dactylifera		Prunus cerasus		Robinia pseudoacacia var. monophylla		Taxus baccata var. fastigiata
Photinia serrulata		Prunus serrulata		Robinia pseudoacacia var. pyramidalis		Thuja plicata
Picea breweriana		Pseudotsuga douglasii		Roystonea regia		Tilia cordata
Picea excelsa		Pseudotsuga douglasii var. pendula		Sabal palmetto		Tilia platyphyllos
Picea excelsa var. pendula		Pterocarya fraxinifolia		Salix alba		Tilia tomentosa
Picea omorica		Puya raimondi		Salix babylonica		Tsuga canadensis
Picea pungens var. glauca		Pyrus communis		Salix caprea		Ulmus glabra
Pinus canariensis		Pyrus salicifolia		Salix caprea var. pendula		Ulmus glabra var. pendula
Pinus cembra		Quercus cerris		Salix daphnoides		Ulmus minor
Pinus halapensis		Quercus coccifera		Salix eleagnos		Washingtonia filifera
Pinus montezumae		Quercus coccinea		Salix matsudana var. tortuosa		Yucca brasiliensis
Pinus mugo		Quercus farnetto		Salix pentandra		Yucca brevifolia
Pinus nigra ssp. laricio		Quercus glauca		Salix triandra		Zelkova carpinifolia
Pinus pinaster		Quercus ilex		Sambucus nigra		Zelkova serrata

NOTES1. The species were source from Ref. [28], and the naming of species follows from the publication, and some nomenclature has since been revised.2. The highlighted ones are the species selected for this study.

2.1.1 Elaboration and Selection of Sub-Traits Specific to Crown

The various architectural characteristics emerging from the analysis and comparison of the (pairs of) illustrations include Shape, Proportion, Texture, and Luminance ^†

Luminance describes the amount of light passing through, emitted from, or reflected by a particular area. It is measured photometrically by the luminous intensity per unit area of light travelling in a given direction. This study established luminance based on human visual perception of the (lightness of the) foliage in illustrations and photographic studies in field laboratories. The Munsell color system was used to establish degrees of lightness (termed "value" in colorimetry). The value system grades lightness from 1 to 10, with 1 the lightest and 10 the darkest. The two other properties in colorimetry include hue (basic color) and chroma (color intensity) (source: Ref. [30]).

^[30]. From the analysis it was concluded that the sub-traits Proportion and Shape were specific to the trait set of "Crown, " and that sub-traits Texture and Luminance were applicable to the trait set of "Foliage." In terms of Crown Proportion, it was found that all crowns in the sample illustrations could be categorized into groups based on the proportional ratio of crown width (measured at the point of maximum horizontal dimension of the crown) and crown height (measured at the point of maximum vertical dimension of the crown). Of the sample trees that were analysed and compared, five proportion groups were identified: < 1:1, 1:1, 1:1.5, 1:2, and > 1:2.5.

Crown Shape is a second important variable within each proportion group. In < 1:1 group, a sole new crown shape was identified from the sample: Ovoid. In the 1:1 group, five new crown shapes were identified: Paraboloid, Irregular, Cylindrical, Spheroid, and Pyriform. In the 1:1.5 group, two new crown shapes were identified: Conical and Ellipsoid. No new crown forms were identified in the 1:2 and > 1:2.5 groups. It is noted that crown shapes are inter-related to the height of the widest point of the crown.

2.1.2 The Elaboration and Selection of Sub-Traits Specific to Wood

The analysis and comparison of the botanical illustrations of sample trees with respect to trunk, branches, and twigs resulted in the identification of several new sub-traits, including Bark Texture, Bark Color, Wood Grain (the relative thicknesses of trunk, branches, and twigs), Wood Density (the total area and concentration of wood in the canopy), and Wood Zoning (the position of branches and twigs in the canopy). Bark Texture and Bark Color were concluded to have negligible impact on shading and therefore were disregarded. Significant differences and associated impacts on the reflectivity, absorptivity, and transmissivity of a tree canopy were identified from the illustrations with regard to Wood Grain. In terms of Wood Density, significant differences in the dimensions of upper trunk parts, branches, and twigs were found in the sample illustrations, with finer branches and twigs resulting in a finer overall density of wood and vice-versa. These were also concluded to have an impact on shading. Further variations were found in the distribution of branches and twigs, with some trees having an even zoning outside of the crown while others demonstrated irregular zonings of branches and twigs, leading to clumping of foliage. As such Wood Zoning was identified as a third critical factor for reflectivity, absorptivity, and transmissivity of solar radiation.

2.1.3 The Elaboration and Selection of Sub-Traits Specific to Foliage

Characteristics of Foliage emerging from analysis of the illustrations included Leaf Size, Leaf Shape and Leaf Color. Other identified relevant sub-traits included Foliage Texture, Foliage Luminance, Foliage Hue, and Foliage Chroma. Further analysis and comparison of other leaf attributes from the botanical illustrations was hampered by incompleteness of the illustration sets; not all of the 101 species in The Architecture of Trees included a description and illustration of leaf sub-traits, and comparing monochrome pen and ink drawings from The Architecture of Trees with the color plates in Illustrated Trees of Britain & Europe proved often too limiting for sound conclusions. As such, further elaboration of Foliage sub-traits occurs in Step Two with the discussion and refinement of the descriptive framework.

2.2 Step Two: Discussion and Refinement of the Descriptive Framework

2.2.1 Selection of Tree Growth Stages

The aim of the research is to develop a descriptive framework of architectural characteristics of trees impacting reflection, absorption, and transmission of solar radiation. Trees grow and develop however, which demands that the framework take this into account. The first section of the framework for crown architecture is thus dedicated to a general description of growth attributes using textual descriptions accompanied by drawings of the overall crown form and related trunk and branching structure of the tree in its various growth stages. Which growth stages to elaborate on is firstly dependent on when trees practically offer shade benefits. Growth stages of trees in cities that offer realistic shade benefits include those in the generative period of tree growth^[31], i.e. the young, adult, and mature stages. Trees in the post-generative growth period may be considered optimal in terms of shading, but due to the low occurrence of these trees in most urban environments this category was omitted.

A further refinement focused on a particular growth stage within the generative period in the following sub-trait description sections. This decision was made based on the most common growth stage at which urban trees have an impact on cooling, i.e. the average life-span of urban trees. Estimates of the average life expectancies of trees in North American cities are between 19 and 28 years^[32], meaning that on average very few trees survive to the mature growth stage. On this basis the focus growth stage for the descriptive framework was determined as the adult phase, which for most trees corresponds to ages between 19 and 28 years. For reference purposes, general descriptions of mature and young tree growth stages were also included in first section on growth characteristics.

2.2.2 Crown Sub-Traits and C-TAT Classes

Further review of the sub-trait Crown Proportion subsequent to the analysis of botanic illustrations involving studies of 75 specimen trees in dedicated field laboratories revealed that variations in width-to-height ratios of the crown was the most critical factor in tree cooling performances. This criticality arises from differences in the proportional area of crown (surface) impacting the degree of reflectivity, absorptivity, and transmissivity of solar radiation. Given that the sun in the warmest hours of the day in summer periods in temperate cities is positioned in the upper hemisphere of the sky (i.e. a high solar incidence angle), tall, thin tree crowns will have effectively less surface area to reflect intercept or transmit radiation than low, wide crowns. In other words, the total surface area for reflection and interception of solar radiation critically varies with the width-to-height ratio of the crown, relative to the solar incidence angle. It was noted at the same time that the solar incidence angle is highly dynamic, varying throughout the day and at different times of the year. The impact of these variables in other climate zones exceeds the scope of this paper and should be further examined in a follow-up study. For the purposes of the descriptive framework, the aforementioned conclusion was considered sound enough to establish Crown Proportion as the primary sub-trait. The section on Crown Proportion included a written description accompanied by an elevation projection drawing of the adult tree of the species. Average crown radius and average crown heights of the adult specimens were also included.

In the definitive framework this proportional key is preceded by a description of growth stages of a tree species accompanied by elevation drawings depicting crown outlines and related branch configurations. The descriptive framework also includes a section on Crown Shape with a written description accompanied by drawings of the crown shape of the adult specimen tree (Tab.2). The total number of crown shapes from the sample was established as eight, but more crown shapes may emerge from analysis of additional illustrations.

Tab.2 Crown descriptions and classification key for Prunus avium (example)

Description		Illustration
Growth Attributes	● Fast-growing, relatively short-lived (100 ~ 150 years), deciduous tree ● Young Prunus avium (5 ~ 30 years) have erect-pyramidal "coniferous" crowns which turn into an ellipsoid form as they become adult ● The adult phase of Prunus avium is between 30 to 60 years with a height of 15 ~ 25 m ● Reiteration in mature stage can lead to clumping of the upper crown and fragmentation of the lower crown; mature specimens have columnar crowns and can reach a height of 30 meters in the Cfb climate zone
Crown Shape	Adult Prunus avium typically have an ellipsoid canopy but some specimens will develop a paraboloid or conical canopy depending on growing conditions
Classification		Illustration
Crown Proportion	Crowns of free-standing adult specimens have a width-to-height ratio of 1:1

2.2.3 Wood Sub-Traits and C-TAT Classes

Observations of specimen trees in dedicated field laboratories confirmed that cooling performances from Wood sub-traits, i.e. Wood Grain, Wood Density, and Wood Zoning, can be expected through the related foliage characteristics of reflection and interception of radiation. Variations in these sub-traits allude to fundamental mechanisms that determine the woody architecture of a tree as elaborated in morphogenetic studies such as by Francis Halle, Roelof Oldeman, and Phillip Tomlinson^[22]. Based on these studies, the revised descriptive framework section on Wood sub-traits includes descriptions of morphological patterns of trunk, branches, and twigs in text and elevation drawings. These are elaborated for the adult tree but also augmented by descriptions and drawings for young and mature trees.

The resulting key for the typology categorization was thereafter established firstly from the observed interrelationship of Wood Grain and Wood Density, which translates into three classes: 1) Coarse Grain–Low Density, 2) Average Grain–Medium Density, and 3) Fine Grain–High Density. Wood Zoning is an additional and relatively independent sub-trait which determines the overall distribution of leaves of the crown affecting reflection, interception, and transmission of radiation through clumping of foliage. An even and concentric zoning of branches and twigs will result in a more continuous foliage structure with even performance of reflection and interception of radiation, while less even, more random branching will lead to a mix of concentrated and less concentrated patches of foliage. Trees in the sample illustrations were found to be able to be classed into either Even or Uneven Wood Zoning classes (Tab.3). In the definitive descriptive framework this pair of keys is preceded by a description of growth stages of a tree species accompanied by elevation drawings depicting its typical trunk, branch, and twig morphology.

Tab.3 Wood descriptions and classification key for Prunus avium (example)

Description		Illustration
Growth Attributes	● The uniform rhythmic growth of branches and twigs give young Prunus avium prominent cone-shaped crowns ● Mature trees reiterate the branching complex of young trees in the top half of the crown, which is a sequential process and gives rise to a more complex crown with a denser wood structure, while lower branches and twigs continue to die off and disappear, resulting in a more open underside of the crown
Trunk	● The main stem of an adult Prunus avium has an orthotropic axis with monopodial, indeterminate growth ● The trunk grows rhythmically (in seasonal phases) and develops a slender, tapering bole ● A dominant apical meristem persists well into the adult phase, but as the tree reaches maturity the trunk splits into two or more meristems in the top third of the crown
Branches	● Main branches of adult trees are predominantly plagiotropic, slanting slightly up or down ● Branching is spiral, with intervals on the vertical axis of 0.2 ~ 2 m, and typically extend out to half the height of the crown ● All branches have determinate growth, with around a third dying off between the young and adult phases ● Die-off occurs mainly in the lower half of the crown, resulting in open patches and less densely vegetated axes
Twigs	● Twigs grow both orthotropically and plagiotropically, and typically also in downward direction ● Twigs' growth is medium-term, determinate and rhythmic, and twig density increases near the top half of the crown where less die-off occurs ● Typical wild Prunus avium are smaller annual twig complexes (brachyblasts), and these very short axes produce a single cluster of leaves every year with no particular growth direction and short-term, determinate growth (5 ~ 7 years)
Classification		Illustration
Wood Grain and Wood Density	Fine Grain–High Density for adult specimens
Wood Zoning	The organization of wood in the crown is Even

2.2.4 Foliage Sub-Traits and C-TAT Classes

In reviewing and elaborating on findings on Foliage from the illustration sets and observations of specimen trees in field laboratories, it was determined that the sub-traits Foliage Hue (basic color) and Foliage Chroma (color intensity) have limited impact on cooling and thus represented characteristics extraneous to the framework. Foliage Texture and Foliage Luminance, however, were concluded to be critical for shading and thus identified as key classification categories for the framework. From field observations, it was established that the contributing characteristics for Foliage Texture and Foliage Luminance are dependent on leaf attributes and their arrangement. The descriptive framework thus elaborated on leaf characteristics identified in the illustrations (Leaf Size, Leaf Shape, and Leaf Color) augmented by characteristics emerging from studies in field laboratories as well as descriptions from benchmark dendrological reference manuals by Gert Krüssmann^[33], Jan de Koning et al.^[34], and Jesus San-Miguel-Ayanz, et al.^[35]. This process resulted in additional relevant leaf sub-traits including Leaf Thickness, Leaf Texture, Leaf Orientation, and Leaf Arrangement. Photographic studies of Foliage sub-traits from controlled field-plot arboreta of selected tree species and cultivars form the basis of two description sections of text and photographs on Leaf Attributes (including Leaf Size, Shape, Color, Thickness, Texture, and Orientation) and Leaf Arrangement. A section on seasonal performances of Foliage and Foliage Phenology, was also included (Tab.4).

Tab.4 Foliage descriptions and classification key for Prunus avium (example)

Description		Illustration
Leaf Attributes	● Size: 6 ~ 15 cm in length, with an average area of 50 cm2 ● Shape: simple, ovate to obovate, with serrated margins and a pronounced pointed apex ● Color: dark-green topside, light-green underside ● Texture: smooth topside, hairy underside; slightly leathery to touch ● Thickness: relatively thin ● Orientation: semi-vertical to vertical
Leaf Arrangement	● Smaller annual twig complexes (brachyblasts) are typical for Prunus avium ● Leaves are pendulous, spaced alternately (distichous) along metamers, with clusters of leaves at the proximal end of the internode ● At the metamer scale both the leaf and the leaf unit are stratified in a pronounced horizontal fashion
Foliage Phenology	● Prunus aviumis a deciduous tree with an average in-leaf season in the Cfb climate zone of 200 days ● Prunus avium attains full leaf cover around mid-May, begins autumn coloration in mid-October and fully sheds its foliage by mid-December
Classification		Illustration
Foliage Texture	Fine
Foliage Luminance	Medium

The resulting classification for use in the typology categorization builds on these descriptions by synthesizing leaf characteristics into two categories: Foliage Texture and Foliage Luminance. Variations in Foliage Texture appeared in all tree illustrations, but significant differences in field studies of specimen trees leading to a large number of categories were not noted. On the basis of the sample sets and through field studies of specimen trees, two primary texture classes were established: Fine Foliage Texture and Coarse Foliage Texture. It should be noted that Foliage Texture is an alternative to existing canopy indices such as leaf area index (LAI)^†

LAI is the total one-sided area of leaf tissue per horizontal ground unit area (source: Ref. [36]).

^[36]. Differences in Foliage Luminance identified in the illustration sources were further validated in the field. Using the lightness value scale from colorimetry, all trees in the field studies were able to be assigned one of three luminance values: 2, 5 or 8, corresponding to luminance classes of Light, Medium or Dark. Using combinations of these two categories, trees can be described and classified into one of six categories for use in the C-TAT typology: 1) Coarse–Light, 2) Fine–Light, 3) Coarse–Medium, 4) Fine–Medium, 5) Coarse–Dark, and 6) Fine–Dark.

3 Conclusions

As the elementary unit of urban forests, trees temper thermal extremes but metrics on the shade performance of different species is currently limited, due to the challenges of measuring the large number of species and cultivars and in knowing exactly what to measure and how. These challenges can be aided by the development of a structured overview of the physical characteristics of trees that impact solar radiation reflectivity, absorptivity, and transmissivity. The development of the descriptive framework is based on the concept of tree architecture, which presents itself as an appropriate term for use on physical characteristics of trees that impact solar radiation reflectivity, absorptivity, and transmissivity and for the development of C-TAT.

The framework describes various architectural sub-traits within the overall categories of Crown, Wood, and Foliage, and offers a structured and comprehensive overview of tree architecture impacting reflectivity, absorptivity, and transmissivity of solar radiation (Tab.5). Sub-traits within these overall categories included in the descriptive part of the framework include: 1) Crown Shape, 2) Trunk Morphology, 3) Branch Morphology, 4) Twig Morphology, 5) Leaf Attributes, and 6) Leaf Arrangement. Given the average maximum age of trees in urban areas (and their corresponding optimum effectiveness for shading urban microclimates) the focused growth stage for elaboration of these sub-traits is the adult phase, but general descriptions of young and mature tree growth stages are also included in the descriptive framework for reference purposes. The various sub-traits are elaborated using text, elevation drawings, photographs, and diagrams.

Tab.5 Descriptive framework

Description			Classification
Crown	Growth Attributes		Crown Proportion	● < 1:1● 1:2	● 1:1●> 1:2.5	● 1:1.5
	Crown Shape
Wood	Growth Attributes		Wood Grain and Wood Density	● Coarse Grain–Low Density● Average Grain–Medium Density● Fine Grain–High Density
	Trunk Morphology
	Branch Morphology
	Twig Morphology		Wood Zoning	● Even		● Uneven
Foliage	Leaf Attributes		Foliage Texture and Foliage Luminance	● Coarse–Light● Coarse–Medium● Coarse–Dark		● Fine–Light● Fine–Medium● Fine–Dark
	Leaf Arrangement
	Foliage Phenology

Classes leading out from these sub-traits for use as a key in C-TAT include five Crown Proportion classes (< 1:1, 1:1, 1:1.5, 1:2, > 1:2.5), three Wood Grain and Density classes (Coarse Grain–Low Density, Average Grain–Medium Density, and Fine Grain–High Density), two Wood Zoning classes (Even and Uneven), and six classes combining Foliage Texture and Foliage Luminance (Coarse–Light, Fine–Light, Coarse–Medium, Fine–Medium, Coarse–Dark, and Fine–Dark). These can be used to organize the many species and cultivars occurring in Cfb climate zone to a lesser number of architectural types for more rapid evaluation and cooling performance calculations. It may also be valuable in assisting tree managers to propose more accurate thermal performance standards for trees in urban projects. The elaboration from an urban microclimate perspective complements existing elaborations and approaches in the field of tree architecture.

References

[1] Wilby, R. L. (2007). A review of climate change impacts on the built environment. Built environment, 33(1), 31– 45.

[2] Zhao, L., Oppenheimer, M., Zhu, Q., Baldwin, J. W., Ebi, K. L., Bou-Zeid, E., Guan, K., & Liu, X. (2018). Interactions between urban heat islands and heat waves. Environmental Research Letter, (13), 034003.

[3] Rogers, C. D. W., Gallant, A. J. E., & Tapper, N. J. (2019). Is the urban heat island exacerbated during heatwaves in southern Australian cities?. Theoretical and Applied Climatology, (137), 441– 457.

[4] Masson, V., Marchadier, C., Adolphe, L., Aguejdad, R., Avner, P., Bonhomme, M., Bretagne, G., Briottet, X., Bueno, B., De Munck, C., Doukari, O., Hallegatte, S., Hidalgo, J., Houet, T., Le Bras, J., Lemonsu, A., Long, N., Moine, M.-P., Morel, T., Nolorgues, L., Pigeon, G., Salagnac, J.-L., Viguié, V., & Zibouche, K. (2014). Adapting cities to climate change: A systemic modelling approach. Urban Climate, (10), 407– 429.

[5] Block, A. H., Livesley, S. J., & Williams, N. S. G. (2012). Responding to the Urban Heat Island: A Review of the Potential of Green Infrastructure (pp. 1–62). Victorian Center for Climate Change Adaptation.

[6] Solecki, W. D., Rosenzweig, C., Parshall, L., Pope, G., Clark, M., Cox, J., & Wiencke, M. (2005). Mitigation of the heat island effect in urban New Jersey. Environmental Hazards, 6(1), 39– 49.

[7] Gill, S. E., Handley, J., Ennos, A. R., & Pauleit, S. (2007). Adapting cities for climate change: The role of the green infrastructure. Built Environment, 33(1), 115– 133.

[8] Gaffin, S. R., Rosenzweig, C., & Kong, A. Y. Y. (2012). Adapting to climate change through urban green infrastructure. Nature Climate Change, 2(10), 704.

[9] Miller, R. W., Hauer, R. J., & Werner, L. P. (2015). (p. 4). Waveland Press.

[10] Livesley, S. J., McPherson, E. G., & Calfapietra, C. (2016). The urban forest and ecosystem services: Impacts on urban water, heat, and pollution cycles at the tree, street, and city scale. Journal of Environmental Quality, 45(1), 119– 124.

[11] Norton, B. A., Coutts, A. M., Livesley, S. J., Harris, R. J., Hunter, A. M., & Williams, N. S. G. (2015). Planning for cooler cities: A framework to prioritise green infrastructure to mitigate high temperatures in urban landscapes. Landscape and Urban Planning, (134), 127– 138.

[12] Oke, T. R. (1989). The micrometeorology of the urban forest. Philosophical Transactions of the Royal Society of London, B, 324(1223), 335– 349.

[13] Mayer, H., Kuppe, S., Holst, J., Imbery, F.,& Matzarakis, A. (2009). Human Thermal Comfort Below the Canopy of Street Trees on a Typical Central European Summer Day. 5th Japanese–German Meeting on Urban Climatology(Vol. 18, pp. 211–219). Meteorological Institute, Albert-Ludwigs-University of Freiburg.

[14] Shashua-Bar, L., Pearlmutter, D., & Erell, E. (2011). The influence of trees and grass on outdoor thermal comfort in a hot-arid environment. International Journal of Climatology, 31(10), 1498– 1506.

[15] Helletsgruber, C., Gillner, S., Gulyás, Á., Junker, R. R., Tanács, E., & Hof, A. (2020). Identifying tree traits for cooling urban heat islands—A cross-city empirical analysis. Forests, 11(10), 1064.

[16] Rahman, M. A., Stratopoulos, L. M. F., Moser-Reischl, A., Zölch, T., Häberle, K., Rötzer, T., Pretzsch, H., & Pauleit, S. (2020). Traits of trees for cooling urban heat islands: A meta-analysis. Building and Environment, (170), 106606.

[17] Beck, H. E., Zimmermann, N. E., McVicar, T. R., Vergopolan, N., Berg, A., & Wood, E. F. (2018). Present and future Köppen-Geiger climate classification maps at 1-km resolution. Scientific Data, (5), 1– 12.

[18] von Goethe, J. W. (1790). Attempt to Explain the Metamorphosis of Plants [Versuch die Metamorphose der Pflanzen zu erklären]. Carl Wilhelm Ettinger.

[19] von Humboldt, A., & Bonpland, A. (1790). Ideas for A Geography of Plants Together With a Natural History of the Tropical Countries: Based on Observations and Measurements Made From the 10th Degree North to the 10th Degree South Latitude in the Years 1799, 1800, 1801, 1802 and 1803 [Ideen zu einer Geographie der Pflanzen nebst einem Naturgemälde der Tropenländer: auf Beobachtungen und Messungen gegründet, welche vom 10ten Grade nördlicher bis zum 10ten Grade südlicher Breite, in den Jahren 1799, 1800, 1801, 1802 und 1803 angestellt worden sind] (Vol. 1). F. G. Cotta.

[20] Raunkiaer, C. (1934). The Life Forms of Plants and Statistical Plant Geography, Being the collected papers of C. Raunkiaer (H. Gilbert-Carter, A. Fausbøll, & A. G. Tansley, Trans.). Oxford at the Clarendon Press.

[21] Rose, J. C. (1958). Creative Gardens. Reinhold Publishing Corporation.

[22] Hallé, F., Oldeman, R. A. A., & Tomlinson, P. B. (2012). Tropical Trees and Forests: An Architectural Analysis. Springer Verlag.

[23] Johnston, R., & Lacey, C. (1984). A proposal for the classification of tree-dominated vegetation in Australia. Australian Journal of Botany, 32(5), 529– 549.

[24] Fordyce, I. R., Duff, G. A., & Eamus, D. (1995). The ecophysiology of Allosyncarpia ternata (Myrtaceae) in northern Australia: Tree physiognomy, leaf characteristics and assimilation at contrasting sites. Australian Journal of Botany, 43(4), 367– 377.

[25] Barthélémy, D., & Caraglio, Y. (2007). Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny. Annals of botany, 99(3), 375– 407.

[26] Sanusi, R., Johnstone, D., May, P., & Livesley, S. J. (2017). Microclimate benefits that different street tree species provide to sidewalk pedestrians relate to differences in Plant Area Index. Landscape and Urban Planning, (157), 502– 511.

[27] Rix, M. (2013). The Golden Age of Botanical Art. The University of Chicago Press.

[28] Leonardi, C., & Stagi, F. (2019). The Architecture of Trees. Princeton Architectural Press.

[29] More, D., & White, J. (2013). Illustrated Trees of Britain & Europe (2nd ed.). Bloomsbury.

[30] MacEvoy, B. (2005, August 1). Modern color models—Munsell color system. Color Vision.

[31] Evstigneev, O. I., & Korotkov, V. N. (2016). Ontogenetic stages of trees: An overview. Russian Journal of Ecosystem Ecology, 1(2), 1– 31.

[32] Roman, L. A., & Scatena, F. N. (2011). Street tree survival rates: Meta-analysis of previous studies and application to a field survey in Philadelphia, PA, USA. Urban Forestry & Urban Greening, 10(4), 269– 274.

[33] Krüssmann, G. (1986). Manual of Cultivated Broad-Leaved Trees & Shrubs (Vols. 1–3). Timber Press.

[34] de Koning, J., & van den Broek, J. W. (2012). Dendrology of the Lowlands [Dendrologie van de lage landen]. KNNV Uitgeverij.

[35] San-Miguel-Ayanz, J., De Rigo, D. Caudullo, G., Durrant, T. H., & Mauri, A. (Eds.) (2016). European Atlas of Forest Tree Species. Publications Office of the European Union.

[36] Watson, D. J. (1947). Comparative physiological studies in growth of field crops. Annals of Botany, 11(1), 41– 76.

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