9. The seasons of the forest
Contents
9.1 The changing seasons
Seasons do not change in regions near the planet's equator. Organisms that live in these tropical areas have adapted to constant warm temperatures and steady amounts of rainfall throughout the year. Even the light part of the day remains approximately as long during the winter as it does in the summer. These conditions are beneficial for many living organisms, which is why tropical regions are home of the most diverse set of species on the planet.
In Northern Europe, the situation is quite different. The yearly conditions for living organisms vary between good and near impossible. The temperate regions of Northern Europe experience four distinct seasons.
Winter is the coldest and harshest season for living organisms. Summer, on the other hand, is the most beneficial time of year for many organisms. During the summer, there is enough sunlight for plants to photosynthesize, and the animals of the forest are at their most active.
Seasons: winter, spring, summer and autumn.
9.2 Birds and the seasons
During the winter, the snowy forests provide less food for animals such as birds. The birds of Northern Europe react to the changing seasons in two different ways: they are either sedentary or migrating birds.
Sedentary birds live in the same region throughout the year. The crested tit (pictured below) is an example of such a sedentary bird. They spend the winter either in or near their home forest.
During the winter, it is more difficult for sedentary birds to find enough food. They need a large intake of food to maintain their body temperature during the cold winter.
The crested tit.
When spring arrives, sedentary birds such as the crested tit are often quick to reproduce. As a result, the crested tit can sometimes have fully-grown offspring when migrating birds such as the pied flycatcher are only returning to their home regions.
During the autumn, sedentary birds like the crested tit do not have to store up energy and fat for a long migration journey. Some sedentary birds spend the winter near birdfeeders that are set up by humans.
Migrating birds complete two long journeys each year. These journeys are connected to the changing seasons. During the spring, migrating birds fly to the north, which is where they nest. They spend the summer in the north, and when autumn arrives, they start migrating back south. For example, the pied flyecatcher can spend its winters in Africa and its summers in Finland.
The reason why migrating birds complete their long journeys is the changing food situation in the northen regions. During the summer, there is plenty of food, such as insects, for migrating birds to enjoy. They nest and raise their young during the summer up north. When autumn arrives and the amount of food decreases, migrating birds begin to prepare for their long journey south. There, they find plenty of food even during the winter.
The pied flyecatcher.
In addition to sedentary and migrating birds, there are also irruptive birds. They do not migrate along the same routes every year, but rather move from place to place depending on the amount of food they can find. Crosbills and nutcrackers are examples of such irruptive bird species.
The nutcracker.
9.3 Spring
During the spring, the light part of the day grows longer and the temperature becomes warmer. The growing season of plants begins when the air temperature rises above +5 °C during the day. This occurs between April and the beginning of June.
In Southern Finland, many plants wake up even before the beginning of the actual growing season. For example, liverleafs often already flower during the middle of April. The first flowers you can find in the spring belong to perennial plants.
The liverleaf flowers early in the spring.
Broadleaf trees begin to develop and open their buds when the temperature rises above 0 °C. The buds develop into photosynthetic leaves and reproductive flowers containing stamens and pistils.
The rising temperatures also cause hibernating animals to wake up. Many reptiles, such as adders, begin to move only when the daily temperature rises well above zero. Similarly, mammals such as bears and hedgehogs wake up when the temperature grows warm enough.
The mountain hare changes its winter coat into a summer coat when the light part of the day grows longer.
Mammals change from their heavy winter coats into lighter summer coats when the light part of the day grows longer. This is why you can sometimes see a white-coated mountain hare even months after the snows have already melted.
9.4 Summer
During the summer, the forest is full of life. Birds take care of their young, mosquitoes buzz around in search of homeothermic animals to feed on, and various invertebrates scutter on the forest floor. You can see green plants and movement everywhere you look.
For plants, summer is the period of growth. As the sunlight is at its most plentiful during the summer, the plants concentreate their efforts into producing energy through photosynthesis. Perennial plants prepare for winter by storing sugars into their roots and rhizomes, whereas annual plants concentrate on reproducing as effectively as they can before they die.
As the summer begins to turn into autumn, plants and animals begin to prepare for the winter. For mammals, it often means eating more food in order to build up a fat layer for the winter, whereas migrating birds begin storing energy for their long journey south.
A birch forest during the summer.
9.5 Autumn
After a lively summer, the following autumn consists mostly of plants and animals preparing for the cold winter. As winter becomes closer, the conditions of the environment change significantly. Days become shorter and colder.
The movement and vital functions of invertebrates slow down. Homeothermic animals, such as mammals, have to use more energy in order maintain their body temperature.
During the autumn, broadleaf trees drop their leaves and store all the excess nutrients and sugars into their trunks. This results in the colorful autumn foliage. Before winter arrives, some trees grow new buds in order to be ready for the arrival of spring. Coniferous trees, such as pine and spruce, retain their needle-like leaves even during the winter months.
As the autumn progresses, igrating birds begin to leave for their southern winter habitats. Some vertebrates begin to hibernate or brumate. Slowly, the forest becomes more quiet. Many annual plants wither away, and the forest loses its green summer color.
However, the life of the forest does not slow down completely. Fungi begin to reproduce during the autumn, as the amount of rainfall increases. This is why mushrooms can be picked from the forest during the autumn months!
Many fungi grow their reproductive fruit bodies during the autumn.
9.6 Winter
When the temperature drops below zero degrees Celsius, water freezes. Plants cannot make use of frozen water in photosynthesis. If the water contained in the cells of living organisms freezes over, the structure of the cells are destroyed.
When the winter arrives, a layer of snow often covers the ground. For many animals, it makes both movement and searching for food more challenging. Invertebrates die or spend the winter in brumation, and the animals that eat them likewise either hibernate or migrate elsewhere.
A birch forest during the winter.
Although wintry forests can look still and lifeless, that is not the case. Both plants and animals have developed various ways to survive the cold, harsh winter. Some plants, for example, adapt to the winter by dropping their leaves and gathering sugar into their roots and rhizomes.
9.7 Overwintering stratedies
The animals of the forest begin to prepare for the winter as soon as autumn begins. Many forest-dwelling animals have to eat more food in the autumn than they normally do in order to build up their fat layer. Fat layers help to keep animals warm during the cold winter months by acting as insulation.
Some animals, such as squirrels, gather food supplies for the winter. Some animals even change their diet completely during the winter. An example of such a change is found in the wood grouse, which eats berries during the autumn but survives mostly on pine needles during the winter.
The stoat grows a white coat during the winter.
Many forest-dwelling animals change their fur coat or feathers during the winter. In most mammals and birds, the winter coat or plumage is significantly thicker and warmer than it is in the summer. In some species, such as the stoat and the mountain hare, the color of the winter coat is white, in order to act as camouflage when the animal roams the snowy forest.
Snow also offers animals protection from the cold air. Small mammals, such as voles, mice and shrew, spend the winter mostly in small tunnels they dig under the snow. These small animals have fast metabolisms, which means that they need a lot of food during the winter in order to stay warm.
The wood mouse spends most of its winter under the snow.
9.8 Sleeping through the winter
Some mammals, such as bears and badgers, spend the winter months in winter sleep. During winter sleep, the animal's body temperature drops a couple of degrees Celsius and its vital functions slow down. Its heart starts beating at a slower rate and its breathing grows slower. The animal does not need any food during its winter sleep.
Animals that sleep through the winter receive energy from their fat supply. They build up this supply by eating a lot of food during the summer and the autumn. The fat layer can take up to one third of the animal's weight when it lies down for its winter sleep. Bears and badgers dig dens to sleep in. They sleep in a curled up position, so that they will stay as warm as they can. An animal's winter sleep is deeper than its normal sleep, but it can still wake up from it, if bothered by people or other animals.
All cold-blooded or poikilothermic animals, such as frogs and snakes, spend their winter in a state of brumation or cold hibernation, where their body temperature drops near zero degrees Celsius. During brumation, their vital functions slow down and almost cease completely. Some insects, such as some species of butterfly, also brumate during the winter.
| Winter sleep | Hibernation | Brumation | |
|---|---|---|---|
| Body temperature | close to normal | just above freezing | close to zero degrees Celsius |
| Species | bear, badger | hedgehog, bats | all poikilothermic animals: e.g. reptiles and insects |