There has been an increase in tree mortality rates across the western US over recent decades.  If tree cover and forest vitality continue to decline, this will have effects on streamflow, animal habitat, lumber production and tourist activities including hunting, skiing and hiking.  It has been shown that the declines in forest cover can be at least partially attributable to warmer summers that have led to a decrease in spring snowpack and higher rates of evaporative demand.  In order to gain new insight into whether the ongoing changes are likely to continue, we looked at how forests changed during the last period in Earth?s history when temperatures were persistently warmer than today.  This period is referred to as the Eemian Interglacial and it occurred 120,000 years ago. We were able to study forests during this period because of access to a unique collection of subfossil wood samples that were collected outside Snowmass, Colorado.   

In order to gain an understanding of the state of the climate during this period, we ran a set of climate simulations for this period with high enough spatial resolution to resolve the effects of complicated topography in the mountainous regions of the western US.  As expected, we found that summer temperatures were a few degrees warmer during the middle of the summer during the interglacial.  We also found that there was a higher amount of summer rainfall in the northern and central regions of the Rocky Mountains.  This was in contrast to the conditions in Arizona and New Mexico, which had drier summers during the interglacial.  We concluded that while the monsoon circulation declined during this period, there were more storm systems coming off the central Pacific into the region.  This led to a complicated regional response of summer precipitation to warming. 

In order to understand how trees responded to these changes in climate, we undertook a comprehensive study on modern trees from the region the fossil wood was recovered from.  We analyzed the stable isotopic ratio (¹⁸O / ¹⁶O) of wood from two common conifer species (Subalpine fir and Engelmann spruce), which serves as an indicator of both the level of water stress and the water source (i.e. snowmelt or summer rain) for the trees. From this analysis, we found that the trees primarily relied on snowmelt to support their water needs.  However, during years when the trees grew more, we found the trees changed to a heavier reliance on summer rain. This result showed that the trees have the capacity to switch between use of snowmelt or summer rain and that snowmelt is insufficient to support growth during highly productive periods. 

We then made a series of identical measurements on the isotopic ratio of interglacial wood.  The results from interglacial wood were highly similar to the modern samples suggesting that the trees experienced overall similar conditions during the interglacial warm period. We concluded, to our surprise, that their stress levels were not likely higher than today?s despite the higher summer temperatures.  However, the results showed evidence that there was a much greater reliance on summer rain during the interglacial. We concluded that despite the higher stress levels from warmer temperatures, the increase in summer rain provided more optimal growth conditions.  This result suggests that the future trajectory of the forests is highly contingent on how summer rain varies with future temperature.  We emphasize that future efforts to predict the response of forests to climate change must prioritize the sensitivity of summer rain to higher greenhouse gas concentrations.  This is a challenge because summer rain in the mountainous regions of the western US is often characterized by highly localized rain events.

This project has provided new mechanistic insight into how forests respond to climate change, which will have an important impact on the disciplines of earth science, ecology and hydrology.  Throughout the duration of this project we generated numerous forms of broader impacts ranging from classroom exercises, research experience for students and public lectures.  The project included the development of two new graduate classes at University of Illinois at Chicago that focus on interactions between climate and landscapes and the use of stable isotopes to study the climate system.  We also developed a multi-week undergraduate syllabus that uses the climate of the interglacial as a test-case for students to consider the multiple processes that lead to global warming. This syllabus has been publicly disseminated.  Over the life cycle of the grant, over 15 undergraduate students received training and worked in the lab on sample processing and isotopic analysis.  This served as a springboard for some students to enter graduate school and provided key training for students going into geological consulting companies.  Lastly, aspects of this project have been discussed in numerous public lectures at Universities, schools and event spaces across the country.

Last Modified: 11/05/2019
Modified by: Max B Berkelhammer