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Assessing the Minimum Number of Time Since Last Fire Sample-Points Required to Estimate the Fire Cycle: Influences of Fire Rotation Length and Study Area Scale

Boreal forest fire history is typically reconstructed using tree-ring based time since last fire (TSLF) frequency distributions from across the landscape. We employed stochastic landscape fire simulations to assess how large a study area and how many TSLF sample-points are required to estimate the fire cycle (FC) within a given accuracy, and if those requirements change with length of the simulated fire rotation (FRS). FRS is calculated from simulated fire-year maps used to create the TSLF map, and is the “true” measure of fire history that FC estimates should equal. Fire-year maps were created by (i) using a spatially homogenous landscape, (ii) imposing large variations in annual area burned, and (iii) having no age-related change in the hazard of burning. We found that study areas should be ≥3× the size of largest total annual area burned, with smaller-scale areas having a bias that cannot be fixed by employing more samples. For a study area scale of 3×, a FC estimate with an error <10% was obtained with 187 TSLF samples at 0.81 samples per 100 km2. FC estimates were not biased in study area scales that were ≥3×, but smaller-scale areas with a short FRS had an overestimated FC and smaller-scale areas with a long FRS had an underestimated FC. Site specific variations in environmental- and age-related variations in the hazard of burning may require more sample-points; site specific simulations should thus be conducted to determine sample numbers before conducting a TSLF field study.