Flowering decisions in Veratrum tenuipetalum: A clonal mast-flowering herb with monocarpic ramets

Background/Question/Methods

Masting, or annually variable and synchronous reproduction, has long been a perplexing biological phenomenon.  Recent work reveals evidence of selective forces behind mast seeding in many plant species, mainly seed predator satiation and pollinator attraction.  Abiotic conditions may synchronize mast flowering, but proximate cues of synchronous flowering are poorly understood, especially in herbaceous species.  Populations of the subalpine herb Veratrum tenuipetalum exhibit extremely irregular mast flowering patterns, and here we used a long-term flowering record to investigate proximal flowering cues.  We also compared the probability of flowering and threshold flowering size among several populations to understand variation in the abundance of flowering individuals.  We used multiple regression to investigate weather variables associated with the percentage of flowering plants in one population recorded over 27 years (1984-2010) in the Colorado Rocky Mountains.  To understand how environmental conditions influence mast flowering, we determined the percentage of flowering individuals in two most recent masting events across populations spanning a 1000m elevational gradient and two habitats, meadow and aspen stands (shaded).  In a subset of these populations, we measured plant size, counted growth lines in rhizomes to estimate age, and used logistic regression to analyze flowering probability and model threshold flowering size.    

Results/Conclusions

Variation in flowering across years (CV= 2.16) was much higher than variation in weather variables (CV_{precipitation}=0.29, CV_temperature=0.07), supporting the hypothesis that environmental conditions alone are insufficient to explain mast flowering in V. tenuipetalum.  As expected, the percentage of flowering plants was higher in meadow compared to understory aspen populations and decreased with elevation.  Cool summers two years before masting and dry summers one year before masting were associated with flowering, consistent with the time frame over which Veratrum pre-forms organs.  Plant size (# leaves) accounted for a significant amount of variation in the probability of flowering, and threshold flowering size was larger in meadow populations compared to aspen populations.  Our results suggest that habitat, elevation, and threshold size interact with weather cues to influence the abundance and timing of flowering in V. tenuipetalum populations.  Conditions suboptimal for growth – namely cool, dry summers, seem to cue flowering, which is consistent with a monocarpic life history strategy.  Changing climate in this region includes increasing rainfall and temperature, which may affect masting dynamics in V. tenuipetalum.