By Ellie Wilding, PhD student in the Department of Plant Sciences
If I say the word “Savanna”, I am sure you – like I did – think of vast, Lion King-esque African plains, bursting at the seams with lions and elephants, Timone and Pumba. Over the course of this summer field season, I have been deep in the long grass, look at the soil of the savanna… in Minnesota, northern US.
Savannas cover many continents, including the US and Africa, and are determined primarily by a continuous grassy underlayer that has sporadic tree growth. In Minnesota, these trees are predominantly of the genus Quercus and are therefore called Oak Savanna. Since European settlement in the US, the savanna of the Midwest region of North America has been in steady and significant decline largely due to land fragmentation, agriculture and fire suppression.
Savannas are dynamic systems which require regular disturbances, such as fire, to regulate a balance between graminoid species (e.g., grasses and sedges) and trees, and so retain the species that exist in the systems. By suppressing fire, trees extend their canopy and shade out grass species and the important and productive habitat on the ground is lost. Many land managers use burning as a mechanism to maintain the savanna system – a practice that is thought to have existed for thousands of years.
Climate and land use change and evolving societal perceptions of fire are causing changes in the frequencies and intensities of fire regimes across the globe, particularly in savanna ecosystems. My PhD research delves into the implications of these fire regime changes on the carbon in these systems. I want to know what the mechanisms driving soil carbon responses to fire regime changes are. How do roots impact the soil carbon response to fires? Do wetter savanna systems hold more carbon in the soil after a fire? How is decomposition affected by different fire frequencies?
In an attempt to answer some of these questions, I flew out to Cedar Creek Ecosystem Science Reserve in Minnesota, USA. Cedar Creek is a branch (pun absolutely intended) of the University of Minnesota and alongside a number of other incredibly important scientific research (see also BioCon, and BigBio), they have one of the longest continuous fire records in the world. It provides the ideal setting for me to sink into my research and figure out what is going on with soil carbon in savannas.
Jet-lagged and exhausted, my introduction to Cedar Creek consisted of a long list of critters to avoid (I had perfectly timed my visit to experience all of the biters) and how to deal with the poisonous plants and resident bison. I drove down a sand track to the lodge I would be staying in, clearing the deer and Sandhill crane from my path as I went. After a few days of adjusting and sussing out the area, I transitioned into the daily grind of the field ecologist. Waking up at 5 am with the hustle and bustle of other researchers rising and mumbling their plans for the day over a pot of coffee. I’d tuck all items of clothing into each other to keep the ticks out, throw a shovel over my shoulder and cycle through the thick sand to find my plots.
The beauty of Cedar Creek is that it is the perfect set up for ecosystem science, and as such, there is a gold mine of data readily available. I was interested, however, in data that hadn’t been collected. My main goal for the field season was to understand the characteristics of roots and the role they played in the soil carbon response to fire (I would soon find out why no one had collected this data before). To do this, I would need to collect data on the following:
- The distribution of coarse and fine roots through the depth of soil;
- The amount of biomass allocated aboveground and belowground;
- The general root traits employed by plants in the system;
- The root traits employed by the dominant plants in the system.
To get an idea of how roots grow at different depths throughout the soil profile, I set about taking soil cores. Across 12 burn units, I took cores of two different depths (at 0-20cm and 20-40cm) at eight sample points across a plot. Taking these cores and washing, drying and weighing the subsequent roots found within them provides us with an idea of the biomass distribution at different depths, the characteristics of the roots at different depths and the soil carbon stored at different depths, and what happens to all these under different fire frequencies.
I unintentionally chose the most glorious plots to sample first. The dawn chorus serenaded me as thwacked a PVC tube into the ground with a mallet and the rising sun gave me warmth as I held onto the rebar and shimmied the core out of the ground. The wildlife in Cedar Creek was fantastic. Bald eagles and turkey vultures would circle overhead as deer jumped through the grass, tricking me every time into believing they were the elusive black bears I’d been told about. With some of the plots, I had to receive special training in case the resident bison got to inquisitive.
Not all plots were as delightful as these early ones though. Some were elusive and would take up to an hour just to find. As the coring started to play havoc on my back, the plots would thicken with vegetation, mostly poison ivy, and ticks and insects. I began to seek comfort only from the head net that I wore over a tie dye bucket hat to stop things biting my face or twigs catching my eyes. The monotonous task of root washing was suddenly incredibly desirable!
Once I had washed and processed all the cores, storing the roots in ethanol in fridges, I headed back out into the field for my second and final sampling stint. In this round, I was collecting samples for biomass allocation measurements and specific root traits from dominant species. Digging square blocks 35cm3 with a shovel, I would then hose away the soil and separate out the roots from the aboveground plants bagging them up ready for drying and weighing. I then identified the dominant species at each plot and – depending on the root system – either cored down 40cm or traced the root system 40cm laterally to get samples of roots from dominant plants. For the grasses, this was a dream. For the large woody species, it was a nightmare. But by this time I had learnt my lesson and chosen the tricky plots first to get them out of the way, knowing that I would end my sampling effort on a dawn morning in a picture perfect oak savanna.
The samples made it back to the UK with me and I write this blog, happily reminiscing of how the efforts it took to get them to the fridge, ready for me to analyse in the Plant Sciences department. My experience in Minnesota was not limited to digging holes and washing roots, however. I was privileged to be surrounded by other PhD students and Postdocs who were delighted to have me help them with their fieldwork on projects from invasive species to tree communication in forests. I was repaid generously in s’mores on the fire in the evening and extra hands with my work.
I also had the opportunity to talk about my research at the Minnesota State Fair, which ended up varying from intense scientific discussions with now-retired researchers to holding a snakeskin up for a toddler to grapple with and having to literally escape the chase of Goldy the Gopher, the University of Minnesota Mascot. The State Fair itself is the epitome of American Midwest culture with butter sculpting competitions and an array of deep-fried foods on a stick, which could be enjoyed whilst watching impressive livestock shows from the “Colosseum” or browsing the vast selection of the most enormous vegetables I have ever seen.
My time in the US has been a pivotal part of my PhD and I have immense gratitude for the people out there who hosted me and allowed me to conduct my research whilst having huge amounts of fun. I am also grateful to the NERC DTP who provided the funding for this fieldwork.
