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Winter Browning of Conifers

winter browning of conifers(Report is courtesy of the Ontario Ministry of Natural Resources' Forest Health Program)

An extensive area of winter browning on conifers (evergreens) became apparent in the latter part of April and early May 2012 in northwestern Ontario. Affected species include white spruce, black spruce, jack pine, white pine, red pine, and balsam fir. Aerial mapping to determine the size of the affected area was done in early May. Initial reports and ground checks indicate the event extends from Nipigon to Geraldton and west to Thunder Bay, Shebandowan, and Fort Frances.

Winter browning is most severe on younger trees up to about 25 years old, although some older trees or older stands also have some browning. Trees in plantations as well as trees on the edges of forest stands are more severely affected. Upper parts of the trees are most severely affected while lower portions are often still green.

SYMPTOMS

Winter browning causes the needles on conifer trees to dry out and turn brown. In some species, the needles may appear red, yellow, or grey, or in the case of blue spruce – purple. In most cases the buds are still alive. Damage is most severe on younger, smaller trees, on the south or west side of trees, and edge trees with westerly or southerly exposures.

CAUSE

Winter browning is caused by several combined factors. During periods of increased sunlight, strong winds, and warm temperatures, conifers lose water from their needles faster than it can be replaced. This is common in February and March. In this situation, dormant roots and the frozen ground prevents the tree from replacing lost water, causing the needles to dry out and die.

Smaller trees are usually more severely affected because larger trees have more moisture to replenish that lost to drying conditions. Smaller trees also have more foliage closer to the ground, where sunlight reflected from snow or ice can increase temperatures and cause increased drying of needles. Foliage below the snowline is protected from the drying conditions, and will survive the event and remain green. If there is no snow, heating of the ground by the sun warms the surrounding air, which in turn warms the parts of the tree closest to the ground,making smaller trees more susceptible to drying.

The damage to needles worsens when warm temperatures and sunlight stimulate the needles to break dormancy and begin photosynthesis (using energy from the sun to convert water and CO2 into carbohydrates). Active needles lose their cold tolerance and can freeze if cold temperatures return. If sunlight, strong winds, and warm temperatures are followed by one or more thaw/freeze cycles, needles can dry out, freeze, and die.

This is likely what occurred in northwestern Ontario in the spring of 2012. Record breaking high temperatures in March (10 - 22) were followed by an extended period of cold temperatures and winter weather. Cold days were sometimes interrupted by short periods of warm temperatures, setting up a cycle of thawing and freezing of the needles. Initial drying of the needles caused cells to lose moisture, damaging the needle tissues. The needles may have been able to recover from this but thaw/freeze cycles in May caused further damage to the tissues before they could repair themselves from drying. Trees would have appeared healthy in March, but cold weather in May killed the damaged needles, resulting in rapid death and browning.

IMPACTS

In most cases, warm weather in March did not last long enough for trees to break bud and start growing and producing new shoots. Buds have likely survived with little damage. Trees may look heavily damaged, but if buds are still alive then the trees are expected to recover from the loss of needles. Existing buds will break dormancy in response to warm spring weather growing into new shoots with a new crop of needles. Ground checks May 10-14 show good bud expansion and healthy growth, indicating the trees are starting to recover.

The trees are likely weakened from losing their older needles. A better growing season (i.e., ample rainfall and moderate temperatures) this year and next, will provide a better the chance for trees to make a full recovery.

Damaged needles will drop from the trees during the summer, leaving the older sections of the branches bare of needles. Within one to two years the damage should no longer be visible. Monitoring will be carried out throughout the season to determine extent and overall severity of this event. It is important to note that dry, flammable needles, are an increased fire hazard.

WHAT YOU CAN DO

Watering trees with a low spraying sprinker system is the best way for these browning Conifers to overcome the stress!