Severe drought restructures the plant community in three major vegetation zones in Northern Arizona

Background/Question/Methods

Both the frequency and intensity of severe droughts in the Southwestern US are predicted to increase in the next century. A consensus of climate change models shows this region is already transitioning to a more arid climate, with potential for widespread landscape changes due to massive vegetation die-off. Pinyon pine, ponderosa pine, and juniper are the major tree species covering the vast majority of Northern Arizona’s landscape, and have all experienced considerable mortality rates, reaching 100% in some sites. The goal of this study was to assess how the plant communities in each of these vegetation zones have changed since the onset of the 2002 drought. We surveyed plant communities in 2002, 2003, 2004, and again in the summer of 2011. These surveys spanned an elevation gradient encompassing juniper-, pinyon pine-, and ponderosa pine-dominated stands, with paired high and low mortality sites within each vegetation type. The data was analyzed within a community phylogenetic framework to determine evolutionary relationships among coexisting taxa, and how these relationships have shifted since the drought. By understanding the community phylogenetics of these ecosystems, we can gain insight into how their diversity and functioning will be impacted with an increasingly arid climate.

Results/Conclusions

Unlike the prediction of species moving up in elevation with hotter, drier climates, we found no evidence of an upward migration of plant communities surveyed in 2002 with respect to those in 2011. Rather, there is a continued trend of plant communities diverging through time (p=0.001).  Elevation, mortality level, and year were all significant factors explaining community composition. This result indicates a non-linear change in understory composition, perhaps due to expansion of niche space with the removal of the dominant tree species. Communities surveyed in 2011 show increased species diversity in all three ecosystems compared to 2002. The Net Relatedness Index for 2011 communities were significantly higher than in 2002, driving a switch from overdispersed communities immediately after the drought to significantly higher phylogenetic clustering nearly a decade later. This phylogenetic clustering is possibly due to a stronger environmental filter since 2002, or a result of the homogenization of the landscape after nearly a decade of tree mortality. In addition, the phylogenetic diversity of all communities decreased since the drought. This study indicates that although species diversity has increased since the drought, phylogenetic diversity is responding in the opposite direction, potentially narrowing the breadth of functions these ecosystems can perform.