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When food runs short, moose suffer across generations
Moose in southern Scandinavia have become lighter and less productive over the past decades. In some areas, there have been dramatic declines in moose health.
For more than 20 years, researchers have done extensive field surveys of moose forage resources in Norway, from Agder county in the south to Trøndelag county further north.
The result is one of Europe’s most detailed datasets on food availability, browsing pressure, and moose condition. This material is now beginning to provide clear answers.
Climate, forestry, parasites, human activity, and a shortage of large bulls have all been suggested as explanations.
But the new and comprehensive study from NIBIO shows that food availability remains a fundamental factor.
What do moose eat – and how much?
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Forage surveys are a method to measure how much food is available in the forest and how heavily it has been browsed.
While traditional methods were designed for rapid monitoring of browsing pressure, the researchers in this study used a more detailed and research-based approach:
“We measured the actual, absolute amount of forage, rather than using the more coarse, indirect indices commonly used,” says Hilde Karine Wam.
She is a research professor at NIBIO.
“We recorded the length of unbrowsed shoots in centimetres, giving a precise picture of available winter forage. We also mapped the coverage of all plant species, not just trees,” she says.
This means that the researchers have a comprehensive overview of moose food resources – both during the growing season and the ‘lean’ winter months.
The amount of food matters – but not equally everywhere
A key finding is a strong link between food availability and the moose's health and condition.
Populations living in areas with abundant and varied forage have cows that produce more calves. The calves also weigh more.
However, the picture is not the same across all of Norway. Natural factors such as snow depth, terrain, and bedrock play a role.
In some areas, moose have to use more energy to find food, while other areas offer a greater diversity of forage plants.
The researchers therefore divided the study areas into four regions with different potential for producing moose forage.
“The same amount of food does not necessarily result in the same condition in different parts of the country,” Wam explains.
The legacy of overbrowsing
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One of the most interesting findings concerns ‘persistent maternal effects’: the impact of past overbrowsing passed down through generations.
Poor condition can therefore be inherited. Cows that grow up with limited food become lighter as adults.
They also produce fewer and smaller calves. This effect can persist for several generations – even if food availability later improves.
“In areas that have been heavily overbrowsed historically, moose are still lighter and less productive than expected based on today’s population density and food availability,” Wam says.
Researchers found this type of historical overbrowsing in large parts of southern Norway, especially west of the Oslofjord. There, several populations showed clearly reduced condition compared to what would be expected from current forage levels.
Calf ratios reveal food shortages more clearly than calf weights
Another clear pattern is that the number of calves per cow is a more sensitive indicator of food shortage than calf weights.
“A cow in very poor condition tends to produce only one calf rather than two, which is more common in healthy cows,” explains Wam.
“The calf that's born may still achieve a reasonable weight because the mother draws on her own body reserves to nurture the calf. For this reason, calf ratios reveal problems with poor food availability sooner than calf weights do,” she says.
Moose prefer certain tree species
Birch is the most common browse tree in most study areas, but it's rarely a species selected by moose, except in areas with few other deciduous trees.
“When moose can choose freely, they prefer rowan, aspen, willow, and oak,” Wam says.
The four species are called ROSE species, based on their Norwegian names.
Research shows that moose condition in most places is more closely tied to the amount of these preferred tree species than to the total amount of available tree forage.
On average, ROSE species make up only about one sixth of the tree forage.
“Moose achieve their best condition where they can afford to be selective,” Wam explains.
Large differences between regions
The amount and type of forage needed to maintain moose in good condition vary greatly geographically.
The western side of the Oslofjord is an area that has historically been overbrowsed. Here, up to ten times more ROSE forage is required to reach the same moose condition as, for example, in Østfold county, the researcher explains.
In Innlandet county, which has the least historical overbrowsing, moose do well on relatively small amounts of high-quality forage.
In coastal Trøndelag, it has been the total amount of forage, rather than ROSE forage, that has limited moose condition. This is partly due to shorter winters and thus a greater importance of summer forage.
“Preferences for certain tree species matter mainly in winter. In summer, a birch can be as attractive as a willow,” says Wam.
This study provides a rare opportunity to quantify how much food is actually needed – and where the risk is greatest that the moose runs a deficit.
A new rule of thumb for management
One of the most useful findings for practical management concerns birch:
“Most moose populations seem to decline in condition when more than 15–20 per cent of birch shoots are browsed in winter over time,” Wam says.
This provides a clear threshold that can be used in monitoring moose forage. Birch is also a better indicator than rowan in many areas, as rowan is often so heavily browsed that it no longer reflects changes in browsing pressure.
Vital knowledge – even in a warming climate
“Although the study tells us what has worked so far, the future is more uncertain,” Wam says.
Climate change, increased land development, more human activity, altered forest management, and rising competition with other deer species will put further pressure on moose.
It will likely become harder for moose to maintain good condition in the coming years.
Regardless, 20 years of forage surveys provide a solid knowledge base for more proactive moose management.
“Forage surveys can signal problems before the moose themselves show clear signs of declining condition," says the researcher.
Reference:
Wam et al. Food for fitness? Insights from 24 Norwegian moose populations for proactive monitoring and preventing overabundance, Ecosphere, vol. 16, 2025. DOI: 10.1002/ecs2.70476
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