The effect of fire on the population viability of an endangered prairie plant

We examined the effects of fire on population growth rate and extinction pr obability of a rare prairie plant. Lomatium bradshawii (Apiaceae) is an end angered species of western Oregon and Washington prairies that were frequen tly burned by Native Americans prior to the late 1800s. Using data from map ped plants in two populations subjected to different fall burning frequenci es over a six-year period, we constructed stochastic transition matrix mode ls to evaluate the effects of fire on the viability of the populations. Sto chasticity was incorporated into the models through two methods performed b y widely used computer programs: randomly shuffling whole annual matrices ( using POPPROJ2), or selecting each matrix element from a distribution with observed mean and variance (with RAMAS/stage). Log-linear analysis was used to test for site and treatment effects on transition rates. Unburned plots had stochastic population growth rates (lambda (s)) of 0.905-0.927, depend ing on the site and stochastic method. Burning twice in six years increased X, to 0.946-1.091, and three burns yielded lambda (s) values of 0.996-1.17 3. The risk of extinction (100 yr, extinction defined as < 10 individuals) was very high (97-99%) in the absence of fire for both methods and sites, b ut with two burns it declined to 57% and 73% for matrix selection and eleme nt selection, respectively, at one site, and less than or equal to 1% (both methods) at the other. Extinction probability was very low (less than or e qual to 1%) for both methods and sites after three burns. Although differen t methods of incorporating stochasticity gave slightly different quantitati ve results, they were qualitatively similar. Both site and treatment had si gnificant effects on transition rates in the log-linear analyses. Elasticit y analysis suggested that burning decreased the sensitivity of population g rowth rate to mid-sized plants and increased the importance of seedlings. F ire was an effective tool for maintaining viable populations of this specie s, although the strength of its effects differed between locations.