I used to be a forest person… that is, until I moved to Colorado State University, started my PhD in grassland ecology, and spent an entire summer exploring some of our wonderfully diverse grasslands here in the US; now I’m a grassland person. I love these systems purely for their aesthetic qualities – I like to watch the prairie move with the wind like waves on the sea; I like to watch them through the growing season to discover new types of flowers blooming every few days; I like the way the hills turn from deep black to emerald green after a burn – but plenty of other people appreciate grasslands for more practical reasons.
Over 25% of the land in the US is composed of grasslands used for cattle forage1making them responsible for a huge proportion of our food supply at an annual production of around 26 billion pounds of beef2. For this reason, conditions in these landscapes controlling how much plant growth occurs (i.e., rain) will affect most of us directly as beef costs are largely controlled by how many cattle can be stocked on a piece of land. Another less intuitive but equally important service grasslands provide us is their ability to take up carbon dioxide (i.e., the molecule mainly responsible for the warming of global temperatures) from the atmosphere, disassemble it, and store it safely belowground or in plant biomass where it can’t act as an insulator for the planet. Currently, photosynthesis done by organisms in the oceans and on land buffer quite a bit of our carbon emissions to the atmosphere, and changes in plant growth across landscapes have the potential to alter this buffering ability. For example, in the 1980s, regrowth of forests where farmland used to exist in the northeastern US took up about 20% of the carbon we emitted to the atmosphere through the burning of fossil fuels3. And while grasslands do not store as much carbon in wood as forests do, the facts that (1) the aboveground grass parts die every winter and much of these are incorporated into soil carbon stocks and (2) most of grass biomass is actually composed of roots, make grasslands really important areas to consider when thinking about how much carbon is being taken from the atmosphere.
Alright, let’s talk very briefly about climate change. Honestly, whether it’s human-facilitated or it’s just happening doesn’t really matter in the context of assessing how landscapes will change under these new conditions. Regardless of the cause, the climate is changing: we’ve documented it clearly in rising sea levels, increases in average global temperatures, and wetter wet and dryer dry years. Also, rainfall is coming in bigger storms in the Midwest (where many grasslands of the US are located) than it used to. According to some research done by the Rocky Mountain Climate Organization using rainfall data from 218 weather stations across the central and mid-western United States, the frequency of 3” or larger storms doubled from 1960 to 20114. However, even if we have a good idea about how climate patterns will change in the future, various lands will respond differently to these alterations depending on what kind of soil is there, how many nutrients are in the soil, and the type of vegetation that is present.
OK… so… how will these changes affect things we care about? Will our prairie hiking trails be devoid of our favorite wild flowers? Will ranchers face higher unemployment in the future? Will our grocery bills start to eat up more and more of our paychecks? Will the carbon dioxide buffering capability of these systems change?
In 2011 and 2012, we conducted an experiment to try and figure out how and why three different types of grassland would respond to changes in the amount of rainfall and the pattern in which it comes in the future5. We added water in different patterns (either in a bunch of small events spaced throughout the summer or added on top of naturally occurring storms) to areas within shortgrass prairie here in northern Colorado, in northern mixed grass prairie up in eastern Montana, and in tallgrass prairie in eastern Kansas. We then looked at two types of plant growth – aboveground (stems and leaves), and belowground (roots) – which impact different aspects of services these systems provide to us; for grazing purposes, aboveground growth is very important, while the sum of above and belowground growth is most important when considering how much carbon a system is taking up from the atmosphere. Our first interesting finding was that northern grasslands responded much, much less than their more southern counterparts… that is to say, added rainfall in southern grasslands made the plants grow more (duh?), but growth wasn’t affected at all in northern prairie despite increasing rainfall by over 50% throughout the summer over two years. We think this has to do with the types of grasses that dominate the landscape in northern grasslands. These are cool-season grasses that do the majority of their growing, you guessed it, during colder temperatures in the spring when soil moisture in the soil is very high from snowmelt. So, adding water doesn’t do a whole lot.
Our second big finding had to do with the Colorado and Kansas grasslands. Researchers in the past have found that when rainfall comes in really big events, aboveground plant growth in dry grasslands is higher, and people have speculated that this is due to coarser soils in these areas so that storm rainfall can filter deeper in the soil where it is protected from evaporation and thus saved for plant use later during drought periods. Alternately, small events in these systems evaporate off very quickly because there is very little shade from grass canopies. The opposite is true for wetter grasslands like the tallgrass prairie in eastern Kansas: more growth has been found to occur with small rainfall events that came often and soil water evaporation was less of an issue due to fuller grass covers. However, all this research was based on studies that only looked at aboveground plant growth. When we incorporated root growth, we found that the pattern in which rainfall came had no effect on total plant growth despite the differences we saw in aboveground growth. And, while this doesn’t change what we know about the effects of precipitation pattern on forage production, it does affect our predictions about how much carbon these systems are likely to store in the future.
So to go back to our original questions and to refer to the title of this post, it’s not just grass: the state of these amazing areas and the amount of grass available for cattle grazing in the future will be dependent on where you are and what types of grasses currently grow there. As far as predicting the amount of carbon that will be stored in grasslands in the future, this is a little trickier as most of what we know is based on aboveground growth (which is much easier to measure than root growth), but we need to incorporate belowground growth if we are to accurately predict the amount of carbon going into and coming out of grasslands. So to keep these systems around – whether it be for their aesthetic qualities or for the stuff they do for us – we need to keep working on understanding how and why grasslands will respond in the future.
1United States Department of Agriculture, Economic Research Service, http://www.ers.usda.gov/data-products/ag-and-food-statistics-charting-the-essentials/land-and-natural-resources.aspx.
2American Meat Institute, http://www.meatami.com/ht/d/sp/i/47465/pid/47465.
3Houghton, R. A., J. L. Hackler, and K. T. Lawrence. 1999. The US carbon budget: contributions from land-use change. Science 285.5427: 574-578.
4Saunders, Stephen, et al. 2012. Doubled trouble: more Midwestern extreme storms. The Rocky Mountain Climate Organization and the Natural Resources Defense Council.
5Wilcox, Kevin R., et al. 2014. Contrasting above‐and belowground sensitivity of three Great Plains grasslands to altered rainfall regimes. Global change biology