Do Ecosystems Have a Memory?

February 16, 2012 by · Leave a Comment
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Black spruce ecosystem in the Boreal forest

Our memories shape who we are and how we perceive the world around us today. Did you know that ecosystems have memories too? It’s obviously not the same as it is for people. But, ecosystems can integrate information about the past into their soils and plants, and it shapes how they look and how they “act” in terms of responding to environmental conditions today.

As an ecologist, one of the things I do is measure the “memory” of ecosystem physiological processes for past environmental conditions. Ecosystems have a physiology, so just like us, they breathe in (photosynthesis), they breathe out (respiration), they grow (put on plant biomass), they break stuff down which makes nutrients available (decomposition).

One of the projects that I’ve recently finished is quantifying the memory of soil respiration (soils breathe out CO2too) for past soil moisture and temperature conditions in the desert. I did this work in the northern part of the Sonoran desert where shrubs are moving into grasslands. I wondered if the shrubs were changing the memory of soil physiology because of how shrubs dump a bunch of food into the soil for the micro-organisms to eat. Shrubs are also really different from grasses in ways that probably change the physiology of the whole desert ecosystem. I found that shrubs lengthen the memory of soil respiration for past soil water. This means that soil water two months ago can affect soil respiration today. Compared to the grasses which need soil water one month in the past, this is a pretty big change.

What does this mean? Imagine if change some part of your life and your memory suddenly gets much longer. Stuff that was going on two months ago is still affecting you today. How would that change your life? For the soil physiology, it means that rain in the past is still important today. So, if the patterns of rainfall change (less frequent events, more tiny events, etc), which is what is predicted to occur in deserts with climate change, then soil physiology will change if there are a bunch of shrubs on the landscape.

In Alaska, I’ve been studying the physiology of plants rather than soils. Instead of looking at respiration, I’m studying transpiration. Like us, when they breathe out, the plants release water vapor (transpiration). Stand outside on a cold morning and watch the puff of vapor come out when you exhale. The plants do the same thing (but in the summer). The amount of water that plants breathe out is related to how wet the soil is (they need to get water from somewhere) and how dry the air is. If you’ve spent time in the desert, you know how it feels to have water sucked out of you into the dry air. Likewise, in the humid southeastern US, water condenses on everything because the air is so wet. If the air is dry, then water is going to get sucked out of the plants more, so the plants want to control this by closing their leaf pores (stomata). The dynamics of stomata opening and closing is an entire field of ecology! This is basically plant physiology at its tiniest scale.

I study the memory of plant physiology for past soil moisture, soil temperature, and atmospheric conditions. So, I ask: how does soil wetness (or dryness), soil hotness or coldness, and the dryness of the air affect the physiology of a plant today? One of the big things I want to know is how changes in the environment, particularly permafrost thaw, affect this memory. This is a work in progress, so I’ll have more to say later on. So, the next time you step outside into nature, take a long look at the black spruce trees and imagine that they remember what was going on last week or even last summer.