Temperature and precipitation are the two abiotic factors that do the heaviest lifting in defining a biome. Most classification systems place them on perpendicular axes and use the combination to predict whether a given patch of Earth will be desert, tropical forest, tundra, or something in between. But that two-variable shorthand, while powerful, leaves out quite a bit. Soil chemistry, elevation, fire frequency, wind exposure, and the timing of rainfall all push vegetation and animal communities in directions that temperature and precipitation alone cannot explain.
Temperature and Precipitation as the Primary Axes
The most widely taught framework for understanding biome distribution plots mean annual temperature on one axis and mean annual precipitation on the other. This approach, formalized in what is known as Whittaker’s Biome Diagram, treats the two variables as independent and maps vegetation types into distinct zones within that two-dimensional space.1Flora. Schimperian World: The foundations of biome ecology The logic is intuitive: a place that is hot and wet tends to grow tropical rainforest, a place that is cold and dry tends to be tundra, and the space in between fills in with grasslands, temperate forests, deserts, and so on.
Temperature matters not just as an annual average but as a set of thresholds that determine which life forms can survive. The presence or absence of trees, for instance, is often controlled by low-temperature limits. Whether tree species can flush new leaves, tolerate freezing at the moment of flushing, and complete their growth cycle within the available warm season determines whether forest can exist at a given location.2Trends in Ecology & Evolution. Low-temperature limits of tree species and the life-form tree That is why treeline exists on mountains and why forests give way to tundra at high latitudes: the growing season eventually becomes too short, or spring frosts too severe, for trees to complete their annual cycle.
Even in warmer regions, occasional cold snaps act as a species filter. In tropical savannas and forests, frost tolerance among tree species strongly predicts how far toward higher latitudes a species can extend its range. Rare frost events effectively reinforce the boundary between savanna and forest by killing back frost-sensitive species at the margins.3Journal of Ecology. Rare frost events reinforce tropical savanna–forest boundaries A biome boundary does not need constant cold to stay where it is; a hard freeze once every few decades can be enough to prevent forest from creeping into savanna.
Seasonality Often Matters More Than Totals
Total annual precipitation is only part of the picture. Two locations can receive the same amount of rain per year and host completely different vegetation if one gets rain steadily throughout the year and the other concentrates it into a few wet months separated by a long dry season. In Africa, deciduous and semi-deciduous forests are highly sensitive to even small changes in both the total amount and the seasonal distribution of rainfall.4Global Ecology and Biogeography. Sensitivity of African biomes to changes in the precipitation regime A slight lengthening of the dry season can tip a region from closed-canopy forest toward open woodland or savanna.
Pollen records from central and eastern Africa illustrate this sensitivity over long time spans. After about 6,000 years ago, pollen from deciduous trees increased in proportion at sites in Burundi, Uganda, and the Congo, suggesting that either total precipitation dropped or the dry season grew longer, or both.5Climate of the Past. Simulated effects of a seasonal precipitation change on the vegetation in tropical Africa The shift in forest composition tracked the change in rainfall timing closely enough for researchers to reconstruct past climate from the plant community alone.
Mediterranean-climate regions offer another example. Places like the Cape region of South Africa, coastal California, and parts of Chile and Australia all share a distinctive pattern of cool, wet winters and hot, dry summers. Despite being thousands of kilometers apart, their vegetation has converged on a similar form: evergreen shrubs with tough, leathery leaves. The summer drought and relatively nutrient-poor soils are what drive this convergence.6PLOS ONE. The Consequences of Precipitation Seasonality for Mediterranean-Ecosystem Vegetation of South Africa That plants on separate continents independently evolved the same growth strategy in response to the same rainfall pattern is strong evidence that seasonality, not just annual totals, is a defining abiotic force.
Evapotranspiration and the Moisture Balance
Raw precipitation numbers can be misleading because they ignore how much water the atmosphere pulls back out through evaporation and plant transpiration. A region receiving 500 millimeters of rain per year might support lush grassland in a cool climate where little moisture evaporates, but only sparse desert scrub in a hot climate where potential evaporation far exceeds rainfall. The Holdridge Life Zone system, one of the more detailed classification frameworks, explicitly accounts for this by incorporating biotemperature, precipitation, and the ratio of potential evapotranspiration to precipitation as variables.7Treesearch. Holdridge Life Zone Map: Republic of Argentina
This ratio captures something that temperature or rainfall alone cannot: the net water availability a plant actually experiences. Two spots at the same latitude receiving the same rainfall will dry out at very different rates depending on temperature, wind, humidity, and solar radiation. That is why biome maps based solely on precipitation tend to blur the distinction between, say, a semi-arid grassland and a dry woodland. The evapotranspiration balance sharpens the line.
Soil as a Biome-Shaping Force
Climate is the broadest filter, but once you zoom in, soil takes over as a surprisingly powerful driver of which species can grow where. In naturally regenerating tropical dry forests, the distributions of tree species are strongly driven by soil chemistry and soil texture. In one detailed study, about 94 percent of tree species responded to soil chemistry gradients and 89 percent to soil texture.8Functional Ecology. Edaphic factors, successional status and functional traits drive habitat associations of trees in naturally regenerating tropical dry forests Species that specialized on nutrient-rich soils tended to have higher leaf nutrient concentrations, while those on poorer soils invested more in tough, durable leaves. Within a single climatic zone, soil differences effectively create distinct vegetation communities that look and function differently from one another.
The type of rock underlying the soil matters too. Phosphorus limitation in plants, for example, is not driven by climate the way nitrogen limitation is. Instead, it tracks tightly to the parent material the soil formed from. Soils derived from rocks with low phosphorus content or high acidity tend to produce phosphorus-limited ecosystems regardless of temperature or rainfall.9PubMed. Soil parent material-A major driver of plant nutrient limitations in terrestrial ecosystems This helps explain why two forests at the same latitude and altitude, receiving the same rainfall, can differ dramatically in their productivity and species composition: the answer is often underground.
Elevation and Topography
Climbing a mountain is often compared to traveling toward the poles. As elevation increases, temperature drops, and you can walk through the equivalent of several biomes in a single day’s hike. The rate at which temperature declines with altitude, known as the temperature lapse rate, varies depending on moisture. Along Himalayan transects, drier western slopes showed a steeper drop of about 0.66 °C per 100 meters of elevation, while wetter eastern slopes cooled more gradually at around 0.50 °C per 100 meters.10Biodiversity and Conservation. Spatial and seasonal patterns of temperature lapse rate along elevation transects leading to treelines in different climate regimes of the Himalaya Moisture, snow cover, and the reflectiveness of the surface all modulate this cooling rate, which means the elevation at which a biome boundary sits is not uniform even within a single mountain range.
Mountains also create rain shadows. On Ecuador’s Chimborazo volcano, the western slopes receive far less precipitation than the eastern slopes, producing a striking difference: lush high-altitude grassland on one side and near-desert vegetation on the other. Researchers attribute this pattern primarily to the unequal distribution of precipitation on opposite sides of the mountain.11Arctic, Antarctic, and Alpine Research. Rain-Shadow in the High Andes of Ecuador Evidenced by Páramo Vegetation Rain shadows like this create biome boundaries that are dramatically abrupt. You can sometimes stand on a ridge and see forest on one side and scrubland on the other.
Fire and Other Disturbance Regimes
Not all biome boundaries are set by the slow, steady hand of climate. Fire, in particular, acts as a powerful abiotic force that can maintain entire biomes in states that climate alone would not predict. Across large portions of the tropics in both the Americas and Africa, savanna and forest exist as alternative stable states under the same climatic conditions. Fire feedbacks maintain this arrangement: savannas burn frequently, which kills fire-sensitive tree seedlings and keeps the canopy open, while forests suppress fire by staying moist and shady enough to prevent ignition.12PubMed. Disturbance maintains alternative biome states
This means that in many tropical regions, whether you find savanna or forest has less to do with the local climate than with the fire history. Remove fire from a savanna, and forest may eventually close in. Introduce fire to a forest edge, and savanna can expand. The same abiotic envelope supports both biomes; fire tips the balance. Other abiotic disturbances, including flooding, landslides, and volcanic activity, play analogous roles in specific regions, resetting vegetation to earlier stages and preventing climatic climax communities from ever fully establishing.
Wind Exposure
Wind is easy to overlook when listing the abiotic factors that define a biome, but it has an outsized effect at biome boundaries, particularly at treeline. A study spanning three continents found that wind exposure had a stronger negative effect on seedling abundance at treeline than elevation-related temperature itself.13Journal of Ecology. Wind exposure and light exposure, more than elevation‐related temperature, limit tree line seedling abundance on three continents Seedlings growing in the lee of existing trees, sheltered from wind, survived at higher rates. The treeline, in other words, is not just a temperature line; it is partly a wind line.
Wind also shapes biomes indirectly. In coastal and alpine environments, persistent wind increases evapotranspiration, desiccates exposed tissues, and mechanically damages stems and leaves. The gnarled, stunted trees you see near alpine treelines or exposed coastlines are responding primarily to wind stress, not cold alone. Coastal dune ecosystems, similarly, owe much of their distinctive character to wind-driven sand movement and salt spray rather than to temperature or rainfall patterns.
Abiotic Factors in Aquatic Biomes
The discussion so far has focused on land, but aquatic environments have their own set of abiotic drivers that define distinct biome-like zones. On land, temperature and precipitation dominate; in water, the key variables shift to light penetration, salinity, dissolved oxygen, nutrient availability, and water movement.
Light controls where photosynthesis can occur and therefore where primary productivity is concentrated. In open ocean, the sunlit zone can extend past 100 meters where phytoplankton are sparse, but in productive coastal waters thick with algae, photosynthesis may be confined to just a few meters.14Limnology and Oceanography. Light attenuation and photosynthesis of aquatic plant communities Dissolved organic matter further compresses this productive layer. When light-absorbing dissolved compounds increase, the sunlit zone can shoal by roughly 100 meters, fundamentally altering where life concentrates.15Ecological Modelling. Sensitivity of euphotic zone properties to CDOM variations in marine ecosystem models The depth of this boundary between light and dark is one of the most important abiotic thresholds in aquatic ecology.
Salinity creates another major divide. In estuaries, the gradient from fresh to salt water structures the entire ecosystem. Species distributions, biogeochemical processes, and community composition all shift along this gradient. A critical transition zone sits at around 5 to 8 practical salinity units, where both biological and chemical processes show abrupt, nonlinear changes.16PubMed. Principal processes within the estuarine salinity gradient: a review This narrow band effectively separates freshwater and marine fauna, acting as a biological barrier as real as any mountain range. Mobile organisms like fish and shrimp distribute themselves according to species-specific salinity tolerances, creating distinct biological zones within a single estuary.17Limnology and Oceanography. Ecosystem variability along the estuarine salinity gradient: Examples from long‐term study of San Francisco Bay
Dissolved Oxygen and Thermal Stratification in Lakes
In freshwater lakes and reservoirs, temperature does not just set the overall climate; it physically layers the water column. Warm water sits on top of cold water, and this thermal stratification directly controls how dissolved oxygen distributes from surface to bottom. In a deep subtropical reservoir in China, oxygen stratification closely tracked temperature stratification, with the two linked through the seasons. During summer and autumn, stable layering trapped oxygen-poor water near the bottom while surface waters stayed well-oxygenated.18PubMed. Dissolved oxygen stratification and response to thermal structure and long-term climate change in a large and deep subtropical reservoir (Lake Qiandaohu, China)
As stratification strengthens through the warm months, oxygen in the deep layer slowly declines because sediments at the bottom consume it and the thermal barrier prevents mixing with oxygen-rich surface water. In some reservoirs, dissolved oxygen in the middle layer can drop to just 1 mg/L by late summer, creating zones inhospitable to most fish and invertebrates.19PubMed Central. Dynamics of oxygen evolution in a thermally stratified reservoir under climate warming These oxygen-depleted zones function as abiotic boundaries within a single water body, confining life to specific depth bands much the way elevation confines life to specific altitudinal belts on a mountain.
Continentality and the Ocean’s Buffering Effect
How close you are to the ocean profoundly influences which biome develops at a given latitude. Ocean water absorbs and releases heat slowly, so coastal areas experience milder winters and cooler summers compared to continental interiors at the same latitude. This difference, sometimes called continentality, turns out to be better captured by the annual range of temperatures than by the absolute coldest month temperature traditionally used in some classification systems. Research comparing different classification approaches found that a simple annual temperature range threshold outperformed the older coldest-month criterion for distinguishing continental from maritime climates.20PLOS Climate. Biome-aligned temperature zones for interpretable climate classification via average monthly temperatures
This matters for biome identity because a continental interior with 40 °C summers and minus 30 °C winters hosts a fundamentally different community than a coastal location at the same latitude that never dips below freezing. The interior is boreal forest or steppe; the coast is temperate rainforest or maritime shrubland. The annual mean temperature might be similar in both cases, but the swing between extremes selects for completely different survival strategies.
Ocean Winds and Marine Biome Boundaries
In the ocean itself, wind is among the most important abiotic forces shaping large-scale biological patterns. Along the eastern edges of ocean basins, persistent winds push surface water away from the coast, drawing cold, nutrient-rich water up from below. This upwelling sustains some of the most productive marine ecosystems on the planet. The rate of upwelling is a fundamental determinant of biological structure and production in these coastal systems, and both the depth of the nutrient-rich layer and surface chlorophyll concentrations track wind patterns closely.21PubMed Central. Influence of ocean winds on the pelagic ecosystem in upwelling regions Without the wind, these regions would be far less productive, and the dense populations of fish, seabirds, and marine mammals they support would not exist.
Upwelling zones are effectively a distinct marine biome type defined almost entirely by one abiotic process: wind-driven water movement. They are strikingly different from the nutrient-poor open ocean on either side, even though temperature and salinity may not differ dramatically. The lesson generalizes: in aquatic systems, water movement and nutrient delivery often replace temperature and precipitation as the primary abiotic axes that define where biological communities form and what they look like.
Permafrost and Frozen Ground
In high-latitude biomes, the ground itself is a defining abiotic factor. Permafrost, soil that stays frozen year-round, controls drainage, nutrient cycling, and which plant roots can penetrate to what depth. Tundra and boreal ecosystems owe much of their character to the presence of permafrost rather than to air temperature alone. When the ground is perpetually frozen just below the surface, trees cannot root deeply, waterlogged soils form bogs and fens, and decomposition slows to a crawl, locking enormous quantities of carbon in the soil.
The sensitivity of permafrost to warming is striking. Measurements from sub-Arctic tundra sites found minimal thawing until mean annual air temperatures rose above freezing, after which thaw depth increased by over a meter for every 1 °C rise in mean annual air temperature. Under projected warming scenarios, permafrost in the upper three meters of soil was not expected to persist through the end of this century.22Journal of Geophysical Research: Biogeosciences. Projecting Permafrost Thaw of Sub‐Arctic Tundra With a Thermodynamic Model Calibrated to Site Measurements Lose the permafrost, and the biome changes: drainage patterns shift, trees can establish where they previously could not, and the stored carbon begins to decompose and enter the atmosphere.
How Climate Change Reshapes Biome Boundaries
Because biomes are defined by abiotic factors, shifting those factors means shifting the biomes themselves. Modeling studies project that tundra and boreal forests are among the most vulnerable, with an estimated 787 million hectares of these two biome types threatened by encroachment from lower-latitude vegetation by 2080 as temperatures rise.23PubMed Central. Climate vulnerability of Earth’s terrestrial biomes The concern goes beyond habitat loss: these ecosystems store massive quantities of carbon in peatland and permafrost soils that could be released as conditions warm and dry.
Projections for other regions show change concentrated along existing biome boundaries rather than wholesale replacement of one biome by another.24Ecography. Divergent projections of future African biome shifts with process‐based and species distribution models In Africa, for instance, models agree that biome shifts will occur but disagree on the magnitude, partly because different modeling approaches handle fire, soil, and species-level responses differently. That disagreement highlights how biomes are not just climate envelopes. They are the product of interacting abiotic factors, and predicting how they will shift requires accounting for the full set of drivers, not temperature alone.
Why Early Biogeographers Got the Big Picture Right
The idea that climate governs the geography of life is old. Alexander von Humboldt, working in the early 1800s, was among the first to systematically connect climate variables to species distributions, including the elevational and latitudinal gradients that structure biomes. His influence on modern biogeography remains strong, particularly in relating climate to species distributions and in the use of tools like species-distribution modeling.25Journal of Biogeography. Challenges and opportunities for biogeography—What can we still learn from von Humboldt?
What has changed since Humboldt’s era is not the central insight but the resolution. We now understand that the broad strokes of temperature and precipitation are modulated by fire, soil, wind, ocean currents, topography, and the physical structure of water columns. A biome is not a line on a two-axis graph. It is the outcome of every abiotic factor that filters which organisms can survive, reproduce, and persist in a given place, from the mineral composition of the bedrock to the timing of a once-in-a-decade frost.