Temperate coniferous forests

Trees with needlelike leaves, known as “conifers,” are an ancient plant group, with fossils that go back 300 million years to the late Carboniferous Period when much of Earth's coal was formed. Coniferous trees are generally more tolerant of drought and fire than are deciduous trees, although some species do inhabit moist forest ecosystems. Coniferous forests of the temperate zone mainly grow in North America, where they occur in the Pacific Northwest, the Rocky Mountains, as stands of eastern hemlock in the northeast (currently disappearing due to an invasive insect pest), and in the Coastal Plain of the southeast. Very small coniferous temperate forests also grow in Japan, Argentina, and Chile. At the highest elevations in the southern Appalachian Mountains in North Carolina coniferous spruce-fir forests occur, but these are ecologically more similar to northern boreal forests than to temperate coniferous forests. With the exception of cypress, which grows primarily in southeastern swamps, the tree species of coniferous forests are almost always evergreen as opposed to deciduous trees, which lose their leaves every autumn. Fire is an important feature of coniferous forest ecosystems, and without fire most of these ecosystems would eventually develop, or “succeed,” into different forest types. Other important environmental determinants of these forests are the amount of soil moisture and, in the Rocky Mountains, elevation. As winters become warmer and growing seasons are lengthened in duration, insect pests and disease pathogens are increasing the mortality of many temperate conifer tree species.

Pacific Northwest Coniferous Forests

The beautiful temperate evergreen forests of the Pacific Northwest hug the coast of the Pacific Ocean, where climates are mild and very moist. These forests grow within 37 to 74 miles (60 to 120 kilometers) of the coast and receive enough rainfall to be called “temperate rainforests.” Massive evergreen conifers are important for ecosystem functioning, wildlife, recreation, and local and regional economies. Giant trees include Douglas fir, western hemlock, western red cedar, Sitka spruce, and coast redwood. These trees occur in diverse tree communities with multiple dominant species and are generally very large and very long-lived. Some of these species, notably the famous redwood (Sequoia sempervirens), include the oldest trees in the world. These moist, mild forests with very long intervals between natural fire events (several hundred years) are unusual on the planet, as most such ecosystems are dominated by deciduous tree species, not conifers. The humid, moist conditions result in exuberant growth of ferns as well as mosses and lichen on tree trunks and branches, leading to names like “Hall of Mosses” and “Elvin Forests” for these temperate rainforest ecosystems.

Major environmental changes over the landscape (“gradients”) that explain differences between forest types in the Pacific Northwest track changes in latitude, distance from the coast, and elevation. The two most important environmental factors explaining differences in forest types are soil moisture and temperature. In coastal regions, temperatures are mild and there are many cloudy days. Differences between day and night temperatures, and differences in temperatures from one day to the next, are mild, making these climates very moist and stable. Most of the precipitation occurs from October through March. As the landscape position moves from south to north, increasing in latitude, the temperature generally decreases and the precipitation increases.

The main forest type in the Pacific Northwest is dominated by Douglas fir and western hemlock and often includes western red cedar and other species of primarily coniferous trees. Forests of these massive trees range from sea level up to 2,296–3,280 feet (700–1,000 meters), with Douglas fir trees that may be 1,000 years old.

Rocky Mountain Coniferous Forests

The eastern slope of the Rocky Mountains rises dramatically from the relatively flat expanse of the Great Plains, reaching peaks of 14,436 feet (4,400 meters) in elevation. Not surprisingly, the elevation where an ecosystem occurs most strongly determines what type of ecosystem it will be. As elevation increases, temperatures decline and precipitation generally increases, as do solar radiation, wind, and snow depth. Although there are important differences in temperature and moisture between, for example, north- and south-facing slopes at a given elevation, lower-elevation climates are generally warmer and drier, whereas higher-elevations sites are colder and moister. The lowest elevations of the Rockies are dominated by grasslands and mixed oak deciduous forests, whereas the highest elevations are evergreen boreal forests grading into alpine meadows. Temperate coniferous forests dominate large expanses of the midslopes of these mountains. Woodlands comprised of juniper and pinyon pine tree species occur upslope from the oak forests, and heavy grazing by livestock has increased the abundance of juniper trees over this landscape by eating grass and reducing the grass-competitive shading of juniper seedlings. Juniper-pinyon grade upslope to Ponderosa pine woodlands and forests and then further upslope to forests dominated by Douglas fir, an important tree for use as a Christmas tree in the United States, that grow at about 5,410 to 8,900 feet (1,650 to 2,700 meters) in elevation. The highest elevation temperate deciduous forest is dominated by lodgepole pine, with grades into spruce-fir boreal forests at higher elevations.

Eastern Hemlock Forests of Northeastern North America

Eastern hemlocks are “climax” species in forests, meaning that they do not require a disturbance event such as fire or the death of canopy trees creating a light gap in order to successfully reproduce and replace themselves in the forest canopy. In fact, hemlocks produce such dense shade that few if any other tree species are able to successfully regenerate under hemlock stands; instead, seedlings of other tree species generally die within a few months or years of germinating. Hence, despite having a dense canopy of evergreen needles, there is little plant ground cover or undergrowth in dark hemlock stands.

Hemlock stands do, however, have some unique hydrological features that enable them to form temporary freshwater “vernal pools” in the spring that are essential for populations of amphibians including frogs, toads, and salamanders. Because these pools are temporary, they do not support populations of fish, which could otherwise eat tadpoles and other offspring that inhabit the pool. And dense, evergreen hemlock forests provide essential wildlife habitat in the winter months, helping to protect white-tailed deer, wild turkey, and other animals from cold winter winds and snow. Hemlock trees are very slow growing and extremely long-lived, but unfortunately they are currently being decimated by an introduced species called the hemlock woolly adelgid.

Southeastern Coastal Plain Coniferous Woodlands and Forests

The Coastal Plain is a flat, sandy region of the southeastern United States with a large amount of biodiversity and also one of the fastest-growing human populations in the nation. Three main types of temperate coniferous forests occur in the southeastern Coastal Plain, and differences among them depend largely on soil moisture and associated fire frequency. The first is the longleaf pine forest, which once accounted for 36 million hectares (90 million acres) of the landscape from North Carolina to Florida to eastern Texas, and occurs in various vegetative structures from quite wet to very dry soil moisture conditions. The vast majority of the former longleaf pine system has been converted to agriculture and forest plantations, making natural longleaf pine forests one of the most endangered ecosystems in the United States, with only 2 percent still remaining. The Nature Conservatory has set a goal to restore 8 million acres of the longleaf pine forest in the Southeast by 2025.

Wet longleaf pine woodlands contain some of the highest numbers of species ever recorded at small spatial scales, including wild orchids and carnivorous plants like sundews, pitcher plants, and Venus flytraps. Like other coniferous forest systems, longleaf pine forests and savannas depend on regular fire events to remain healthy. The second type of temperate coniferous forest in the southeast is very wet bald cypress and pond cypress swamp forests, or “bayous,” which can have standing water several meters deep. These wet forests are less prone to fire, but during especially dry periods or when humans alter groundwater aquifers, these forests, too, can burn. Unlike most other conifer trees, cypress trees lose their needles in the autumn, and these swamp forests are important for freshwater fisheries, including crayfish and shrimp. The third type of temperate coniferous forest in the southeast is the sand pine woodland, or “scrub,” that occurs on very deep, very infertile, and extremely dry white sands in central Florida. Many of these woodlands have been converted to citrus plantations. Sand pine scrub woodlands are the habitat of rare animal species, such as the gopher tortoise and scrub jay, as well as 12 types of endangered plant species, including wildflowers.

Fire in Coniferous Forest Ecosystems

Fire, set either by natural lightening strikes or by human land managers, is an important feature of almost all coniferous forests, with the exception of those dominated by eastern hemlock. Evidence of past fires is found in fire scars on tree trunks and in charcoal layers in soils. There is a major difference between ground- or surface-level fires, which burn less intensely and generally do not kill large trees, and crown or canopy fires, which do. When fires are suppressed by people, these coniferous forests can develop into deciduous hardwood-dominated stands or change into overly crowded tree conditions that burn much more intensely with more extreme damage than natural fires when a burn finally does occur. The latter happened in the Yellowstone fire event of 1988, which formed the largest recorded fire in the history of Yellowstone National Park.

Fires had been suppressed for decades, and normally widely spaced trees became crowded by smaller, younger trees that would have otherwise been burned as seedlings when ground-level fires occurred naturally. Instead, due to fire elimination, these younger trees were allowed to grow taller and when fires did ignite, they formed a “ladder effect” and the flames burned up into the large trees, resulting in a canopy fire. Thus land managers now set controlled, or “prescribed,” fires to mimic natural fire frequencies and promote the health of these ecosystems.

In the wet and foggy Pacific Northwest, forests have fire intervals of 500–800 years as evidenced by fire scars and soil charcoal. When a fire does occur, it is a catastrophic fire burning up the giant canopy trees as fuel. In contrast, other temperate coniferous forests typically have more frequent and less severe fire events. Douglas fir forests of the Rocky Mountains have typical fire frequencies of 50–100 years, while the more arid Ponderosa pine forests burn more frequently every 40 years or so. Natural fire frequencies can be as often as every few years in moist longleaf pine forests and woodlands of the southeastern Coastal Plain. Florida sand pine scrub systems are so dry that plant cover is extremely limited and fuel that forms from dead plants and leaves takes a long time to accumulate. Hence, fire intervals in these systems may be 20–50 years, whereas nearby moist longleaf pine woodlands could burn every few years. The cones containing the seeds of sand pine are serotinous, meaning they require the high heat of being burned in a fire before they can open and release the seeds to produce the next generation of trees.

Frequencies and sizes of natural and accidental arson-caused fires have been going up in the United States over the last several decades, largely as a result of earlier spring snowmelt and higher spring and summer temperatures. For example, the National Interagency Fire Center finds that the size of the area burned by an average fire in the United States was about 47 acres between 1987 and 2000, and increased to 100 acres during 2001–07. The size of the area burned in the United States continued to increase into the 2020s, with 58,985 fires burning more than 7 million acres in 2021, according to the National Interagency Fire Center.

Insect Pests and Disease Pathogens

Like fire, climate is another important controller of the insect pests and pathogens that shape the health of all ecosystems, including temperate coniferous forests. With warming winter temperatures, disease pathogens and insects like pine bark beetles as well as human pets' outdoor flea populations are better able to survive and even increase their distribution over regions like mountaintops that were formerly too cold. This is occurring now as the mountain pine bark beetle that is causing over 90 percent tree mortality rates in western lodgepole pine stands has been able to cross over the Rocky Mountain Continental Divide and now is killing jack pine trees in eastern North America. The near synchronous death of these standing trees forms fuel for extreme canopy-level fires, further hastening change in these forest ecosystems and having major ecological and economic consequences. Longer growing seasons also enable insects and pathogens to produce more generations per year, which exacerbates negative effects on tree health. For example, the hemlock woolly adelgid beetle, which invaded the east coast of North America from Asia, is wiping out trees and stands of eastern hemlock. In New England, this insect generally produces two generations per year and takes 15 years or so to kill the trees. In the warmer climates of the southern Appalachian Mountains, this insect can produce three generations per year and typically kills mature trees in just four years.