Abundance of secondary structure in lodgepole pine stands affected by the mountain pine beetle in the Cariboo-Chilcotin

The extent and abundance of secondary structure was studied in the Cariboo-Chilcotin region of the central interior of British Columbia. Plots were selected from age class four and higher pine stands in the Quesnel, Williams Lake and 100 Mile House Timber Supply Areas. A total of 1,649 plots were obtained of which 1,109 were determined to be pine-leading. Secondary structure is understory and overstory trees that survive the current mountain pine beetle epidemic. All lodgepole pine trees 7.5 cm in diameter or greater were assumed to die in the epidemic. The forest health status of a large dataset of individual trees was also summarized. Secondary structure was abundant in all biogeoclimatic zones of the Cariboo-Chilcotin region and was consistent with results from other regions of British Columbia where pine-leading stands have been examined. The median density of conifer seedlings and saplings in the Cariboo-Chilcotin was 1800 stems per hectare (spa). This varied from a high of 4,700 sph in the ESSF zone to a low of 1019 sph in the SBS zone. Across all ecological units about 70 % of sample plots in pine-leading stands exceeded a 1000 sph threshold for understory conifer seedling and sapling density. The species composition of the understory tree layer varied considerably in the individual biogeoclimatic zones. Lodgepole pine was by far the most common understory tree species in the MS and SBPS zones (79 and 74%, respectively). About 34 % of all plots had or exceeded 5 m 2 ha