Ecology of temperate forests

"Temperate" forests occur at the mid-latitudes, between 23.5 and 66.5° N and S, where they cover approximately 20 % of the available land area and are characterized by distinct seasonal climate cycles. Temperate forests are dominated by plants with a woody, treelike growth form, and they produce relatively closed canopies (60-100 % areal canopy coverage). Temperate forests occur across a broad range of climate zones, including those with moist, warm summers (e.g., deciduous forests in North America and Europe) and dry, cool summers (e.g., montane and subalpine forests in North America, South America, and Europe). Temperate forest ecosystems exhibit carbon to nitrogen (C:N) ratios that are higher (often >100-200) than other temperate-latitude ecosystems, due to Classic, historical concepts in ecology, such as "succession," have been developed from studies of temperate forest ecosystems. Forest succession refers to decadal-scale transitions in community composition. Each shift in community composition causes changes in the forest microenvironment, which in turn causes further changes in community composition. Traditionally, this pattern of progressive change in community composition and associated feedbacks to forest microenvironment was viewed within a highly deterministic framework. More recently, ecological concepts, such as "gap theory," have emerged from the older concepts of succession and have been developed with greater emphasis on stochasticity. Both succession and gap theory has contributed greatly to our understanding of the causes of natural and anthropogenic changes to the species composition of temperate forest ecosystems. Nitrogen and phosphorus (N and P) are cycled through temperate forest ecosystems through a process of coupled recycling involving serial relationships between plants and soil microorganisms. N or P that is deposited to the soil through litter production is transformed from organic to inorganic forms through microbial mineralization, producing nitrate and phosphate ions, which can then be re-assimilated by plants and used to construct new organic biomass. Leaching of phosphate and nitrate from forest soils (especially nitrate in temperate forests) prior to re-assimilation by plants represents an important nutrient loss process and often limits forest biomass production. Root-fungal symbioses, called mycorrhizae, are well developed in temperate forest ecosystems. The hyphal biomass from the fungus radiates from associated roots and increases the capacity for trees to capture nitrate and phosphate prior to leaching and, in some cases, allows trees to take up organic nitrogen (such as small proteins or single molecules of amino acids). The acquisition of organic forms of nitrogen (and to some extent phosphorus) "short-circuits" the conventional form of biogeochemical cycles (alternating between plants and microbes) and increases the efficiency of nutrient retention in the ecosystem. Most water that is cycled through forests is used to sustain a favorable energy balance. Evapotranspiration from forests facilitates the loss of heat that is absorbed as net radiation (from the sun and sky) and returns water to the atmosphere, thus sustaining the terrestrial water cycle. Trees in temperate forests (especially in North America and Europe) have been exposed to increasing physiological stress in recent decades due to the increased frequency of drought and high temperatures. These stresses have the potential to reduce forest growth and may be responsible for the observed weakening of forest carbon sinks globally. Climate-induced stress, in turn, exposes temperate forests to an increased frequency of epidemic insect outbreaks and associated high rates of herbivory, as well as shorter fire return cycles. The combination of abiotic and biotic stress is likely responsible for an increase in observed mass tree mortality in temperate forests of the Northern Hemisphere. Greater frequencies of alternations between years with extreme climates (e.g., wetter-than-average years followed by drier-than-average years) have the potential to convert less damaging surface fires into more damaging crown fires due to the buildup of beneath-canopy fuels and greater connectivity of lower-elevation grasslands that border higher-elevation forests, during wet years, followed by greater ignition potential during dry years. Given changes in the Earth's climate system that increase the threat to fire-and insect-induced mass tree mortality in temperate forests, effective management of these ecosystems has become even more urgent and important to responsible stewardship of our natural resources. Future management efforts should be designed on a solid foundation of scientific knowledge about forest succession, forest biogeochemistry, and the natural relations of forests to fire return cycles and cycles of higher and lower levels of insect herbivory.