Forest ecosystems regulate the planet’s climate and water cycles, storing vast amounts of carbon in their biomass and soils. Climate change, driven by rising global temperatures and altered precipitation patterns, directly threatens the health and function of these forests. This environmental shift acts through a combination of physical, biological, and ecological mechanisms. These mechanisms undermine the stability of forest communities worldwide. Understanding these specific mechanisms is necessary to grasp the risks facing these terrestrial biomes.
Altered Growth Conditions
Rising temperatures directly alter forest health and productivity by accelerating the metabolic rate of trees, leading to higher respiration rates. Elevated respiration consumes the energy reserves (carbohydrates) produced through photosynthesis. This consumption can lead to a net carbon deficit that slows growth and reduces vitality. This stress is compounded by warmer nights, which inhibit a tree’s ability to replenish carbohydrate stores overnight.
Temperature and water availability interact to create stress at the cellular level. During drought, trees close the tiny pores on their leaves (stomata) to minimize water loss, which restricts the intake of carbon dioxide necessary for photosynthesis. Prolonged drought can cause “carbon starvation,” forcing the tree to deplete energy reserves. If stomata remain open, the tree risks “hydraulic failure,” where water-transporting tissues (xylem) become blocked by air bubbles, leading to rapid dieback.
Rising temperatures can induce photorespiration, especially when water is limited, causing trees to release carbon dioxide back into the atmosphere instead of sequestering it. This inefficient carbon use begins when average daytime temperatures exceed approximately 68°F (20°C) for some forest types. The combination of accelerated metabolic consumption and restricted carbon intake compromises the tree’s ability to grow, allocate resources for defense, and survive.
Exacerbated Natural Disturbances
Physical stress on forests is frequently followed by an amplification of destructive agents, transforming local disturbances into regional catastrophes. Warmer, drier conditions are changing fire ecology by extending fire seasons and increasing the aridity of forest fuels. This increase in heat enhances the drying of organic matter, which doubled the number of large fires in the Western United States between 1984 and 2015.
The combination of higher temperatures and drought creates fire weather conditions, significantly increasing the risk of stand-replacing megafires. A 1°C rise in annual temperature could increase the median burned area by as much as 600% in some forest types. This new fire regime is characterized by longer, more intense burns that incinerate entire forest communities, unlike the lower-intensity surface fires many ecosystems tolerate.
Climate-driven stress also fuels pest and disease outbreaks, particularly involving bark beetles. Drought-stressed trees cannot produce enough resin to repel boring insects, leaving them vulnerable to mass attack. Warmer winter minimum temperatures reduce the cold-snap mortality that historically controlled insect populations, allowing more larvae to survive the winter. This dual effect accelerated tree mortality by 30% during an extreme drought event in California, demonstrating the synergy between drought and insect outbreaks.
Shifts in Species Distribution
The cumulative effects of physiological stress and severe disturbances lead to long-term changes in the geographic range and composition of forest ecosystems. Tree species attempt to track preferred climatic conditions by shifting distributions poleward in latitude or upward in elevation to find cooler, wetter habitats. This has been observed along the southern margins of the boreal forest, where coniferous species are being replaced by more temperature-tolerant, broad-leaved species.
The speed of current climate change far outpaces the natural migration capabilities of most tree species, leading to a widespread “adaptation lag.” While post-glacial migration rates averaged up to one kilometer per year, the projected rate of climate change is estimated to be 10 to 100 times faster. This disparity means the suitable climate zone is moving away from established forest populations faster than the species can colonize new areas.
The result is often a contraction of the species’ range, with dieback occurring in the warmer, drier southern or lower-elevation portions of their distribution. This ecological restructuring changes forest composition, where remaining or newly dominant species are often more drought-tolerant but slower-growing. This shift alters the established species mix, potentially reducing biodiversity and leading to the collapse of forest types unable to adapt or move quickly.
Impact on Carbon Storage Function
The decline in forest health and increased large-scale mortality compromise the forest’s role in regulating global climate by altering its capacity to store carbon. Healthy forests act as a global carbon sink, annually removing atmospheric carbon dioxide from human activities. When trees die from drought, fire, or insect attack, the carbon they accumulated is transferred from living biomass into dead biomass.
This transfer leads to a “sink-to-source flip,” where the forest releases more carbon back into the atmosphere through decomposition or burning than it absorbs through new growth. The carbon sink strength of tropical forests has been weakening; modeling suggests the Amazon rainforest could transition to a net carbon source as early as the mid-2030s. Disturbances in the Western U.S. have resulted in the death of trees containing significant carbon stores, with fire and bark beetles killing trees holding up to 5–11 and 2–24 teragrams of carbon per year, respectively, between 1984 and 2010.
Warming also affects the massive carbon pool stored in forest soils, particularly in northern boreal regions. Boreal forests contain hundreds of gigatons of carbon stored in cold soils, protected from decay by low temperatures. Warming accelerates the microbial decomposition of this organic matter, increasing CO2 from the soil. Experimental warming of forest soils by 4°C has caused a significant loss of subsoil carbon, with a 30 percent increase in CO2 emissions, confirming a positive feedback loop that intensifies global warming.