Certain plant families are characteristically more scented than others. Of course, the range of scents is much greater when flowers or fruits are included; but some families are notable for being scented in stems and leaves, such as the Annonaceae (custard apples), Lauraceae (laurels and cinnamon), Myristicaceae (nutmegs) and Piperaceae (black pepper family). Among the monocots, the Zingiberaceae (gingers) are particularly notable for their scented leaves and stems. Although most grasses are unscented, a few are widely cultivated for their scent, such as lemon grass relatives and patchouli. In
Plant characters suitable for field guides 137
BOX 6.8 SMELLS AND TASTES
Smell and taste are related senses. Some elements of taste are detected primarily in the mouth; but most of the subtleties of taste – for instance, in food – are derived from the aroma or smell (thus, the sense of taste declines if the nose is blocked). There are also certain other mouth-related sensations, such as the cooling effect of peppermint oil and the heat of chilli pepper, which are not strictly taste, but are usually discussed with it.
Some plants are poisonous, so the tasting of unknown plants is not to be recommended wholeheartedly. On the other hand, poisons generally taste very bad – if not immediately, then as an aftertaste. When tasting bark or leaves, first nibble the tiniest amount possible with your front teeth and always spit fragments out after a few seconds.
Never swallow after tasting plant parts, and have a bottle of water to wash your mouth out if you are left with a bad aftertaste. The author, who has tasted most Ghanaian forest barks and leaves, nevertheless disclaims any liability should you die after eating any plant parts!
There are some poisonous or allergy-causing groups that should certainly be kept well away from your skin and mouth, including the Anacardiaceae, Euphorbiaceae (especially those with hairs or latex) and Urticaceae. For leaves, it is advised that you only consider a small taste once you have ascertained that there are no other field characters which can distinguish it, and there is no bad reaction from handling the crushed leaves.
Our noses can distinguish more than 10,000 different smells, and it is thought that there are hundreds of different olfactory receptors, each recognizing different odours. For example, the distinctive smell of bananas is due to isoamyl acetate, and the primary smell of an orange comes from octyl acetate. These chemicals account for the basic ingredients of a smell; but plants blend such molecules to different degrees, creating endless possibilities. Some people are ‘blind’ to particular chemicals. Although you should not primarily rely on smells in a field guide for this and other reasons, smells are a highly useful source of information to support plant identification (see Worms, 1942; Moskowitz, 1975; Classen et al, 1994; Civille and Lyon, 1996; de Garine, 1997; Bubondt, 1998; Casagrande, 2000).
addition to these, scents are common in some of the families with translucent dots, or oil sacs, in their leaves, such as the Rutaceae (citrusy in all parts) and Myrtaceae (including, and often scented like, cloves; the bark usually does not smell strongly, though). There are a few plants with scents so unusual that they deserve special mention in your field guide, such as the foliage and bark of Afrostyrax lepidophyllus (Huaceae) and Cordia alliodora (Boraginaceae), and the bark of Cedrela odorata (Meliaceae), all of which emit a fetid smell like onions or garlic; or Chione and Chiococca (Rubiaceae) and most Montiniaceae, with peculiar ammonia or floor polish-type scents:
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Always smell the bark slash. Try to develop your powers of description, like a wine expert. Always ask others from your user group what they think a smell is like.
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Smells are useful for identifying many fungi – for instance, agarics produce a wide range of subtle fragrances, from aniseed to almonds. Your challenge will be to describe them to your audience using words for smells that they can relate to.
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As any universal smell vocabulary is not very subtle, the potential to make the use of scent clues is almost lost. A standard reference set of chemicals in small bottles, with common bark, essential oil and other smells (‘vic’, ‘cedar’, ‘onions’, ‘eau de cologne’,
‘lily of the valley’, ‘mushrooms’, ‘citrus’, etc.) would be a good start for helping
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technical guide writers and users to express their experiences, and would probably not challenge the skills of modern odour chemists or a budget unduly. Such smell sets might even enhance the saleability of field guides and interest in eco-tourism.
CHARACTERS OF WHOLE PLANTS OR PLANT POPULATIONS
Habit or life form
Habit is the word used to describe the general appearance of a plant – for example, whether it is a tree, herb, shrub or climber. Although this is obviously a very significant field character, it is difficult to apply the terms precisely. There is no agreement about their definitions; gardeners may refer to a shrub as a low (say < 5m) and many-stemmed plant; but this is the definition others prefer to reserve for bush. The American Society of Foresters (http://soilslab.cfr.washington.edu/S-7/EcolGlos.html) defines a shrub as:
… a woody perennial plant differing from a perennial herb in its persistent and woody stem, and less definitely from a tree in its lower stature and the general absence of a well-defined main stem.
However, the society does not then define tree. Although the first decision in Keller’s (1996) key to tropical families is ‘tree or shrub’, as against ‘liana or weakly prostrate plant’, these terms are not defined in that book, and there is a considerable grey area from shrub to ‘rambling shrub’ to liane. Habit categories are very vague, and we advise users to define their use of terms carefully, perhaps choosing an arbitrary cut-off, possibly in height or diameter, between shrub and tree (see Box 6.9).
Raunkiaer (1934) defined life forms according to how and where the growing points of plants are protected. However, these are not very suitable categories for rainforest, nor do the words roll smoothly off the tongue – for instance, trees > 30m tall are
‘megaphanerophytes’. The plain English is shorter and clearer:
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Look for links between functional or morphological groups and habitat preference.
Any links you find might help you to choose appropriate divisions in your keys – for example, ‘Leaves < 1cm wide; plant always along rivers.’
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Apart from the extreme differences (such as bromeliad-, grass-, palm- or cactus-like plants), do not expect an easy and clean division of rainforest plants by habit unless you have a limited number of species and they cover diverse habits. However, stem texture (woody, herbaceous and succulent) is a useful trait to concentrate on.
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Maximum recorded size, or minimum size of fertile plants, can be expressed as height or DBH (diameter at breast height, taken as 1.3m above the ground or above buttresses) recorded for fertile herbarium specimens of larger trees and lianes.
Include other descriptive adjectives – for example, ‘liane to 15cm DBH, capable of looping between ground and 30m canopy several times over’; or ‘small spreading tree to 20cm DBH, often with straggling, half-climbing branches’.
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For a field guide to trees, define at the outset a minimum diameter for trees to be included – perhaps 5cm or 30cm DBH – which can be easily checked as fieldwork progresses. Then perhaps also include (with a note to this effect) species that have only been found just below this diameter, say 4cm or 28cm DBH, as one may expect
Plant characters suitable for field guides 139
BOX 6.9 HABIT CATEGORIES IN THE WOODY PLANTS OF
WESTERN AFRICAN FORESTS
The following quote from the Introduction to Hawthorne and Jongkind’s (2006) field guide to Western African woody plants summarizes the problems faced by the authors, where the 2130 species include a similar proportion of trees, climbers, shrubs and herbs: We use the terms liane to mean large woody climber (as distinct from barely woody or herbaceo
us small twiners, such as the garden bean). We do not distinguish very precisely the terms tree, treelet, shrub and shrublet, but use the terms descriptively as follows.
A tree is an erect woody plant with a well-defined main bole or trunk. A treelet is a small type of tree that is always in the lower layers of the forest; if pressed, we could suggest an upper limit of 5cm trunk diameter for treelets; but we have not been able to apply this definition rigorously. Shrub is used variously for other, smaller woody plants that are not well described by the previous terms for various reasons: either they show little tendency to have a single straight main stem; or some of their branches sprawl around in a half-climbing way, but other branches behave as a small tree; or because they are too small even to be a treelet, perhaps never exceeding 2m in height [other habit terms are discussed similarly]… We initially struggled with the proper application of these terms; but there is so much scope for error, lack of accurate records, and so many species change their habit depending on growing conditions that we have ended up using the terms in an explicitly vague way. Note that in all cases the terms refer to the typical plant when mature, so immature trees are referred to as saplings rather than shrubs or treelets.
Source: Hawthorne and Jongkind (2006)
the larger sizes to be discovered eventually. Users will be more likely frustrated if you miss out species that should be included than if you include a few too many. It is useful to check forestry organizations for inventory data, at least to see how many common species are known in various size classes.
Architecture and crown form
The overall form of a whole plant depends, to some extent, on their architectural ‘rules’
or branching pattern, but also on other factors. Hallé (1971, 1995), Hallé et al (1978) and Keller (1994, 1996) have approached the issue of tree architecture formally.
However, although the extreme types of architecture, such as ‘Corner’s model’ can be distinguished easily, these characters are, in general, hard to assess, more of academic than practical interest, and often involve analysis of where flowers develop. For the easily observed architectural patterns, it is less confusing to use in a field guide terms such as ‘ Terminalia-type branching’, or ‘un-branched, papaya-like habit’ rather than Aubréville’s model or Corner’s model, respectively. Similarly, related characters of the buds and leaf arrangement can usually be specified and used more easily (Cullen, 1978; Keller, 1994).
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Source: William Hawthorne
Figure 6.3 Distinctive forms of two species of Bombacaceae in West Africa (left: Rhodognaphalon brevicuspe ; right: Ceiba pentandra ) It is often possible to see in a distant crown, either across fields or in the canopy above, distinctive shape, pattern and textural effects; but these are hard to put into words. One solution is to photograph tree silhouettes and convert them to high-contrast black-and-white photographs (see Figure 6.3; Figure 6.4 shows some of the variety that can be captured in silhouettes). But, apart from a few species with unusual crowns, most species are not distinctive enough to allow identification by this method in a practical guide (see http://herbaria.plants.ox.ac.uk/VFH).
The architecture of lianes (Putz and Mooney, 1991) tends to go unnoticed, intermin-gled as they often are among tree crowns. Hawthorne and Jongkind (2006) review liane architectural terms useful for West Africa field use, and these apply elsewhere in the tropics. Little-branched or small trees or shrubs may be distinctive looking; but their distinctiveness is generally easily describable with common language, perhaps helped with a few key architectural adjectives.
Older woody stems and wood anatomy
Rosayro (1953) and Empire Forestry Association (1953, 1957) summarized some of the characters of tree stems; but there are many more to be found in numerous specific field guides to tropical trees. Wood anatomy is rarely of any use in a tree field guide, unless perhaps the guide is aimed at loggers where there is a close working relationship
Plant characters suitable for field guides 141
Source: Hawthorne and Jongkind (2006)
Figure 6.4 A minority of species have crowns that can be illustrated with silhouettes
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between fieldworkers and the sawmill. However, consideration of liane wood anatomy can be useful, firstly, because some of the patterns are extremely distinctive, even without a lens, often with repercussions visible from the profile of uncut stems, and also because cut lianes are more likely to be available than cut tree stems. Some foresters spend a lot of time cutting down lianes.
Although lianes frequently have very distinctive wood, there is not much published other than the works of Caballé (1993) and Carlquist (1991), and some publications for specific plant groups (Villiers, 1973; Vliet, 1979). For Celastraceae, a globally important liane family and one of the most interesting for unusual wood, Hallé (1962) and Mennega (1988) have remarked on the great taxonomic and diversity value of wood structure. Hall and Lock (1976) discuss Salacia stems in this family in Ghana, recognizing four main types (normal, annular, lacunose and sulcate). More recently, Hawthorne and Jongkind (2006) have summarized what is known for all liane families in West Africa. See Plate 4 for a sample of the spectrum. Some sample terms and definitions for liane wood are summarized in the same book.
Many of the extremes of liane wood pattern are manifested in unusual stem shapes (such as the ribbon-shaped stems of various Bauhinia, Millettia and Dalbergia); so even if you do not consider wood anatomy useful, it is worth considering its external manifestation forms as a ‘stem shape’ character.
Bole and liane stem shape and tree bases
It is generally easy to distinguish irregularities on the surface of the tree due to a combination of patterns in the underlying wood and more superficial patterns within the bark.
The tree trunk or bole may be cylindrical or variously distorted by fluting, sinews, knee-like bumps or knobs and various more bizarre conditions. In some cases, the spines or prickles of the younger stems are carried on the main bole, as well. See Plate 3 for the main types of bole and base, and Plate 4 for some of the range of stem forms found in West African tree boles and bases:
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Sometimes the utter lack of irregularities or special characters is as striking as their presence, and this applies when the main trunk of large trees is completely cylindrical to the ground. In Africa, these strikingly cylindrical-when-huge boles occur in genera from many families (for example, Daniellia, Gilbertiodendron, Petersianthus and Hannoa).
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Small buttresses or other outgrowths around the base of trees are age dependent, so buttressing can be unreliable for identification unless the guide is meant to work reliably only for very large trees. However, they can be very distinctive, and should be recorded in plant descriptions and photographs even if not in keys.
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In a general field guide, consider making one key for all species and another, perhaps, for very large trees (see, for example, Hawthorne, 1990). In the latter case, detailed descriptions of buttresses are almost indispensable; but in the former they are useless if the guide has to work for small trees.
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For buttress details, note whether the top edge of the buttress is straight or curved, concave or convex. Buttress height and width obviously increase with age; but their ratio (extent from tree at ground level/height reaching up bole) is more constant, distinguishing tall and narrow from low and spreading forms. It is particularly useful to estimate the thickness of a buttress, halfway between the bole and the
Plant characters suitable for field guides 143
buttress edge: ‘plank buttresses’ are thin, not much thicker than floor boards; but there is no common term for thick buttresses. You may be able to define a very clean-cut division between species with plank, or normal and thick, or no buttresses in your area.
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The more subtle bole irregularities can be very hard to photograph – the best w
ay of describing them – as it is important to illuminate the bole from the side in order to create shadows. This can be achieved by selecting a time of day when the sun is appropriately positioned relative to the camera, or more conveniently by obtaining a
‘remote flash’ or a very long flash cable, positioning it with the help of an assistant to shine across the tree and downwards. Some degree of trial and error is generally needed, so a digital camera with a preview function is very useful.
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Be alert for trees on the forest edge where the buttresses are clearly visible and not obscured by others, and for lone trees on recently cleared hill tops where crown form is obvious. Take their photographs at any opportunity.
Roots
Roots are rarely of much interest in a field guide, except where they are found above ground level, as in swamps and on epiphytes, where stilt, strangling, breathing and other unusual aerial roots are typical (see Figure 6.5):
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Look around your trees for any surface or breathing roots that are partly exposed in the soil. These are diagnostic for some species.
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In a few species with medicinal bark or wood, the roots are collected since the tastes, smell and presumably chemical content are often stronger there. If your plant is supposed to be scented or medicinal tasting, but is not convincingly so in the main stem, dig a root from the foot of the tree and encourage your guide’s users to do so.
Bark
Bark is the protective outer coating of old stems and roots characteristic of shrubs, trees, and lianes (‘woody dicots’); it is for these that bark is a most useful field character (Foxworthy, 1927, 1932; Wood, 1952; Chattaway, 1953; van Wyk, 1985). Bark should be described from two points of view: by external appearance and by slash (= blaze), which is the appearance of the oblique section of the bark soon after being cut. Whitmore (1962a, 1962b, 1962c, 1963) classified bark primarily by external appearances, or manifestations, subdivided by the internal anatomical structure that gives rise to that pattern, defining seven major bark types for 29 manifestations of dipterocarp bark. A particular manifestation of bark – such as roughness – is associated with a limited number of anatomical or slash patterns. However, this approach involves a heavy burden of jargon and microscopic detail inappropriate for most field guides, even technical ones.
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