Andy Bach

Research interests include glacial geomorphology, environmental change on Mt. Baker, Channeled Scablands, and soil genesis.

I've been researching the retreat of the Easton Glacier on the south face of Mt. Baker. Using repeat imagery and determining the establishment of trees on deposited glacial till, my students and I have been refining and extending the history of the glacier fluctuations back into the Little Ice Age.   We have found that Easton Glacier advanced down the mountain and into an old-growth forest stand during the Little Ice Age (a period of Northern Hemisphere cooling between approximately 1600-1850AD).  Starting about 1850 the glacier began to slowly retreat back up slope, with a rapid period of retreat between 1900 and 1940, zooming a mile up slope (zooming by glacial standards).  The retreat slowly continued until 1956, then the glacier advanced (grew) about a quarter mile down valley, and then began its current retreat in 1990.   As of 2023, the glacier is at its smallest documented extent, over two miles shorter than it was in 1850.  After the glacier retreats, a pile of loose rocky ruble covers the valley floor, known as glacial till.  This material has very low fertility for plant establishment and remains vegetation free for a decade or two.  During this time of ecesis, organic matter (i.e. needles, leaves and seeds) from the surrounding forests blow onto the till along with microorganisms which build the fertility of the till to a point which plants become established. Once the plants are established on site, especially nitrogen fixing lupine, the fertility of the surface begins to rapidly increase, allowing tree species (mostly Mountain Hemlock and a few Pacific Silver Fir) to become established.  These tree seedlings grow extremely slowly given the cold temperatures and deep winter snowpack (typically >8 m), remaining as short 0.5-2 m tall trees for decades to a century.  Eventually the trees will grow tall enough that their tops will be above the snow pack year round and they will experience growth release, where they grow rapidly in size.  As these trees grow in size, the amount of organic litter they produce increases, thusly increasing the amount of organic matter stored in the soil underneath them. We found that over a hundred year period, soil organic matter increases from 0% to 12% in the upper centimeter of the mineral soil, with a leaf-litter layer over the top of that. 

Whelan, P., and A.J. Bach (2017) Retreating Glaciers, Incipient Soils, Emerging Forests:  One-Hundred Years of Landscape Change on Mt. Baker, Washington, U.S.A., Annals of the Association of American Geographers: 107:  336-349.  http://www.tandfonline.com/doi/full/10.1080/24694452.2016.1235480         

I've also worked on tree invasion of lowland prairies in western Washington. Specifically, the Ozette Praires near Cape Alava on the Olympic Peninsula, where tree invasion is threatening the habitat of a potential endangered species.  The endemic Makah Copper Butterfly was petitioned for listing on the U.S. Endangered Species List in August 2024.  The petition summarizes the work we did on the habitat loss:  https://xerces.org/publications/petitions-comments/esa-petition-makah-copper-butterfly         Here is my full report to the National Park Service on the natural history of the prairies:  https://npshistory.com/publications/olym/ahlstrom-roose-prairies.pdf