Wind, salt spray and freeze-thaw cycles
In Isanti County, there are two major highways that move traffic, lots of traffic, to and from Cambridge. Red pines planted thirty to fifty years ago bear witness to this in the form of red needles and declining and dying lower twigs on branches facing the roads. On some trees, all of last year's needles are 50 to 75% dead. Yet, needles produced this spring are green and the new shoots are vigorous. These trees were planted in two to five rows paralleling the roadways and are within twenty to fifty feet of the pavement. They are overcrowded now, as are many trees of this vintage. The extent and severity of needle reddening is worse on pines near road curves and trees along stretches of road where pines line both sides of the road.
Red pines have a very low tolerance to salt spray that is revealed by needle tip dieback and reddening in late winter. Salt spray is made up of chlorine and sodium ions in liquid water. It's the chlorine that causes the problem. Chlorine ions accumulate to lethal concentrations in needle tips. Wind enters the picture as micro-wounds due to needle abrasion allow entry of the toxic chlorine ions and loss of water vapor. And that's not all that the salt spray does. Salt injury can impair cold hardiness and recurrent salt damage causes twig, branch and even tree death.
Along these highways there's always some winter/ salt injury. What was so different about last year? To begin with, more road salt was applied. These highways were pre-treated with a liquid salt solution before the snow storms and then treated afterwards with granular salt. Salt sprays are created as vehicles travel over the roads. Pine needles are very efficient collectors of salt spray droplets. So when eddies of salt sprays are swirling around they are quickly gleaned by needles close to the ground and close to the road.
Secondly, the winter was mild with many episodes of freezing-thawing. Needle tip dieback results from the deprivation of internal water when the needles are temporarily thawed and water vapor leaves the needles. In the presence of salt spray, more needle tissues are killed. This winter, all of the exposed needles had tip dieback on upwards of 50% of the pines growing along these highways.
A number of calls have come in to the Metro Region concerning ash and maple trees that either have not leafed out and were very late doing so, even though their buds look fine. Whether the damage was due to the sudden change last December or one later this spring isn't clear, but the damage is a common response to cold injury that occurs after a sudden change in temperature. Some trees, particularly the ash display severe die-back with suckering from the lower trunk. Others, particularly maple, show no die-back, but all of the buds come out one to two months later than usual. Some of these are just now coming out and their leaves are small and somewhat stunted. Other than a little TLC and perhaps some pruning to remove the dead wood, there is little we can do now to help these trees. In the future, avoid fertilizing late in the season and try to redirect street and house lights that may be keeping them active longer in the fall than is good for them.
Casting of freeze-damaged white pine needles
On June 20th, last year's white pine needles were yellowing on a few trees in the Brainerd area. Examination of collected needles did not show any sign of mechanical injury or infection. Needles nearest the apical buds were chlorotic or mottled yellow and by early summer they were being shed. Buds were usually not affected and new growth developed normally. Within a week of first noticing this, similar reports began coming in via area foresters north of Brainerd and as far as Beltrami County. They were receiving reports from homeowners concerned about the yellowing and subsequent shedding of needles on white pine.
An extensive survey of white pines showing these symptoms was conducted on June 28th and 29th in Crow Wing and southern Cass Counties. With few exceptions the following conditions were noted: only the needles from the past years' growth were affected, new needles and shoots were healthy, trees were growing in "frost pockets" or on the down slope of ridges, and only lower branches up to perhaps thirty feet were involved. Winter freeze injury can occur on the previous seasons needles of eastern white pine when a period of warm weather in late winter or early spring is followed by a sudden drop in temperatures at the beginning of the growing season. The youngest needles, those nearest the branch tip loose cold hardiness and they are vulnerable to sudden temperature drops. By the time the new shoot is elongating, the affected needles are discolored and are being shed.
This injury differs from white pine needle blight caused by the fungus, Canavirgella banfieldii. With needle blight, only some of the needles are infected, and they turn grey and are retained on the twig as fruiting bodies develop.