Scale Issues in the Fire History of a Fine-grained Landscape
© Eva Riccius 1998
SFU

Research project submitted in partial fulfillment of the requirements for the degree of master of natural resources management in the School of Resource and Environmental Management. Report No. 225.

Abstract

The scale of an ecological study determines the ability to detect patterns and processes on the landscape. I demonstrate the importance of a multiscale analytical framework, using evidence of historical surface fires in a dry interior forest of ponderosa pine and Douglas-fir in southwestern British Columbia. The study area comprises 28 ha in a set of seven riverside terraces along the Lower Canyon of the Stein River. I use three spatial scales within a narrow range to test five alternative hypotheses about factors influencing fire regimes and their spatial distribution. The coarsest scale encompasses the entire study area and examines regional climate as a factor influencing fire. The intermediate scale is at the level of riverside terraces ranging from 2 ha to 16 ha. This scale analyses the influence of local topographic features such as, the river and rocky ridges which separate the terraces from one another as a factor influencing fire. I defined the finest scale as 2-ha sub-terrace units which includes the small terraces. The factors influencing fire at this scale include fuel continuity and fuel moisture. The first two hypotheses use the parameters mean fire interval, seasonality and extent, to distinguish between coarse scale factors and intermediate and fine scale factors that influence the fire regime. The remaining three hypotheses use the synchrony in fire dates to test for coarse, intermediate and fine scale factors, that influence the spatial distribution of fire .

Fire history data were collected using a fine scale sample design which provided an even spatial distribution of samples through the study area. I prepared and crossdated fire scars according to standard dendrochronological techniques. For statistical purposes, I identified a subset of fire dates which fall within a Period of Reliability from 1863 to 1936. Most analysis are also conducted using a longer record (1748 to 1972) to maximize the temporal depth of the data.

Mean fire intervals, seasonality and extent, determined from 102 samples, were similar through the study area at each scale indicating that coarse scale factors dominate the regime. Mean fire intervals were 5 years for the whole study area, and ranged from 5 to 28 years with a grand mean of 14 years at the scale of 2-ha sub-terrace units. Seasonality of fires was also similar among fire scarred samples for each scale of analysis. Extent was analysed only within the study area, recognizing that many fires probably also burned outside the study area. Most fires were small. Seventy-five percent of fires within the Period of Reliability recorded evidence on only one or two sub-terrace units, covering 2 or 4 ha. The largest fire burned 24 ha.

The spatial distribution of fires in the study area seems to be driven by factors at the finest scale. Synchrony in fire dates was not observed at any scale of analysis which reinforces that fires were small. Fires were random in their spatial locations among the 2-ha sub-terrace units. At the finest scale, 21% of fires burned in a disjunct pattern; most of these fires occurred on 2 sub-units only. These results provide evidence that fuel continuity and moisture play a key role in the spatial distribution of fire.

Management practices for dry interior Douglas-fir and ponderosa pine forests in southwestern British Columbia should reflect the natural disturbance regime. A selection silviculture system is appropriate for timber extraction to reflect the patchy nature of fire as a disturbance. For management of protected areas, historical stand structure should be restored through vegetation management and prescribed fire.