Schwinning Lab

Areas of high juniper mortality in the Texas Hill Country as seen from the air

In New Mexico, Arizona and Utah, pinyon pine trees have died off in large numbers since 2002

Widespread tree mortality in forests and woodlands brought on by drought and fire has been a recurrent phenomenon over the past decades. Global warming has likely ramped up tree mortality world-wide and will continue to do so. The sudden loss of live tree cover alters ecosystem functions and services in the short term, but little is known about what it will do in the longer term, and whether or how forests recover.

In collaboration with Dr. Marcy Litvak and Dr. Will Pockman (University of New Mexico), and funding from the NSF DOE program, we looked at what happens in the aftermath of tree mortality in the pinyon-juniper forest of the southwestern United States. In this replicated experiment, either pinyon or juniper overstory trees were killed by girdling in 1000-sqm plots. Graduate student Aarin Sengsirirak conducted seedling/sapling surveys to determine how the girdling treatments affected their growth and mortality over two years.

Previously, graduate student Beth Crouchet completed an extensive survey of tree mortality in Central Texas. This project was partially funded by the Texas Ecolab program. The purpose was to correlate rates of mortality to local stand structure and climate variables, so that potential risk factors can eb identified. As other studies have concluded, exposure to high temperature during drought is an important amplifier of local tree mortality. Somewhat paradoxically, sites with more soil cover had higher rates of mortality, probably because trees had build up to higher density on such sites. High density of conspecifics was more closely correlated with tree mortality than overall tree density. Thus, Ashe juniper, which is a woody plant encroacher in Central Texas, suffered relatively high mortality rates, but high densities of Ashe juniper did not increase the mortality of other species.

Schwinning, S., Litvak, M.E., Pockman, W.T., Pangle, R.E., Fox, A.M., Huang, C.W., McIntire, C.D. 2020. A 3-dimensional model of Pinus edulis and Juniperus monosperma root distributions in New Mexico: implications for soil water dynamics. Plant and Soil 450: 337-355. pdf

Crouchet, S.E., Jensen, J., Schwartz, B.F., Schwinning, S. 2019. Tree mortality after a hot drought: Distinguishing density-dependent and -independent drivers and why it matters. Frontiers of Forests and Global Change 2:21 pdf