Pollination strategies

Forest plants face a number of problems in respect of pollination, from the sheer size of the larger trees to the still, dark conditions inside a forest hampering pollinators. Trees in northern and temperate forests, especially the conifers, tend to be wind pollinated (anemophilous) but towards the tropics the trend is towards animal pollination, particularly by insects (entomophilous). Wind pollination is often seen as primitive and wasteful of pollen (a single birch catkin can produce 5.5. million pollen grains) and yet is remarkably prevalent. The easy answer would be to say that in northern areas there are fewer insects and higher wind speeds, leaving wind as the best pollen transporter. While this is undoubtedly true, it appears to be so successful due to low species diversity of trees. Wind can transport pollen long distances but is unspecific as to where the pollen goes. In northern areas where forests are dominated by relatively few tree species this is not necessarily a problem since the pollen has a high chance of landing on a flower of the same species. The high species diversity of tropical forests, where individuals of a species are few and widely scattered, would make wind pollination too unreliable, and the higher costs of attracting animal pollinators are clearly worth the investment. Wind-pollinated flowers have no need to attract animals and so have dispensed with bright petals, nectar and scent. Various adaptations help to ensure the pollen goes as far as possible. Most deciduous wind-pollinated trees flower while the canopy is bare of leaves; conifers and evergreens improve pollen dispersal by placing the flowers on the ends of branches.

Since trees are so large and produce huge quantities of pollen, self-pollination is a likely problem whether wind- or animal-pollinated. Even if the tree is self-sterile, the female flower may still be swamped with its own pollen physically preventing pollen from other trees reaching the stigma. In wind-pollinated trees the most frequent way of avoiding this is to have separate male and female flowers on different parts of the trees, sometimes separated on the same branch (e.g. oaks) and sometimes on separate parts of the tree (as in many conifers, males at the bottom of the canopy, females at the top).

Plants buried in the dark, still conditions of the forest have their own share of pollination problems. As outlined in Section 1.4.2, many herbs of temperate woodlands flower early in the year before the canopy closes. Those that flower after canopy closure are usually insect-pollinated and tend to have pale or white flowers which increase visibility to pollinators or are heavily scented. More information on pollination in trees can be found in Thomas (2000).

4.2.2 Regenerative strategies and methods of seed dispersal

In 1942, Salisbury put the study of plant reproduction on a firm footing in his classic work The Reproductive Capacity of Plants. Five strategies are widespread in terrestrial vegetation, and all occur in woodlands and forests. Vegetative expansion (V) is found most often where disturbance is at a low level. In habitats subjected to seasonally predictable disturbance by climatic or biotic factors that reliably creates openings seasonal regeneration (S) is common, seeds or vegetative propagules being produced together in a single cohort. In contrast, species with persistent seed or spore banks (Bs) enjoy a selective advantage in places where the timing of disturbance is unpredictable: the seeds are present whenever an opening occurs (see Section 4.6.2). Numerous widely dispersed seeds or spores (W) are common in species growing in habitats that are relatively inaccessible or subject to spatially unpredictable disturbance. Such species are often opportunists (or r-species - see Section 4.1.1), indeed both fireweed Chamerion angustifolium and the moss Funaria hygrometrica, which frequently develop on burnt areas, are widespread invaders. Persistent juveniles (Bj) are seedlings or sporelings which persist for long periods in unproductive habitats in which levels of disturbance are low. This last strategy is that of the seedling bank (Section 9.2.2), and is employed by a number of shade-tolerant trees such as the Norway spruce Picea abies whose dwarf trees persist for many years in primeval Scandinavian forests. These seedlings can quickly grow into a gap in the canopy ahead of a seedling that starts from seed. It is a risky strategy, however, since many seedlings will eventually die before a suitable gap appears.

The majority of the strategies outlined above depend upon seed dispersal. Consequently, the evolutionary pressure to find effective means of transport has been strong, resulting in diverse and fascinating mechanisms. Methods of seed dispersal vary considerably; many fruits are spread by wind, animals or explosive mechanisms. This last system, ballistic dispersal, is employed by violets and geraniums, including Herb Robert Geranium robertianum, a

Sycamore Seed Dispersal Mechanism

Figure 4.4 Wind-dispersed fruit (samaras) of (a) the Pride of Bolivia Tipuana tipu (b) sycamore Acer pseudoplatanus and (c) common ash Fraxinus excelsior. Convergent evolution has resulted in remarkably similar samaras in the Fabaceae (a), the maples (Aceraceae - b), and the ashes (Oleaceae). All spin as they fall, carrying them away from the parent tree when winds are strong. The samara of sycamore is double, and sometimes even treble; the line of separation can be seen in the figure. Many gymnosperms are also spread by winged seeds and those of the noble fir Abies procera (d) fly particularly effectively. (Drawn by John R. Packham.)

Figure 4.4 Wind-dispersed fruit (samaras) of (a) the Pride of Bolivia Tipuana tipu (b) sycamore Acer pseudoplatanus and (c) common ash Fraxinus excelsior. Convergent evolution has resulted in remarkably similar samaras in the Fabaceae (a), the maples (Aceraceae - b), and the ashes (Oleaceae). All spin as they fall, carrying them away from the parent tree when winds are strong. The samara of sycamore is double, and sometimes even treble; the line of separation can be seen in the figure. Many gymnosperms are also spread by winged seeds and those of the noble fir Abies procera (d) fly particularly effectively. (Drawn by John R. Packham.)

widespread woodland and coastal species in Britain and a widespread introduction in North America, especially western forests. Many legumes (Fabaceae) have pods each of which splits into two halves which dry, twist and expel the seeds explosively. Relying on wind, members of different families have, by a process of convergent evolution, produced similar winged fruits (Fig. 4.4). Many animal-dispersed fruits, such as those of enchanter's nightshade Circaea lutetiana and goosegrass or cleavers Galium aparine are covered in hooks, others are succulent and the seeds they contain may be transported considerable distances before being wiped off a bird's beak or egested. Some plants use two or more seed dispersers (diplochory), which increases the chance of successful germination. For example, a number of Mediterranean plants have explosive fruits and an elaiosome (an oily body) on the seed which attracts ants to carry the seed away. Pines with big seeds such as the coulter pine Pinus coulteri and sugar pine P. lambertiana of California, have wings on the seeds which aid spread but being big they tend to fall near the parent. But they are then gathered by secondary dispersers such as chipmunks, mice and corvids, are carried away and buried (scatter hoarded) in the soil (Box 4.1). Some will be eaten by the animals, but others will escape unharmed and germinate.

Vander Wall and Longland (2004) generalize that the first phase of dispersal moves seeds away from the parent plant and so reduces losses to seed

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