How we measure snow, a key driver of wildflower phenology

 

Plot 8 on Reflection Lakes Trail, July 18th, 2017

The growing season in wildflower meadows at Mt. Rainier, like many high mountains in the western US, is extremely short – with snow lingering in the mountains well into July. Our data indicates that the disappearance of that snow (i.e. the date that snow melts) strongly affects when wildflowers bloom at Mt. Rainier: when and where snow melts early, wildflowers bloom early. We needed a quantitative measure of snowmelt date in order to understand that link between climate and wildflower phenology. And because snow amounts vary from year to year and are extremely patchy (see picture above), we needed to measure snowmelt date at each individual plot every year. How did we do this? We use small sensors called Hobo Pendant loggers (shown in the upper right of the picture above) programmed to measure temperature every two hours all year. Each fall, we bury these small temperature sensors just below the surface of the soil in each of our MeadoWatch plots (the pink flagging above marks the location of one of these sensors), and retrieve them the following summer after they've measured soil temperatures all year. How does this tell us about snow? When plots (and sensors) are covered by snow, they measure the temperature of snow, which is zero degrees Celsius. Because snow is a very good insulator, soil temperatures also stay constant when plots are snow covered, and temperatures don't fluctuate on a daily basis like they otherwise would (being affected by air temperature). This means that we can use temperature data to pinpoint the date that snow melts: essentially by identifying the point in time when soil temperatures go from zero degrees Celsius with no daily variation to warmer temperatures that fluctuate during the day (see the two graphs below). We can then statistically link these snowmelt dates to the phenology data our volunteers collect, which allows us to predict how the timing of the peak wildflower season may shift when climate change causes snow to melt earlier. Please help us collect one more valuable year of snow and wildflower phenology data by supporting our citizen science program MeadoWatch (see link here)!

 

May - July soil temperatures measured at one plot in two separate years. Temperatures measure zero degrees Celsius when snow-covered, and follow daily cycles in temperature after snowmelts. This allows us to pinpoint the date that snow melts at each plot in each year.