Local climate adaptations in two ubiquitous Mojave Desert shrub species, Ambrosia dumosa and Larrea tridentata

Abstract

Abstract Widely distributed species are often locally adapted to climate gradients across their ranges. But little is known about the patterns of intraspecific adaptation in desert shrubs. We examined the questions of local adaptation in multiple populations of two common shrub species of the winter‐wet Mojave Desert in North America in a multiple common garden experiment. Plants were raised in the greenhouse and transplanted at the age of 1 year. Ambrosia dumosa is a drought‐deciduous low shrub and Larrea tridentata is an exceptionally long‐lived evergreen. Over 4 years, we monitored growth, survivorship, leaf and reproductive cover and once measured leaf N content, δ13C and SLA. We hypothesized that populations of both species would be differentiated along a growth–survivorship trade‐off according to homesite aridity. Both species exhibited previously undocumented population differences along gradients of winter precipitation and temperature. In general, populations from more winter‐mesic regions had faster growth in more mesic gardens and lower survivorship in the most arid garden. Homesites with more variable summer precipitation had greater growth for A. dumosa populations, but lower growth for L. tridentata. Among L. tridentata populations, leaf cover correlated positively with growth and negatively with survival time. For A. dumosa populations, growth and survival could not be attributed to specific traits across gardens. However, larger transplants had generally lower growth rates and higher survival rates across gardens, except in the driest garden, where the population averages of intrinsic water use efficiency (iWUE) and stem growth rate were positively correlated. Synthesis. Two dominant species of the Mojave Desert adapted locally to variation in winter and summer precipitation and temperature. They did so in different ways, suggesting that L. tridentata mitigated the risk of hydraulic failure, while A. dumosa optimized carbon assimilation for growth.