Tree damage after fertilization of thinned lodgepole pine, Douglas-fir and spruce stands in the British Columbia interior: synthesis report on up to 18-year responses from EP886.
As a silvicultural practice, forest fertilization has emerged to mitigate risks and reduce the effect of damage agents on timber supply due to climate change and on subsequent extensive landscape-scale natural disturbances in British Columbia. In total, 61 installations were established in the central and southern interior of British Columbia to quantify fertilization responses of major commercial tree species. One-time fertilization with two to five kinds of fertilizer blends was applied, and the stand ages at fertilization ranged from 9 to 58 years. Plots were repeatedly measured at 3-year intervals, although a few plots were measured 2-4 years after fertilization. The repeated measurements (up to 18 years after fertilization) indicated that trees were damaged by various damage agents in the experimental sites, and that the fertilization may have been associated with the damage agents' activities. This report provides a descriptive overview of the damage that occurred after fertilization in the EP886 installations. First, the major damage agents after fertilization were identified by comparing proportions of damage records for each plot. Second, to account for different measurement years since fertilization, annual damage rates were calculated. The damage record proportions and damage rates were summarized at different scales (i.e., plot and installation), and by size classes, age, and fertilization treatments. The results of the analyses indicated the following: 1. The four most prevalent damage agents after fertilization were squirrels (accounting for 11.8% of total damage records), western gall rust (16.4%), mountain pine beetle (19.5%), and white pine weevil (10.8%). 2. Squirrels, western gall rust, and mountain pine beetle were observed only in lodgepole pine stands, whereas white pine weevil was exclusively observed in spruce stands. The four major damage agents were not observed in Douglas-fir stands. 3. The squirrel attack was concentrated in the stand age class of 25-30 years, which implied a potential association between susceptibility and stand age. 4. Throughout the study periods, fertilized plots showed slightly lower average plot-level damage than control plots for squirrel (0.1%) but higher damage rates for western gall rust (1.0%), mountain pine beetle (5.2%), and white pine weevil (6.6%). 5. Annual damage rate throughout the study periods after fertilization for squirrel, western gall rust, mountain pine beetle, and white pine weevil were 1.1, 1.2, 1.8, and 3.3%, respectively. Those annual damage rates for control plots were 1.0, 1.1, 1.2, and 2.9%, respectively. The results presented are purely descriptive. Further analyses are required to assess more detailed associations between fertilization and tree damage. The findings in this study will provide insights for further research, and subsequent research efforts will provide useful information about the relationship between fertilization and forest health for managing interior British Columbia forests sustainably.