Recent, record droughts in the American Southwest have resulted in high mortality of long-lived pinyon pine (Pinus edulis) that represents a foundation species of the 3rd largest vegetation type in the U.S. We examined the hypothesis that this mortality is both a major ecological event as well as an evolutionary event that is changing the genetic structure of pinyon pine and a diverse dependent community of arthropods, decomposers, and mycorrhizal fungi. Methods include observations in the wild, long-term insect removal experiments, and pinyon greenhouse and common garden experiments. We also discuss a newly funded facility, the Southwest Experimental Garden Array (SEGA) that will use 10 experimental gardens along an elevation and temperature gradient to examine gene x environment x community interactions.
Results/Conclusions
We found: 1) Pinyons growing in chronically high stress sites suffered significantly lower mortality than pinyons growing in low stress sites. 2) Mortality during drought is non-random; trees resistant to insect attack suffered 70% mortality, whereas trees susceptible to attack suffered only 20% mortality demonstrating that drought can be an evolutionary event. 3) Seedling regeneration was substantially lower in chronically high stress sites than low stress sites. 4) Tree susceptibility to herbivory also influenced seedling growth and survival in the greenhouse and in a common garden, suggesting these traits are heritable. 5) Arthropod community richness and abundance was significantly higher in low stress sites than in high stress sites. Mycorrhizal fungal diversity was similar across sites, but shifted towards a drought tolerant community in both low and high stress sites. 6) Because resistant and susceptible trees support different communities (about 1000 species affected), changes in the ratio of resistant to susceptible trees will alter a diverse community microbes to vertebrates. In combination, our findings show that climate change is an agent of selection on a foundation tree species. As selection results in differential tree mortality and changes the genetic structure of the population, the interactions, composition, and diversity of organisms from microbes to vertebrates will be affected. For future studies the Southwest Experimental Garden Array (SEGA) will become a powerful tool for studying gene x environment x community interaction in a region that is suffering among the highest rates of climate change in the U.S. Such trials are crucial to separate genetic and environmental effects and to experimentally evaluate the roles of key players in this community.