Diversity and Life Forms

Diversity is also given by the variability of life-forms. The life-form concept is defined rather loosely. Many attempts have been made to categorize the diversity of plant types by distinction of life forms which are morphologically more or less conspicuous. Life forms represent morphological adaptations to environments or towards a given stress factor or set of stress factors. An original example illustrating an approach to such a classification is Raunkiaer's crisp distinction of five life forms of vascular plants according to the life time of shoots and the position and protection of regenerating buds, namely:

• the phanerophytes with regenerative buds higher than 50 cm above ground,

• the chamaephytes with buds closer to the ground (10 - 50 cm above ground),

• the hemicryptophytes having a close contact of the buds with the ground,

• the cryptophytes with below ground regenerative organs (rhizomes, onions, bulbs, storage roots etc.),

• the therophytes or annuals.

These life forms are particularly conceived for analysis of vegetation in mesic and temperate climates as they describe strategies for over-wintering. However, they may also be used to describe adaptation to regular seasonal drought periods and, thus, are useful with respect to the tropical environments. Moreover, of course, the distinction between trees (macrophanerophytes), shrubs or bushes (microphanero-phytes) and herbs of different forms inherent in these definitions is always applicable. However, this scheme is hardly sufficient to come to grasp of diversity in a tropical forest.

Vareschi (1980) has noted that as a basis for schemes defining different life forms, morphological modifications of any of the major plant organs could be chosen, e.g. life forms based on roots, shoots, leaves, flowers or propagules. Another approach would be to derive life forms which use morphological modifications at the whole plant level with distinct mechanistic relations. Both shall be illustrated here by giving a few specific examples.

3.3.4.1 Root Categories

Root categories of trees frequent in tropical forests are stilt roots and buttress roots (Fig. 3.21). It has been debated whether such roots have mainly mechanical functions or serve aeration and O2-supply to below ground root tissues. Presumably both are important. Buttress roots, in particular, may function like ropes with effective anchoring of the trunk to the ground (Mattheck 1992) and it was shown experimentally that they indeed have a clear anchorage function (Crook et al. 1997). In addition, the increase in above-ground root surface brought about by these root types may facilitate aeration. In wet tropical soils, where gas diffusion is limited and where vigorous soil-respiration will lower O2-concentration, this may be a particularly important aspect. Indeed, immediately below ground the buttress roots show much branching and produce many fine absorptive roots, which can be supplied with O2 via pores in the bark of the above ground buttress (Fig. 3.21E,F). Stilt roots also are often covered with lenticels facilitating gas exchange with the atmosphere (Fig. 3.21G,H).

3.3.4.2 Leaf Categories

The most variable plant organ in form is the leaf. Vareschi's (1980) leaf analyses of plants in the cloud forest of Rancho Grande in Venezuela reveal more than 300 forms, most of which are reproduced in Fig. 3.22 for the sake of their graphic attractiveness. Raunkiaer's size classes of leaves offer a more systematic approach, where leaves are distinguished by their area:

megaphyll

>

1 , 500 cm2

macrophyll

1,500 -

180 cm2

mesophyll

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Renewable Energy 101

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  • faye wright
    What is Phanerophytes life form of tropical plant?
    2 years ago

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