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Open grassland

50 m2 leaf area per node

Forest edge

Forest shade i

May June July August Sept.

C Month

Fig. 2.4.28, Annual progression of A growth in biomass and B changes in storage products starch, amino acids and nitrate in the rhizome of the stinging nettle, Urtica dioica. It is interesting that rhizomes of nettles function in summer as a starch store and in winter as an N store for amino acids. C Schematic of leaf development in Urtica. Height of stems and size of leaves are shown. With growth of stems, new leaves are formed at their tips, and at the same time leaves at the base of the stem die, so the number of leaves per stem remains constant. In shaded areas fewer but larger leaves are formed. In open areas there are more leaves of smaller area. In this way, more stems per unit area can exist in brightly illuminated locations. (Stitt and Schulze 1994)

meander through the soil and new shoots are formed at their tip; eventually, the end of the rhizome dies off. On nutrient-rich sites, rhizomes (more than 12 years old) are formed which are probably only destroyed by burrowing animals. On dry sites taproots with secondary growth in thickness are formed, supplying a whole cohort of shoots with short rhizomes with water. Therefore, it is not surprising that it is very difficult to eradicate a bed of nettles (Urtica dioica) or ground elder (Aegopodium podagra-ria) in a garden. Couch grass (Agropyron repens) is an example of a successful perennial rhizomatous weed in crops.

Perennial herbaceous species are successful where there are continuously poor growing conditions for growth. They occupy shady forest floors, salty sites, dry sites and cold habitats. It is these plants that occur at greatest elevation in Alpine regions (Stellaria decumbens in the Himalayas above 6000 m) and go furthest north in the Arctic (Saxifrage oppositifolia, 83' north). Alio casia macrocarpa is the species in which the lowest light saturation of photosynthesis was measured (80 (.imol m~2 s_1 photons). Some perennial herbaceous species only flower once and require several years to store sufficient resources for flowering. These "monocarpic" species not only include Agava, but also bamboo species and tropical alpine giant rosette plants (Lobelia, Senecio; see Chap. 1.3.6.7).

For herbaceous perennial species there is a maximum size of the vegetative body, when respiration in non-photosynthetically active organs consumes the C gained by leaves. Bananas have to be cut back to maintain the development of flowers and fruits. Old shrubs do not develop flower panicles. Some species cope with this limitation of structure by secondary lignification of the shoot (e.g. sunflower). Secondary thickening is rather rare in perennial herbaceous species, but does occur, for example, in the root of nettles. Many herbaceous temperate species have close relatives in the woody flora of tropical regions.

The fact that a balance of resources between respiring and assimilating biomass occurs in perennial herbaceous species becomes particularly obvious in grasses where maximum production is reached with grazing or mowing. Grasses are predestined for such a use as they are able to regenerate new leaves quickly after herbivory, because of the intercalar growth of the leaf sheath. Grasses of the "old world" are better adapted to grazing than those of the "new world". The typical cluster grass of the American Prairie, the genus Bouteloua, was completely eradicated after introduction of horses, as Bouteloua does not root firmly enough in the soils and the clumps were ripped out by horses.

2.4.5.4 Woody Plants

Woody plants, trees and shrubs, are always perennial. They often flower and fruit only after decades. They differ from herbaceous species particularly by the fact that the growth of the woody body is balanced by the transition of living sapwood into dead heartwood. This does not have to occur synchronously, but is influenced by environmental conditions and species-specific characteristics. In the whole plant the capacity of the xylem determines the leaf area, and the leaf area in turn determines growth and area of sapwood. This was shown in the "pipe model" (Shinozaki et al. 1964), starting from a balance of growth processes in the stem, branches and leaves which is regulated by the conductive area of the xylem. The ring-porous oak only has three to five functioning annual rings in contrast to spruce which has over 30, as spruce wood consists of tracheids with low hydraulic conductance and little medullary ray parenchyma. It is important for the C balance that the respiring woody body remains constant, independent of growth and changes with the leaf mass (see also Chap. 2.2.2.2). Using dead, that is non-respiring biomass (heartwood), as the supporting apparatus, the functional limitation of the size of the vegetative body no longer applies, removing the limitation to growth in plants that do not lignify. However, even in trees there is a functional maximum size. This is achieved in the tallest trees on earth (Eucalyptus marginata, South West Australia; 140 m) and probably the thickest trees (Agatis australis, New Zealand, 7 m diameter, 50 m height; Fig. 2.4.29 A) or in trees with the greatest volume (Sequoia gigantea, California, General Sherman tree, 96 m height, 6 m di ameter at the root, volume 5500 m3). The maximum size at which respiration of non-green parts is in balance with assimilation of green parts is not only achieved in large trees, but also in shrubs. For Calluna, for example, flowering depends on grazing which regulates the ratio of green to non-green biomass. For ungrazed plants the non-green biomass increases, the amount of shoots remains constant and the investment in flowers decreases with age. Similar effects have been shown for fruiting trees (apple), which need to be "cut back" in order to increase the yield of fruits.

Storage of carbohydrates and nitrogen-containing substances is also very important in trees. Leaf emergence and early tree ring growth occur from the mobilisation of amino acids and carbohydrates from the woody parenchyma. In spruce, N concentration in the sap water increases to > 5 mM in spring. Bottle trees (e.g. Adansonia digitata; separating pages to Chap. 1.6) do not store water, but carbohydrates and amino acids for dry periods (Schulze et al. 1998). In vegetation which regenerates by fire (Mediterranean vegetation), there are so-called seeders, fruits of which open only with fire allowing seeds to fall into the fresh ash and germinate. These species do not have storage parenchyma in roots and the starch concentration in the root is small (1.9±0.5 mg starch per g dry weight of root). This contrasts to so-called sprouters which emerge after the fire from the root (Fig. 2.4.29 B); these have 14.1 g starch per dry weight root (Bell et al. 1996). Obviously, in cold winter climates, most N is stored (as the soil is still cold at the time of emergence), whilst in fire climates predominantly carbohydrates are stored, as the ash contains sufficient nutrients for the new vegetation.

A particularly impressive C economy exists in trees which become very old; Pinus aristata is the longest living tree species (Fig. 2.4.29 C, D; disregarding species with runners) with more than 4000 years. This species occurs in the high Alpine dry regions of Nevada. In this climate, wood is only slowly degraded and conditions allow only very slow growth. Most of its cambium dies within a 1000 years. Only a small strip of the living bast remains, keeping the ratio of green biomass to respiring biomass almost constant. Trees grow like a board, 1-2 m wide and only 10-15 cm thick. Without external interference (lightning, storms) these trees may potentially live forever (LaMarche et al. 1984; Fritts et al. 1991).

| Fig. 2.4.29. A Barrel-like trees: Agatis australis, Kauri, in the subtropical forest of New Zealand's North Island. The base of the stem is branchless and may achieve 45 m in height with 6-7 m diameter. Dimensions become clear with Waltraud Schulze climbing a liana. B Eucalyptus pauciflora as an example of sprouting plants, i.e. tree species which re-generate from hypocotyl buds after fire. C Pinus arsitata in the Snake Mountains of Nevada at 3500 m above sea level; an example of extreme longevity. Trunks may be more than 4000 years old, resulting from death of the cambium which, except for a narrow strip, grows in balance with the resource supply from the shoot. Thus an equilibrium is achieved between assimilation and respiration and between shoot and root. The trunk grows only in width on the side with the living cambium, i.e. the trunk develops in the shape of a plank over a long period. A trunk in broad aspect with E.-D. Schulze and D side aspect, with Valmore LaMarche in the picture

E Aerial roots of Ficus, growing through the canopy of a rain forest in Costa Rica. F After reaching the ground with its aerial root and thus securing water, Ficus enmeshes the host with a root net, Lamington, Queensland, and thus kills the host. G Ficus superba with stilt roots which grow from the canopy to the soil, supporting the canopy and supplying it with water and nutrients, Yakushima, Japan. H Central hole in the trunk of a Ficus, showing the position of the original host. (Photos E.-D. Schulze)

Amongst the many life-forms termed "tree" (Vareschi 1980) are figs, which are masters in carbohydrate economy They germinate as epiphytes in the canopy of the rain forest and grow with a large aerial root through the canopy (Fig. 2.4.29 C,F). As soon as the roots have reached the soil and thus water, they enclose the host with a mesh of roots. The host dies as its phloem transport is interrupted by strangulation from the fig, which then uses nutrients released from the decomposing wood of the host. In the end only a chimney-like structure remains of the host (Fig. 2.4.29 H). The fig then increases in size by forming new aereal stilt-like supporting roots and conquers a new habitat for the tree. Ficus benegalensis (Fig. 2.4.29 G) produces many hundreds of stilt roots covering an area of > 20 ha, under which "an army of 20,000 men could camp in the shade", according to Warburg (1913). Similar areas are covered by Populus deltoides, with underground runners, in the southern boreal forests of Canada.

Trees and shrubs are distinguished from each other by height (greater or less than 2 m, respectively, according to Ellenberg (1978), or greater or less than 5 m according to the FAO (2000), and are functionally distinguished by the different arrangement of regenerating buds (Fig 2.4.30). In trees there is an apical dominance, also called acrotony (e.g. in spruce) in which the terminal bud remains. The term amphitony is used if the terminal bud remains, but only one of the lateral buds develops (Fagus). In some trees the terminal bud dies off and the next bud at the upper side of twigs takes the lead, which is called hypotony, e.g. in Acer or Thilia. Some deciduous trees have acrotonic growth in the juvenile period and hypotonous growth when older (Acer pseudoplatanus). In contrast, shrubs have renewal buds predominantly in the lower region of branches (hazel, basitonic dominance) or on the underside of branches, also called epitonic growth (e.g. Rosa, Rubus). Obviously, there are many transitions between these extremes, but it is usually possible to determine the tree form from the dominance of bud development along the main axis of the stem and along the branches. Thus, true trees are monopodial (one stem) while shrubs and tree-like shrubs are mostly sympodial (many stems). These different types of growth determine competition and succession in hedgerows (Küppers 1989).

In woody plants the shoot-root communication depends on the species and is tuned so that shoot and root growth are tightly correlated (Fig. 2.4.31; Heilmeier et al. 1997) - even if, under extreme stress conditions, very different tree sizes are reached. For the almond, Prunus dulcis, there is tight correlation between root and shoot biomass as well as between leaf area and length of fine roots. Such regulation is also observed in basitonic shrubs. Evenari et al. (1982) called the phenomenon of a root-shoot regulation "survival through dieback" which is common in arid regions. In a dry year part of the shrub dies back and only a few shoots survive which are in balance with the root.

In addition to the regulation of the respiring mass of their woody body, trees have another means of regulation, the life span of assimila-tory organs. Distinction is made between:

• deciduous species (trees of temperate forests and rain-green savannahs) with seasonal changes in leaves;

• semi-evergreen species with a life span of as-similatory organs of about 12-14 months, when the old leaves are shed with the emergence of new leaves (Eucalyptus and many tropical species);

• evergreen species with a life span of their assimilation organs of up to 35 years (Pinus aristata, California).

Deciduous and evergreen assimilatory organs show functional differences:

• Evergreen species have a larger dry weight per area because they usually are forced to survive very unfavourable conditions (cold, drought).

• A smaller rate of C02 assimilation in evergreen species than in deciduous species correlates with the larger dry weight per area. Short-lived leaves have a greater C02 gain per time than long-lived leaves, which experience so-called "diminishing returns" (smaller returns per unit of investment with time). This investment should be compared with a possible maximisation of physiological activity with lower investment in protecting cell walls.

• Evergreen species are able to use intermittently favourable weather conditions to assimilate (spruce is able to assimilate C02 on warm days during the European winter). Assimilation starts earlier in spring than for deciduous species and lasts longer in autumn. For deciduous species the growing season is often limited by early or late frosts.

Longitudinal symmetry i

Growth form Î

Lateral symmetry

Basitonic

Acrotonic Sympodial -► Monopodial

Shrub

Tree-Shrub

Shrub-Tree

Tree

Epitonic Hypotonic Amphitonic -

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