Faculty Highlight: Paul Bolstad

As the “Land of 10,000 Lakes,” Minnesota is identified with water. We take ample, clean, natural supplies for granted, until there is too little (the 2007 drought in the Upper Midwest) or too much (the 2012, ten-inch megastorm in Duluth). Forests are an integral part of the water story because they buffer extreme flows, and because trees can use over half of the rain and snow that falls in a year. Unfortunately, we cannot predict the trajectory of future water supplies because it depends on complicated processes that play out across broad landscapes. The source areas for even mid-sized rivers like the Rum or Snake River cover hundreds of square miles, encompassing a broad range of forest types. 

Paul and Sheryl Bolstad in New Zealand

Forest Resources faculty Paul Bolstad has spent the last two decades working at landscape scales on forests, carbon and water cycles, and climate change. Forests are aging, experiencing changes in species composition and structure, and we are transitioning to a warmer world.  Observations across the eastern half of the U.S. point to significantly higher temperatures; rising and more variable precipitation; longer, dryer fall seasons; and perhaps more frequent, longer droughts. How will these changes affect forest water, and is there significant interaction among changing forest structure and changing climate? Professor Bolstad and his collaborators have sought to answer these questions in Minnesota, the U.S., and abroad.

Salli Dymond collecting soil moisture readings

Bolstad’s research has focused on measuring and modeling forest water cycles, often on U.S. National Forest lands which are the source of much of our nation’s drinking water. Bolstad worked with the recent Ph.D. graduate Salli Dymond, and U.S. Forest Service scientist Randal Kolka, to analyze more than sixty years of records of forest soil moisture in northern Minnesota, and reconstructed growth of aspen, red pine, and northern hardwoods. Their work noted a significant decline in average soil moisture since the 1960s, caused primarily by an increase in evapotranspiration. Lower soil moisture and higher evapotranspiration caused decreased aspen growth. Red pine growth primarily increased due to increasing temperatures and showed limited sensitivity to changes in soil moisture. Collaboration with scientists at the Coweeta Hydrologic Lab in North Carolina has resulted in better quantification of how forest water use and water balance changes based on forest age. Other work has focused on how new tools, like LiDAR imaging and satellite systems, help us measure forest leaf area, density, age, and other factors that influence forest water cycles.

Bolstad spent the last year in Melbourne, Australia, to share and improve his skills estimating resource vulnerability in a water-limited environment. Southeastern Australia is one of the wetter, cooler parts of a warmer, drying continent, and although substantially different than Minnesota, is under similar threats. An increasing population and changing climate are likely to alter future water supplies that are derived largely from upstream forest lands. While they are much closer to the limits of sustainable use, we may not be but a few decades behind, given expected trajectories, in having to drastically change our attitudes toward and use of Minnesota’s water resources. Bolstad and other members of the Department of Forest Resources hope to develop key knowledge over the interim, so that we can make the best choices about water resources when the time comes.