Viability and Usefulness of Broadcast Burning
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Introduction
There was a time that broadcast burning was a popular method of forest management to control fuel buildup, control competing vegetation, and as a site preparation technique. Despite the benefits that broadcast burning has shown, its popularity in BC has declined for three main reasons.
1. Increased use of other techniques for reforestation (e.g., mounding, or disc trenching) and other forest management systems (e.g., management of riparian
zones and wildlife tree patches), which restrict the applicability of prescribed burning (Lepine, Opio, and Ayers 2003). Some silvicultural systems, such as selection or shelterwood harvesting, are also not compatible with the application of prescribed burning (Weber and Taylor 1992).
2. Increased concern about air quality and related health issues in many areas of the province. As a result, smoke management is a critical factor that requires planning in burn operations, and this sometimes reduces the number of days when fire can be applied (Haeussler 1991; Weber and Taylor 1992).
3. Changes in public policy in the 1980s eliminated the forest industry’s limited liability for prescribed fire (B.C. Ministry of Forests 1988). Therefore, the company or agency bears all the costs of an escaped burn.
There is still evidence to prove that broadcast burning, if done properly, can be a viable and economical choice for forest management.
The sites that were used for this study were in Sub-boreal Spruce Zone (SBS), Engelmann Spruce—Subalpine Fir Zone (ESSF), and Interior Cedar-Hemlock Zone (ICH).
Broadcast burning as a Site-Preparation Technique
For many Silviculture foresters looking to use broadcast burning as a site preparation treatment the problem is simple — they don’t have any experience lighting up an entire block in a controlled manner. This creates the worry that costs may run high and surrounding blocks may be adversely affected. According to the Ministry of Forests’ fire records, 351 wildfires resulted from silvicultural burns from 1970 to 1997 (Lepine, Opio, and Ayers 2003). With the burden of having to pay for suppression costs, it is easy to see why many are shying away from broadcast burning.
However, in the interest of seeing how seedlings perform on sites that were prepared by various techniques, there is evidence to show that broadcast burning produces the best results. Study results show how effective broadcast burning can be in plantation establishment and growth (Yole, Macadam, and Kranabetter 1997). Figure 1 shows how well seedlings performed 3 and 5 years after treatment.
Figure 1. White spruce and lodgepole pine growth at years 3 and 5 (Yole, Macadam, and Kranabetter 1997).
In comparison to the control or other methods of site preparation, broadcast burning allowed for the best seedling growth through the first five years. This is encouraging to Silviculture foresters who are looking to get stands to free growing status as quickly as possible.
Effects on Fuel and Soil Properties
In the early 1980’s, concerns were raised about the short and long-term impacts of broadcast burning on organic matter and soil properties (Macadam and Kranabetter 1997). Broadcast burning was suspected of causing losses in soil fertility and loss of nitrogen through leaching and volatilization. Although many agencies were seeing very good initial growth on their stands, there was an idea that fire intensity would dictate to what extent nutrient loss would occur. The key was to keep the burn at a low to medium intensity by utilizing the Fire Weather Index and Muraro’s Prescribed Fire Indicator to provide foresters with information as to when they should burn.
Burning with high indices was a mistake that some foresters have made in the past to disastrous results. I have already presented data that shows how many prescribed fires have escaped during a twenty-seven year period, and this number would attest to the fact that many foresters are aware of burning indices, yet are not heeding the information being presented.
Burning at too low of an intensity can be a waste of time and resources if slash and forest floor consumption isn’t sufficient, therefore defeating the purpose of burning in the first place.
In looking at studies of fuel consumption on several sites burned at low to medium intensities, the data presented shows the effective consumption of slash. Broadcast burning was found to reduce forest floor mass by 29% on average, ranging from a low of 14% to a high of 39% (Macadam and Kranabetter 1998).
Pre-burn
Post-burn
Consumed
Slash size class
(tonnes/ha)
% loss
Forest floor
< 3 cm
3-7 cm
> 7 cm
Total slash
Table 1 Average slash and forest floor consumption (Macadam and Kranabetter 1998)
The burn on this site reduced total slash by 55% and forest floor debris by 29%. The burn has achieved success, but at what cost to the soil properties?
Burning effects on forest soils have been studied in many parts of the world, and soil impacts are often tied site factors such