BC’s Inland Rainforest – Conservation and Community

Conference Proceedings


Old-growth and fire hazard in the Interior Cedar Hemlock Forests in and around Glacier and Mt. Revelstoke national parks


Michael Feller (UBC Forest Sciences Department, Vancouver) 1 and Gregg Walker (Parks Canada, Revelstoke) 2

Introduction

Fire has been a major disturbance agent in ICH forests, often causing complete stand replacement. Despite this, some ICH forests have remained undisturbed by fire for periods well in excess of the age of the oldest trees present. Why this has occurred is not completely clear. Due to the uncertainties in maintaining old-growth ICH forests in the prevailing forest fire and fire management environment, Parks Canada initiated a study in 2006 into the fire hazard present in ICH forests in and around Mount Revelstoke and Glacier national parks. The present report of this ongoing study focuses on the historical distribution of old-growth forests and the surface fire hazard of different aged forests.

Methods
1. Old-growth forest distribution

The GIS forest age databases for the ICHmw3, ICHvk1, and ICHwk1 variants, the 3 ICH variants on land in and around Glacier and Mt. Revelstoke parks, were obtained. One database was obtained for B.C. provincial lands from the B.C. Ministry of Sustainable Resource Management, while another database was obtained from Parks Canada for the two national parks. Both databases were updated to January, 2006, and were analyzed to determine the distribution of different aged forests during the last 300 years. Only those polygons which could support forest were included.

Old-growth forests in the ICH are considered to be those > 140 years old, as defined by Mackinnon and Vold (1998). The GIS databases contained no information about the age of a forest in a polygon prior to the disturbance which created the current forest. Uncertainty over the age of this preceding forest precluded the estimation of a single area of old growth in any given year. Instead, a range in which the area of old growth is likely to lie was calculated. Two old-growth areas were calculated – one assuming that the preceding forest was old-growth and the second assuming that all forests > 50 years of age were disturbed and that all such forests had an equal probability of being disturbed.

2. Surface fire hazards and fine fuel moisture contents

Study stands (123 as of May, 2008) were selected, based on ease of access, stand age, and disturbance history. The coarse woody surface fuels in each study stand were assessed using the line intersect method. The quantity of fine (< 1 cm diameter) surface fuels was estimated using a calibrated visual assessment. The relative surface fire hazard of each stand was estimated using the Surface Fire Hazard Index (SFHI) described by Feller and Pollock (2006). This is – SFHI = FF(1+CWD) where FF is the fine fuel quantity (kg/m2) and CWD is the coarse fuel quantity (kg/m2).

Fine fuel moisture contents were assessed during the 2007 summer by placing fuel moisture sticks in 6 replicated forest types and measuring their moisture content, by weighing, throughout the fire season. The forest cover types were – clearcut (age 0-1 year), young (age 10-40 years), mature (age 60-120 years), old-growth, ancient old-growth, and thinned mature.

Results
1. Old-growth forest distribution

Currently old-growth forests average around 20% of all forest land in the three ICH variants. There has been a general decline in old-growth in the ICHmw3, possibly a slight, but fluctuating decline in the ICHwk1, and a general increase in the ICHvk1 variant from around 1800 until around 1970 (Figure 1). Old-growth forest abundance has decreased in all variants during the last approximately 40 years. Old-growth forest abundance within Mt. Revelstoke and Glacier national parks has followed very similar trends as it has on provincial land (Figure 2).

Figure 1. Trends in old-growth forest abundance in 3 ICH variants on B.C. provincial land, from 1705 to 2005, using two methods of calculation.


Figure 2. Trends in old-growth forest abundance in 2 ICH variants within Mt. Revelstoke and Glacier national parks, from 1705 to 2005, using two methods of calculation.


2. Surface fire hazards and fine fuel moisture contents

The SFHI initially followed different trends, depending on whether an area had been burned by a forest fire, or had been clearcut (Figure 3). Following clearcutting without slash removal, the SFHI was initially very high – the highest of all age classes – then declined rapidly with time, primarily due to a decline in fine fuels. Following forest fire, the SFHI was initially very low – the lowest of all age classes – then increased with time up to age 11-20. This likely reflects initial loss of fuel to fire, then gradual fuel build-up again as fire-killed trees fall to the ground surface. Old-growth forests, including ancient old-growth forests, generally had relatively low SFHI values.

Figure 3. The Surface Fire Hazard Index (SFHI) plotted as a function of stand age for 9 age classes of forest (OG is old-growth, AOG is ancient old-growth). The red line is for stands that follow fire, while the blue line is for stands that follow clearcutting).


Surface fine fuel moisture contents were lowest in clearcuts, being significantly lower (P<0.05) than in young, old-growth, and ancient old-growth forests on every sampling occasion (Figure 4). Fuel moisture was highest in young and the two types of old-growth forests on all sampling occasions.

Figure 4. Mean fuel moisture stick mass on each of 6 occasions during the 2007 fire season for each forest cover type. Forest cover types include CC = clearcut and OG = old-growth.


Conclusions

The high SFHI, together with the low fine fuel moisture content, in recent clearcuts suggest that these present the greatest surface fire hazard in the ICH forests studied. Old-growth and ancient old-growth forests generally had the highest fine fuel moisture contents and some of the lower SFHI values, suggesting that their surface fire hazards were relatively low. Since relatively few young and old-growth stands have been sampled, the trends reported here must still be considered tentative.

References

Feller, M.C., and S.L. Pollock. 2006.

Variation in surface and crown fire hazard with stand age in managed Coastal Western Hemlock zone forests. P. 367-380 in B.W. Butler and P.L. Andrews (Compilers) Fuels management – how to measure success: Conf. Proc. U.S. Department of Agriculture, Forest Service, Proceedings RMRS-P-41.

MacKinnon, A., and T. Vold. 1998.

Old-growth forests inventory for British Columbia, Canada. Natural Areas Journal 18: 309-318.

Contact Information


1 Forest Sciences Department, University of B.C., Vancouver, B.C. Email: feller@interchange.ubc.ca

2 Mount Revelstoke & Glacier National Parks, Parks Canada, Revelstoke, B.C. Email: gregg.walker@pc.gc.ca

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