by Denny Olson

In the summer of 2017, Flathead High School International Baccalaureate Students Conrad Hedinger, Molly Adams and Sarah Randolph did increment borer fieldwork aging black cottonwood trees at 16 plot points in a grid pattern at Owen Sowerwine Natural Area (OSNA). The casual observation suspicion that cottonwoods were not regenerating very well was confirmed by the study. The youngest tree in the study was 24 years old, and most cottonwoods were in the 80 to 140-years old range. The oldest was 162 years. A majority of the sampled trees had broken tops and many held stagnant water in hollowed trunks.                      

A few saplings were observed while traveling between plots in the low, seasonally flooded areas of fossil river channels and along the Stillwater River. The 8 – 10-foot higher “bench” areas of the area had virtually none, and in fact were undergoing a succession pattern toward shade-tolerant spruce and fir.

Since cottonwoods are dependent on the scouring effects of flooding for seed germination, and also dependent on a gradually falling water table post-germination, it was fairly easy to conclude that flooding rarely happened on the higher plateau areas (with the historical records showing that probably the last flooding that high had happened in 1964, and possibly 1975. It could very well be that the damming of the South Fork of the Flathead River in 1959 had moderated flooding to the extent that it allowed a nearly constant downcutting of the river — within the confines of its banks. It left behind a higher plateau of land higher and drier, relative to the river, no longer suitable for cottonwood regeneration through flooding.

The other potential issues in the seasonally-flooded areas were sunlight competition by invasive species, specifically a non-native highbush cranberry shrub/tree. A removal program by FAS volunteers did increase the numbers of cottonwood seedlings and saplings in those areas newly sunlit.

Pat and Denny putting up the enclosure

Most of the saplings and seedlings in those areas are short and clumped, probably reflecting heavy browsing by a generous population of white-tailed deer in the area. We want to get some data on how much effect deer-browsing has on the new cottonwood survival, so this fall we constructed a temporary eight-foot-tall deer exclosure, 25 feet by 25 feet, in an area that is seasonally flooded and has a good supply of the cottonwood seedlings. We staked a “control” area of the same size right next to it , with roughly the same number of seedlings. We will be training high school and junior high students to not only count cottonwoods and measure growth in those areas year-by year, but other plants as well.

We are also going to be training some students to do yearly spring breeding bird surveys (with an accompanying FAS birder chaperone) at GPS plot points started by biologists at MT Fish, Wildlife and Parks in past years.

A third student project will examine the natural restoration rates of native plant species of areas where we removed invasive buckthorn (2000 trees removed so far, and only about half done!). So, we are answering our own questions about OSNA, educating students in scientific process, and restoring a river corridor that is a superhighway of bird migration spring and fall. And having fun doing it!