Terrestrial Biomes

Differences in temperature or precipitation determine the types of plants that grow in a given area (Figure 1). Generally speaking, height, density, and species diversity decreases from warm, wet climates to cool, dry climates. Raunkiaer (1934) classified plant life forms based on traits that varied with climate. One such system was based on the location of the perennating organ (Table 1). These are tissues that give rise to new growth the following season, and are therefore sensitive to climatic conditions. The relative proportions of different life forms vary with climate (Figure 2). In fact, life form spectra are more alike in similar climates on different continents than they are in different climates on the same continent (Figure 3). Regions of similar climate and dominant plant types are called biomes. This chapter describes some of the major terrestrial biomes in the world; tropical forests, savannas, deserts, temperate grasslands, temperate deciduous forests, Mediterranean scrub, coniferous forests, and tundra (Figure 4).

Tropical forests are found in areas centered on the equator (Figure 4). Central and South America possess half of the world’s tropical forests. Climate in these biomes shows little seasonal variation (Figure 5), with high yearly rainfall and relatively constant, warm temperatures. The dominant plants are phanerophytes - trees, lianas, and epiphytes. Tropical rainforests have an emergent layer of tall trees over 40 m tall, an overstory of trees up to 30 m tall, a sub-canopy layer of trees and tall shrubs, and a ground layer of herbaceous vegetation.

Tropical forests have the highest biodiversity and primary productivity of any of the terrestrial biomes. Net primary productivity ranges from 2–3 kg m^-2 y^-1 or higher. This high productivity is sustained despite heavily leached, nutrient poor soils, because of the high decomposition rates possible in moist, warm conditions. Litter decomposes rapidly, and rapid nutrient uptake is facilitated by mycorrhizae, which are fungal mutualists associated with plant roots.

The tropical forest biome is estimated to contain over half of the terrestrial species on Earth. Approximately 170,000 of the 250,000 described species of vascular plants occur in tropical biomes. As many as 1,209 butterfly species have been documented in 55 square kilometers of the Tambopata Reserve in southeastern Peru, compared to 380 butterfly species in Europe and North Africa combined.

The tropical forest biome is composed of several different sub-biomes, including evergreen rainforest, seasonal deciduous forest, tropical cloud forest, and mangrove forest. These sub-biomes develop due to changes in seasonal patterns of rainfall, elevation and/or substrate.

Located north and south of tropical forest biomes are savannas (Figure 4), with lower yearly rainfall and longer dry seasons (Figure 6). These biomes are dominated by a mix of grasses and small trees. Savannas cover 60% of Africa and represent a transition from tropical forests to deserts. Trees in savannas are usually drought deciduous. Several savanna types associated with differing rainfall patterns, height of the water table and soil depth can be distinguished by their relative abundance of trees and grass.

Repetitive dry season fires have occurred in the African savanna over the last 50,000 years. Fire plays a major role in the balance between trees and grasses in savannas. With long periods between fires, tree and shrub populations increase. Fires release nutrients tied up in dead plant litter. Soil provides a good thermal insulator, so seeds and below ground rhizomes of grasses are usually protected from damage.

Net primary productivity ranges from 400–600 g m^-2 yr^-1, but varies depending upon local conditions such as soil depth. Decomposition is rapid and year-round, and the annual turnover rate of leaf material is high; up to 60–80%. This turnover is aided by the rich diversity of large herbivores found in savannas, where up to 60% of the biomass can be consumed in a given year. Dung beetles are important components of the nutrient cycle due to their role in breaking down animal droppings. The high herbivore diversity and production is mirrored by the great variety of predators and scavengers found in savannas.

Deserts generally occur in a band around the world between 15–30° N and S latitude (Figure 4). They cover between 26–35% of the land surface of the Earth. The climate of deserts is dominated by low precipitation, generally below 250 mm yr^-1 (Figure 7). However, there is a lot of variability in desert types, with hot deserts, cold deserts, high elevation deserts, and rain shadow deserts. Consequently, there is a great deal of variation in the biodiversity, productivity and organisms found in different types of desert.

The dominant plant biomass in most deserts is composed of perennial shrubs with extensive roots and small, gray or white leaves. However, in warm deserts, therophytes (annual plants) can make up most of the species diversity (Figure 2). Desert annuals can survive unpredictable dry periods as seeds. Seeds may remain viable in the soil for several years, until the appropriate rainfall and temperature conditions occur, after which they will germinate. These annuals grow rapidly, completing their life cycle in a few weeks, then flowering and setting seed before soil water reserves are depleted. Winter desert annuals in North American deserts can generate over 1 kg m^-2 of biomass in a wet year.

With the exception of large blooms of annuals, net primary productivity in most deserts is low and extremely variable. There is a positive relationship between productivity and precipitation, and values can range from near 0 to 120 g m^-2 yr^-1. Just as with savannas, productivity will vary with soil depth and local drainage patterns (e.g., washes).

Grassland biomes occur primarily in the interiors of continents (Figure 4) and are characterized by large seasonal temperature variations, with hot summers and cold winters (Figure 8). Precipitation varies, with a strong summer peak. The type of grassland community that develops, and the productivity of grasslands, depends strongly upon precipitation. Higher precipitation leads to tall grass prairie with a high biodiversity of grasses and forbs. Lower precipitation leads to short grass prairies and arid grasslands.

Net primary productivity in dry grasslands may be 400 g m^-2 yr^-1, while higher precipitation may support up to 1 kg m^-2 yr^-1. Grasslands grade into deciduous forest biomes on their wetter margins, and deserts on their drier margins. The borders between grasslands and other biomes are dynamic and shift according to precipitation, disturbance, fire and drought. Fire and drought will favor grassland over forest communities.

Three major selective forces dominate the evolution of plant traits in grasslands, recurring fire, periodic drought, and grazing. These factors have led to the dominance of hemicryptophytes in grasslands with perennating organs located at or below the soil surface. Many grasses have below ground rhizomes connecting above ground shoots or tillers. Grass blades grow from the bottom up, with actively dividing meristems at the base of the leaf. Thus when grazers eat the grass blade, the meristem continues to divide and the blade can continue to grow. Grasses are often decay-resistant, and recurring cool, fast moving surface fires started by lightning at the end of summer aid in nutrient recycling. Fires stimulate productivity and the germination of fire resistant seeds.

Many of the world’s largest terrestrial animals are found in grasslands. Animals such as gray kangaroos (Macropus giganteus) in Australia, Bison (Bison bonasus) and horses (Equus spp.) in Eurasia and North America were part of species rich assemblages of grazing animals, their predators, and scavengers. Remnant herds in North America suggest that disturbances due to grazers increased local biodiversity by creating openings that rare species could colonize. Large grazers also accelerated plant decomposition through their droppings, creating nutrient hotspots that altered species composition.

Temperature deciduous forests occur in mid-latitudes (Figure 4) where cool winters, warm summers, and high year round precipitation occurs (Figure 9). Net primary productivity ranges from 600–1500 g m^-2 yr^-1 with high litter production. Litter serves as a major pathway for nutrient recycling. This biome is named for the dominant trees that drop their leaves during the winter months. These forests may have an overstory of 20–30 m tall trees, an understory of 5–10 m trees and shrubs, a shrub layer around 1–2 m in height, and a ground layer of herbaceous plants. Biodiversity is relatively high in this biome due to the niche partitioning allowed by the multiple forest layers. More complex forests are associated with a greater number of animal species; for example, bird species diversity shows a positive correlation with forest height and number of layers.

This small biome (about 1.8 million square km) is separated into five separate regions between 30–40 degrees N and S latitude (Figure 4) with hot, dry summers, and cool, moist winters (Figure 10). Unrelated evergreen, sclerophyllous shrubs and trees have evolved independently in each of these areas, representing a striking example of convergent evolution. Net primary productivity varies from 300–600 g m^-2 yr^-1, dependent upon water availability, soil depth, and age of the stand. Stand productivity decreases after 10–20 years as litter and woody biomass accumulates. Recurring fires aid in nutrient cycling and many plants show fire-induced or fire-promoted flowering. Some species are able to resprout from buds protected by the soil, while others germinate from decay-resistant seeds that lie dormant in the soil until a fire promotes their germination. Therophytes make up a large component of the flora, and their appearance is associated with openings created by fires.

Located at higher latitudes is a biome dominated by needle-leaved, drought tolerant, evergreen trees (Figure 4), and a climate consisting of long, cold winters and short, cool summers (Figure 11). Biodiversity is low in this two-layered forest made up of an overstory of trees and a ground layer of herbs or mosses. The overstory in much of the boreal forest is made up of only one or two species. The low biodiversity is mirrored by low net primary productivity of 200–600 g m^-2 yr^-1. Productivity varies with precipitation, the length of the frost-free period, and local soil drainage. In flooded areas, sphagnum bogs may develop. The acidic tissue of sphagnum, and the anoxic, flooded conditions, slows decomposition, resulting in the production of peat bogs.



Biomass in tree trunks and long-lived evergreen leaves results in nutrients being stored in the plants. Low temperatures lead to slow decomposition and high litter accumulation. Up to 60% of the biomass may be tied up in litter and humus. Soils are heavily leached, and permafrost underlies much of the soil. Consequently, trees have shallow root systems and rely on extensive mycorrhizal associations for nutrient uptake.

At latitudes beyond the boreal forest tree line lies a marshy area (Figure 4) where growing seasons are very short and temperatures are below zero degrees Celsius for much of the year (Figure 12). Because of these low temperatures and short growing seasons, net primary productivity is very low in the tundra, between 100–200 g m^-2 yr^-1. Productivity varies with snowfall depth and local drainage. Rocky fields and dry meadows will have lower productivity than moist, low-lying areas and wet meadows.

Biodiversity in the tundra is low and dominated by mosses, lichens, and low-growing perennial shrubs. The tundra biome contains only about 3% of the world’s flora. Up to 60% of the flora can be made up of long-lived hemicryptophytes. Windy conditions and low temperatures select for low growing shrubs, often with tightly-packed, rounded canopies with closely spaced leaves and branches. Wind and ice damage help form this shape by pruning branches. The canopy morphology reduces wind speeds and absorbs solar radiation, resulting in canopy temperatures on sunny days more than 10° C above air temperature.

Soils are low in nutrients due to slow decomposition rates and plants retain nutrients in long-lived evergreen tissues. Nitrogen fixation by lichens with cyanobacterial components is a major source of soil nitrogen. Animals have extended hibernation periods or migrate seasonally.