Disappearance of Tundra Ponds

Observation by Scott Richardson:

This is just one of many examples in Denali National Park in which these ponds are being reabsorbed into the ground. Kettle ponds are maintained by natural barriers of ice--permafrost--in their base soils. But with longer warming trends, that permafrost thaws and the water of the ponds follows the water cycle into the soil. I have been watching this particular pond for 23 years as its circumstances lessens and lessens. Many kettle ponds manifest seasonal fluctuations, but this is the first time in decades that this pond has shown no water in May! It was a very warm spring this year, so many seasonal ponds are showing little to no surface water. And more long-lasting ponds are dramatically smaller or dry. An additional observation and problem is that the grasses that grow from the base of the pond will slowly be replaced by the willows, dwarf birch, and other plants, leading to greater shrubification, a dramatic change in plant succession in a shorter amount of time.

Katie Spellman, Plant Ecologist at the University of Alaska Fairbanks, writes:

Hi Scott,

Your observation has gotten a bunch of scientists into a wonderful conversation. The guests on your tours are so lucky to have you, with such an amazing perspective on how the land is changing in the park.

I first passed your observation on to Carl Roland, Plant Ecologist at Denali National Park and Preserve, who thinks about succession a lot like me, but knows Denali's plants and soils better than I do (I am a little further north and don't work much in higher elevation systems). Carl shared some neat repeat photos of rapidly colonizing woody vegetation in Denali Park lakes. He said:

"Once a pond dries, there would generally be a fairly rapid colonization by vegetation (first herbaceous and then woody) as it is frequently nutrient-rich substrate. You can see matched pairs of historical/modern photographs documenting different stages of these processes here (for example) and here."

He then passed your observation on to Amy Larsen, the Park Service expert on drying lakes, who commented on your observation, and the linked Chistochina observation from a prior year. She wrote:

"I think there are several factors that are causing lake drying in the Healy area. First, water level in these lakes is largely controlled by precipitation, most importantly snowmelt. This year there was very little snow in the Healy area. The second is infiltration rates. With warmer soils, seasonal active layer is getting deeper, allowing water to infiltrate the soil deeper (equates to a drawdown at the lake surface), and permafrost is thawing, allowing water to move outwards away from the lake basin more easily. The third is the encroachment of terrestrial vegetation, particularly woody vegetation. Terrestrial vegetation has a much higher evapotranspiration rate, and uses much more water for structure and photosynthesis. The woody vegetation sucks lots of water out of the soil and out of the pond. Once woody vegetation is established it reduces the amount of water that is able to impound in these small basins each year. The water is used to create more woody/terrestrial vegetation. I'm guessing similar processes are happening in Chistochina. Both the Healy and Chistochina regions have significant gravel and course rock below them. Once the permafrost degrades and seasonal active thaw depth deepens there is little to trap the water near the surface. This is a natural successional process in these moraine environments, but it is likely be amplified by dramatic soil warming."

I also think that the depth of thawed soil (soil active layer, as Amy called it) could come back a bit as the shrubs encroach on the lake. There has been some very interesting research coming from up here at UAF on how succession and shrub growth can actually make permafrost come back (a process called permafrost aggradation) in a drying lake. The deciduous shrubs can shade the soil, decrease summer soil temperatures, and take up some of the water that transfers heat into the soil (as Amy mentioned, the shrubs suck a lot of water out of those spots due to evapotransipration!). All these things have been documented building the permafrost back up after the shrubs come into a drying lake. This may help slow permafrost loss down a bit, but as Amy pointed out, it may not be enough if the summer temperatures continue to warm dramatically, and so much depends on the snow.

Your Denali tourists will have plenty to talk about the next time you stop to look at a drying lake in Denali!!

Rick Thoman, Climate Scientist at the Alaska Center for Climate Assessment and Policy, writes:

From the climate side, we can say for sure that

1) it was a very mild spring, with an average spring temperature at Headquarters of 36.5F, which is x above normal and 2nd warmest spring on record (only 1926 warmer); 2) low snowfall over the winter (38.2" is the 3rd lowest on record for years with nearly complete data) and very early meltout of the winter snowpack. Earliest on record at Headquarters (March 30).