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Influences of mountain pine beetle (Dendroctonus ponderosea), fire, and ungulate browsing on forest stand structure in the southern Canadian Rocky Mountains Dordel, Julia

Abstract

Forests in the montane and lower subalpine ecoregions of the southern Canadian Rocky Mountains may have been more open and structurally diverse at the beginning of the 20th century than today. Today, mature Pinus contorta var. latifolia Dougl. (lodgepole pine) causing mountain pine beetle (MPB, Dendroctonus ponderosae) infestations, infrequent high severity fires, and herbivory appear to have increased in Banff and Kootenay National Parks. Based on a review of the literature it was hypothesised that 1. MPB infestations increase forest stand structural diversity; 2. Browsing intensity decreases with increasing amounts of coarse woody debris; and 3. Disturbances such as MPB infestation and low severity fire lead to decreased MPB stand susceptibility. Stand structure parameters were investigated in mesic montane and lower subalpine stands 15, 25, and 65 years after MPB outbreaks. Parameters measured were tree density, diameter, height class, species, and age distributions as well as coarse woody debris mass. Also, influences of fire frequency, time since fire and fire severity on the current stand structure were analysed. A MPB susceptibility index was calculated for stands with different MPB and fire disturbance histories. The Shannon-Wiener index indicated higher stand structural diversity on plots 15 years but not 25 and 65 years after MPB infestations. Influences of fire on stand structure were limited to increasing tree density with increasing time since the stand initiation fire, and higher proportions of understory vegetation at lower fire severities. Also, there was an indication that high amounts of coarse woody debris resulted in reduced browsing intensity. Finally, the MPB susceptibility index was significantly lower on stands with previous MPB infestations. The MPB susceptibility index also tended to be lower with increasing number of fires. Consequently, none of the above hypothesis could be rejected. Low intensity fires and a reduction of herbivory might be crucial to promote other early successional species such as Populus tremuloides (trembling aspen). Management strategies allowing for MPB and fire disturbances would benefit the ecosystems in the study area.

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