J. Stephen. Shelly

Effects of cryptogamic soil crust on the population dynamics of Arabis fecunda (Brassicaceae)

-Cryptogamic soil crusts are reported to have both positive and negative effects on recruitment and survival of vascular plants. At four sites in SW Montana, Arabisfecunda (Brassicaceae), a rare regional endemic, occurred more often than expected in cryptogamic soil crust compared to bare soil. At all four sites older larger plants were also more common on soil crust than expected. However, plants in the smallest life-stage class were not more common on soil crust than expected at three of the four sites. Measures of fecundity were independent of substrate at all four sites. These results demonstrate that the presence of soil crust is beneficial to populations of A. fecunda at some sites and suggest that this relationship is likely due to increased survival of older plants rather than enhanced recruitment.

Competitive effects of Centaurea maculosa on the population dynamics of Arabis fecunda1

plots for two years at one site and four years at the other. We analyzed differences between treatment and control in recruitment, survival, individual growth and fecundity separately. Estimates of equilibrium population growth (X) were obtained from matrix projection models to evaluate the overall competitive effects of C. maculosa on A. fecunda. Over the course of the study X was significantly higher in treatment plots compared to controls, indicating that C. maculosa had a negative effect on A. fecunda populations. Recruitment was significantly higher in treatment plots during the first two years of the study. Survival, growth and fecundity did not differ between treatment and control. These results indicate that the main effect of removing C. maculosa on populations of A. fecunda was enhanced recruitment resulting from increased seedling establishment. Competitive effects of C. maculosa on A. fecunda varied between sites and among years, indicating that long-term studies are required to fully understand the effects of competition on population dynamics.

Matrix population models from 20 studies of perennial plant populations

Demographic transition matrices are among the most commonly applied population models for both basic and applied ecological research. The relatively simple framework of these models and simple, easily interpretable summary statistics they produce have prompted the wide use of these models across an exceptionally broad range of taxa. Here, we provide annual transition matrices and observed stage structures/population sizes for 20 perennial plant species which have been the focal species for long-term demographic monitoring. These data were assembled as part of the “Testing Matrix Models” working group through the National Center for Ecological Analysis and Synthesis (NCEAS). The data represent 82 populations with >460 total population-years of observations. It is our hope that making these data available will help promote and improve our ability to monitor and understand plant population dynamics. The complete data sets corresponding to abstracts published in the Data Papers section of the journal are publ...

Divergent growth rates of alpine larch trees (Larix lyallii Parl.) in response to microenvironmental variability

ABSTRACT In this study we explore radial growth rates and climatic responses of alpine larch trees (Larix lyallii Parl.) growing in high elevations of the northern Rocky Mountains of Montana, USA. We examine responses between two stands of alpine larch that are separated by less than one kilometer and are growing at similar elevations, but with different aspects. Radial growth rates from trees sampled on the southern aspect of Trapper Peak (TPS) were largely controlled by January snow-water equivalent, while summer maximum temperature was the principal radial-growth driver for trees sampled on the northern aspect of Trapper Peak (TPN). Following the coldest summer (1993) in the century-long instrumental climate record, the radial growth at TPN became greater than at TPS and was the reverse of what occurred pre-1993. We posit that an upward trend in maximum summer temperature is preferentially benefitting the trees growing on the north-facing TPN site by extending the growing season and causing earlier snowmelt, and this has caused the growth rate divergence during the past two decades. As such, our study illustrates that the growth-divergence phenomenon noted in other high-elevation species, whereby macroenvironmental changes are eliciting responses at the microenvironmental level, occurs within stands of alpine larch growing in western Montana.