What is an old-growth forest?

Old-growth forests that have developed in the absence of human influence are extremely rare nowadays. The original definition of an old-growth forest suggests the complete absence of human influence, implying that such forests are almost non-existent. Hence the concept of old-growth forest is slightly modified and has been defined as follows: a forest that has developed and is maintained with very limited human influence. This implies the soil has not been tilled and the living elements (plants, animals) have not been introduced, planted or sown by man. These living elements consist exclusively of native species with trees of all ages from seedlings to old and large trees. In Europe only small areas can still be considered old-growth forest, including the old European forest remnants. These forest complexes show the most complete and complex ecological patterns, including beech (Fagus sylvatica) under a variety of environmental conditions.

More characteristics of old-growth forests

During the Quaternary (current geological period), forest development increased dramatically. During the current geological period, cold times alternated with warmer times (respectively glacial and interglacial periods), which resulted in shifts of forest vegetation from the north to the south. The trees that always appear first are pine and birch (pioneer tree species), followed by various other tree species. Deforestation started at least in the Bronze Age. Over thousands of years there were multiple waves of deforestation, although the scope was such that old-growth forests largely persisted. With population growth, industrialization, the rise of agriculture and cattle breeding and the further intensification of land use, man’s influence on the old-growth forests has continued to increase. The influence of man creates a changeable environment with higher chances of tree mortality due to, among other things, the felling of trees, higher chances of fire, pollution etc. Forests with trees older than 200 years are scarce. They provide numerous habitats for indigenous animal and plant species and undergo several complex life cycles with, among others, soil fauna and fungi. They are important in terms of biodiversity, species diversity and the general functioning of natural ecosystems. Often, they are the last resources for preserving our archaeological and geomorphological heritage in a modern landscape. The extraordinary value of these old-growth forests forms an important reason to protect and restore them.

 Mensing, S. A., Schoolman, E. M., Tunno, I., Noble, P. J., Sagnotti, L., Florindo, F., & Piovesan, G. (2018). Historical ecology reveals landscape transformation coincident with cultural development in central Italy since the Roman Period. Scientific reports, 8(1), 1-9. https://edepot.wur.nl/270309
Waar vind je nog eeuwenoude oerbossen in Europa?
http://www.redbosques.eu/resources https://nl.wikipedia.org/wiki/Oerbos https://www.unesco.nl/nl/erfgoed/oerbeukenbossen-van-de-karpaten-en-andere-regios-in-europa https://www.foret-pro-bos.eu/nl/organisaties-en-beleidsmakers/duurzaam-bosbeheer-en-ontwikkeling-2 https://www.kuleuven.be/metaforum/pdf/visie-en-beleidsteksten/visietekst-2011-behoud-beheer-en-duurzame-ontwikkeling-van-bossen
On average, old-growth forests have larger, older trees than their regrowth counterparts. Old-growth forests have a multi-aged structure, especially when they encompass larger forest areas. Old-growth forests can be recognised based on the following criteria: -Old-growth forests show no or less signs of human use in the past. -The soil in an old-growth forest will be deeper and richer than that of younger forests which have been regrown by logging, agriculture and grazing. -The canopy consists of several complex vertical layers. This structural complexity gives us much more specialised habitats for plants and animals, including little light reaching the ground, predominance of shade-tolerant species. -There are small clearings or gaps due to disturbances (falling trees): they increase heterogeneity and allow regeneration. Species such as yellow birch, sugar maple, American beech and eastern hemlock have strong root systems. When large trees of these species are blown over, the hole they leave behind can be enormous. Besides small clearings, there are also occasionally larger disturbed areas from rare or episodic disturbances (for example, violent storms or fire). This introduces large pulses of sun-exposed deadwood and other key habitats that early successional species require for their maintenance on a landscape scale. -The forest canopy consists of varied species and age classes: it promotes a high species diversity and increases resilience. – There are large amounts of dead wood, dead trees and coarse wood debris. This results as a chaotic pattern of logs in all stages of decay, what represents a habitat for a large variety of dead wood-dependent species (saproxylic) -There are very thick trees, both dead and alive. They contain a large variety of microhabitats (cavities, cracks) suitable for specialised species. -In old-growth forests, mosses, liverworts, lichens and fungi are much more abundant. These soils retain more water.

How does one recognize an old-growth forest in the Adirondacks?
The answer to this question remains enigmatic. In the 1970s, researchers started using the term “old-growth” to describe complex, biodiverse forests with the oldest trees of at least 150 years old. Nowadays, these restrictions have been relaxed such that forests only need to be at least 120 years old in order to be considered an old-growth forest. It’s difficult to determine which old-growth forest in Europe is the oldest one and how old it would be… An example of an “old” old-growth forest in Europe is the Bialowieza Forest in Poland. Bialowieza has been covered by trees for more than 1000 years. This seems very “old-growth,” but there have been some human interventions in the forest (logging,…). The forest has multiple old and famous trees and is an important habitat for bison, the heaviest land animals in Europe. Overall, the oldest old-growth forests in Europe are located in the Balkans, Spain, Italy. However, there are other, much older old-growth forests in the rest of the world. The oldest forest is the Daintree Rainforest on the northeast coast of Queensland, Australia, and it is approximated to be 180 million years old. The value of old-growth forests arises from their unique old trees. Recently, scientists examined the lifespan of beech and other species (maple, spruce, fir) in old-growth forests of the Balkan and the Carpathes. Among the 4 species, beech had the longest lifespan. A few beech trees surpassed 500 years !

Pavlin et al.: Disturbance history is a key driver of tree lifespan in temperate primary forests, Journal of Vegetation Science, 2021
Trees continue to play an important role even during their death. Dead wood is the result of storms, fire, competition for nutrients, water and light, drought or high-water levels and senescence. Thanks to dead wood, the forest comes to life. Old-growth forests are characterised by the presence of trees of different growth stages, in contrast to managed and cultivated forests. With time, old trees are subject to a slow and unremitting decay. This decay manifests itself in the accumulation of large quantities of dead wood in various forms, including fallen trees of various sizes, broken branches, pieces of trunks and stumps and standing dead trees. In an old forest, the presence of a certain quantity of dead wood is considered an indicator of environmental health, as it is essential for the improvement of biodiversity. It sets processes in motion and creates crucial microhabitats for a myriad of vertebrate and invertebrate species that play important functional roles in the forest ecosystem and that speed up the decomposition process. It is estimated that 20 to 40% of living organisms in a forest depend on dead wood during at least one part of their lifecycle.

In beech forests, dead wood is used by various species of invertebrates, fungi, bryophytes, lichens, amphibians, birds and mammals as a source of food or shelter. Some examples are: xylophage insects (dead wood feeders) living in the upstanding dead trees or trunks, Rosalia Longicorn (Rosalia alpina) in which the females lay their eggs in the crevices of the bark and the larvae feed on decomposing wood, Fungi, bacteria and other organisms that break down plant waste and lignicolous fungi that break down the woody debris and release in the forest litter nutrients and organic substances useful for the life and development of all plant organisms. All these processes make forests the ideal ecological niche for the germination and development of numerous tree species.

Dead wood plays a key role in the nutrient cycle. It represents an important carbon reservoir and, at the same time, a reserve of energy that is made available again. Moisture is also stored in dead wood, protecting it from erosion and frost. The key roles and different relationships that dead wood possesses must therefore be protected and maintained.

Bauhus et al., Dead wood in forest ecosystems, 2018
Handbook on sustainable forest management : Miha Varga, Bojan Kocjan, Domen Kocjan, Špela E. Koblar Habič, Urban Prosen- SLOVENIA FOREST SERVICE, Slovenia
Habitat trees are large trees that are not economically profitable, but are very important for biodiversity (foraging, nesting, resting, …). They provide micro-habitats for many endangered and specialized species (animal and plant species so highly adapted to a specific habitat, food source, climatic condition, etc. that they are very prone to changing conditions). Therefore, forest managers can indicate 10-15 trees per hectare as habitat trees with a variety of characteristics (shape, size, species, among others). Characteristics of good habitat trees are nesting trees with nests and cavities, resting and singing trees, rare and fertile tree species, large trees with irregular shapes, trees with injuries (cracks, fractures, loose bark), trees with dead wood or decay and trees with epiphytes (ferns, fungi, lichens, mosses). Habitat trees should not be cut down because of their extremely important value.

Handbook on sustainable forest management : Miha Varga, Bojan Kocjan, Domen Kocjan, Špela E. Koblar Habič, Urban Prosen- SLOVENIA FOREST SERVICE, Slovenia