Jack pine seedling mortality
It's tough world out there, especially if you're a jack pine seedling. To begin with, jack pines are one of the few trees that can withstand the droughty, nutrient-poor sandy soils found in many areas of Minnesota. Sometimes, seedlings are planted and then must go ten or more days without rain. After surviving that, there are hungry deer and insects and diseases to contend with. This just doesn't sound like a recipe for success.
Backus Area Foresters are concerned about jack pine seedling survival, noting mortality levels in plantations in northern Wadena County that occasionally exceed their expectations. In most cases, however, the plantations are still considered adequately stocked. In May and June of this year, twenty plantations less than eight years old were surveyed for establishment losses and incidence of gall rust. Collections were made to determine which gall rust species were infecting the seedlings and saplings. Most plantations have been annually bud capped to prevent deer browse.
Establishment losses are losses during the first one or two years that seedlings are in a plantation. Seedlings die if they are planted improperly, if the roots are trimmed off, if the roots dry out during planting, if the microsite is unfavorable, if rains aren't timely, if the soil is cold and the air is hot, and so on. For the last eight years, establishment mortality in these plantations has averaged 22%. Deer mortality is less than five percent due to a program of bud capping otherwise it would be much, much higher.
Gall rust is not a problem for tall saplings or pole sized trees but can cause mortality in seedlings. A gall on the main stem of seedling can girdle it or can be a point of fracture, in both cases, killing the seedling. A rule of thumb for seedling mortality is that about 25% of the main stem galls kill the infected seedlings. The other 75% of the galls do not kill the seedling because the galled tissues did not completely encircle the stem and, ultimately, the tree will be able to grow over the gall.
In 1984, a University of Minnesota/ DNR-Forestry survey found that 100% of the galls (n=30) found in Wadena County were caused by Cronartium quercuum, commonly called pine-oak gall rust. In May and June of 2000, all the sampled galls (n=100) were still pine-oak gall rust. The alternate host for this rust is oak and there are plenty of oaks on and near these sandy, plantation sites. And, in fact, gall rust is very widespread in the county since jack pine and jack pine/oak are the main covertypes there. The recent survey also found that the bulk of the infections occurred during the "wave years" of 1996 and 1998. Up to fifteen galls could form on the elongating main stem during a wave year; the average being three galls. Woody galls developed but do not fruit (bear spores) for at least three years so the galls from 1998 and 1999 were not collected. Gall rust was also found on the lowest portion of the main stem and in one year old plantations, indicating that a small proportion of the trees are infected with gall rust while still in the nursery.
Plantation surveys showed that 50% of the jack pine seedlings had one or more galls on their main stems. Gall rust losses were estimated to be 13%. This gall rust estimation is based on the average incidence in Wadena County of 50% main stem galls and the estimation that about 25% of infected trees die. Mortality of trees with multiple galls, such as those formed during wave years, may be higher.
On average, establishment mortality accounts for 22%, deer browse less than 5% mortality and gall rust is estimated to cause 13% mortality. So, the total average mortality of seedlings is about 40%. Seedlings are planted at 800 trees/ acre, and, doing the math, about 480 trees/ acre survive. Foresters would like have 400 to 500 trees/acre to consider the plantation well-stocked.
Plantations become poorly stocked if establishment, deer browse or gall rust mortality exceeds the average. Gall rust incidence, as you might expect was quite variable, ranging from 33% to 84%. And this is where the problem lies, especially if establishment or deer browse mortality are also high.
What can be done? Here are some suggestions to boost the chances of plantation
1. Plant jack pine at higher densities to allow losses yet still have acceptable stocking levels.
2. Add red pine seedlings to the plantations, either at establishment or as interplanted seedlings in "failed" or browsed areas.
3. Accept lower stocking and encourage oaks after about 10 years.
4. Wave years are unavoidable, work around them with interplantings.
5. Where feasible, plant red pine instead.
6. Investigate mortality factors of natural jack pine stands, seeded stands, containerized stock, etc. to see if 2-0 stock is more prone to mortality than other types of seedlings.