Conifer density increases in semi-desert habitats of British Columbia in the absence of fire
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We determined whether or not conifer density has changed over time in the hottest and driest areas of the south Okanagan and lower Similkameen valleys in British Columbia, Canada. These grassland and savannah habitats are important for biodiversity and species at risk in Canada, and there is some discussion as to whether conifer establishment is occurring. Densities of ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) were compared between airphotos from 1938 to 1985, and from 1985 to 1996 in study sites 10- to 183-ha in size. Air photos were scanned and georeferenced to ensure spatial congruency between years. The location and area of wildfires in the intervening periods were also recorded. A three-way ANCOVA, with site area as the covariable, tested the effects of fire, latitudinal category (north versus south), and biogeoclimatic (ecological) zone (ponderosa pine versus Bunchgrass) on changes in conifer density between years. The results showed an overall significant increase in density in unburned sites from 1938 to 1985, and from 1985 to 1996. Despite some inconsistent sites, fire resulted in an overall decrease in density. Neither biogeoclimatic zone nor latitude affected conifer density change over time. Stem densities in 1938, 1985 and 1996 were similar between latitudinal category and biogeoclimatic zone, except for the hottest and driest combination, in the south, in the lower elevation Bunchgrass biogeoclimatic zone, which had less than half the stem density of the other areas. However, the rate of change in stem density of conifers in unburned sites did not differ because of latitude or biogeoclimatic zone. Our results show that coniferous trees are capable of establishing in even these dry semi-desert sites, and prescribed burns might be considered for restoring grassland and savannah habitat.