Instant Notes: Plant Biology

with it nutrient supply, becomes limiting (Topic I1). In these environments,

bryophytesbecome particularly common since they absorb water from the

atmosphere and have no internal conduction system (Topic P3). In ever-wet cool

or cold conditions on seaboards in temperate zones, in mountains and in sub-

polar conditions, bryophytes, notably Sphagnummosses, dominate and dead

plant material does not decay fully, forming peat.

Where water, aeration and temperature are not limiting factors, as in parts of

the tropics,rain forestsgrow. The tropical rainforests are among the most

diverse terrestrial environments in plant species and life form. In these environ-

ments biotic interactions play an important role in limiting plant distribution

(Topic K2).

In the sea there are very few highly specialized flowering plants, such as

eel-grasses (Zosteraspp.), the great majority of photosynthesis being done by

unicellular algal plankton and, inshore, by large algae (Topic P2).

The nutrient and ion status of soils is variable, with different elements limiting

in different places and some elements being toxic (Topic I4). In many soils,

nitrogenis one of the main limiting nutrients although some plants, notably

legumes, have nodules in their roots filled with nitrogen-fixing bacteria (Topic

M2). In many soils more than one nutrient may be in short supply and plants

compete for them. The ion status of a soil will depend in part on the underlying

Nutrients and
ions

K1 – Physical factors and plant distribution 163

Tundra

Coniferous forest

Temperate deciduous forest

Montane forest, alpine tundra complex

Mixed hardwood-conifers

Mediterranean chaparral

Tropical forests

Semidesert, arid grassland, tree savannah

Tropical savannah, thorn forest

Desert

Grassland, Steppe

Fig. 1. Distribution of the major biomes of the world. (Redrawn from A. Mackenzie et al.(1998),Instant Notes in Ecology,
BIOS Scientific Publishers Ltd.)