Summer precipitation synchronizes mast flowering with a four-year lag in a long-lived monocarpic gentian

Background/Question/Methods

Mast flowering is an interesting phenomenon from ecological and evolutionary perspectives.  Proximal cues triggering flowering may have little influence on the ultimate factors selecting for the phenomenon.   Frasera speciosa (Gentianaceae; monument plant) is a long-lived monocarpic , mast-flowering gentian found commonly in montane meadows in the Rocky Mountains.  Minimum observed age at flowering is 22 years, and it is likely that some individuals live 75+ years before flowering and dying.  An annual count of flowering plants along 14km of County Road 317 in the East River valley of Colorado’s West Elk Mountains since 1979, supplemented with qualitative observations since 1961, was used to investigate environmental variables that could trigger and synchronize flowering events.  Because some other mast flowering species are known to use precipitation as a cue for flowering, and because preformation of leaves and inflorescences occurs in Frasera speciosa, I investigated summer precipitation (measured at a nearby NOAA weather station) as a possible trigger using different numbers of years of lag time between the precipitation and flowering.   

Results/Conclusions

From 1979 – 2010, flowering individuals have ranged from 2 – 17,682. Major flowering events (> 1,000 stalks observed, mean = 10,286) have occurred at intervals of 2 – 7 years (mean 3.7).  Number of stalks in other years ranged from 2 – 511, with a mean of 133. 1961, 1969, 1973. and 1977 were also major flowering years preceding my study.  The correlation between July+August precipitation and flowering four years later (r² = .747) was much higher than any other lag time from 1 – 6 years (next highest was r² = .191).  Mean precipitation was 13.1cm in years that triggered flowering, vs. 9.5cm in other years. Thus the plants preform their inflorescences for almost four years before they flower.  Only one other (alpine) species has been reported to have such a long period of preformation.  The ultimate factor selecting for synchronous flowering is probably the benefits related to pollination. The mast flowering occurs over hundreds of miles, and perhaps much of the species’ range, as might be expected if an event like an unusually wet summer is also geographically widespread.  I predict that the next mast flowering will be in 2014, as the summer of 2010 had significant July and August precipitation.  An effect of climate change, to the degree that it affects summer precipitation, may be a change in the pattern of mast flowering in this species.