Tough Paddling

Droughty weather, again

To say it's been dry would be, well, an understatement. Since last July, 7/8ths of the state is two or more inches short of rainfall. For large sections of the western and southern counties, precipitation fell six or more inches below normal. In the extreme southeast, deficits exceeded eight inches. See map. For the northern third of the state, this is the third consecutive droughty spring.

But don't just take our word for it, the State Climatologist's Office released this information in early May:

  • When compared to other July to April time periods, the July 2004 to April 2004 combined rainfall totals ranked among the driest on record for many areas of western and southern Minnesota.
  • The National Drought Mitigation Center indicated that most of Minnesota was judged to be in the moderate and severe drought categories.
  • MN Ag Statistics reported that the state's topsoil moisture was 10% very short, 32% short, 57% adequate and 1% surplus.
  • US Geological Survey indicates that stream discharge values for nearly one half of Minnesota's rivers ranks below the 25th percentile for the date.

But that's not all. Most of the state has been running a rainfall deficit since late 2001, with some areas in the Arrowhead and extreme Southeast pushing twenty inches shortfall over a twenty-four month period. Looking at water year precipitation, from Oct 2002 to Sept 2003, you can see that most of the state has experienced significant rainfall deficits. See map.

Does Arthur Murray's Studio teach the steps for a Rain Dance?

Drought mortality and recent winter injury

The southeast experienced a severe drought during the later half of the 2003 growing season. As a result, this spring we have seen scattered mortality of arborvitae and young spruce in windbreaks and ornamental plantings. Additionally winter burn is evident in red pine plantations. The good news is that we have seen some rainfall which should reduce the potential for bark beetles. Hopefully we will return to seasonal patterns of precipitation.

Winter-injured conifer needles turn red in the spring

By now you have probably noticed all the red needles on white pines and other conifers that are growing along highways. Although the needles look terrible, the buds, twigs and trees are not dead. The needles had a rough winter and they were discolored by winter injuries, but resist the urge to prune them away or remove discolored trees. Chances are very good that these trees are live and healthy beneath their mask of red needles. Buds are well protected during the winter and will grow once spring arrives. Winter injury was enhanced by strong, dry winds, many days of bright sunshine and by low relative humidity. Humans also added to the injury equation by applying road de-icing salts which are toxic to plants.

Even during the dormancy of winter, tree needles need and use liquid water. When water, stored in twigs and needles, is gone, cells and tissues become progressively more dehydrated and start dieing. Water is lost faster when the relative humidity is low, when dry winds are blowing and when warm, sunny days occur. Affected needles, turn red or brown from the tip down and, often have dark bands or a mottled appearance. In late winter, the needle discoloration intensifies and becomes more noticeable. Buds are usually not killed. Normally, snow cover prevents winter injury of young conifers by providing shelter from drying winds and from the glare of the sun. In some years it is common to see young conifers with a strong line of demarcation separating the brown, desiccated tops from the healthy, green branches that were covered by snow.

Needles loose their internal water three ways:

  • through evaporation from tiny ruptures in the needle's surface which are caused by wind and ice crystal abrasion.
  • through the rupturing of cells and tissues due to rapid freezing. After slowly warming up and becoming metabolically active on sunny days, the needles quickly refreeze when the sun sets. The water inside the cells and cell walls freeze which ruptures and kill's them.
  • through transpiration on sunny, warm winter days. Water stored in needles was lost during the few (very few) days when temperatures were above freezing because cells were actively using water.
  • In our climate, water uptake and resupply, to replace water the needles have lost, are prevented by the continuous freezing of roots and stems.

Some trees or groups of trees seem to get winter injury every year. In these situations, it is likely that the trees are under stress and do not have the resources to tolerate any internal desiccation and therefore suffer winter injury every year. For example, some clumps of roadside red pines show winter injury symptoms every year because they are growing offsite, either in soils that are too wet or in soils that restrict rooting depth. This stress predisposes the trees to needle desiccation and ultimately to repeated winter injury.

Winter injury is the most important factor limiting the northern range of conifer species. Temperate species, such as red and white pines, are much more vulnerable to injury than are the boreal species, such as black spruce and jack pine. Native tree populations are adapted to their locality. Moving them ( seeds or seedlings) 100 miles north or south of their site of origin can result in damage due to winter injury. Exotic species, like Austrian or Scots pine, should be planted in climatic zones similar to their site of origin in Europe.

Is there anything people can do to prevent winter injury? Here are some suggestions.

  1. When selecting trees to plant, choose species and cultivars that are adapted to your local growing conditions.
  2. Avoid planting white and red pines, balsam fir and white spruce within 150 feet of a highway to prevent salt damage.
  3. Avoid planting yew and arbor vitae on south or southwest sides of buildings or in sunny and windy locations.
  4. Erect temporary barriers around conifers susceptible to winter burn. They can be made of plywood, burlap, tar paper or plastics. Recycle your Christmas boughs and tree by propping them up on susceptible conifers. They will act as a barrier and also hold snow for more natural insulation and protection.
  5. Just after the snow melts and prior to bud break, rinse de-icing salts off both conifers and hardwoods.
  6. Reduce or eliminate the use of de-icing salts.
  7. Replace trees that have severe winter injury year after year. They are not in the right location and will only decline due to needle and twig loss over a period of many years.
  8. Keep conifers properly watered throughout the growing season and fall.

Conifers growing in Minnesota have had a long, long winter with plenty of opportunities for winter injury. But in spite of their appearance, chances are good that your trees are live and healthy beneath their mask of red needles. Buds were well protected during the winter and will be elongating before the end of the month.