Common garden experiments, a type of study in which individuals from distinct populations of a species are grown side-by-side, have provided a great deal of evidence to support the position that populations adapt their phenology and resource allocation strategy to local conditions. Environmental conditions that may vary across a species’ range include average annual temperature, minimum and maximum temperatures, timing and amount of precipitation, ratio of precipitation to evaporative losses, associated plants and animals, and soil characteristics. Some aspects of phenology and resource allocation strategies that may evolve include timing of germination, flowering, fruiting, or senescing, number of flowers, seed number and mass, and ratio of aboveground to belowground biomass. Adaptation to a given set of conditions may hinder survival under a different set of conditions, hence the need to collect and make available to restorationists seed from a wide variety of locally adapted populations of a given species.
[Lewisia rediviva, the bitterroot]
The bitterroot, Lewisia rediviva, offers a good case study of a plant that lives in widely separated areas which receive precipitation in very different amounts and at very different times. While the plant is reported from all western states and British Columbia and Alberta, I’ll make a short study of only three reported populations (Coconino co., AZ, Humboldt co., NV, San Joaquin co., CA) that experience a range of conditions across the southwestern United States. The Arizona population, reported from near the south rim of the Grand Canyon, receives roughly 16 inches of rain per year spread over a winter and a late summer rainy season. The Nevada population, reported from near Winnemucca in Humboldt Co., receives roughly 8 inches of rain per year, almost entirely in the winter. The California population, reported from northeast of Stockton in San Joaquin co., receives roughly 14 inches of rain per year, almost entirely in the winter. While it does not appear that any common-garden studies of this species have been conducted, it does seems unlikely that an individual from a population that is adapted to high rainfall (i.e. an individual from the south rim of the Grand Canyon) would prosper under the much more arid conditions around Winnemucca, NV. In the same vein, an individual transplanted from San Joaquin co., CA, which lacks a summer monsoon, might not prosper when planted among the south rim population due to a lack of adaptation to the local timing of rainfall.
[L. rediviva on high, dry ridge – the small white forms in the center are Lewisia flowers]