Beneath the canopy of our woodland habitats
There are many intricate connections in British deciduous woodland. Interactions between fungi, plants, insects and animals enable the whole system to survive. In particular plants on the forest floor have to survive under a dense canopy of leaves, that not only reduce light levels but also generate a cooler daytime environment that pollinators are reluctant to visit.
Next time you visit a woodland area, take a closer look for plants, insects and animals living together and think about the interconnections which enable them all to survive.
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Seasonal woodland with coppiced gaps and closed canopy
Woodlands often have a patchwork structure with a mixture of light airy gaps and closed tree canopy. On the dark woodland floor plants have difficulty surviving due to lack of light. Gaps in the canopy can be due to fallen trees or man made clearings.
Spring wildflowers such as primroses Primula vulgaris and violets Viola sp. use the time before leaves come on the trees to grow and flower.
In order to succeed, primroses need to get their flowers pollinated very early in the year, before most insects are around, and get their seeds transported to other suitable areas.
Individuals of the familiar yellow flowered primroses Primula vulgaris can be as much as 50 years old. Plants hang on as the woodland gaps close up, slowly growing and producing occasional vegetative offsets. They do have a strategy to move around more though, when they finish flowering their capsules bend down so the seeds are within easy reach of ants which carry seeds short distances around the gaps.
Small mammals are also keen on the seeds and they can be involved in much longer distance dispersal into new areas.
Early flying bee-fly on violet flower
Some early flying bumblebees and bee-flies Bombylius sp. have long tongues capable of getting nectar at the base of the deep primrose or violet flowers. The bumblebees often pollinate a variety of these early flowering plant species as they are around before other pollinating insects emerge.
Bee-flies are actually parasites of solitary bees and so are looking for their nests to lay eggs as well as feeding on the flowers. The bee-fly may also be termed a flower thief as it takes the nectar that the plant uses to attract them but avoids touching the pollen so cheating the flower out of pollination.
Small black ant carrying seed with white eliasome
Ants are persuaded to help some of the spring woodland plants to disperse.
Plants have evolved seeds with an oil-bearing elaisome.
Ants are attracted to this rich food source and collect the seeds and carry them to their nest where they strip off the elaisome and discard the seed among nutrient-rich organic waste from the nest.
Duke of Burgundy butterfly caterpillar on its food plant the primrose leaf
The Duke of Burgundy butterfly, Hamearis lucina was a common coppice woodland species, but the butterfly has declined as this type of woodland with gaps and closed canopy has declined.
The caterpillar may grow to 16 mm, the one shown is smaller, perhaps 12 mm.
Unlike the primrose, the butterfly can’t hang on as the canopy closes and the woodland becomes dark and cool.
Passing on the skills of coppicing
TRANSCRIPT: Richard Wellings and Ron Smart have been coppicing this patch of woodland in Worcestershire for almost 15 years. Ron's passing this ancient art onto his grandson, and later he'll show me some of the skills that you need. Coppicing is basically controlling the growth of trees by cutting them when the stems get to the required size. The wood can be used in lots of traditional craft products and if it's left to rot in the forest it makes for a rich habitat for a variety of wildlife. Having trees at different stages of development means there's more chance for other plants to survive beneath their canopy, which attracts invertebrates and the creatures which feed on them.
The clip is reproduced with permission from the BBC.
Oak woodland with a tree suffering defoliation
Oak trees support hundreds of different species of insects and other wildlife, however there is a delicate balance. If the caterpillars emerge and start voraciously eating the succulent young leaves, without predators to control their numbers, they can completely defoliate the tree. Sometimes a second flush of leaves, known as Lammas growth, occurs later in the season partly offsetting this negative effect on trees. Lammas is derived from loaf mass, the first day of wheat harvest, and occurs on 1 August. However, it is likely that severe defoliation does reduce the ability of trees to defend themselves from attack by other agents and can contribute to ‘oak decline syndrome’.
With a changing climate the caterpillars are more likely to be able to keep in sync with the tree compared to the birds that eat caterpillars and which migrate thousands of miles to get there.
Winter moth caterpillar eating oak leaves
Two of the most important early season herbivores are the caterpillars of oak leaf roller moth Tortrix viridana and winter moth Operophtera brumata. In both species the eggs hatch at around the time of oak bud burst.
Winter moth caterpillars attack a range of trees and shrubs not only oak and can be a serious pest in orchards. After hatching the young caterpillars produce silk threads that catch the wind and carry them to other plants so that they still have a chance of survival even if their ‘home’ tree has not yet produced leaves.
Lammas growth in oak leaves
TRANSCRIPT: Oak trees are very unusual in having recurrent flushes . . . interspersed with dormant periods. Once the terminal bud opens the shoot starts elongating very rapidly, this will stop elongating around mid June and become dormant. It may stay dormant over the following winter or it may after a dormancy period of four or five weeks start growing again. This is known as Lammas growth and will produce a growth period of a further four to five weeks before becoming dormant again. The Lammas growth enables it to reproduce its leaf area and continue producing resources for the growth of the tree. [Birdsong] Throughout the range of the winter moth adult emergence is always in early winter. This ensures that the eggs are laid on the tree and experience the same climatic conditions as the buds and this helps to ensure synchronicity between the two.
The clip is reproduced with permission from the BBC.
Pied flycatchers Ficedula hypoleuca are migratory birds arriving back in the UK to start raising young when there is peak food abundance in May. However their preferred food, caterpillars, is more able to respond to changing climate, so the birds may not be able find enough food.
As a result, some of the chicks may starve. There are suggestions that the decreasing numbers of pied flycatchers is one of the first effects of climate change on our bird population.
Tachinid fly that parasitizes winter moth caterpillars
As usual in nature, if there is a large food source, such as the winter moth caterpillars, then there are several creatures that eat them. Cyzenis albicans a fly that looks a little like a house-fly, parasitizes the caterpillars and can reduce their numbers somewhat in UK.
However in North America, where winter moths have been introduced and are an invasive pest, the fly is used as a form of biological control and can substantially reduce winter moth populations.
Two native species of oak: English oak
There are two native species of oak in the UK, pedunculate or English oak, Quercus robur (illustrated here) and sessile oak, Quercus petraea (illustrated on the next slide). It's quite easy to tell them apart, English oak has leaves without stalks and long-stalked acorns, whereas sessile oak has stalked leaves and acorns without stalks.
Just to make things a little more complicated the two species of oak can hybridise giving trees with intermediate characteristics. Oaks can be very long-lived trees and have been very important for construction in the past, for example building Nelson’s navy.
Two native species of oak: sessile oak
There are two native species of oak in the UK, pedunculate or English oak, Quercus robur (illustrated on the previous slide) and sessile oak, Quercus petraea (illustrated here). Sessile oak is sometimes called Welsh or Cornish oak, since it's the type of oak most commonly found in western areas.
Oak is used for furniture making and is coming back into fashion for traditional timber-framed buildings. It takes a long time for the tree to grow large enough for use in building.If you plant an acorn now your newborn baby's great, great, great, great-grandchild might be able to use it to build their house. Actually, might be a good idea to plant that acorn, in future the oil will have run out, so no plastics and we might come back to using wood more extensively.
iSpot woodland observations
Share your woodland observations
Woodland habitats in Britain range from ancient oak, beech and lime woodlands in the south through to Caledonian pine forests in the Scottish highlands. There are also many types of planted woodland and urban forests, all have a wide variety of plants and animals.
Why not go out and see what you can find, take a photo and add it to iSpot.
Dark woodland floor
White helleborine orchid on dark woodland floor
White helleborine orchid Cephalanthera damsonium can grow on the dark woodland floor because it is not just relying on photosynthesis. Instead it is also getting carbon from the trees via ectomycorrhizal fungi.
Photosynthesis, literally means ‘putting together with light’. It is the process most plants rely on to convert carbon dioxide from the air into the sugars they need for growth.
The orchid only has this ectomycorrhizal relationship with a few species of fungi and they, in turn, need a specific type of host tree to be present. This set of relationships also tends to be restricted to a limited range of soil types.
Large fungi, 8cm across
Many Cortinarius fungi are ectomycorrhizal, some interact with both the orchid and forest trees, nutrients may be passing between all three.
Fungi in woodland have a range of different actions some parasitize trees others rot down dead material. However there are also more complicated relationships where carbohydrates from plants are passed to the fungus and the plant gets inorganic nutrients from the fungus. The white helleborine case is particularly unusual as the plant gets carbohydrates as well as inorganic nutrients from the fungus. These ectomycorrhizal networks between fungi, trees and other plants are very important for the overall health of the forest.
Orchid seed scattered on a ruler marked in mm.
White helleborine orchids Cephalanthera damsonium are self pollinated so they do not rely on insects in the dark environment where few bees ever visit.
The seeds themselves, like all orchid seed, are absolutely tiny like dust and have virtually no food reserves to help them start into growth.
Instead orchids again get the help of a fungus to germinate and grow, this may be a different species of fungus to the one they use later to help them gather nutrients.
Bluebells sometimes flower too early
Beech trees form some of the darkest woodland, the dense canopy and deep layer of leaf litter can result in almost no ground flora in summer.
However, there is a brief window of opportunity in spring before the leaves are fully out.
Bluebells Hyacinthoides non-scripta race to produce their flowers so that they can be pollinated before the woodland is dark and cool. But they must not flower too early or they risk being snowed on.
Some plants that are not green, such as birdsnest orchid Neottia nidus-avis, or yellow birdsnest, Monotropa hypopitys grow in very dark woodland and rely entirely on their fungal ‘partners’ to provide food.
Birdsnest orchids do contain chlorophyll but in an inactive form. If you heat the plant up then it goes green. Do not attempt this yourself as it will damage these rare plants!
There is a debate as to whether or not these plants are just parasites on the ectomycorrhizal network between trees and other plants in the woodland.
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