Boreal forests play a critical role in the global carbon cycle and provide important macrofaunal habitat and ecosystem services. Humans depend on forests for paper, timber, wood, medicine and outdoor activities such as hunting, skiing and hiking. However, there has been an increase in tree mortality rates across the western US over recent decades. The decline in forest cover has been, at least partially, attributed to warmer summers and associated changes in water availability and supply. In order to gain new insight into whether the ongoing changes are likely to continue, we assess changes in North American monsoonal climate and controls on Rocky Mountain forest health during the Eemian-interglacial (around 125,000 years ago), which is the most recent period in Earth's history when growing season temperatures were warmer than today. 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.   

To better understand the climate during the Eemian, we ran a set of climate simulations with initial conditions reflecting orbital parameters and atmospheric greenhouse gas concentrations for this time period and implemented a spatial resolution that was able to resolve the effects of complicated topography in the mountainous regions of the western US. Our model confirms warmer June/July temperatures during the interglacial and suggests an increase in summer rainfall throughout most of the southwestern US, including the northern and central regions of the Rocky Mountains. In contrast, Arizona experienced significantly drier summers during the interglacial. Our simulations suggest a decline in monsoon circulation during the Eemian, but storms from the Gulf of Mexico and the central Pacific transported more moisture into the southwestern US, leading to a complicated regional response of summer precipitation to warming. 

In order to understand how trees respond to 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. 

A series of identical measurements on the isotopic ratio of interglacial wood was conducted.  Results indicate strong similarities between interglacial and modern wood samples, suggesting that the trees experienced overall similar conditions during the interglacial warm period and present. We concluded, to our surprise, that stress levels for Eemian woods were not likely higher than today?s despite the higher summer temperatures.  However, the results show 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 increases.  We emphasize that future effort 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 multiple climate modules at NEIU that are integrated into our undergraduate syllabus and that use the climate of the interglacial as a test-case for students to consider the multiple processes that lead to global warming. Over the life cycle of the grant, over 15 undergraduate students (from NEIU and UIC) received training and worked in the lab on sample processing and isotopic analysis or climate modeling.  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: 01/30/2020
Modified by: Nadja Insel