Demography, reproduction, and dormancy along altitudinal gradients in three intermountain Allium species with contrasting abundance and distribution

Abstract

Little is known about the variation in demographics, flower number, fruit set, and summer dormancy in Allium species native to the Intermountain West region of the United States. The purpose of this study was to investigate demographic patterns, summer dormancy traits, and variation in reproductive characteristics among habitats. Three populations of each of three species, Allium acuminatum, A. brandegei, and A. passeyi, were studied along an altitudinal gradient in northern Utah, USA, from 2004 to 2006. Demographic data were collected from individuals in permanent plots established in each population. Individual flowering plants were periodically monitored and used to calculate reproductive characteristics. Leaf area measurements throughout the 2005 growing season and soil temperature sensors were used to assess the relationship between habitat and dormancy characteristics. Populations of the widely distributed species, A. acuminatum and A. brandegei, were dominated by individuals in seedling and flowering stages while the restricted A. passeyi had populations containing a majority of individuals in the post-seedling, but non-flowering stage-class. There was a strong directional trend in A. acuminatum reproductive traits, with flower number and fruit set decreasing with increasing elevation. The mid-elevation A. brandegei populations were reproductively more successful than the high and low-elevation populations. The high-elevation A. passeyi population had more flowers per plant and higher fruit set in comparison to the lower elevation populations. The onset of summer dormancy as indicated by the timing of leaf senescence, was associated with high soil temperatures specific to each species. In general, the timing of leaf senescence was later in higher elevation populations in all species. A smaller proportion of non-flowering individuals in A. acuminatum and A. brandegei may indicate a high seedling mortality rate or a shorter juvenile phase in comparison to A. passeyi. The A. passeyi populations exhibit demographic characteristics of rare taxa; low seed production and low seedling recruitment. The high number of non-flowering plants in A. passeyi likely includes many reproductively mature individuals which are influenced by the unique habitat and environment of this species. Temperature is likely the key environmental cue inducing summer dormancy in all three species.