Biomass production and fire regimes

• Background
• Objectives
• Research activities
• Abstract
• Approach
• Results
• Conclusions
• Publications

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Abstract

The way in which the stability of forest age distribution is related to fire regimes was investigated. The first step was deriving theoretical negative exponential equations for forest age distribution and using three models to explore the conditions under which a stable age distribution could be expected. The results suggested that (1) a stable age distribution can always be achieved as long as forest age-specific mortality is constant over time, and (2) the shape of a stable age distribution is mainly determined by forest age-specific mortality. However, the stability of forest age distribution is reduced when a small variation in age-specific mortality is introduced. The simulation results under various fire regimes indicated that a variety of age-distribution curves could develop, including negative exponential curves and curves with one or multiple peaks. A stable forest age distribution might never be achieved if the forest landscape is subjected to large and irregular fire disturbances.

Different concepts and methods of estimating fire frequency and fire cycle in models of forest dynamics were examined from a computational perspective. Fire frequency and fire cycle can be defined in point-based and area-based ways, and the analytical results indicate that the different definitions are interrelated, except the fire frequency definition based on fire number. The point-based definitions can be seen as special cases of area-based definitions in which the area is reduced to a single site. The generation of an ideal historical fire data set allowed comparison of bias among different methods of estimating fire frequency and fire cycle. An overestimate of fire cycle could be expected if stand-origin maps were used, and recent fire history could influence the estimate significantly. The number of fire maps required to obtain a relatively stable fire cycle estimate is also influenced by fire history. Point-based data obtained from random and systematic sampling designs displayed fire cycle estimates for the tested data set that were nearly as good, if the sample sizes were large relative to the total number of possible sample units.

Historical forest fire regimes in Alberta, Canada, were examined. The ways in which the long-term spatial and temporal dynamics of fire regimes and forest ecosystems could be influenced by fire suppression were investigated by modeling with a revised SEM-LAMD model. The results indicated that the assumption of random fire ignition could be suitable for lightning-caused fires, but might not be suitable for human-caused ignitions. Fire suppression could reduce the number of large fires, while enabling a large quantity of fuel to accumulate, as indicated by higher mean forest ages; thus, the probability of uncontrollable fires likely increases in later years. The simulation results also suggested that a faster fuel accumulation pattern could be favored by the evolutionary process in Canadian boreal forests, if natural selection favors survival and reproductive strategies by which organisms reach higher biomass and reproduction to assure species continuation and expansion.

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