Forest and tree line
The Varanger Peninsula is located right where sub-Arctic forest transitions into low-Arctic tundra. As the climate warms, we expect this boundary to move northward.
However, the forest is also threatened by outbreaks from several species of insect defoliators. What will the forest in Varanger look like in the future?
The forest on the Varanger Peninsula consists almost entirely of mountain birch. This birch forest is found mainly in the southwestern part of the peninsula, although there are also scattered occurrences of forest further north and east.
Photo: Geir Vie
The map on the left shows the distribution of high-Arctic (pink), low-Arctic (light green), and sub-Arctic (green) zones. The tree line (dark green line) towards the Arctic stretches 13,000 km across the entire Northern Hemisphere and crosses, among other places, the Varanger Peninsula.
Source: Arctic Monitoring and Assessment Programme (AMAP)
A warmer climate may make it possible for trees and shrubs to grow in areas that were previously too cold for them, such as the tundra on the Varanger Peninsula.
However, it takes a long time for new trees to grow, and the tree line moves very slowly.So far, limited upwards movement of the tree line on the Varanger Peninsula has been documented.
The birch's worst enemy
Climate changes are not only an advantage for the birch forest, as a warmer climate also provides better living conditions for the birch's worst enemies – insect defoliators.
The most common species of defoliators on birch are the autumnal moth and the winter moth. Both are butterflies in the family Geometridae. Moth populations have periodic fluctuations, and sometimes the larvae become so numerous that they consume all the leaves on the birch trees.
Severe moth outbreaks can kill the forest over large areas.
Larvae of the autumnal moth.
Photo: Moritz Klinghardt
Larva of the winter moth.
Photo: Moritz Klinghardt
Defoliators gain new ground
Outbreaks of the autumnal moth have probably occurred in the birch forests of Varanger far back in time.
However, outbreaks of the winter moth are a newer phenomenon, which was recorded for the first time in Varanger around 2005.
The species has thrived in Eastern Finnmark and has even spread beyond the birch forest range and into the willow thickets on the tundra along the east coast of the Varanger Peninsula.
Willow thickets damaged by the winter moth east of Vadsø.
Photo: Ole Petter Laksforsmo Vindstad
The winter moth is traditionally a more southern species than the autumnal moth.
Larva of the winter moth. Photo: Moritz Klinghardt
The establishment of the winter moth as an outbreak species in Varanger is therefore most likely mediated by a warmer climate.
Double trouble with defoliators
The establishment of the winter moth has had large consequences for the birch forest in Varanger.
Outbreaks of the winter moth often come a couple of years after outbreaks of the autumnal moth, so the birch trees can be defoliated for as many as 4-5 years in a row when the two species of defoliators occur together.
Outbreaks of this magnitude are often more than the trees can tolerate. Such intensified outbreaks have left large amounts of dead birch forest in Varanger over the last 20 years.
Birch tree killed by defoliators in Polmak.
Photo: Jakob Iglhaut
This happens when the autumnal moth and the winter moth have outbreaks in the same area:
The peak outbreak of the autumnal moth comes first, while the peak of the winter moth outbreak comes a couple of years later.
Thus, the combined outbreak lasts 4-5 years, even though each species only has outbreaks for a couple of years.
The life cycle of spring feeding geometrid moths
Insects, including geometrid moths, are ectothermic organisms, which means that their body temperature is governed by the temperature of their surroundings.
Therefore, the survival and development of geometrid moths are strongly influenced by temperature throughout their entire life cycle.
Higher temperatures mean that the moth develop faster. Climate change may thus allow these species to cope further north and at higher elevations that has historically been the case.
Spring (Mar-May): When temperatures are high enough, the eggs hatch.
Summer (Jun-Aug): The larvae grow faster in warm summers and pupate when they have grown large enough from eating birch leaves.
Autumn (Sep-Nov): The adult moths are more active when the autumn is warm.
Winter (Dec-Feb): The eggs have a greater chance of surviving in mild winters.
What happens to the forest after an outbreak?
The birch is well adapted to moth outbreaks and is generally able to recover. Surviving trees can replace dead trunks by sending up new shoots from the root. Such basal shoots grow quickly and can become more than 2 meters tall in just ten years.
Birch with basal shoots. Photo: Jane Uhd Jepsen
Severe outbreaks can also kill the entire root system of a tree, and then it cannot produce basal shoots. If this happens, new trees must be established from seeds. Seedlings grow slowly and are vulnerable to browsing by both rodents, reindeer, and moose. In this way, it takes several decades for the forest to grow back. Read more about reindeer and moose.
Reindeer are fond of fresh birch shoots. Photo: Geir Vie
The new combined outbreaks of the autumnal moth (on the right) and the winter moth (on the left) increase the risk of tree mortality and hence the need for rejuvenation by seeds.
Photo: Moritz Klinghardt
How we study and monitor forest and tree line dynamics?
To document and understand changes in the birch forest in Varanger, we need to gather information about the forest's health. For example, it is important to know what proportion of the trees are dead, and whether regrowth is progressing mainly via basal shoots or seedlings.
We also need information about the grazing and browsing animals that affect the forest.
How does the amount of defoliators vary from year to year, and how is regrowth in areas with dead forest affected by moose and reindeer?
To answer these questions, the forest and tree line module in COAT has the following regular activities in Varanger:
- Annual counts of moth larvae.
- Regional mapping of forest health approximately every 5 years.
- Experiments with animal exclosures in dead forest to see how the forest regenerates in the absence of rodents, reindeer and moose.
Recording of dead trunks in forests damaged by outbreaks..
Photo: Jakob Iglhaut
Measuring basal shoots from a birch exposed to moth outbreaks.
Photo: Moritz Klinghardt