Assessment of Armillaria Root Disease on the Ice-Road Trial in Nakusp BC

This work assesses a trial that was established in 1996-97 in the area of Nakusp, BC, to look at the efficacy of pushover logging (root removal technique with large machines) over handfalling type logging to reduce the severity of post-harvest Armillaria root disease. Pushover logging did not seem to reduce levels of root disease over handfalling type harvesting. High variability within treatments lowered the power of the trial. However, in addition to being statistically insignificant the mean differences between the two main treatments were small and biologically uninteresting. The trial also included four retention levels as treatments and there seems to be a trend of declining root disease with increased retention of stems. This phenomenon could be further investigated for its potential to be part of a protocol for harvesting high Armillaria stands. The relationship between soil disturbance, tree damage and Armillaria root disease should be studied further. There was a very strong difference between susceptibility of natural and planted seedlings to Armillaria root disease. Natural regeneration should be favoured in Armillaria affected areas. Western red cedar seems to offer much better survivability than other species in areas affected by Armillaria, whether planted or natural regeneration. INTRODUCTION Armillaria root disease is one of the most important tree disease issues in the Southern Interior of British Columbia (SIBC) (Cleary et al, 2008). The primary disease organism in SIBC is Armillaria ostoyae (Romagnesi) Herink. In badly affected stands, mortalities can be 3% per year starting at about three years after harvest (Chapman, unpublished data) and can continue to 12 or 15 or 20 years depending on site conditions, after which time mortality levels tend to decline (Reeves et al, 1993; Cleary et al, 2008; Wargo and Kile, 1992). For young trees in SIBC, most commercially valuable species, except, perhaps, Western Red Cedar ((Thuja plicata Donn ex D. Don) have the potential to suffer significant losses in the correct conditions. In British Columbia one of the recommended procedures for treating to mitigate losses to Armillaria root disease is to remove roots after harvesting and prior to planting (Cleary et al, 2008). The usual procedure for doing this is to pull stumps from the ground using an excavator. Some concern has been raised that this approach is costly and has the potential to create large amounts of soil disturbance (Mike Curran, personal communication). An alternate approach to root removal that has been proposed and used on an operational scale, is called pushover logging. In this technique an excavator pushes the trees over, using the bole as a lever to assist lifting the root. In theory, this should reduce the shear stresses on the soil. Soil disturbance measurements were carried out as part of the Ice Road Trial, though they are not dealt with here. One of the major reasons for establishing the Ice Road Trial was to evaluate the efficacy of the pushover type of root removal technique. Root removal techniques have gained a fairly wide degree of acceptance in Southern British Columbia. However, there remains a dearth of published peer reviewed literature establishing the efficacy of the techniques, while to the contrary, a number of peer reviewed publications question the efficacy of the techniques (Leach, 1939; Reaves et al, 1993; Roth et al, 2000). The technique is based on the logical argument that reduction of inoculum should necessarily reduce the incidence of disease. Two pieces of evidence argue against this deduction. The first is that Armillaria ostoyae is known to affect a very wide range of hosts (Kromroy et al, 2005) and so on many sites it is probable that high levels of inoculum will remain even after removal of the roots of harvested trees. The second objection is that even very tiny pieces of inoculum are able to re-establish infection and once established, the infection grows exponentially and quickly reaches untreated levels (Reaves et al, 1993. Bruhn et al, 1998). In addition, the techniques are costly to apply and the ecological ramifications of removing root systems from large areas have not been studied, including the possibility of destroying existing fungi in soils that may have anti-Armillaria tendencies. Because of the cost and potential for unforeseen consequences from root removal, it would seem imperative to at least confirm the efficacy of the technique for mitigating losses due to Armillaria root disease. The Ice Road Trial was initiated in 1993 with harvesting taking place in spring 1996. The Ice Road Trial is paired with another trial with similar treatments, called the Mt. Seven Trial. That trial does not have Armillaria root disease present and only has a very small amount of Tomentosus root disease to datein the spruce of the root disease mortality plots and so is not discussed here. The Ice Road Trial is a factorial design with treatments consisting of a cross of four retention levels and three harvest methods. One of the four retention levels is full retention and so the only harvest treatment in those replications is unharvested. Not all measurements can be undertaken in the unharvested treatment units as there is limited regeneration in those treatments and one of the major, monitoring protocols in this studythe Armillaria mortality plots were not established there. In addition to the Armillaria mortality plots, a number of different tree species were planted and a variety of natural regeneration established in a separate network of Silvicultural plantation plots. The Armillaria mortality plots consist of Douglas-fir only. The trial was established by the Nelson Region Forest Sciences section in conjunction with the Arrow Forest District and Golden Forest District. In 2003, the BC Ministry of Forests underwent restructuring and the Nelson Region Forest Sciences group was absorbed into the Southern Interior Forest Region Forest Sciences. People involved with establishing the trial left the MoFR and responsibility for the trial passed over to Teresa Newsome and Michaela Waterhouse of the Southern Interior Forest Region Forest Sciences. Portions of the trial to monitor Armillaria root disease were originally installed under the supervision of Eric Allen of the Pacific Forestry Center. Dr. Allen retained certain records of this trial including the Establishment Report for the root disease portion of the study and he has assisted in clarifying details regarding the experimental design and establishment. METHODS These establishment methods were extracted from Extension Note 30 by Angela Hawe (1996). Site description:The Arrow Forest District study site is located on Ice Road, about 50 km south of Nakusp, B.C., in the Interior Cedar-Hemlock moist warm (ICHmw2) subzone. The timber type on the site was CwFdLw 633G7. Soils are predominantly silty loam and fine sandy loam. The site is north facing with slopes ranging from 25 to 35%. Pre-harvest root disease surveys indicated that Armillaria ostoyae is dispersed throughout the site. Phellinus weirii and larch dwarf mistletoe were also reported to be present on the sight but the presence of P. weirii was not confirmed during reasssessment. Study design: Eight treatments were established in a 2 x 4 factorial design with two replicates, for a total of 16 one-hectare (100 x 100 m) blocks. The first factor is harvesting method, and the second factor is level of basal area retention (Figure 1).