A comparison of three techniques to date stand-replacing fires in lodgepole pine forests

We examined the effectiveness of three sampling techniques and five sample sizes to date stand-replacing fires in lodgepole pine forests in the southern Rocky Mountains, USA. Transects (5 m X 250 m) were established in a relatively young (120 yrs old), nearly pure lodgepole pine forest ('road' transect) and an uneven-aged, older, mixed-species subalpine forest ('forest' transect). The forest transect was divided into an upper and lower forest transect at a fire boundary. All trees were mapped, and age, species, and dbh were recorded for trees > 5 cm dbh. Fire scars were collected to verify stand-origin dates. Random sampling, dbh-based, and spatial systematic sampling were performed using sample sizes of 5, 10, 15, 20, and 25 trees. Random sampling resulted in a higher estimate of mean maximum age, but always lower than the actual maximum, as sample size increased. Sampling based on dbh located the oldest tree for sample sizes of 10, 15, 20, and 25 trees for the road transect, 15, 20, and 25 for the upper forest, and all sample sizes for the lower forest. The spatial systematic technique obtained the oldest tree in the road transect for sample sizes of 15, 20, and 25. Spatial systematic sampling did not find the oldest tree for the lower forest. Our results indicate the most effective method to find the oldest tree in a stand to estimate fire dates is to use sample sizes of at least 10 trees in young forests and 15 trees in older forests, based on the largest trees in a stand. The identification of possible survivors of fire is also important to determine fire history. Fire scars alone are insufficient to understand lodgepole pine landscape history.