Copyright © John A. Vucetich — All rights reserved
Balsam fir has a morphology (shape) that allows one to measure - with relative ease - how much a balsam fir has grown in each of the past several years. Each year we measure hundreds of fir trees. The calipers to the left show height growth from the previous summer.
The effect of moose on fir is dramatic. A fence protects the trees on the left. Moose browse where ever they like on the right side of the image.
The tall conifers on the left are balsam fir trees that are about 35 years old and about 30 feet tall.
The scraggily sticks in the foreground are also living balsam fir trees. Some are 30, 40, and even 50 years old.
Balsam fir is a strictly North American species.
Balsam fir has not always been present on Isle Royale. When Isle Royale first emerged from Lake Superior about 8,000 years ago - shortly after the glaciers of the last ice age retreated - balsam fir lived in southeastern North America. For the past several thousand years, balsam fir has been slowly migrating north.
Moose eat balsam fir only during the winter.
Balsam fir is not very nutritious. While moose feed on balsam fir, they lose weight. Moose eat balsam fir, because during the winter it is one of the most abundance sources of food.
Compared to other trees, balsam fir is not especially long lived or especially tall. However, for Isle Royale moose balsam fir is especially important.
Trees that make it to the canopy typically grow to be 40-80 feet tall and live 100 to 150 years.
source: USGS
Below are two images of the same balsam fir. One is labeled to highlight growth and browse. The images depict the top 2-1/2 feet of a 4-ft tall tree. Red arrows point to places where a moose had browsed. This tree shows moderate levels of browsing. To be out of the reach of a moose, a balsam fir branches must be 9 or more feet (3 meters) from the ground.
We use dendrochonology (tree-ring analysis) to examine past growth patterns in balsam fir. But what do tree rings have to tell us about the dynamics of wolves and moose?
Each year moose abundance fluctuates. In some years abundance increases, and in some years abundance decreases. The fluctuations are sometime minor and sometime major. What are the most important influences affecting annual fluctuations in moose abundance? Annually fluctuating wolf predation would seem to be important. But, what about annual fluctuations in food supply?
image by G. Desort
image by G. Desort
The size of a tree ring produced in a particular year is an indication of how much the tree grew in diameter that year. Tree ring size also indicates how much foliage the tree produced that year. That foliage is winter moose food, in the case of balsam fir.
example of large annual growth
example of small annual growth
This particular tree was sacrificed in 1995. count the rings to learn in what year it had been born. It had been growing since about the time our research began.
moose
wolves
40 YEARS OF BALSAM FIR GROWTH
ULTIMATELY, ANNUAL FLUCTUATIONS IN FOOD SUPPLY HAVE AT LEAST AS MUCH TO DO WITH MOOSE POPULATION DYNAMICS AS DO ANNUAL FLUCTUATIONS IN WOLF PREDATION.
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height growth from previous summer
height growth from two summers ago
A heavily browsed tree never gets a chance to grow out of the reach of a moose. Once a tree is about 1 foot tall, it still takes about 10-15 years of good height growth (without being top-browsed) to grow out of the reach of moose.
The tree on the right suffered heavy browsing in the past. By about 2001, Isle Royale moose began to decline. By about 2005, many shorter fir trees responded like this one by exhibiting greater annual height growth.
