Can a native Mojave Desert shrub, Ambrosia salsola, adapt to increasing atmospheric nitrogen deposition and annual grass invasion in Joshua Tree National Park?

Background/Question/Methods

Anthropogenic emissions have increased atmospheric nitrogen deposition nearly four-fold since preindustrial times. Joshua Tree National Park has seen a similar increase, creating a gradient from high deposition in the west to near preindustrial rates in the east. Greater N availability, although usually beneficial for individual plants, can increase susceptibility to invasion and have detrimental effects on native plant communities. We aimed to determine if populations of the native Mojave Desert shrub Ambrosia salsola growing along this N deposition gradient differ in their capacity to perform well under both increasing N deposition and annual grass invasion. First, we asked whether local adaptation to these conditions has evolved in populations experiencing these disturbances. Because invasion and anthropogenic N deposition are relatively recent developments, we also asked whether these populations possessed heritable variation in response to these conditions, which may indicate potential for further adaptive evolution. We collected seed from individual plants at six sites representing three levels of N deposition and invasion in Joshua Tree National Park and grew plants in a greenhouse with and without Bromus rubens competition and at three N levels. We analyzed differences in above and below ground biomass as well as root length and plant height using a nested ANOVA.

Results/Conclusions

As expected, nitrogen addition increased A. salsola biomass and Bromus rubens competition decreased biomass. None of the measured growth parameters differed between sites or differed by the N deposition level of the collection sites. However, there were significant interactions between the N deposition level of the collection sites and competition treatments: plants from high N sites fared better with competition than those from low N sites. There were no differences in response to N treatment based on the N deposition level of the collection sites. This suggests that there was an adaptive response to competition in populations that have higher exposure to annual grasses, but that there has not been an evolutionary response to higher N deposition. There was some heritable variation within populations for growth traits, indicating that there may be the capacity for further adaptation. If native plants have the capacity to adapt to the changing N deposition and invasion levels, then we may see greater resilience in these Mojave Desert communities over time. Ambrosia salsola is also commonly used in restoration work, and finding and using seed from adapted populations could increase the success of these projects.