Forests cover roughly a third of Earth’s land surface, but the word “forest” describes an enormous range of ecosystems that look, function, and feel nothing alike. The dense, dripping canopy of a tropical rainforest and the sparse, cold-adapted spruce stands of the boreal taiga are both forests, yet they differ in nearly every measurable way: temperature, rainfall, tree diversity, soil chemistry, and the species that depend on them. Ecologists broadly sort forests into a handful of major types based on climate, latitude, and the dominant tree species, though the boundaries between types are often blurry and shifting.
Tropical Rainforests
Tropical rainforests are the archetype most people picture when they hear the word “forest.” They have tall, closed, evergreen canopies, often exceeding 25 meters in height, and they grow where annual rainfall tops roughly 2,000 millimeters with little to no dry season.1Current Biology. Tropical savannas and dry forests These forests support the highest tree species richness of any biome. Research using data from dozens of large forest plots around the world has confirmed that warm, moist, aseasonal climates directly support more tree species, both because fewer species are excluded by climatic stress and because higher productivity supports larger populations that can be partitioned among more species.2PubMed Central. Direct and indirect effects of climate on richness drive the latitudinal diversity gradient in forest trees
Tropical rainforests also harbor extraordinary complexity underground. The networks linking tree roots to fungi are highly specialized in these forests, with host-symbiont associations that are significantly more specific than you might expect from a system with so many potential partners.3PubMed Central. Structural diversity across arbuscular mycorrhizal, ectomycorrhizal, and endophytic plant-fungus networks These fungal partnerships help trees access nutrients in soils that are often surprisingly poor, since high temperatures and rainfall break down organic matter quickly.
Tropical Dry Forests and Savannas
Where rainfall in the tropics is lower and distinctly seasonal, with four to nine months of virtually no rain, the landscape shifts to savannas and dry forests rather than dense rainforest.4Current Biology. Tropical savannas and dry forests Tropical dry forests share the warmth of rainforests but have evolved entirely different strategies for surviving prolonged drought. Trees in these systems are often deciduous, shedding their leaves during the dry months to conserve water.
The boundary between tropical dry forest and rainforest is not a clean line. Research in the Americas shows that this transitional zone, the ecotone, tends to settle at mid-elevations where topography channels moisture. Interannual shifts in rainfall push the boundary upslope or downslope from year to year, and there is evidence that the dry-forest side of this boundary is less resilient to future reductions in precipitation.5PubMed. Climate and topography control variation in the tropical dry forest-rainforest ecotone If droughts become more frequent, the dry forest edge may retreat, effectively shrinking the transition zone.
Defining exactly what counts as a tropical dry forest has proven surprisingly contentious. Different classification systems, based on different bioclimatic thresholds, disagree substantially about where tropical dry forest exists. When tested against field plots worldwide, the most widely used definition from the UN’s Food and Agriculture Organization agreed with ground truth about four-fifths of the time, but some competing definitions matched barely a third of field-verified sites.6PLoS ONE. Global tropical dry forest extent and cover: A comparative study of bioclimatic definitions using two climatic data sets This matters for conservation planning: if you cannot agree on where dry forests are, you cannot protect them effectively.
Boreal Forests
The boreal forest, also called the taiga, forms a vast belt across northern North America, Scandinavia, and Siberia. It is the largest terrestrial biome on Earth. Dominated by cold-hardy conifers like black spruce, the boreal forest is an environment where low temperatures govern almost everything: the length of the growing season, the severity of frost damage, and even the pace of nutrient cycling. Black spruce forests are generally nitrogen-poor, and in North America and Siberia their distribution closely mirrors the distribution of discontinuous permafrost.7Soil Biology and Biochemistry. Soil amino acid turnover dominates the nitrogen flux in permafrost-dominated taiga forest soils Although much of the northern boreal is underlain by continuous or discontinuous permafrost, the southern stretches are entirely permafrost-free.8Alaska’s Changing Boreal Forest. Climate and Permafrost Dynamics of the Alaskan Boreal Forest
Temperature drives the boreal forest’s yearly rhythm. The timing of snowmelt and the start of the growing season determine how much photosynthesis happens in a given year, and severe frosts can damage needles and reduce the total leaf area available for growth.9PubMed. The importance of phenology for the evaluation of impact of climate change on growth of boreal, temperate and Mediterranean forests ecosystems: an overview This makes the boreal forest especially sensitive to warming: a few extra weeks of growing season can substantially change how much carbon these forests absorb.
Temperate Forests
Temperate forests occupy the mid-latitudes, stretching across eastern North America, Europe, and East Asia. They are predominantly mixed-species deciduous forests, and like boreal forests, their phenology is driven primarily by temperature. But because multiple broadleaf tree species coexist and respond to temperature differently, warming or cooling can tip the competitive balance between species.10PubMed. The importance of phenology for the evaluation of impact of climate change on growth of boreal, temperate and Mediterranean forests ecosystems: an overview A species that leafs out earlier in spring captures more light, potentially outcompeting neighbors that respond more slowly.
A special subset of temperate forests deserves separate attention: temperate rainforests. Found in coastal regions with extremely high rainfall, places like the Pacific Northwest of North America, southern Chile, New Zealand, and Tasmania, temperate rainforests are the most carbon-dense forest ecosystem on the planet, storing several times more carbon per hectare than most other forested biomes.11Canadian Journal of Forest Research. Watershed-scale forest biomass distribution in a perhumid temperate rainforest as driven by topographic, soil, and disturbance variables Their massive old-growth trees, thick mossy understories, and waterlogged soils all contribute to this exceptional carbon storage.
Mediterranean Forests
Mediterranean forests grow in regions with hot, dry summers and mild, wet winters, a climate found not just around the Mediterranean Sea but also in parts of California, central Chile, South Africa, and southwestern Australia. Unlike boreal and temperate forests, the key driver here is water, not temperature. Water availability governs how much leaf area trees can develop, and the timing of drought shapes when and how vigorously trees grow.12PubMed. The importance of phenology for the evaluation of impact of climate change on growth of boreal, temperate and Mediterranean forests ecosystems: an overview
Trees in Mediterranean forests have evolved remarkably varied drought survival strategies, even among species that look superficially similar. In one study of three sclerophyllous (tough-leaved) tree species growing side by side in the same conditions, each used a completely different approach: one tolerated severe internal water loss, another spent water freely to keep its tissues hydrated through deep root access, and the third conserved water aggressively while developing the ability to recover from even tiny losses by pulling water from the soil with exceptionally low leaf water pressure.13PubMed. Different strategies of drought resistance in three Mediterranean sclerophyllous trees growing in the same environmental conditions
This diversity of strategies is not random. Studies using common garden experiments, where seeds from different populations are grown in the same location, have found that differences in drought tolerance and cold hardiness among populations track the precipitation and temperature gradients of their home environments, indicating that natural selection has driven adaptation to local climates within species.14Journal of Ecology. Adaptation of Mediterranean forest species to climate: Lessons from common garden experiments The large plant diversity that characterizes Mediterranean ecosystems also relies on species dividing up soil resources in space and time, exploiting different soil layers and different seasons to avoid direct competition.15PubMed Central. Plant-soil interactions in Mediterranean forest and shrublands: impacts of climatic change
Peat Swamp Forests
Not all forest types are defined by temperature and rainfall gradients. Peat swamp forests, found mainly in Southeast Asia, are shaped by waterlogging. These forests grow on thick layers of partially decomposed plant material (peat) that accumulates because saturated, oxygen-poor conditions slow decomposition to a crawl. One site in Sumatra’s Tripa Peat Swamp Forest began accumulating peat roughly 2,360 years ago on lowland terrain between two rivers, initially fed by river flooding and rising groundwater, then sustained by rainfall alone as precipitation increased in recent centuries.16Palaeogeography, Palaeoclimatology, Palaeoecology. Late Holocene vegetation and environmental dynamics in Tripa Peat Swamp Forest – Leuser Ecosystem, Aceh, Indonesia Peat swamp forests store enormous amounts of carbon in their soils, making their drainage and conversion to plantations a major source of greenhouse gas emissions.
How Secondary Forests Recover
Most of the world’s forests are not pristine old-growth. A large fraction are secondary forests, regrowing after logging, farming, or other disturbance. Understanding how these secondary forests recover, and where they fall short, is critical for conservation.
The headline finding from tropical research is that secondary forests recover species counts faster than they recover species identity. Across Neotropical secondary forests, species richness bounces back to about 80% of old-growth levels within 20 years, and it typically takes around five decades to reach full recovery. But the species composition, the actual mix of species present, recovers much more slowly, reaching only about a third of old-growth composition after 20 years and requiring centuries for full recovery.17PubMed. Biodiversity recovery of Neotropical secondary forests In other words, a 20-year-old secondary forest may have a lot of species, but they are not the same species as in the original forest. The rare, specialized species that define old-growth communities are the slowest to return.
Biomass recovery follows yet another trajectory. In a study of secondary forests in southwestern Costa Rica, aboveground biomass accumulated rapidly in the first years of succession, reaching about half the biomass of old-growth forests after 20 years. Species richness in that same study, however, had reached only about 31% of old-growth levels after 20 years.18Forest Ecology and Management. Recovery of aboveground biomass, species richness and composition in tropical secondary forests in SW Costa Rica This means secondary forests can look impressively lush and tall while still missing the majority of species found in undisturbed forest. Protecting existing old-growth alongside regenerating secondary forests is the dual strategy researchers consider essential for maintaining biodiversity in tropical landscapes.
Animal recovery adds another dimension. A meta-analysis of paired primary and secondary tropical forest sites found that forest-specialist bird species gradually returned to secondary forests as they aged. Interestingly, the recovery of bird community composition over time was evident in Old World tropical forests but not clearly so in the New World, hinting that regional history and species pools shape recovery patterns as much as forest age alone.19Journal of Applied Ecology. The effects of tropical secondary forest regeneration on avian phylogenetic diversity
Why Plantations Are Not Forests in the Ecological Sense
Tree plantations are often counted as forest cover in national statistics, but ecologically they function very differently from natural forests. A global analysis found that tree plantations supported lower species richness and abundance across multiple groups of organisms compared to primary forests. In terms of biodiversity, plantations performed about as well as naturally regenerating secondary forests, but they fell clearly short of old-growth.20Global Ecology and Biogeography. A global meta‐analysis of the impacts of tree plantations on biodiversity
Monoculture plantations are especially damaging. In the Italian Prealps, spruce monocultures planted about a century ago still harbored roughly 39% fewer plant species than native mixed forests and 56% fewer than the pastures they replaced. The diversity of different plant growth forms, from trees to understory shrubs and herbs, also dropped in the plantations. Researchers linked these losses to lower light levels and cooler ground temperatures under the dense spruce canopy.21Ecological Solutions and Evidence. Tree monoculture plantations decrease plant diversity in the Italian Prealps The persistence of these effects a full century after planting is striking and suggests that simply planting trees does not restore the ecological functions of a natural forest.
The damage extends belowground. Converting rainforest to plantation leads to roughly a 50% loss of soil organic carbon, with cascading effects on nitrogen cycling that can increase greenhouse gas emissions.22Soil Biology and Biochemistry. Conversion to agroforestry and monoculture plantation is detrimental to the soil carbon and nitrogen cycles and microbial communities of a rainforest So while plantations can sequester carbon in their growing trees, the net picture is more complicated once you account for what happens to the soil.
Edge Effects and Forest Fragmentation
When forests are broken up by roads, farms, or development, the newly created edges fundamentally alter the remaining forest. Forest near edges is warmer, drier, receives more light, and is more exposed to wind and other disturbances than intact interior forest.23PubMed Central. A unifying framework for understanding how edge effects reshape the structure, composition and function of forests These changes ripple inward and reshape what grows there.
Studies using lidar (laser-based remote sensing of canopy structure) have shown that the link between edges and degradation works through changes in canopy structure. Plots near forest edges have shorter canopies with more gaps, which drives higher temperature extremes, lower biomass, and lower taxonomic and functional diversity.24Journal of Ecology. UAV‐Lidar reveals that canopy structure mediates the influence of edge effects on forest diversity, function and microclimate What is adjacent to the edge matters too. In the northern Amazon, forest edges next to cattle pastures had lower humidity, higher temperatures, and less species richness than edges next to rubber plantations, with generalist species characteristic of disturbed environments dominating the pasture-adjacent edges.25Forest Ecology and Management. Edge influence on the microclimate and vegetation of fragments of a north Amazonian forest
This is a practical concern for conservation in every forest type. A protected area that is long and narrow, with a high ratio of edge to interior, provides less functional habitat than its acreage would suggest. Fragment shape and connectivity to other forest patches are as important as total area.
How Forests Shape Water and Climate
Forests do not just respond to climate; they actively create it. Trees pull water from the soil and release it as vapor through their leaves, and this transpiration feeds moisture back into the atmosphere. On average, at least 40% of rainfall over land originates from evapotranspiration, with far higher contributions in some regions. In the Río de la Plata basin, for instance, moisture recycled from the Amazon contributes more than 70% of rainfall. Satellite observations show that across most of the tropics, air that passes over forests for ten days produces at least twice as much rain as air passing over sparse vegetation.26Global Environmental Change. Trees, forests and water: Cool insights for a hot world Losing forest upwind can reduce rainfall thousands of kilometers downwind.
Forests also cool the landscapes around them. Research on oasis systems in arid regions found that evapotranspiration from a reservoir and surrounding riparian forest reduced peak daily temperatures by 7°C and the daily temperature swing by 6°C compared to nearby areas with sparse vegetation, while also boosting local moisture recycling enough to increase precipitation.27Water Resources Research. The Cooling Effect of Oasis Reservoir‐Riparian Forest Systems in Arid Regions This cooling function is relevant far beyond deserts: urban forests, riparian buffers, and even farm windbreaks all leverage the same principle.
For carbon storage, management approach matters. Old-growth forest conservation stores more carbon in soils than any management strategy. Intensive management using fast-growing species and soil preparation can enhance the rate of carbon being pulled from the atmosphere, but it trades soil carbon for aboveground growth. Extensive management sits between the two extremes.28Current Forestry Reports. Forest Carbon Management: a Review of Silvicultural Practices and Management Strategies Across Boreal, Temperate and Tropical Forests
Climate Change Is Redrawing Forest Boundaries
All of these forest types exist in climate envelopes, ranges of temperature and moisture where their dominant species can thrive. As those envelopes shift under warming, the forests shift too, though rarely fast enough to keep up. Human-caused climate change has already driven extensive tree mortality across West Africa and the western United States and has caused biome shifts around the world.29Parks Stewardship Forum. Tree Mortality, Biome Shifts, and Living Sustainably to Halt Human-Caused Climate Change
At high elevations, treelines are expanding upslope in response to rising temperatures, a process that sounds benign until you consider what it replaces. Alpine meadows and tundra ecosystems above the treeline harbor specialized plant communities that have no higher ground to retreat to. Evidence suggests this treeline advance is already reducing alpine diversity and altering functions like carbon cycling and nutrient processing.30Arctic, Antarctic, and Alpine Research. Consequences of Treeline Shifts for the Diversity and Function of High Altitude Ecosystems
In the tropics, as noted earlier, the dry-forest side of the rainforest-dry forest boundary appears less resilient to precipitation declines than the rainforest side.31PubMed. Climate and topography control variation in the tropical dry forest-rainforest ecotone If drought frequency increases, the net result could be a contraction of dry forests from both sides: losing ground to savanna where it gets drier, while failing to colonize rainforest zones where conditions remain wetter.
The Deep History of Forest Types
It is easy to think of the world’s major forest types as ancient and permanent, but the fossil record tells a different story. During the Cretaceous period, the tropics were forested, but those forests bore no resemblance to modern rainforests. They were dominated by gymnosperms and ferns, lacked a closed canopy, and had no modern equivalent. The mass extinction event at the end of the Cretaceous wiped the slate clean. Modern-style tropical rainforests, with their multi-layered canopy dominated by flowering plants like legumes, first appeared at the start of the Cenozoic, roughly 66 million years ago. Tropical dry forests emerged by the late Eocene, while savannas and montane forests are much younger, expanding dramatically only in the last few million years at the expense of rainforest.32PubMed. The evolution of extant South American tropical biomes
The boreal forest may be the youngest of the major biomes. Nothing resembling today’s taiga existed during the warm greenhouse climates of the Mesozoic and Paleogene. The conifer lineages that now dominate boreal latitudes likely originated in montane forests of the western North American mountains, spreading to lower elevations and then eastward and into Eurasia via the Bering land bridge as global temperatures cooled after the Eocene.33Global and Planetary Change. Global greenhouse to icehouse and back again: The origin and future of the Boreal Forest biome In Africa, the earliest solid evidence for angiosperm-dominated tropical rainforest dates to roughly 39 to 26 million years ago, based on pollen assemblages from Cameroon. Grass-dominated savannas began expanding around 16 million years ago and became widespread by about 8 million years ago.34PubMed Central. Palaeobotanical studies from tropical Africa: relevance to the evolution of forest, woodland and savannah biomes
Understanding this history reframes how we think about forest conservation. The biomes we are trying to protect are not static features of the landscape; they are contingent outcomes of evolutionary events, tectonic shifts, and climate oscillations spanning tens of millions of years. The tropical dry forests at risk from increasing drought today have been expanding and contracting for epochs. What is new is the speed of change and the additional pressures from habitat fragmentation, plantation conversion, and direct human use that leave forests less room to adapt or migrate.