Few studies have comprehensively linked population dynamics of exotic species with associated environmental factors, though the spread and persistence of exotics is most fundamentally a population ecology problem.  Our broader objective is to assess impacts of land use, loss of native foundation species, and associated changes in soil water to success of tap-rooted forbs in sagebrush steppe.  We hypothesized that persistence of exotic forbs following land use disturbances result from loss of sagebrush and corresponding increases in soil water.  We are testing this hypothesis experimentally with six replicate blocks of three treatments: 1) undisturbed controls, 2) sagebrush removed to simulate disturbance, or 3) sagebrush removed and winter/spring recharge of deep soil water blocked with rainout shelters.  For the current study, we assessed how estimates of population dynamics for the annual or biennial Tragopogon dubius in treatments are affected by temporal and spatial aggregation among life history stages or treatments.  Despite having 100 m² plot sizes, scarcity of Tragopogon in some treatments and replicates was an analytical issue.  Thus, we compared transition matrix outcomes developed on an annualized or periodic basis (1 or 3 times/year, respectively), and on a lumped or averaged basis (1 or 6 matrices). 

Results/Conclusions

All modeling approaches agreed that population growth of Tragopogon was enhanced by loss of sagebrush and corresponding increases in soil water following disturbance.  Also, there was agreement that life stages expected to rely on relatively deep soil water were significant to population changes.  Lumping versus averaging did not appreciably affect matrix outcomes, and so we proceeded with data pooled over replicates.  The annualized model predicted less variation in population responses to the treatments than the periodic model.  Population growth was most sensitive to maturation of juveniles (having only basal leaves) to reproducing plants, in both models.  The periodic model moreover specified sensitivity to maturation of first-year (vs. second year) juveniles to reproducing plants.  Fertility and maturation of juveniles to reproducing plants were most elastic and thus contributed most to population growth, in the annual model and in all treatments.  Elasticity varied between treatments in the periodic models, with seed production most elastic in the shrub removal treatment, and germination and progression to reproducing plant contributing most to population growth in the other two treatments.  The periodic model better explained mechanisms of population responses, which appear related to the reliance of individuals on deep water as they matured to reproductive stages and seed production.