ECOLOGY OF DIFFERENT GROWTH FORMS
Variety of form
Plants can be mechanically independent and woody, i.e. trees or shrubs, or herbaceous. Mechanically dependent plants are climbers, epiphytes or stranglers. There are also parasites and saprophytes.
Ecology of woody plants
Trees dominate many terrestrial habitats where there is sufficient rain and a warm temperature for at least part of the year. These are mainly dicotyledons and conifers. Shrubs dominate heathlands and extend to the tundra as dwarf shrubs.
These dominate the ground layer in many habitats, and grasslands are entirely dominated by them. Many in seasonal environments die down in the dry season or winter and some are short-lived.
Mechanically dependent plants
All these are most common in tropical rainforests and they depend mainly on trees for their support. Climbers include woody and nonwoody forms. Epiphytes include some specialist families and are much less common in seasonal environments with only bryophytes occurring as common epiphytes in temperate climates.
There is a great variety of growth forms of plants but they can be classified into a few main categories with different ecological characteristics. Most plants grow without any mechanical support and these may be woody, forming trees or shrubs, or they may be herbaceous without any woody parts. Trees and woody shrubs range from giants of over 100 m height to many-stemmed shrubs and some undershrubs that remain prostrate on the ground but retain their woody stems. Tree ferns, cycads and most palms have a single trunk with one enormous bud and leaf rosette at their tip, while other trees have a few branches, sometimes dichotomous . A majority of the larger mechanically
independent woody plants have a single main stem with side branches, though often flattening out to a domed crown in which there is no one dominant stem. Some woody plants, particularly shrubs, have underground rhizomes from which several woody stems grow, forming thickets.
Herbaceous plants are low-growing without lignified stems. Frequently, the above-ground stems are ephemeral and many in seasonal regions lose their above-ground parts during the dormant season. Some are short-lived (Topic K4). Many have rhizomes or stolons and spread vegetatively.
Mechanically dependent plants include climbers which can reach the tops of tall trees and may have woody stems. Other climbers persist without much secondary thickening and some in seasonal climates grow like herbaceous plants and die back each dormant season. Epiphytes grow attached to other plants, usually trees, by their roots or stems but not extracting nutrients directly from the supporting tree. These are light-demanding plants that obtain water and nutrients directly from rain and run-off along branches. Stranglers start life as epiphytes but send some roots to the ground to become partially or totally independent. Heterotrophic plants depend on other plants or fungi for all or part of their nutrients (Topics M6 and M7).
These growth forms have no direct relationship with plant classification, though some plant families are specialized to one particular form. Some growth forms include plants from widely different families and some plants are intermediate. Some individual species can adopt a different growth form in different conditions.
Trees dominate most ecosystems where the climate is warm and moist for at least part of the year. They cease to dominate where the soil is too thin for their roots, where it is too dry in deserts, where there is permafrost in tundra regions, in permanently waterlogged places, places dominated by salt or heavy metals and in many environments modified by people. They can live for a few years or decades to as long as a few thousand years setting seed most years, or only for a few years. The majority of woodland and forest ecosystems are dominated by dicotyledonous angiosperms except for the great coniferous forests of the northern hemisphere. Tree ferns, cycads and monocotyledonous trees may be common but are rarely dominant except occasionally in swamps or in colonizing situations. In many forests the trees form strata. They have a canopy layer, with taller emergent trees projecting through it in some rainforests, and understorey trees and shrubs often with more than one layer. Different speciesof tree are frequently involved, with the shade-tolerant species forming theunderstorey and faster growing canopy trees being more light-demanding,though there is overlap, with saplings of larger trees present in the lower layers.At the edges of forests and where there is a gap, other pioneer, fast-growingspecies grow.
Shrubs and smaller woody plants occur at the edges of many forests and dominate ecosystems in the arctic regions and in some deserts where the environment is too harsh for trees. Heathland is dominated by low-growing shrubs mostly less than 1 m high as well as some arctic shrubs, such as willows (Salix spp.), rising no more than 1 cm above ground level having all their woody parts below ground.
Many herbaceous plants in seasonal environments die down at the end of each growing season leaving only their roots and sometimes leaves to survive until the following season. These plants, along with small shrubs, form the groundlayer in many habitats, and dominate in savannahs,temperate woodlands and grasslands throughout the world. Some herbaceous plants persist above ground, particularly in less seasonal areas but these mostly remain less than about 3 m tall. Those that live for less than a year are the quick colonizers of newly opened areas or take advantage of the rare rains in deserts to grow quickly to set seed before unsuitable conditions return.
Grasses are all herbaceous, except for the bamboo group, and they are particularly well adapted to withstand grazing pressure since they have growing meristems at nodes rather than at the tip, and produce stems above these nodes. They frequently dominate in places with intense grazing pressure, and those that have meristems below the soil surface can withstand fire.
Climbers, epiphytes and stranglers all require the presence of other plants, normally trees, for their growth and all are much more abundant in tropical rainforests than elsewhere. Climbers occur throughout woodland habitats with many woody climbers (lianas) in tropical forests. Many fewer occur outside tropical rainforests but most temperate woodlands have some, e.g. Clematis spp. Herbaceous climbers can be small and occur throughout the world, some persisting in grasslands. In tropical rainforests, some of the tallest climbers do not produce woody stems, or only a little secondary thickening, e.g. members of the arum family, Araceae.
Epiphytes are dependent on a high rainfall since they only get water from it and run-off along branches. They are abundant in tropical forests. Many ferns and members of certain families, notably the orchids, Orchidaceae, andbromeliads, Bromeliaceae, are specialist epiphytes but many other families are represented. Tropical mountains have a particularly abundant epiphyte flora. Epiphytes are common in parts of the seasonal tropics but become much rarer and less diverse in drier climates. In temperate regions epiphytic flowering plants only occasionally occur, although there are a few ferns. Bryophytes are abundant epiphytes in the wet tropics, even occurring on the leaves of rainforest trees. They are the only common epiphytes in temperate woodlands, along with lichens. In tropical rainforests some epiphytes are shrubby, producing woody stems and some of these develop roots which reach the ground to become independent. The figs (Ficus spp.), are the most prominent of these stranglers and can become self-supporting, killing the tree on which they started life.
The mistletoes (two families, Loranthaceae and Viscaceae) and a few other plants are epiphytic in growth form but are partial parasites, penetrating their hosts and extracting nutrients and sugars.