Trees are essential for human life on Earth. they capture energy from the sun and convert it into seeds, fruits, leaves, and roots, which we use directly or indirectly in so many ways around the globe. Trees provide many – so-called ecosystem services – such as air pollutant removal, timber, carbon storage and sequestration, urban heat island reduction, stormwater runoff reduction as well as other socio-economic benefits. Furthermore, trees are central to many ecological processes and to the protection of biodiversity – especially in the Alpine Space, where trees are fundamental to the landscape, forests, woodlands, and urban green infrastructure. Over the course of human history, some trees have been found more attractive and useful than others, which resulted in the introduction of new tree species in an area outside of their natural range. Some tree seeds, also, followed and still follow us unintentionally along with trade and travel routes to new areas. As you know from the previous chapter, we call those tree species – outside of their native areas – non-native trees.
The distinction between non-native and native is important because some non-native tree species are considered invasive and can alter the ecosystem of the area they grow in. Observing nature and being able to identify potentially invasive species can help to better evaluate the dissemination of non-native species and their invasiveness in forests and cities. In general, we believe it is worth the effort to know the characteristics of the trees that surround us, in order to develop a better understanding of nature. In this chapter we will not teach you, how to recognize every tree species in the Alpine Space – that would take very long, considering that we identified 135 native tree species and over 500 non-native tree species in the Alpine space. But this chapter will provide you with the basics and first tricks you need to identify tree species and hopefully also enable you to increase your knowledge on tree diversity day by day – tree by the tree!
The definition of a tree
For someone, a banana, a bamboo, a tree fern, or a large palm tree are trees because they are perennial and large plants with a trunk, branches, and leaves (needles). However, experts such as botanists have a stricter definition for a tree: a tree must reach a certain size, produce woody tissue, and show thickness growth. The latter is also called secondary growth and is based on the activity of the cambium, a formation tissue in the shoot and roots. The cells formed by the cambium are discharged both outwardly and inwardly. Wood is formed from the cells discharged inward and the bast, also called the bark, is formed from the cells discharged outward. From the xylem cells formed (i.e. wood cells), one can read the tree age, as they form the typical annual rings. This is not the case with bamboo or palm trees.
Trees belong to the group of woody plants, which include trees, shrubs, and vines of different plant families. All woody species have at least one characteristic in common: a woody stem that is perennial or that lives for many years. Usually, a bark forms the outer covering on the trunk, twigs, and woody roots. The outer bark we are familiar with is a layer of dead corky cells protecting the rest of the stem. The inner bark, or phloem, is a live spongy layer, that moves sugars and other substances from the leaves to the stem, roots, and other places where they are needed.
A tree is generally understood to be a woody plant consisting of a root, a tall trunk rising from it, and a leafy crown. The branches divide in the crown or branch into deciduous or needle-bearing branches on the trunk. Trees are very developed plants and are among the largest organisms. They differ from other plants primarily in their strong growth in height, with which the assimilating leaf mass is raised above the layers to compete for light. Above all, the outstanding static properties of wood and the associated ability of trees to raise their crown far above all other plants have made trees the most competitive life form among plants.
Tricks, tools & Apps for the identification of tree species
About 250 years ago a Swedish mathematician, Carl Linnaeus (also known as “Carl von Linné” or the “father of modern taxonomy”) came up with a smart system to classify plants and animals using a binomial nomenclature, which names each tree species in by giving each a name composed of two parts: the generic name – identifies the genus to which the species belongs, and – the specific name – that helps to distinguish the species within the genus. For example, the Tatarian maple, a non-native tree species from Eastern Europe and Western Asia, belongs to the genus Acer and within this genus to the species Acer tataricum. Until today this system provides us with stability when working with species, that we can name in a unique and international name. However, thanks to new technologies and huge research efforts, the taxonomy has been constantly improved, which leads to reordering and name changes. But to learn the binomial name, which can be also called the scientific name or Latin name, of the tree species you want to identify, will help you to see the common characteristics and differences within species, genera, and families.
Another tool that also seems to be more complicated at the first sight than it really is, is the dichotomous key. Ecologists came up with ways to identify species according to their morphological and anatomical characteristics, a long time before Apps and Identification algorithms were developed. The dichotomous key gives you two choices in each step, which present two opposite statements describing a detail of the species’ form, organs position, flower, fruit, bark, arrangement of leaves, towing, etc. For example, to identify a tree, first decide whether it is a conifer or broadleaf? (A) Conifers have needlelike or scale-like leaves and usually bear seeds inside woody cones, or (B) Broadleaves usually have wide, flat leaves and bear their seeds inside of soft fruits. Very often a dichotomous key for trees starts with basic binary differentiations such as the following examples:
Evergreen vs. Deciduous: An evergreen tree does not lose all leaves every year but rather keeps the leaves all year long and replaces the leaves by dropping only the injured or old ones. In opposite, a deciduous tree loses all leaves every year. The process of leaf drop, which is called “abscission” involves complex physiological processes.
Broadleaf vs. Conifer: Broadleaf trees describe a class of trees that produce seeds inside of fruits, and have broad, flat leaves in many different shapes. In the forestry sector, this group of trees is often referred to as “hardwood”. While broadleaves belong to the taxonomic clade of Angiospermae (also called “angiosperms” or simply “flowering plants”), conifers belong to the taxonomic subset of Gymnospermae (also called “gymnosperms” or ” cone-bearing seed plants”). Conifers are mostly evergreen and have a needle or scalelike foliage and conelike fruits.
Perhaps the use of a dichotomous key, which follows the description of anatomical and morphological criteria of a scientific diagnosis process is not the most comfortable tool for beginners. Illustrated identification handbook, which is available in most bookshops, presents the trees in groups following simplified botanical criteria. Be aware, that illustrated identification handbook and can be an excellent start to identifying the most common tree species, but you might miss identifying fewer common trees, which is often the case when you want to identify non-native trees in the Alpine Space. Besides illustrated identification handbooks, many Apps were developed, to assist you in the tree identification process. The most common Android and iOS plant recognition apps, such as PlantSnap, iNaturalist, or Flora Incognita, are suggesting an identification based on a certain probability, that the picture you take of a plant matches with a growing database. The use of Apps empowers you to easily explore and identify tree species, but be aware, that any identification tool can sometimes lead to a misidentification. If you have a chance to interact with other explorers on your observations and identification of tree species, take the opportunity to learn from each other.
Once you have trained your tree identifier skills, you will recognize that not all tree species grow everywhere. In fact most trees can be linked to certain habitats. A habitat describes an area or environmental condition in which an organism or population normally occurs, grows, and reproduces well. Physical factors such as soil moisture, elevation, temperature, and light availability as well as biotic factors shape the conditions of habitat. By knowing the habitat preferences of a tree species it is possible to estimate whether it is likely that the tree species you identified can occur here. Non-native tree species, which occur outside of their natural geographical range, often also follow certain habitat preferences, which may differ from the habitats in their native range. The invasive tree species Tree of heaven ( Ailanthus altissima), for example, requires in its native range in East Asia optimal growing conditions of deep and fresh soils, but outside its native range, it is commonly growing on moderately rocky or sandy soils and tolerates long-term drought and city pollution, but avoids humid and compact soils.
In order to understand the ecology of a tree and to be successful in its identification, it is helpful to remember some simple but essential elements:
The tree is divided into four parts: roots, trunk, branches, and leaves.
Roots have two essential functions: to fix the tree to the soil and to nourish it with water and mineral elements. The fine roots have small hair formed by thin and elongated cells near their ends, which allow them to extract water and nutrients from the soil. This explains why trees that are transplanted with well-branched roots contained in a ‘root ball’ are more likely to succeed, and that those that have been badly damaged will find it difficult to grow again in the new planting area. Depending on whether the roots advance deep into the soil or extend more horizontally just below the surface, a distinction is made:
- Taproot system: a vertically growing, thickened the main root dominates (oaks, pines, firs, elms).
- Heart-root system: several roots of varying strength are formed on the rootstock (beeches, birches, hornbeams, larches, lime trees).
- Horizontal root system: The main root is only present in young trees. There are several horizontally growing lateral roots (poplars).
- Sinker root system: in which sinker roots grow vertically into the ground from strong horizontal roots (ash, spruce in the age phase).
The main stem or trunk supports the branches, and the vessels of its tissues allow the circulation of the sap between them and the roots. The trunk consists of five main parts: the bark, inner bark, cambium, sapwood, and heartwood. Most of the trunk is formed by dead tissues that have mainly a stabilizing role for the living tissues, through which the sap circulates under the bark that protects them. It is easy to understand why bark stripping or wounds to the bark can cause serious damage and even death of the tree. Essential for the identification of a tree species are differences in the color, texture, and shape of the bark as well as the height or dimension of the trunk.
Branches and Twigs
Branches are the prolongation and extension of the trunk, which are generally arranged so that all the leaves can receive light. Their internal structure is similar to that of the trunk. In urban areas, it is advisable to remove dead or deteriorated branches that appear at the top of the tree for plant health and safety reasons. The branching pattern – opposite or alternate – is a reliable indicator for the identification of tree species. Further, the pattern of buds, leaf scars lenticels or pith can help to identify a tree species.
Leaves are considered the engine of a tree, as these organs are specialized for photosynthesis: they receive water enriched with nutrients that arrive through the root and trunk and convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism’s activities. The diversity of leaves is enormous given the endless combination of leave types, compounds, patterns, shapes, margins, textures, colors, and venation. To begin with, you can differentiate between the following leave types: (1) a simple leaf (e.g. Alnus cordata), with one leaf per bud, (2) a pinnately compound leaf (e.g. Ailanthus altissima), where several smaller leaves are arranged along one main stem, and (3) a palmately compound leaf (e.g. Aesculus x carnea), with many leaves per stem radiating out.
IUCN/SSC Global Tree Specialist Group
The IUCN/SSC Global Tree Specialist Group (GTSG) is a global network of experts who, working in their own regions and institutions, all contribute to the conservation of globally threatened trees. More information is availabe at: https://globaltrees.org/iucn-ssc-global-tree-specialist-group/
Global Tree Assessment
he Global Tree Assessment (GTA) is assessing the conservation status of every known tree species. More information is availabe at: https://www.bgci.org/our-work/projects-and-case-studies/global-tree-assessment/
Flora Helvetica – Illustrated Flora of Switzerland with species descriptions and distribution maps of 3200 wild growing ferns and flowering plants, including important cultivated plants [in French and German]. Avilable at: https://flora-helvetica.ch/index
Exkursionsflora für Österreich, Liechtenstein, Südtirol
The Excursion Flora is currently the only scientific identification book for all wild-growing (including the impermanent / ephemerophytes) and frequently cultivated agricultural, forestry and horticultural ferns and seed plants (flowering plants) in Austria, Liechtenstein and South Tyrol. The work was edited by three members of the Verein zur Erforschung der Flora Österreichs. It is available in the third edition of 2008 and can be ordered online via the website www.xflora.info.
ALPTREES Handbook for the Identification of NNT in the Alpine Space