How quickly do plants replace melting glaciers?

Climate change means the world is warming, and one very visible manifestation of that is the retreat of glaciers and ice-sheets. But how rapidly will they disappear, and how long will plants take to reclaim the naked ground revealed as they go? This matters not just for aesthetic and biological reasons but because plants also alter the energy balance of the land they grow on, and that in turn alters the temperature and therefore the pace of further deglaciation: data that could prove very valuable for climate and other models. And although we often describe the present circumstances as unprecedented, the Earth has been through many warm spells in the past, which means there is already a template laid down in the geological record for how this tends to happen. And the key is DNA from ancient species trapped in ancient lake sediments. Speaking with Chris Smith, David Harning…

David - One particular question is, what happens when ice sheets recede? And how quickly do plants come in and colonise those areas? This is a really important question for understanding large scale energy balance of the earth. So ice sheets reflect energy that comes from the sun back to space; whereas plants absorb that energy. So you can imagine if energy's being reflected back, that's keeping the planet relatively cool, versus if plants are there absorbing it, that's gonna keep it relatively warm. So sort of like sitting in a car in the middle of the summer if the seats are black.

Chris - What can you study to give you insights though? Because obviously we're not seeing this other than in real time at the moment. So how do you go back in time to look at this?

David - We look at past sedimentary records. So in particular we go to lakes where mud that is comprised of all the living material in and around the lake at any given time. When that organic material dies, it sinks to the bottom of the lake and it accumulates, layer by layer. So the deeper you go down into the lake, the further back in time you go. And we've just identified a number of places in the northern hemisphere where glaciers are present and being able to address the question we're after about plants replacing glaciers at high latitudes, these go back to the last glaciation 10,000, 12,000 years ago. To identify basically what's there, you can use a number of available tools which we call proxies. So proxies for a past environment or a past climate state. And in particular in this study we use DNA, so DNA that is being produced by plants living in and around the lake. And just as I said before, when that material dies, it goes to the bottom of the lake and old DNA is preserved, see which plant was actually growing there. And then if you have a nice way to date your lake sediment, so be able to say like, well, this step was 5,000 years ago and this step below that, it's 10,000 years ago; you can actually say when certain plants were there and maybe when they weren't.

Chris - So the DNA tells you who's around? Mm-Hmm. The mm-Hmm. <Affirmative> dating of the mud via various methods can tell you who is around when. So how do you know what the climate was doing at each of those time points?

David - One of the major things that we use in our group is looking at the molecular structure of lipids or fats produced by bacteria and algae. A great example is if you have a plate of margarine and a plate of butter, both at cold temperature, so you stick 'em both in the fridge, you take 'em out. Butter is really, really hard to spread. Margarine is really easy to spread, and that has to do with number of double bonds or unsaturations in the actual chemical structure of those fats. So just the same way that those saturations have been modified by humans in margarine, different animals, different bacteria, different algae, they also adjust their saturations at a cellular level in the membranes to adapt to that environmental temperature. And by extracting and looking at these molecules, or we know who's producing them through time, we can use that, the changes in the saturations of the molecules to actually reconstruct temperature.

Chris - And when you do that, what picture emerges of how the plants do come back and, and recover or recolonise a previously glacial area? And is it just random or are there specific founder species that come in as pioneers first and then they're replaced? What does the picture look like?

David - Yeah, so there are pioneer species first. Those are often grasses, which, you know, can establish and root themselves in pretty desolate landscapes. So you imagine, you know, after an ice sheet retreats, there's really nothing there. It's a fresh slate, if you will, minimal soil development. So there's minimal nutrients available for the plants. So there's plants that are adapted to that low nutrient landscape. And then as those pioneer species establish themselves, the soil begins to develop organic content increases, nutrients increase, the soil becomes richer and more mature. And that allows plants that require that richer soil to move in. And many of those plants are actually influenced by the temperature.

Chris - And going back to the temperature effect of plants versus ice, what is this now revealing to you about how the dynamics of that work?

David - As you can imagine, plants absorb more energy than the ice. So that will actually elevate the temperatures more than what they would be otherwise if the plants weren't there. And going back to the original question about trying to understand the pace of plant colonisation, when ice retreats, it's generally been assumed to be very rapid. All these plants move in very quickly, particularly the woody taxa. So trees that really are absorbing a lot more energy, say than grasses. What we found in our study was a little different, that the trees, their arrival was spread out over several thousand years versus all of them coming in together. And this suggests that where ice sheets are currently retreating, such as in Greenland and Antarctica, the migration of these woody plants in particular may not be so quick. So as temperatures continue to rise and the ice sheets melt, and that's replaced with these higher energy absorbing plants, the warming may dampen the ultimate magnitude of future warming. So it may not be as rapid.