Seasonal changes and aspect societies

Phenology is the study of the onset and duration of the activity phases of animals and plants throughout the year (Sections 4.6.1, 11.3.2). These are largely synchronized to the weather so the dates on which they occur differ from year to year. Though the order in which the various plant species unfold their buds, flower, fruit and senesce is much the same; different individuals of the same tree species frequently commence leafing out (flushing) earlier or later than their fellows. This influences, amongst other things, their liability to defoliation by insects (Section 5.3). However, as described in Section 1.4.1, this sequential activity of different species leads to the formation of aspect societies where the plant community is dominated by one or several species at a particular season and whose appearance changes markedly over just a few weeks in spring.

Although the total number of species is much greater, the broad pheno-logical groups of the damp oak-hornbeam woodlands of central Europe (Ellenberg, 1988) correspond well with the periods of active vegetative growth and flowering of those in an oak-hornbeam wood in Hertfordshire, southern England by Salisbury (1916a,b) long ago. Salisbury noted that several species (see Box 3.1) had pre-vernal flowering that was almost completed before the leaves of hornbeam Carpinus betulus, which formed part of the deciduous tree canopy, were fully expanded in mid-May. Aerial parts of lesser celandine Ranunculus ficaria and wood anemone Anemone nemorosa die down completely by the end of June, but dog's mercury Mercurialis perennis, a summer-green plant, remains active throughout the summer and shoots are commonly present during the winter in sheltered British woodlands. Bluebell Hyacinthoides non-scripta has vernal flowering, continuing to flower until mid-June, but then dies down very abruptly although its seed capsules are held high above the ground until early autumn. Pignut Conopodium majus commences vegetative growth at the beginning of March, has summer (aestival) flowering starting in May, and continues active growth through the summer. The influence of climate on flowering time is illustrated by lesser celandine, which usually flowers in late February in Hertfordshire, early April in Germany and late April in the Ukraine, which has a continental climate. It is also interesting to see the same niche occupied by different species in different

Box 3.1

Variations on the theme of understorey plants growing before and after the deciduous tree canopy casts the deep shade of summer, using European examples. See also Fig. 3.9 which shows the timing of spring growth of understorey plants in an oak-beech wood in relationship to the leafing-out of the trees and shrubs.

Pre-vernal ('before spring') and vernal ('spring') plants produce their leaves sometimes as early as January and by the beginning of July have completely died back above ground.

Bluebell Hyacinthoides non-scripta Lesser celandine Ranunculus ficaria Spring snowflake Leucojum vernum (shown in Fig. 3.9) Wood anemone Anemone nemorosa (shown in Fig. 3.9)

Summergreen plants start with the pre-vernal plants and do most of their growing before the tree leaves appear but keep their leaves through the summer using what little light is available.

Dog's mercury Mercurialisperennis (shown in Fig. 3.9) Meadowsweet Filipendula ulmaria (shown in Fig. 3.9) Honeysuckle Lonicera periclymenum

Wintergreen plants keep at least a few green leaves year round, usually as a new basal rosette formed before the winter, so that growth can start as soon as spring conditions allow and can continue into a warm late autumn after leaf fall.

Wood-sorrel Oxalis acetosella (shown in Fig. 3.9) Primroses Primula spp.

Yellow archangel Lamiastrum galeobdolon (shown in Fig. 3.9)

Evergreen plants are the true evergreens that keep all their leaves year round.

Stinking iris Iris foetidissima

Ivy Hedera helix (shown in Fig. 3.9)

Holly Ilex aquifolium (shown in Fig. 3.9)

Parasites and myco-heterotrophs solve the problem by acquiring their carbon from living green plants or fungi, so doing away with the need for light and photosynthesis.

Toothwort Lathraea squamaria (parasitic) Bird's-nest orchid Neottia nidus-avis (myco-heterotroph) Coralroot orchid Corallorhiza trifida (myco-heterotroph) Yellow bird's-nest Monotropa hypopitys (myco-heterotroph)

Myco-heterotrophs were, for over a century, incorrectly described as sapro-trophs and were believed to live on soil organic matter. In fact they are parasitic upon fungal carbon, a reversal of the normally accepted relationship between vascular plants and fungi (Leake, 2005). Over 400 plant species belonging to 87 genera, which lack chlorophyll are parasitic upon fungi, which they exploit as their principal source of carbon. It is also estimated that over 30 000 species depend upon myco-heterotrophy for establishment from dust-like seeds or spores during critical early phases, but produce green shoots on emerging into light from the soil or growing as epiphytes on other plants.

Recent research has yielded much useful information regarding the early stages of the plant/fungal relationships and the very specific associations between members of the two groups. Yellow bird's-nest, for example, when growing on the dune slacks at Newborough Warren NNR, Anglesey, Wales, is associated with the Sa/ix-specific fungus Tricholoma cingulatum when present under its autotrophic co-associate creeping willow Sa/ix repens. Virtually all the nutrients taken up by the yellow bird's-nest are acquired from the fungal sheaths on its roots, so it is thus indirectly parasitic on the creeping willow. When this herb is growing under Scots pine in the same area, the roots of both vascular plants are linked by T. terreum, a closely related fungus.

places. Hollow corydalis Coryda/is cava, for example, grows on good soil in southern Sweden and central Europe but is not native in Britain where its place is taken by bluebell. Hollow corydalis flowers at the same time as lesser celandine, but its foliage, though produced slightly later, dies back earlier.

The conspicuous chasmogamous flowers of violets Viola spp. and wood-sorrel Oxa/is acetose/la (see Section 4.3), which are open and can be visited by insects, develop in spring. Later in the year both form cleistogamous flowers that are closed, much reduced and self-pollinated. Those of wood-sorrel have hooked peduncles and are often buried in plant litter; it is probable that most seeds of this species are produced by such flowers. Cleistogamy is an efficient means of reproduction, but its adoption reduces gene flow.

Phenological studies investigating dry-matter allocation to the organs of species characteristic of different aspect societies have been made both under cultivation (Packham and Willis, 1977, 1982), and in the wild. The geophytes ramsons A//ium ursinum (Ernst, 1979) and bluebell Hyacinthoides non-scripta have been studied in this way in their native woodlands (Fig. 3.12). In both species the bulb is renewed annually. At the beginning of October bluebell bulbs, which are hidden well below the soil, have sloughed off the roots active in the previous summer. The new roots beginning to emerge through the side of each bulb arise from the base of a newly initiated 'daughter' plant in the centre of the bulb. These make their way by enzyme action through the leaf and bud scales of the 'parent' bulb, and by late autumn the bulb scales, specialized tubular scale leaves, foliage leaves, flower stalk (scape) and flowers of the new

Rain Forests

Figure 3.12 Percentage dry matter allocation to plant organs in bluebell Hyacinthoides non-scripta growing amongst bracken Pteridium aquilinum on an acidic sandy brown earth in an open region of Himley Wood, near Dudley, West Midlands, England. Note the high allocation to bulb weight. Each of the seven harvests (H1-H7) is based on the plants within a 0.1m2 area. Samples included plants of all the age classes present, but in this site there was a preponderance of mature bulbs, over 90% of bulb biomass occurring at a depth of 20 cm or more. (Reprinted from Grabham and Packham, 1983. Biological Conservation 26, with permission from Elsevier.)

Figure 3.12 Percentage dry matter allocation to plant organs in bluebell Hyacinthoides non-scripta growing amongst bracken Pteridium aquilinum on an acidic sandy brown earth in an open region of Himley Wood, near Dudley, West Midlands, England. Note the high allocation to bulb weight. Each of the seven harvests (H1-H7) is based on the plants within a 0.1m2 area. Samples included plants of all the age classes present, but in this site there was a preponderance of mature bulbs, over 90% of bulb biomass occurring at a depth of 20 cm or more. (Reprinted from Grabham and Packham, 1983. Biological Conservation 26, with permission from Elsevier.)

plant can be clearly seen. They increase in size as the old bulb withers, and by mid-February the shoot will often have risen above the soil and begun to exploit the relatively high light intensities found at the forest floor before the tree canopy expands. The plant rapidly accumulates the food reserves needed for flowering and a new, usually larger, bulb is formed. Flowering ends in June, capsules form and the now flaccid leaves come to rest on the soil surface, where they soon decompose. The cycle is complete by autumn and the seeds, which under favourable conditions take five years to develop into flower-bearing plants, have been discharged from the capsules.

In the European wild daffodil Narcissus pseudonarcissus establishment of new plants by daughter bulbs is high in comparison to that by seedlings, but in both bluebell and ramsons there is usually a high rate of seed output and seedling establishment per unit area of stand. Vegetative propagation is rare in both these species.

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