Fire and Harvest Residual (FAHR) Project

The Fire and Harvest Residual Project compares the early successional trajectories for forest structure and biodiversity between wildfire- and harvest-origin aspen-dominated stands within northcentral Alberta. This retrospective study compared the most common wildfire residual pattern (<1% live merchantable timber) to a harvest strategy leaving 5–6% live merchantable timber over the first 28 years of succession.

This section is taken from the Executive Summary of the FAHR project final report: Lee, P. 1999. FIRE AND HARVEST RESIDUAL (FAHR) PROJECT: The Impact of Wildfire and Harvest Residuals on Forest Structure and Biodiversity in Aspen-Dominated Boreal Forests of Alberta. Edmonton, AB, Canada.

 Key Findings

1. The greatest difference between wildfire and harvest stands occurs immediately after disturbance. Any differences in forest structure or biodiversity noted in this study were greatest immediately after disturbance. These were the result of the very different processes involved with wildfire and harvest.
2. Snags, downed wood, understory plant, mammal, and bird communities converged in the first 28 years. Despite very disparate differences immediately after disturbance, forest structures and biotic communities from wildfire and harvest stands exhibited significant convergence during the first 28 years. Biological legacies associated with disturbance diminished. Stand development was increasingly set by the cohort of regenerating trees, with structure and biota becoming more similar between disturbance types.
3. Differences after 28 years of succession were largely restricted to the relative abundances rather than the presence or absence of species. Although few plant and animal communities were identical between wildfire- and harvest-origin stands after 28 years. The differences were at the level of relative species abundance, rather than the presence/absence of species. This result notwithstanding, it is important to note that the study only assessed common species. Rare species may have somewhat different thresholds of response to disturbances and subsequent recovery.
4. The removal of carbon from harvest stands manifests itself as a loss of snags, then downed logs, and eventually in a lower carbon content within the soil organic layer. One clear difference between wildfire and harvest stands was the removal of carbon from harvested stands. In comparison, wildfire stands lost carbon through consumption of the forest floor. However, the retention of most standing materials provided a source of carbon unavailable in harvest stands. Given the relatively short rotation of aspen harvest, it was unclear whether harvest stands will accrue pre-disturbance levels of carbon prior to the next rotation.
5. Early fire communities are unique and cannot be replicated by any practical harvest plan. An unexpected result of the present study was the uniqueness of early wildfire communities. Plant and animal communities present immediately post-fire were very different from all other communities. It is unlikely that any economically feasible plan for leaving harvest residuals will replicate this community.

Management Implications

1. Retention of trees at harvest provides a continuing source of advanced forest structure for future stand development. The convergence of wildfire and harvest stands was due, in part, to the continued growth of trees left at harvest and their eventual death and decay. Retention of a continuous distribution of tree sizes, including pulp-merchantable diameters, allowed for a relatively constant recruitment of large tree and deadwood resources post-harvest.
2. Increasing the amount and patch size of residuals leads to more old stand attributes. This study examined the impact of leaving 6% residuals at the point of harvest. It did not directly investigate the impact of leaving varying amounts of live residuals. The logic of the results and correlations with other studies suggested that leaving more live residuals tended to speed the convergence between wildfire and harvest stands and provide some older seral stage habitats. However, there is a limit to the amount that should be left behind since a very large amount of live residuals tends to suppress aspen regeneration. With convergence partially dependent on the regeneration of trees, leaving too many residuals may delay convergence by hindering regeneration. The spatial pattern of materials left on the block also affects biota. Larger patches of materials retain more of the biodiversity found in the pre-cut stand. Nonetheless, this should not preclude the use of smaller patches. For any set level of retention, one can place a greater number of smaller patches throughout the block. Even though smaller patches have less initial impact, these patches may eventually provide a wider dispersal of old growth communities throughout the block in comparison to a few large patches. In general, a variety of patch sizes distributed over the cutblock may be appropriate for most harvest areas.
3. Harvest potentially leads to a reduction of deadwood resources and soil carbon with an ensuing decline in biodiversity. As with other forested ecosystems, deadwood resources are a significant feature in aspen-dominated boreal forests. Our results indicated that even with a 5–6% residual, there was a potential reduction of deadwood and soil carbon. Although losses within a single rotation may be relatively small, those losses may compound over successive cycles of harvest and regeneration. Since losses may accrue with time, maintenance of deadwood resources is a potential long-term management issue. Clearly, it will be difficult to compensate for all the carbon removed from harvested stands. However, we may be able to extend the rotations on a percentage of stands and develop regional strategies that provide a base level of snags, DWM, and soil carbon over broad areas.
4. Maintenance of wildfire communities requires some protection from salvage logging. A percentage of mature or old stands burned in wildfires should be allowed to escape salvage harvesting. This would ensure the perpetuation of the very unique fire-dependent aspen community in Alberta. These stands would not have to be “protected” in perpetuity, rather 15–20 years would be adequate. Currently, burned stands escape harvest by happenstance rather than design. A clear policy should be crafted to set aside a percentage of the landbase that bums. To be of value, the unsalvaged stands should be large and mature- or old-aged at the time of disturbance. A policy of high residual salvaging should not be substituted for setting aside whole stands.
5. Careful assessment of management issues should precede selection of traditional research, adaptive management, or status monitoring as the appropriate knowledge gathering platform. This project identified a number of gaps in the knowledge base and areas of management concern. These included further work on understanding functional relationships between residuals and different taxonomic groups, integration of different cutblock residuals, and long term losses in large tree and deadwood resources. We recognized that a number of knowledge gathering techniques are available for dealing with these issues. Objectives, scope, spatial and temporal scale, cause-effect relationships, and use of deliverables should be carefully assessed prior to decisions on the use of traditional research, adaptive management, or status monitoring models.