Trunk splitting is the result of rapid drying of the tree.  The bark is like a band around the wood.  As the band dries it compresses the wood tighter and tigher.  When the band of Bark stretches to the limit, it splits.  The wood underneath bursts as pressure is released, splitting as well.  Trunk splitting occurs more during rapid drying conditions and than when the trees were already dry when they wee cut.  Trunk splitting may be worse in fields with high levels of tissue nitogen, relating to the amount and type of wood formed in the tree.

Trunk splitting may aggravate the decorating of a tree, it in no way increases the fire potential of that tree.  They dehydrate the same as they would without the split.  It is also related to the rapid drying and heating that can occur in full sunlight.  The needles adjacent to the string are compressed and possibly under greater moisture stress than the rest of the tree.  String burn can occur in less that 30 minutes in full sunlight under temperatures about 70 degrees f.  It can also occur when frozen or frosted trees areexposed to sunlight.  If trees are quickly put in shade, string burn can be totally avoided.

The risk of all this can be avoided by carrying for your trees when the arrive at the retail lot.  Cut 1/4 inch and place the trees in water and rehydrate them.  In addition put these trees under shade and finally when you put the tree out on the lot the ultimate is to drill the trees and display them on stand with water.  Country Cove Choose and Cut Christmas Trees does all these things with the cut Fraser Firs they buy from Cool Springs Nursery and they have NEVER HAD A SPLIT TRUNK IN TEN YEARS OF BUSINESS.

The primary force involved in spit trunks is the capillary tension for the water in the cell walls of the wood.  Capillary tension is the force that holds liquid in small tubes such as a glass thermometer.  As moistureis lost from foiage of a cut tree, the remaining water in the wood is still stretched from foliage to trunk by capillary tension.  This increases the inward pull of the water in the tree trunk-much like a vacum in a soaked container-and increases the force on each vessel in the wood.

When the forrce of capillary tension exceeds the internal strength of the wood a crack develops.  The tensile strength of Fraser fir wood is only about 180 psi.  The forces involved in capillary tension have been measured as high as 515psi-much greater than the strenght of the wood.  When the cell wall of one vessel collapses, integrity is lost, more cells collapse, and the crack splits open.

Trunk splitting is a function of rapid moisture loss from cut trees.  Cut trees can lose some moisture from the cut end of their trunks, but lose most from the foliage as it respires or breathes.  Therate at which tree foliage respires is closely linked to climatic conditions and tree dormancy.

Full sun, high temperatures, and dry winds can pull moisture from the foliage of cut trees.  While drought may be a contributing factor to the stresses that initiate cracks, the primary factor is exposure to conditions tha dry the tree out rapidly.   Cracks  can develop the day after a rain if newly cut trees are subjected to drying conditions.  Cracks have occurred in cold temperature accompanied by dry winds, but exposure to bright sun and temperatures above 70 degrees are the conditions most likely to result in split trunks.  Cracks will develop during a period of severe exposure whether it occurs in the field, in storage, during transportation, or on the retail lot.

In a normal fall season Christmas trees experience enough cold temperature to go dormant before tree harvest begins.  Dormant trees shut down for the winter.  Their stomates, the openings on the undersides of needles, do not oen as much.  Respiration slows.  less air is exchanged and less moisture is lost than if the trees were actively growing.  Dormant trees can be exposed to drying conditions and not lose much water.  Dormant trees are less likely to develop cracks in their trunks.

If fall temperatures remain warm with few nights below freezing, trees will fail to achieve dormancy.  Trees will still be actively respiring and have minimal defense against drying conditions.  This influences all aspects of tree freshness, not just formation of cracks in the trunk.  Needleshed and stringburn are also more likely in trees that have not undergone the physical changes of dormancy.  in short, if autumn is unusually warm, extra measures will be needed to maintain tree freshness.

Recommendations:  Since trunks can split during any period of exposure during harvest or the retail season, all handlers must share in the responsibility for tree care:

Growers should: 

-limit cutting in the niddle of very hot days to minimize exposure of unbaled trees.

-transport trees from fields to storage areas as quickly as possible to avoid excessive exposure to sun, heat, and wind.  This is very important when trees are cut during sunny, warm weather.

-store trees upright with trunks in contact with the ground in a cool, dark, and moist location. 

Retailers should:

-build a storage area prior to tree delivery that is cool, dark, and provides some means of watering the trees.  Delivered trees should be stored immediately to minimize exposure.

-display trees with some means of irrigation such as a wet mulch or watered sod, or preferably use tree stands with water reservoirs.  Misting displayed trees with water can cool them down and slow drying.

-consider putting up a tent or other shade structure to protect displayed trees or relocate to a lot naturally shaded by mature trees.

-cut a 1/4 to 1/2 inch disk off the trunk to improve water uptake prior to display in water or sale to a consumer.

-educate consumers regarding care and safety of fresh cut Christmas trees.

Information from:Jeffrey H. Owen, Area Extension Specialist, N.C. State university and Dr. Eric Hinesley