ANALYSIS OF A TRANSITION MATRIX MODEL FOR GAURA-NEOMEXICANA SSP. COLORADENSIS (ONAGRACEAE) REVEALS SPATIAL AND TEMPORAL DEMOGRAPHIC VARIABILITY

The fates of individuals in nine fixed plots in three areas were recor ded for a population of Gaura neomexicana ssp. coloradensis on F. E. W arren Air Force Base in Cheyenne, Wyoming, U.S.A., from September 1992 to September 1994. The data were used to construct a stage-based tran sition matrix model for the population across two transitions. The spe cific goals were to determine if the population size is increasing, de creasing, or stable; if there is significant spatial and/or temporal v ariation in demographic parameters; which demographic transitions have the most impact on population growth; and the probability of extincti on. An additional objective was to compare Gaura with other plant taxa that have been similarly modeled. The population is increasing, altho ugh some plots experienced a decline, as evidenced by the asymptotic g rowth rates (lambda) calculated for the individual plots and for the p opulation as well as the actual numbers of plants in the plots. The po pulation stage structure varied across the two transitions and was dif ferent from that predicted by the matrix model. Log-linear analysis re vealed significant temporal and fine-scale spatial variation in demogr aphic transition frequencies for Gaura. Elasticity analysis indicated that growth from one stage to another had the largest effect on popula tion growth rate and that the most important single (nonsummed) transi tions were from large rosette to flowering and recruitment into the sm all rosette stage. The population as a whole will persist, but there i s a chance for local extinction under certain conditions. The mean gro wth (G), survival (L), and reproduction (F) elasticity values placed G aura in the range of other semelparous herbs.