Woodland Diversity Factors: Light, Species, Habitat, Age

Diversity Importance

Woodlands provide trees for lumber, fuel, carbon dioxide scrubbing and carbon sequestering, and produce oxygen; the rate of climate change and its effects are reduced, soils are stabilized, water is filtered, and folks just plain feel better walking around in woodlands. All these functions and benefits are increased within a more biologically diverse woodland, and a higher level of diversity promotes a higher activity level of these functions.

LIGHT 

Light is one of the most important factors in determining which plants grow where in a woodland and their level of success. Biodiversity in a woodland is greatly influenced by variations in light intensity. The amount of light available beneath a woodland tree canopy depends on tree spacing and canopy density. Minimal spacing causes tree-canopy overlap, increasing total canopy density, and thus decreasing light penetration. 

All of the plants growing beneath the main tree canopy in a woodland are adapted to light conditions from deep shade, through to open, well-lit clearings. Various light intensities will provide environmental conditions favorable for the development of a wide range of species-specific communities beneath the woodland canopy. The resultant plant diversity supports animal diversity, because each plant species generally has its own range of animal species associated with it and these animals also supply food for yet other animal species.

The fruits and seeds of woodland plants are another important source of food. Different plants flower and set seed at different times. The greater the variety of plants present, the longer the period when flowers and or seeds/fruits will be available to feed the woodland animals and birds. Increased diversity also decreases the risk of food supply interruption due to the many alternatives during a particular plant species collapse from disease or competition.   

Passive forest management practices often result in stagnant, dark, dense areas within a woodland, with few openings that allow the light penetration necessary 

for greater plant diversity. But these areas support a variety of other plant and animal species which cannot survive in more open areas. The damp, cool, sheltered habitat thus created supports thriving communities of bugs and worms, mosses, ferns, and liverworts. Rotting wood in this environment holds more species communities compared to wood left to decay in full sun. Restoring a more vibrant diversity may require the selective removal of trees to open the canopy, and/or tree limb pruning, to decrease canopy density…both allowing more light penetration.  

     NATIVE VS. NON-NATIVE

A plant or animal introduced with human help is considered nonnative, and this species introduction to an environment can be devastating. They often leaf-out earlier than natives and retain their foliage longer, thus able to shade out native understory vegetation such as spring ephemerals…eventually out-competing native tree species in the forest canopy. This reduces the natural food supply chain; since the native species have not evolved to subsist upon the non-native, biological diversity is reduced, resulting in a disruption in the structure and natural functions of the native woodland habitats. Devastation happens if the non-native thrives to the level of displacing natives; this non-native then becomes invasive, and can eliminate species’ communities within the local ecosystem’s habitats, severely depleting bio-diversity. 

Non-natives aren’t necessarily deleterious to a woodland. The honey bee is a crucial pollinator. This non-native was brought to North America by European settlers around 400 years ago for honey and wax. It turns out they were a perfect fit for our crops and climate, and now they are an essential part of pollination. 

MICRO-HABITATS

Structural diversity in woodlands is all about microhabitats. A microhabitat is simply an identifiably different portion of a larger discrete habitat such as a woodland. Thus, a bird nesting in a tree-hole would be occupying a different microhabitat to a millipede living in the leaf litter on the woodland floor. The more different microhabitats there are available within a woodland, the greater biodiversity will be. Different microhabitats offer differing microclimates, food opportunities, architectural structure, camouflage and sheltering opportunities. These allow a great range of organisms with very different requirements, or even only subtly different requirements, to live side by side in the same general area. Structural diversity is enhanced through having a variety of different tree species, of differing ages, making up the woodland.Natural environmental variations within the woodland, such as areas of differing soil type (for example, acidic, or lime) and degree of water saturation, will provide conditions suitable for the growth of a wide range of different types of plants. Glades and rides within the wood, provide areas suitable for less shade-tolerant plants. All of the variety of plants within a wood will offer different microhabitats and food opportunities for an enormous number of other organisms. The decaying products of all these plants, from leaf litter to rotting wood, provide food sources and microhabitats for thousands of other different kinds of organisms. Large individual rotting logs, piles of logs and old tree stumps provide a staggering number of microhabitats, as will old trees. If these are left to die naturally in the woodland, rather than being harvested at an appropriate time, they will acquire a value for wildlife which is usually proportionate to their age.

TREE AGE

As trees age, they get bigger. With increased size comes increased surface area for other plants to colonize, more gnarled and fissured bark, rot-holes and caverns in the trunk, together with the production of flowers, fruits and seeds. Past a certain size, the tree will begin to senesce and bits of it will die off, remaining attached to provide standing rotting wood microhabitats. Old trees therefore develop a wealth of microhabitats for other organisms and a large surface area for colonization. By extension, the more old trees a particular woodland contains, the greater its biodiversity is likely to be.

In general, biodiversity within woodlands increases both with the age of the individual trees which it contains and the historical age of the wood. 

Oak is an example of a tree with thick bark. It will increasingly develop networks of fissures and cracks in the bark as it grows and expands in girth. Therefore, the older the tree, the more sheltered hiding places and microhabitats it will provide for small invertebrates in its bark. All of this enormous surface area provides a substrate for other life to colonize, from algae, mosses, ferns and lichens, to climbing plants such ivy. These plants offer a wealth of microhabitats in their own right. Many of them are very slow-growing and may require up to hundreds of years to develop a significant presence.

Periodically, the tree will also flower and set seed, providing seasonal abundance for many different animals. The enormous number of leaves produced are fodder for a staggering number of individual invertebrates: aphids, leaf miners, caterpillars, etc. When the leaves die and fall, they will slowly rot away on the woodland floor, providing yet another microhabitat for woodland life. The older the tree, the more leaves are produced and so the more of this resource that is available.

Old, dead branches, still attached, will slowly rot away, providing food for the many species inhabiting standing dead wood. The heart wood may begin to rot away, opening up holes and caverns within the trunk, which can be occupied by many opportunistic creatures. Knot holes may have developed higher up, providing nesting sites and rain water traps. These will often develop their own very characteristic fauna, capable of completing their life cycle in the short life span of this lofty temporary pool.

The value of ancient trees for biodiversity is now well recognized. Because of the individual biodiversity value of ancient trees, the more old trees a woodland contains, the greater its overall biodiversity is likely to be. In addition to this, the length of time an area has been continuously wooded is also relevant. Thus ancient woodlands will often contain species which are peculiar to them and which are rarely found in newer woodlands. Many of these species are slow to colonize new habitats, as well as slow-growing. Newly planted woods will predominantly contain those species which are quick colonizers, or which have survived in the seed-bed from previous land use.

FOREST AGE

Ancient woodlands are habitats which can have enormous biodiversity. They may contain much of the same biodiversity present in more recent woodlands. However, the great age of many of the trees and their resultant large size, thick, cracked, fissured bark and rot-holes, all provide a great many additional microhabitats for other species. The sheer age of the habitat itself and the absence of major physical disturbance also gives rise to a continuum of conditions which favour a variety of rarer species. In particular, this would include species which are either slow to establish and, or which require very particular conditions in order to survive. This means that ancient woodlands have potentially far greater biodiversity than more recently established woodlands. 

Individual old trees provide an amazing number of microhabitats for other plants and animals. Many different kinds of weird and wonderful beetles and other insects live in the cracks in gnarled and fissured old bark. Knotholes and hollow centers in the heartwood of the tree, caused by wood-rotting fungi, can support a great variety insects, as well as providing nesting holes for birds and small mammals. Many insects have very specific requirements and are found nowhere else. For example, a small black beetle is only found only inside hollow beeches, in the fruiting bodies of a specific fungus.  

Lichens are very susceptible to air pollution; where air pollution is not a problem, many different species of lichens may attach to the branches of old trees in an ancient woodland. Lichens often have very particular requirements for their establishment and growth and are very slow-growing. Their diversity in ancient woodland is favored by the great range of different physical substrates provided by very old trees and also by the lack of physical disturbance. This gives slow-growing species the opportunity to grow without sudden changes in microclimate or removal of habitat.

Ferns and mosses, which mostly require fairly damp conditions, are common woodland plants. Different kinds of mosses grow on the woodland floor, as well as on the bark of tree branches and trunks. Together with the lichens, they provide another entirely different range of microhabitats for small invertebrates such as mites and spiders. The presence of particular species of ferns, mosses and lichens can be used to indicate ancient woodland status.

Thick, springy layers of leaf, accumulated over centuries, enrich and modify the woodland soil. The leaf litter will host innumerable invertebrates feeding on the leaf litter itself. In their turn, they will provide food for centipedes, spiders, and others.

Hundreds of different types of fungi will infiltrate and decompose standing, fallen, and rotting tree trunks and branches. Tunnels in the rotting wood are inhabited by a wide range of insects which live in this microhabitat. The only indication of their presence may be small entrance or exit holes, with no hint of the labyrinths within.

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