5.1 Plant communities and biomes

Plant communities are groups of plant species that grow together in specific environments, shaped by factors like climate, soil, and species interactions. Understanding these communities is the foundation for grasping how ecosystems function, how they change over time, and why conservation matters.

Types of plant communities

A plant community is an assemblage of plant species that occur together in a particular environment and interact with each other and their surroundings. The composition and structure of any given community depend on abiotic factors (climate, soil, topography) and biotic factors (competition, herbivory, pollination). Different community types have distinct characteristics and ecological roles.

Forests

Forests are communities dominated by trees that form a closed canopy, shading the layers below. They can be classified by leaf retention (evergreen vs. deciduous), leaf type (broadleaf vs. needleleaf), and climate zone:

• Tropical rainforests (e.g., the Amazon) have year-round warmth and rainfall
• Temperate deciduous forests (e.g., eastern United States) have distinct seasons with leaf drop in fall
• Boreal forests/taiga (e.g., Russia and Canada) have cold temperatures and short growing seasons

Forests provide critical ecosystem services: carbon sequestration, water regulation, and habitat for a huge range of species.

Grasslands

Grasslands are dominated by grasses and other herbaceous plants, with few or no trees. They occur where precipitation is moderate and are maintained by grazing, fire, and periodic drought.

• Temperate grasslands (prairies in North America, pampas in South America)
• Tropical savannas (African savannas with scattered trees)
• Steppes (Eurasian steppes with cold, dry winters)

These communities support diverse herbivores like bison, zebras, and kangaroos, and many have been converted to agricultural land because of their deep, fertile soils.

Deserts

Deserts are communities adapted to arid conditions with low precipitation and high evaporation. Desert plants have evolved water-conserving adaptations: thick cuticles, reduced leaf size, and deep or widespread root systems.

• Hot deserts (Sahara)
• Cold deserts (Gobi)
• Coastal deserts (Atacama)

Despite harsh conditions, deserts support unique flora like cacti and succulents, along with specially adapted animals like kangaroo rats and camels.

Tundra

Tundra occurs in cold regions with very short growing seasons, either at high latitudes (Arctic tundra in northern Alaska and Siberia) or high elevations (alpine tundra in the Rocky Mountains). Vegetation is low-growing: mosses, lichens, sedges, and dwarf shrubs, all adapted to extreme cold, wind, and a thin active soil layer above permafrost.

Tundra ecosystems are especially sensitive to climate change and support species like caribou, musk oxen, and migratory birds.

Wetlands

Wetlands form where soils are saturated or covered by standing water. Plants here are adapted to waterlogged conditions and fall into three categories: emergent (rooted in shallow water with stems above the surface), floating (on the water surface), and submerged (entirely underwater).

• Freshwater marshes (Everglades)
• Saltwater marshes (coastal Louisiana)
• Mangrove swamps (Florida Keys)

Wetlands punch above their weight in ecosystem services: water purification, flood control, and habitat for both aquatic and terrestrial species.

Characteristics of plant communities

Every plant community has measurable characteristics that define its structure, composition, and function. These traits are shaped by environmental factors and ecological processes, and they determine the community's role in its ecosystem.

Species composition

Species composition refers to which plant species are present and their relative abundances. It's determined by climate, soil, disturbance history, and biotic interactions.

Three useful categories for thinking about composition:

• Dominant species make up the bulk of the community's biomass (e.g., oaks in an oak-hickory forest)
• Keystone species have outsized ecological influence relative to their abundance (e.g., saguaro cactus in the Sonoran Desert, which provides food and shelter for many animals)
• Indicator species signal specific environmental conditions (e.g., sphagnum moss indicates acidic, boggy conditions)

Shifts in species composition often signal environmental changes or ongoing succession.

Vertical structure

Vertical structure describes how vegetation is arranged in height layers. A tropical rainforest, for example, has a tall emergent layer, a main canopy, one or more understory layers, and a ground layer. Tundra, by contrast, has almost no vertical layering since everything grows close to the ground.

Light availability is the main driver of vertical structure. The canopy intercepts most sunlight, so understory species must tolerate shade or exploit gaps. This layering creates different microclimates at each level and allows more species to coexist by partitioning resources vertically.

Horizontal structure

Horizontal structure describes the spatial arrangement of vegetation across a landscape: patches of dense growth, gaps, and transition zones (ecotones) between different communities.

• Topography creates variation (e.g., a south-facing slope vs. a north-facing slope in the Northern Hemisphere)
• Soil heterogeneity produces patches of different vegetation
• Disturbance patterns like windthrow or fire create gaps and mosaics

Horizontal structure affects how organisms move through a landscape, how disturbances spread, and how diverse the community can be.

Factors influencing plant communities

Plant communities are shaped by a complex mix of abiotic and biotic factors operating at different scales, from local microhabitats to regional climate patterns, and from short-term disturbances to long-term evolutionary processes.

Climate

Climate is the single biggest factor controlling which plant communities occur where at regional and global scales. Temperature and precipitation patterns determine which species can grow, reproduce, and survive.

Key climatic variables include mean annual temperature, length of the frost-free period, total annual precipitation, and how rainfall is distributed across seasons. Climate change is already shifting the distribution and composition of plant communities worldwide, as species ranges move poleward or to higher elevations.

Soil properties

Soil texture, fertility, pH, and moisture all influence which plants can establish and compete successfully. Different species have different soil requirements, which is why distinct plant communities form on different soil types.

• Sandy soils drain quickly and support drought-adapted vegetation
• Clay soils retain moisture and support plants that tolerate wet roots
• Serpentine soils are low in essential nutrients and high in heavy metals, supporting a specialized flora found almost nowhere else

Soil degradation through erosion, salinization, or nutrient depletion can dramatically alter community composition and productivity.

Topography

Elevation, slope angle, and aspect (which direction a slope faces) create local environmental gradients. These affect microclimate, soil development, and water availability.

A classic example: on mountains, you'll see distinct vegetation zones as you climb from base to summit, each with its own plant community. In temperate regions of the Northern Hemisphere, south-facing slopes receive more direct sunlight and tend to be warmer and drier than north-facing slopes, supporting different plant communities just meters apart. Topographic variety increases overall diversity by providing many different microhabitats.

Biotic interactions

Plants don't exist in isolation. Their interactions with each other and with animals, fungi, and microbes shape community structure.

• Competition for light, water, and nutrients determines which species dominate
• Facilitation occurs when one species helps another, like a nurse plant shading a seedling from intense sun in a desert
• Herbivory by insects and mammals can suppress some species and release others from competition
• Mutualism with pollinators, seed dispersers, and mycorrhizal fungi supports plant reproduction and nutrient uptake

These interactions can lead to species coexistence, dominance by certain species, or exclusion of others, depending on their strength and direction.

Succession in plant communities

Succession is the process by which the composition and structure of a plant community changes over time, typically after a disturbance or on newly available habitat. It's driven by differences in how quickly species colonize, grow, and modify their environment.

Primary succession

Primary succession occurs on brand-new substrates where no soil or vegetation previously existed: volcanic lava flows, glacial moraines, newly exposed rock, or sand dunes.

The process follows a general pattern:

1. Pioneer species (lichens, mosses, hardy plants) colonize the bare substrate
2. These pioneers begin breaking down rock and accumulating organic matter, slowly building soil
3. As soil develops, new species that need more nutrients can establish
4. Over decades to centuries, later-successional species gradually replace the pioneers

Classic examples include vegetation colonizing lava flows in Hawaii and plant communities developing on glacial moraines in Alaska. Primary succession is slow because soil must be built from scratch.

Secondary succession

Secondary succession occurs on sites that were previously vegetated but experienced a disturbance: abandoned farm fields, logged forests, or burned areas. Because soil and a seed bank already exist, secondary succession is much faster than primary succession.

1. Surviving roots, seeds in the soil, and seeds from nearby areas begin to regenerate
2. Fast-growing, sun-loving species (often called "early successional" species) dominate first
3. Over time, slower-growing, shade-tolerant species establish and eventually overtake the early colonizers

The speed and trajectory depend on the type and severity of the disturbance, the availability of seeds and root fragments, and local environmental conditions.

Climax communities

A climax community is the relatively stable, self-perpetuating community that develops at the end of succession when no major disturbance intervenes. Examples include old-growth forests of the Pacific Northwest and tallgrass prairies of the Great Plains.

That said, the climax concept has been debated among ecologists. Many ecosystems experience periodic disturbances (fire, storms, insect outbreaks) and may never reach a true stable endpoint. A more modern view treats climax as a useful concept for understanding successional direction, while recognizing that most real communities exist somewhere along the successional continuum.

Major terrestrial biomes

Biomes are large-scale ecological regions characterized by distinct plant communities, climatic conditions, and soil types. They represent the broadest way to categorize Earth's terrestrial ecosystems.

Tropical rainforests

Found near the equator, tropical rainforests experience high temperatures (averaging 25-28°C year-round), abundant rainfall (typically over 2,000 mm per year), and minimal seasonality. They have the most complex vertical structure of any biome, with a tall canopy (30-50 m), emergent trees poking above it, multiple understory layers, and a relatively sparse ground layer due to low light penetration.

Tropical rainforests contain the highest species diversity of any terrestrial biome. Major examples include the Amazon, the Congo Basin, and the rainforests of Southeast Asia.

Temperate forests

Temperate forests occur at mid-latitudes with moderate temperatures, distinct seasons, and adequate precipitation (750-1,500 mm per year). Two main types exist:

• Deciduous forests are dominated by broadleaf trees (oaks, maples, beeches) that drop their leaves in winter. Found across eastern North America and much of Europe.
• Temperate evergreen forests are dominated by conifers like Douglas fir and western red cedar. The Pacific Northwest is a prime example.

The structure and composition of temperate forests are heavily influenced by soil fertility, disturbance history, and centuries of human land use.

Boreal forests

Boreal forests (taiga) stretch across high-latitude regions of Russia, Canada, and Alaska, forming the largest terrestrial biome on Earth. Winters are long and harsh; growing seasons are short (about 3-4 months).

Conifers like spruce, fir, and pine dominate because their needle-shaped leaves and conical form help them shed snow and conserve water during frozen winters. The understory is often sparse, with a thick carpet of moss and lichen on the ground. Wildlife includes moose, lynx, wolves, and many boreal bird species.

Savannas

Savannas are tropical or subtropical grasslands with scattered trees or shrubs. They occur where there are distinct wet and dry seasons, and their open structure is maintained by a combination of fire, grazing, and soil conditions that prevent a closed canopy from forming.

Savannas support some of the highest concentrations of large herbivores on Earth: antelopes, zebras, elephants, and their predators. Major examples include the Serengeti in East Africa, the Cerrado in Brazil, and the tropical savannas of northern Australia.

Temperate grasslands

Temperate grasslands occur at mid-latitudes with moderate precipitation (250-750 mm per year), cold winters, and hot summers. Grasses and forbs dominate, with trees largely absent due to drought, fire, and grazing pressure.

Their deep, fertile soils have made them prime targets for agriculture. The North American prairies, South American pampas, and Eurasian steppes have all been extensively converted to cropland, making intact temperate grasslands one of the most endangered biome types.

Deserts

Deserts receive less than 250 mm of precipitation per year and experience high evaporation rates and often extreme temperature swings. Vegetation is sparse and highly specialized: cacti, succulents, and drought-resistant shrubs with deep taproots or shallow, widespread root systems.

Major deserts include the Sahara (hot), the Gobi (cold), and the Sonoran (with its iconic saguaro cacti). Community structure varies with soil type, topography, and the timing and amount of the little rain that does fall.

Tundra

Tundra occurs at high latitudes (Arctic tundra) and high elevations (alpine tundra). Temperatures are cold, growing seasons are very short (6-10 weeks in the Arctic), and precipitation is low. Permafrost, a permanently frozen soil layer, underlies most Arctic tundra and prevents deep root growth.

Vegetation is dominated by mosses, lichens, sedges, and dwarf shrubs, all hugging the ground to avoid wind exposure. Tundra stores large amounts of carbon in its frozen soils, making it a critical biome in the context of climate change.

Aquatic plant communities

Aquatic plant communities are found in lakes, rivers, oceans, and wetlands. They face unique challenges: obtaining light underwater, anchoring in shifting substrates, and exchanging gases in a liquid medium. These communities play important roles in providing habitat, stabilizing sediments, and regulating water quality.

Freshwater communities

Freshwater plant communities occur in lakes, ponds, rivers, and streams and include submerged, floating, and emergent plants. Their composition depends heavily on water clarity (which controls how deep light penetrates), nutrient availability, and water level fluctuations.

• Submerged macrophyte beds in clear lakes provide habitat for fish and invertebrates
• Floating plant communities (like duckweed mats) thrive in nutrient-rich, still water
• Emergent vegetation (cattails, bulrushes) grows in shallow margins

Marine communities

Marine plant communities include seagrasses, macroalgae (seaweeds), and phytoplankton, each filling a different ecological role.

Seagrass meadows grow in shallow coastal waters and serve as nursery habitat for many fish and invertebrate species. Kelp forests are formed by large brown macroalgae in cool, rocky subtidal areas and support exceptionally high biodiversity. Phytoplankton are microscopic algae that drift in open water, forming the base of marine food webs and playing a major role in global carbon cycling.

Marine plant communities are threatened by coastal development, pollution, ocean warming, and acidification.

Estuarine communities

Estuaries are transition zones where freshwater meets saltwater, creating environments with variable salinity, water levels, and sediment conditions. Plants here must tolerate these fluctuations.

• Salt marshes (e.g., along the Atlantic coast of North America) are dominated by salt-tolerant grasses
• Mangrove swamps (tropical coastlines) feature salt-tolerant trees with specialized root systems
• Estuarine seagrass beds (e.g., in the Mediterranean) grow in shallow, brackish water

These communities provide shoreline stabilization, nutrient cycling, and nursery habitat for commercially important fish and shellfish. They're threatened by land reclamation, pollution, and sea-level rise.

Human impacts on plant communities

Human activities are reshaping plant communities worldwide, altering their composition, structure, and ability to provide ecosystem services. The main drivers are land-use change, habitat fragmentation, pollution, invasive species, and climate change.

Deforestation

Deforestation is the conversion of forest to other land uses like agriculture, pasture, or urban development. It's driven by population growth, economic development, and global demand for commodities like timber, palm oil, and soybeans.

The consequences extend well beyond the loss of trees: deforestation reduces biodiversity, releases stored carbon into the atmosphere (contributing to climate change), disrupts water cycles, and degrades soils. Tropical deforestation is especially concerning because tropical forests hold the majority of terrestrial species diversity. Between 2001 and 2020, the tropics lost roughly 411 million hectares of tree cover, an area larger than the entire European Union.