In the White Mountains of California, climate and land use change has led to the altitudinal expansion of a dominant subalpine shrub species, Artemisia rothrockii (Rothrock's sagebrush) into alpine grasslands. This range expansion has coincided with declined abundance of native alpine bunchgrass (Koeleria macrantha) and cushion plant (Eriogonum ovalifolium), however the mechanism(s) of these species’ declines are unknown. In addition to direct competition, resident natives may experience negative 'plant-soil feedbacks' (PSFs) in soil conditioned by range expanding species, as has been shown for invasive plants. This study sought to determine how sagebrush expansion affects the demography and population dynamics of E. ovalifolium and K. macrantha and to test the relative impacts of PSFs and competition. We measured demographic vital rates (growth, flowering, survival, reproduction) of tagged individuals over a three-year period in experimental plots where aboveground biomass of sagebrush and herbaceous (grass/cushion) plants have been manually removed as well as plots where plant cover has been left intact. Differences between removal (+soil conditioning, -competition) and non-removal plots (+soil conditioning, +competition) represent the effects of soil alone (i.e. PSFs). Experimental plots were repeated across three elevations (3100m, 3500m, 3700m) spanning the observed gradient of sagebrush expansion from subalpine to alpine zones.
Results/Conclusions
Demographic vital rates responded differently within and among species to experimental treatments and across elevation. E. ovalifolium growth and probability of flowering was highest in herbaceous plots with competition and significantly higher than sagebrush plots with competition and sagebrush removal plots. Probability of flowering and seed production were highest at 3100 m elevation and significantly higher than 3500 m and 3700 m elevation for both species. K. macrantha seed production was also higher in in herbaceous plots with competition and significantly higher than sagebrush competition plots. Overall, sagebrush competition had a stronger impact on vital rates than PSFs, as growth, flowering, and seed production were higher in grass competition than sagebrush competition plots, but not different between sagebrush removal and competition plots. Across elevation, probability of flowering, survival and seed production were highest at 3100 m, as would be expected based on larger plant sizes in the subalpine (3100 m) vs alpine (3500 and 3700 m) sites, however plant growth did not differ in this way. Population models will combine multiple demographic vital rates to determine the net effects of competition and PSFs and to predict how future population sizes of these species will change as a result sagebrush expansion.