Reading Four: Plant Communities
Impact: Soil destabalization
The ice in the permafrost (permanently frozen ground) helps maintain the structure of the soil. When it melts, trees can start to fall over or sink-holes can develop which then seasonally fill with water and kill trees living there.
Impact: Wetland drainage
In some Arctic wetlands, ponds and lakes, the water is perched on top of a layer of permafrost. The permafrost acts like the countertop in your kitchen and the wetlands are like a sponge that is completely full of water sitting on top of the counter. If the permafrost melts, then the water can drain out of the wetlands and ponds just like the water would drip out the bottom of the sponge if there were no countertop. When wetlands and ponds drain, not only are the plants that live there affected, but also the fish and other animals that rely on the water.
Impact: Potential desertification
Even though the total amount of precipitation is projected to increase in the Arctic, precipitation may come at times of the year when plants do not need it, or it may come in extreme events where most of it runs off to the rivers quickly. Also, as the temperatures get warmer, more water will evaporate and plants will transpire more water. Both processes acting together, known as evapotranspiration, send water back into the atmosphere. It is possible that in certain areas the increased precipitation may not be able to keep up with the increased evapotranspiration. If this happens, areas can dry out and become polar deserts.
Impact: Thriving inscects
When winters are long and very cold and when summers are short, as they traditionally have been in the Arctic, numbers of pests like the spruce bark beetle are kept in check. Spruce bark beetles can kill spruce trees. Warmer winters mean that more bark beetles survive each year. Also, the bark beetle usually needs two years to complete its life cycle. When the summers are unusually warm and long, however, bark beetle lifecycles can be accelerated and take only one year. This means that there will be many more beetles. Also, healthy spruce trees have natural defenses against bark beetle attacks. When the beetles try to bore into the tree to lay eggs, the tree can push pitch (sap) out against the beetle and keep her from being able to get into the tree far enough to lay eggs. When trees are stressed from drought and warmer than normal temperatures, however, they do not have enough pitch to fight the beetles.
Similarly, spruce bud worms, another pest that can kill spruce trees, lay more eggs when it is warmer. Also, warmer temperatures make spruce bud worms change the time of their reproduction. When this happens, the natural predators of the spruce bud worm are not available or ready to eat them, so bud worm numbers increase.
Impact: Competition from invading species
As temperatures warm, plant species begin to shift their ranges northward, invading areas previously inhabited by Arctic species. Many of the adaptations that allow Arctic species to survive in such cold conditions also limit their ability to compete with invading species. For example, when the temperature gets above about 60 degrees F (16 degrees C), black and white spruce trees are not able to grow as well. If temperatures get too hot, the black and white spruce will not be able to grow at all.
Impact: Increased forest fires
As climate warms and forests dry, forest fires increase. The average area of North American Boreal (northern) forests that burns each year has more than doubled since 1970.