PHYSICAL FACTORS AND PLANT DISTRIBUTION
The requirements for plant life’
All plants have the same basic requirements for solar energy, water and nutrients. Their distribution is determined by adaptations to withstand environmental stress and their ability to spread, along with biotic interactions.
The number of plant species in a community increases with increasing temperature, given sufficient water. Frost is a barrier for many plants and requires adaptations to prevent tissues from freezing. The distribution of some plants follows isotherms.
In most parts of the world water is limiting at some time of year and plants must resist drought. In waterlogged conditions plants may be limited by the availability of air, though aquatic plants may derive dissolved gases
from the water. High rainfall and humidity on tropical mountains leads to reduced transpiration and stunted growth. Bryophytes dominate in some wet environments. Very few flowering plants occur in the sea.
Nutrients and ions
The overall and relative quantities of elements in soils varies greatly, both with underlying geology and age and thickness of the soil. Many are essential and some toxic, and plants differ in their requirements and abilities to withstand toxicity; communities differ under different conditions. Plants tolerant to sea salt occur around all coastlines.
Periodic or occasional disasters such as fire, hurricanes or landslides can dominate plant communities. Fire may occur frequently in savannahs and less than one per century in conifer forests, but still have an overriding influence.
Some plants have good seed dispersal and colonize easily but many do not and barriers of unsuitable habitat preclude colonization or migration. Globally, the world is divided into several phytogeographic regions, these regions reflecting the current distribution of the continents and their drift over the past 120 million years or so.
Glaciation and plant migration
Over the last million years, glaciers have expanded and contracted over the northern hemisphere making many of these areas suitable only for tundra-like vegetation. The plants have migrated differentially leading to a flexible community structure. In the tropics, glacial periods are dry and rainforests were more fragmented. There is good evidence from plant fossils, mainly pollen, of post-glacial changes.
Plants are dependent on external temperature and solar radiation, water supplies, and nutrients, normally in the soil, for their survival (Topics I4 andI5). The basic requirements are the same for all plants. No plant can survive when its growing temperature remains below 0°C all the year round (plants frequently have a higher internal temperature than the ambient air temperature) or in the driest of deserts where the soil is often unstable. The overall geographical distribution of plants is partly determined by their relative abilities to withstand different climates and soil conditions. These major differences across the world give rise to different biomes, i.e. groups of plant communities dominated by plants of similar form, such as tropical rainforest or temperate grassland, which may have a totally different species composition in different areas but look similar and function in similar ways (Fig. 1).
For many plants, dispersal across unfavorable environments, such as the sea or a desert, is limited, giving rise to different plant communities in different regions of similar climate and frequently a different flora on islands from continental areas. Interactions between plants, such as competition, and interactions with other organisms, have an overriding influence on the distribution of a species within one geographical region and can affect major distribution patterns (Topics K3 and K4, and Sections L and M).
Temperature and incoming solar radiation have a profound influence on the distribution of plant communities. In general, given sufficient water, the number of plant species present increases with increasing temperature. One of the major limits is frost; many plants cannot tolerate the presence of frost at any time of year. Frost limits not just species but whole biomes. Temperate and boreal (northern, between temperate and arctic) environments are characterized by more or less frequent frosts in one season of the year and during this period the plants become dormant, having developed mechanisms to stop their tissues from freezing. These mechanisms include becoming deciduous, dying down completely or concentrating their cell contents.
Many plants appear to be limited in their distribution by temperature at a particular time of year, following the line of a maximum or minimumisotherm, though the reasons for this are not clear. Many plants are able to live outside their observed natural distribution in cultivation. The reasons are probably competition with other plants or other biotic interactions.
On land, the quantity and distribution of rainfall or other sources of water such as fog or snowmelt have a major influence on overall plant distribution. In most parts of the world water is limited at some season, either by a dry period in the seasonal tropics or a dry summer or frosty winter outside the tropics. In deserts and steppes it is limited most of the time. In all these environments there are specialized plants that have adaptations to limit water loss during the dry periods and most plants become partially or totally dormant during the dry periods.
At the other extreme, in permanently waterlogged conditions and in aquatic environments, aeration is often the limiting factor for plant growth and onlyplants specialized to withstand waterlogging can survive, although there are aquatic plants with submerged leaves that derive all their photosynthate and nutrients from the water. On tropical mountains there is frequently a high rainfall and frequent fog and high humidity, low solar radiation and cool temperatures. These conditions lead to a stunted forest, since transpiration, and
with it nutrient supply, becomes limiting . In these environments, bryophytes become particularly common since they absorb water from the atmosphere and have no internal conduction system . In ever-wet cool or cold conditions on seaboards in temperate zones, in mountains and in subpolar conditions, bryophytes, notably Sphagnum mosses, 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 forests grow. The tropical rainforests are among the most diverse terrestrial environments in plant species and life form. In these environments biotic interactions play an important role in limiting plant distribution .
In the sea there are very few highly specialized flowering plants, such as eel-grasses (Zostera spp.), the great majority of photosynthesis being done by unicellular algal plankton and, inshore, by large algae .
The nutrient and ion status of soils is variable, with different elements limiting in different places and some elements being toxic . In many soils,nitrogen is one of the main limiting nutrients although some plants, notably legumes, have nodules in their roots filled with nitrogen-fixing bacteria . 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 underlyinggeology and certain elements, such as calcium, are most common in alkaline conditions, but in these soils iron may be limiting. Likewise in many acid soils, many nutrients are in limited supply but aluminum becomes available and is a toxic element for many plants. These and other differences between soils lead to quite different plant species and communities occurring on different soils. In places rich in heavy metals, many of which are toxic (e.g. lead), only certain specialized plants can grow. As a soil ages, elements will gradually leach away and it will become poorer in nutrients. In these conditions the presence of animal dung or carcases, ant nests or other plants can lead to great variation in nutrient status on a small scale within the community.
Areas dominated by saline conditions occur around all coastlines and estuaries. The great majority of flowering plants cannot grow in the presence of salt in concentrations found in sea water, with its high osmotic potential. Specialized plants with adaptations for excluding or excreting salt occur, mostly succulent and low-growing . Tidal estuaries which are inundated for part of each day with brackish water are often rich in other nutrients and can be highly productive for those plants that can tolerate the conditions: specialized mangrove trees in tropical estuaries; herbs and low shrubs in temperate saltmarshes.
Some parts of the world are prone to periodic or occasional destructive forces that can dominate a plant community. Fire is the most common, started naturally by lightning, but in many places becoming more frequent with human influence. In some savannahs the grass which dominates the ground layer is burned most years but regrows when rain returns. This maintains the savannah community. In other places, such as coniferous woodland and heathland, fires are less frequent, even as few as one per century or less, but still are one of the overriding influences on the composition of the plant community.
High winds are frequent near many coasts and on mountains, and tallgrowing trees frequently cannot grow through mechanical instability. Periodic or infrequent storms or hurricanes, developing over the sea, can cause havoc in restricted areas, usually near coasts. Though each hurricane ploughs a different path, certain parts of the world, such as the Caribbean and the Philippines, are hurricane-prone. Some plants resist these winds better than others and in these places communities can be dominated by occasional hurricanes.
Tsunamis (tidal waves), volcanic activity or landslides can also devastate communities, sometimes maintaining them as permanent pioneer communities.
The dispersal of plants is normally dependent on seeds. In those plants with light, wind-dispersed seeds and some of those with seeds dispersed by birds or other means, dispersal can cover long distances over unfavorable habitat and the plants can colonize new places. Other plants are much more restricted so the spread of species across the world is limited. If a plant is restricted to a particular environment which has a discontinuous distribution, e.g. mountain ranges, dispersal between one range and another may be impossible, restricting a plant’s distribution and resulting in different species occurring on each range.
Most plants cannot disperse from one continent to another and there are differences in the floras of the major continents at the genus and family level. The major phytogeographic regions of the world are given in Fig. 2. The distribution of these regions is a reflection not only of the existing configuration of the continents, but of their histories. It is inferred from geological and fossil
evidence that about 120 million years ago there were only two continents and the first flowering plants were appearing. The subsequent evolution of flowering plants has been dramatically affected by the shifts in continents that have occurred since, and consequent isolation or joining together of, large land masses. The regions are mainly similar to vertebrate geographical regions but are less well defined owing to the fact that flowering plants evolved before modern vertebrate groups and migrated across the world earlier, maintaining affinities across regions. They differ particularly in the southern temperate zone.
The whole north temperate zone is regarded as a single region, the boreal. The north Atlantic formed about 50 million years ago and North America and Eurasia have periodically been joined at the Bering Strait. The whole region has had great climatic changes in the last million years throughglaciations, probably extinguishing many species so that only adaptable plants with good dispersal powers remain. In the tropics there are three regions: South America with central America; tropical and subtropical Africa with tropical Asia forming the palaeotropical region; and Australia. The Cape region of South Africa is a separate and exceptionally diverse small region and the Antarctic region comprises southern South America and New Zealand.
There are strong affinities between all the tropical floras; New Guinea and north-eastern Australia have a similar flora to Asia. The southern super-continent, Gondwana, split up about 100 million years ago to form the southern continents of today and all drifted north except for Antarctica. The Antarctic region retains a remnant flora of what was once widespread across Gondwana, and the Cape and Australian regions have rather different Gondwanan floras of drier Mediterranean-climate and semi-desert floras that have evolved separately. They have not been subject to the major glaciations and climatic shifts of the northern hemisphere. Minor phytogeographic regions are formed by some oceanic islands, particularly in the Pacific.
Over the last million years, periodic cooling and warming of the world has led to glaciers spreading and retreating in the northern hemisphere and the tropics becoming drier during glacial periods and wetter during interglacials. These fluctuations have led to major changes in the range of plant communities in the northern hemisphere with large areas of Europe and North America containing temperate broad-leaved forest as its natural community dominated by Arcticlike tundra. In the tropics, rainforest became more restricted and fragmented during the drier glacial periods. There were many extinctions during these glacial times. The spread and retreat of the glaciers affected plant distribution throughout the north temperate, plants with good colonizing ability surviving better than others and some tundra plants reaching their current disjunct distributions, i.e. distributions consisting of several quite separate areas, through their retreat northwards and into mountains as the glaciers retreated.
In post-glacial times (the last 10 000 years or so) we have a better record than for any previous time from the analysis of pollen and other fossil fragments preserved mainly in peat and lake sediments. These show that plants have spread at different speeds north and often west as well, following the ameliorating climate, and that the plant communities in the northern hemisphere have changed markedly in extent and composition throughout this time.