Stand-replacing Fire Research Articles

Decomposition, litter fall, and forest floor nutrient dynamics in relation to fire in eastern Ontario jack pine ecosystems

Decomposition, litter fall, and nutrient and organic matter turnover rates were determined in five eastern Ontario jack pine (Pinusbanksiana Lamb.) stands having various burning histories, including wildfire. The stands included a 65-year-old age-class (stand No. 1), two stands within this age-class that were treated with nonlethal understorey fires in 1962 and 1963 (stand Nos. 2 and 3, respectively), a 21-year-old age-class (stand No. 4), and an 8-year-old age-class (stand No. 5) created by experimental burning plots within the 21-year-old age-class. Overstorey and understorey litter decomposition was assessed separately using the litterbag (1-mm mesh size) technique over a 2-year period. Overstorey litter weight loss did not vary among stands and understorey litter lost significantly more weight (P < 0.05) in the older age-classes (stands 1,2, and 3) compared with the younger stands (stands 4 and 5). Litterbag nutrient dynamics between overstorey and understorey were significantly different (P < 0.05) for P, K, and Cain all stands. Magnesium and N dynamics were the same in both litter types on all treatments, as was Fe, except in the 65-year-old stand where significantly more Fe was accumulated in understorey litter (P < 0.04) at the end of the litterbag exposure period. Three-year averages of annual litter fall ranged from 119 kg•ha−1•year−1 in the 8-year-old age-class to 4182 kg•ha−1•year−1 in the older stands. Nutrient inputs through litter fall reflect the developmental stage occupied by the younger stands along a continuum leading to equilibrium conditions of the 65-year-old age-class. Forest floor nutrient and organic matter residence times (or annual fractional turnover) were longest (least amount cycled) in the 8-year-old stand (57.6 years for organic matter), indicating harsh environmental controls over nutrient dynamics. Recovery for the 21-year-old age-class to turnover rates approaching equilibrium conditions (10-year residence time for organic matter) was rapid, demonstrating ecosystem stability in its interaction with fire. Detrimental effects on ecosystem processes can be expected if a stand-replacing fire recurs during early stages of jack pine ecosystem development.

Forest soil respiration in eastern Ontario jack pine ecosystems

Forest soil respiration insitu was used as a comparative measure of the metabolic activity of substrate in eastern Ontario jack pine (Pinusbanksiana Lamb.) ecosystems that had been exposed to various burning treatments, including wildfire. The five burning treatments consisted of a 1920 wildfire, experimental understorey burning (nonlethal to the overstorey) of this age-class in 1962 and 1963, a 1964 wildfire, and experimental burning of this age-class in 1977. Seasonal respiration trends were similar on all treatments. Carbon dioxide evolution increased in the spring (4000 mg•m−2•d−1) in response to ambient warming (5000 mg•m−2•d−1 in August) and decreased in late fall as seasonal temperatures declined (4000 mg•m−2•d−1 in November). Precipitation and autumnal litter fall apparently acted as secondary modifiers of this general trend by affecting substrate moisture content and nutrient quality, respectively. Highest metabolic activities were measured on the 1963 understorey burning treatment followed in decreasing order by the 1920 wildfire, the 1964 wildfire, the 1962 experimental understorey burn, and the 1977 burn of the 1964 age-class. Multiple comparisons of overall seasonal respiration means revealed lower rates (P < 0.01) on the latter two treatments compared with the 1963 treatment. Effects of understorey burning treatments on respiration activity appeared to depend on depth of burn and subsequent forest floor development. Stand-replacing fire, reoccurring during early stages of jack pine ecosystem development, significantly lowered metabolic activity of the site.