Understanding Old-Growth Forests

Old-growth forests are not merely collections of large, old trees. They are complex, self-regulating ecosystems that have evolved over centuries—often millennia—without catastrophic human intervention. These forests exhibit a layered canopy, abundant deadwood, and a mosaic of gaps created by natural tree falls, which together create unique microhabitats. Typical characteristics include a high diversity of tree ages and sizes, standing dead snags, fallen logs, and deep, undisturbed soils rich in organic matter. Key structural attributes include multiple canopy layers, large-diameter living trees with broken tops, pit-and-mound topography from root throws, and a continuous supply of coarse woody debris. Iconic examples include the temperate rainforests of the Pacific Northwest (USA), the ancient beech forests of Białowieża (Poland/Belarus), and the towering mountain ash forests of Australia. Understanding what makes a forest truly “old-growth” is critical because restoration efforts must aim to recreate these complex structural and functional attributes, not just a veneer of tall trees.

Ecological Benefits of Restoring Old-Growth Forests

Biodiversity Enhancement

Old-growth forests serve as irreplaceable refuges for species that depend on mature structural elements. Many birds, such as the northern spotted owl and the marbled murrelet, require large-diameter trees for nesting and are absent from younger stands. The presence of dead wood—both standing snags and fallen logs—supports an astonishing array of wood-decay fungi, insects, amphibians, and cavity-nesting mammals. Restoration of old-growth conditions in conservation areas can reconnect fragmented habitats, allowing gene flow and population recovery for threatened species. For example, re-establishing a continuous canopy and diverse understory in the Great Bear Rainforest in British Columbia has helped maintain viable populations of grizzly bears, salmon, and the coastal gray wolf. In the Pacific Northwest, restoration thinning in second-growth stands has accelerated the development of old-growth characteristics, leading to recolonization by red-backed voles and Pacific tree frogs that depend on moist, debris-rich forest floors. Similarly, in the ancient forests of Tasmania, restoration of logging coupes to old-growth structure is essential for the survival of the iconic Tasmanian devil, which relies on large hollow logs for denning. Beyond vertebrates, old-growth forests harbor an immense diversity of epiphytic mosses, lichens, and liverworts that are sensitive to disturbance and require stable microclimates found only in mature stands. The restoration of these complex habitats is a cornerstone of biodiversity conservation.

Carbon Sequestration and Climate Mitigation

Contrary to earlier assumptions that old-growth forests are carbon-neutral, research shows they continue to accumulate carbon in soils and living biomass over centuries. A study in Nature (Luyssaert et al., 2008) demonstrated that old-growth forests store significantly more carbon per hectare than younger forests, particularly in the deep mineral soil layers. Restoration that accelerates the development of old-growth characteristics—such as encouraging large legacy trees and enriching soil organic matter—can enhance this carbon sink. Moreover, by preventing deforestation and degradation, restoration avoids the immediate release of stored carbon. The Intergovernmental Panel on Climate Change (IPCC) recognizes the protection and restoration of old-growth forests as a high-priority nature-based climate solution. Recent studies using eddy covariance towers in tropical old-growth forests show that these ecosystems can continue to sequester carbon for hundreds of years, with soil carbon accounting for up to 60% of total ecosystem carbon. Restoration efforts that focus on rebuilding deep forest soils—by retaining logging residues, adding wood chips, and promoting root growth—can accelerate carbon storage in the mineral soil horizon, which is less vulnerable to fire than surface carbon. For instance, in the Douglas-fir forests of the Pacific Northwest, active restoration through variable-density thinning and prescribed fire has been shown to increase total ecosystem carbon storage over 50-year rotation cycles compared to even-aged management. See the Nature study.

Soil and Water Conservation

The deep root systems and extensive mycorrhizal networks in old-growth forests bind soil, preventing erosion on steep slopes and stabilizing watersheds. The thick forest floor—composed of leaf litter, mosses, and decaying wood—acts as a natural sponge, reducing surface runoff and recharging groundwater. Restoring these structural layers in degraded conservation areas can improve water quality by filtering pollutants and sediments. In tropical old-growth forests, such as those in the Amazon, evapotranspiration from the canopy generates rainfall that supports regional agriculture and hydropower. Losing that function increases the risk of drought and wildfire. Active restoration, including replanting native understory species and reintroducing woody debris, rebuilds these hydrological services. Mycorrhizal networks in old-growth soils also play a critical role in nutrient cycling and water uptake; these fungal hyphae can extend hundreds of kilometers per gram of soil, connecting trees and transferring water from deeper soil layers to shallow-rooted plants. Restoration of these networks often requires inoculation with native mycorrhizal fungi, especially in areas where topsoil has been lost. In the coastal redwood forests of California, restoration of old-growth structure has been shown to increase fog drip by providing more surface area for fog interception, adding up to 30% more water input to the ecosystem during summer drought.

Microclimate Regulation

Within old-growth forests, the dense canopy creates a stable microclimate: cooler, more humid, and less variable than surrounding cleared areas. This buffering is crucial for amphibians, lichens, and many invertebrates that cannot tolerate temperature swings or desiccation. Restoration that closes canopy gaps and reestablishes vertical layering helps recreate this microclimate. In coastal old-growth forests, the persistent fog interception by tall trees provides a critical water source during dry seasons. For conservation areas, restoring these microclimatic refugia can help species survive climate change—a concept known as “climate-smart” restoration. Detailed studies in the Hoh Rainforest of Olympic National Park show that old-growth stands maintain daytime temperatures up to 5°C cooler than adjacent second-growth stands, with relative humidity consistently above 80%. This buffering effect extends downward to the forest floor, where leaf litter remains moist even during summer dry spells. Restoration of vertical structure—through planting of mid-story and understory species—can accelerate the development of these microclimatic benefits. In montane cloud forests, old-growth restoration is especially critical because the persistent cloud immersion supports unique epiphyte communities and provides a habitat for species such as the resplendent quetzal, which requires cool, humid conditions year-round.

Supporting Ecosystem Services

Beyond the well-known benefits, old-growth forests contribute to pollination (via abundant flowering plants and pollinator habitats), nutrient cycling (through decomposition and mycorrhizal networks), and pest regulation (by maintaining populations of natural predators). The structural complexity offers roosting and feeding sites for bats, which control insect populations. Restoration of old-growth forest buffers in agricultural landscapes can enhance crop pollination and reduce pesticide needs. These services are often undervalued but are vital for both ecological integrity and human well-being. Seed dispersal by birds and mammals in old-growth forests maintains genetic connectivity across landscapes; restoration that establishes corridors of old-growth structure facilitates the movement of these dispersers. Additionally, old-growth forests produce non-timber forest products such as wild mushrooms, medicinal plants, and honey, which provide sustainable income for local communities when harvested responsibly. In the Pacific Northwest, the restoration of old-growth understory has been linked to increased yields of wild huckleberries, which are culturally and economically important for indigenous tribes.

Resilience to Natural Disturbances

Old-growth forests exhibit greater resilience to disturbances such as fire, windstorms, and drought compared to younger, even-aged stands. The heterogeneous structure—with gaps, patches of different tree ages, and abundant deadwood—creates a mosaic that dampens the spread of fire and reduces vulnerability to insect outbreaks. Large old trees with deep root systems are better able to withstand drought and wind. Restoration that reintroduces heterogeneity and legacy elements can enhance the resilience of conservation areas under climate change. For example, in the ponderosa pine forests of the American West, restoration of old-growth forest structure through thinning and prescribed fire reduces the risk of catastrophic crown fires, allowing fire-adapted species to persist. Research from Yosemite National Park shows that old-growth stands with intact understory and scattered large trees experienced lower tree mortality during the severe 2012–2015 drought compared to adjacent, historically logged stands.

The Restoration Process: Challenges and Strategies

Invasive Species Management

In many former old-growth landscapes, invasive plants (e.g., English ivy, Japanese knotweed) and animals (e.g., feral pigs, deer overpopulation) outcompete native species and prevent natural regeneration. A first step in restoration is controlling these invasives through mechanical removal, targeted grazing, or carefully applied herbicides. In the coastal forests of New Zealand, aggressive eradication of introduced rats and possums has allowed native tree seedlings to recruit and old-growth structure to return. Learn about New Zealand’s pest control efforts. In the Great Lakes region of North America, restoration of old-growth hemlock–northern hardwood forests requires controlling white-tailed deer populations through culling or exclusion fencing, because dense deer herds eliminate tree regeneration and understory plants. Biocontrol methods, such as introducing specific pathogens or insects that attack invasive plants without harming natives, are being developed for species like garlic mustard and buckthorn.

Land-Use Conflicts and Community Engagement

Restoring old-growth forests often requires setting aside areas from logging, mining, or agriculture, which can create conflict with local communities that rely on these resources. Successful restoration projects use participatory planning, alternative livelihood programs, and payments for ecosystem services to build support. In the Atlantic Forest of Brazil, community-led restoration of degraded slopes into old-growth-like corridors has improved water quality and provided agroforestry income. Engaging indigenous knowledge is equally critical; many indigenous groups have stewarded old-growth ecosystems for generations and can guide restoration practices that mimic natural disturbance regimes. For example, the Karuk Tribe in California uses traditional burning to maintain oak woodlands and reduce fuel loads, benefiting old-growth forest resilience. In Indonesia, community-based restoration of peat swamp forests, including the re-establishment of old-growth tree species such as ramin and jelutong, has reduced fire risk and improved hydrological function while providing non-timber forest products like rattan and honey. Conservation organizations are increasingly using conservation easements and carbon credit payments to compensate landowners for forgone timber income, making restoration economically viable.

Fire Regime Restoration

Many old-growth ecosystems are fire-adapted, with frequent low-intensity surface fires that maintain open understories and promote fire-resistant tree species. Suppression of these natural fires over the last century has led to fuel buildup and increased risk of catastrophic fires. Restoration of old-growth forests in fire-dependent landscapes requires reintroducing prescribed fire and allowing some wildfires to burn under controlled conditions. In the ponderosa pine–bunchgrass forests of the western US, restoration treatments include thinning small-diameter trees to reduce ladder fuels, followed by low-intensity burns every 5–20 years. Studies show that such treatments restore understory diversity, increase soil moisture, and enhance carbon storage in large trees. In the longleaf pine savannas of the southeastern US, restoration of old-growth structure is dependent on frequent fire; prescribed burning every 2–4 years maintains the open, grassy understory that supports endangered species like the red-cockaded woodpecker. Fire regime restoration must be carefully planned to avoid harming sensitive species and to mimic historical fire patterns.

Genetic Considerations in Restoration

Successful old-growth forest restoration requires using planting stock that is genetically diverse and adapted to local conditions. Many restoration projects have failed when they used seeds or seedlings from distant sources that were ill-suited to the local climate or soil. Seed sourcing should prioritize local provenances, but with climate change, some managers are exploring “assisted migration” of genotypes that might be better adapted to future conditions. In the Pacific Northwest, restoration of Douglas-fir old-growth forests uses seed from multiple elevations to maintain genetic diversity. Genetically diverse populations are more resilient to pests, diseases, and climate stress. Additionally, restoration should aim to preserve the genetic legacies of remnant old-growth trees, which may harbor unique adaptations developed over centuries. In some cases, cloning or micropropagation of exceptional individuals can help restore their genetic contribution to the regenerating forest.

Long-Term Monitoring and Adaptive Management

Forest restoration is not a one-off replanting. Recreating old-growth structure takes decades to centuries, requiring continuous monitoring of tree growth, understory development, deadwood abundance, and species recolonization. Adaptive management—adjusting techniques based on results—is essential. For example, if natural regeneration fails, managers may enrich planting with shade-tolerant, late-successional species. In the Hoh Rainforest of Olympic National Park, restoration efforts after logging included “variable-density thinning” to accelerate structural complexity, followed by long-term monitoring of soil carbon and bird populations. Read a Forest Service study on thinning for old-growth development. Modern monitoring tools, such as LiDAR remote sensing, allow managers to assess canopy complexity and deadwood distribution across large areas. Permanent sample plots and repeated measurements of carbon flux (via eddy covariance) provide data to fine-tune restoration techniques. Citizen science programs also contribute by tracking bird and mammal recolonization. Adaptive management frameworks ensure that restoration activities are iteratively improved based on ecological outcomes.

Case Studies in Old-Growth Forest Restoration

Several large-scale initiatives illustrate the potential. In the Pacific Northwest, the Northwest Forest Plan (1994) shifted management from timber production to old-growth restoration across millions of acres of federal land. Active restoration includes removing roads, thinning young plantations, and reintroducing fire where ecologically appropriate. Early results show increased spotted owl habitat and carbon storage. In Europe, the Białowieża Forest restoration—despite political controversy—has focused on deadwood retention, exclusion of logging, and recolonization of bison, creating a living laboratory of near-natural forest dynamics. In Costa Rica’s Monteverde Cloud Forest Reserve, restoration of abandoned pasture through corridors has reconnected old-growth patches, allowing a spectacular return of birds and amphibians. Read about Monteverde’s restoration. In Australia, the restoration of mountain ash forests in Victoria following the 2009 Black Saturday fires has focused on retaining large hollow-bearing trees and promoting natural regeneration, with active enrichment planting of understory species that were missing after salvage logging. These case studies demonstrate that long-term commitment and adaptive management can restore functional old-growth forests even in heavily modified landscapes.

Conclusion

Restoring old-growth forests within conservation areas delivers profound ecological benefits that extend far beyond the forest edge. From safeguarding biodiversity and storing massive amounts of carbon to regulating water cycles and local climates, these ancient ecosystems are pillars of planetary health. The challenges are significant—invasive species, conflicting land uses, fire management, and the long time scales required—but the strategies of community engagement, genetic care, fire reintroduction, and scientific monitoring are proven effective. Investing in old-growth forest restoration is not an option but a necessity if we are to meet global biodiversity and climate goals. By restoring these irreplaceable woodlands, we ensure that future generations inherit not just trees, but fully functioning, resilient forests capable of sustaining life. Policymakers and conservation organizations must prioritize funding for restoration projects, incorporate indigenous and local knowledge, and commit to decades of adaptive management. The time to act is now—every year of delay diminishes the chance of preserving these vital ecosystems for the planet and its people. Learn more about IUCN’s forest restoration guidance.