In this post Nick Smith and Mike Schuster discuss their recent paper with Jeffrey Dukes ‘Rainfall variability and nitrogen addition synergistically reduce plant diversity in a restored tallgrass prairie

Prairies once covered great swathes of North America, from Texas to Canada and the foothills of the Rocky Mountains to the hardwood forests of the Eastern United States. These grasslands were home to a diverse array of plant, animal, and insect species. While some historical prairies remain, human activity has diminished this ecosystem to a fraction of its original extent. The tallgrass prairie, which is found on the Eastern edge of the American prairie, has been particularly hard hit. Tallgrass prairie remnants likely constitute less than 20% of the original range, with some estimates reaching lower than 5% (Samson et al. 2004). Thus, efforts to restore tallgrass prairies are vital to their continued existence. However, co-occurring global changes, such as those associated with climate change and nitrogen deposition, may threaten these restoration efforts.

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Overhead view of a plot with large abundance of Solidago canadensis (yellow flower). Photo credit: Mike Schuster.

We asked how increased rainfall variability and nitrogen addition might influence plant diversity in a restored tallgrass prairie. While projected changes in total rainfall amounts are relatively uncertain, climate models indicate rainfall patterns are likely to become more variable (IPCC 2012; IPCC 2013). Since water, nitrogen, or both limit productivity in many temperate systems, changes in rainfall variability and nitrogen deposition are likely to strongly influence plant communities. More diverse systems tend to be more productive and provide habitat and forage for a greater variety of wildlife and insect species. Thus, the impact of global change on prairie diversity might have important consequences for the success of future restorations and the ability of prairies to provide essential ecosystem services.

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Rainout shelter at the Prairie Invasion and Climate Experiment (PRICLE). Photo credit: Mike Schuster.

To design our experiment was no small feat. We needed to apply manipulations that were strong enough to illicit a response at the plant community scale, but that were not so large as to be unrealistic. For the nitrogen addition, this was relatively simple: we added an amount of slow-release nitrogen fertilizer to half of our plots, simulating nitrogen deposition levels in heavily industrialized regions of the world. The rainfall manipulations were a little trickier since few field studies have examined intra-annual rainfall variability. After entertaining multiple concepts, we decided to install rainout shelters that would constantly remove 50% of ambient rainfall from half of the plots. The shelters were built using wood frames mounted above the prairie. On top of these, we installed slats of corrugated plastic that covered 50% of the frame and gutters that diverted the intercepted rainfall away from each plot. While the rainout shelters created a constant drought treatment, we wanted to manipulate variability. To do this, we calculated the total amount of rainfall excluded from each plot and applied it in a single, large event once every 30 days. Thus, all plots received the same amount of water, but those under rainout shelters experience a much more pulsed regime. Rainout shelters also removed some of the ambient sunlight reaching the plants. So, in order to control for this, we installed similar structures in plots not exposed to increased rainfall variability. These “control” structures were covered with a wide mesh that blocked an equivalent amount of sunlight, without altering rainfall.

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Purple coneflower (Echinacea purpurea) and indiangrass (Sorghastrum nutans), among other species, at the Prairie Invasion and Climate Experiment (PRICLE). Photo credit: Mike Schuster.

What we found with our experiment was unexpected and stood out amongst previous studies. Interestingly, the rainfall manipulation generally acted to increase soil moisture, likely as a result of deep soil infiltration during heavy rainfall events. This, combined with nitrogen addition, provided a substantial increase in resources in the plots receiving both treatments. However, increased resource availability was not equally beneficial to every type of plant in our study. In fact, many native tallgrass prairie species, such as C4 grasses (adapted to withstand dry conditions) and leguminous forbs (adapted to withstand low soil N concentration), showed decreases in cover under the rainfall variability and nitrogen addition treatments. The reason? Competition. Instead of allowing all species to grow better, the global change treatments favored a small number of fast-growing C3 forbs. Solidago canadensis, which was not part of the original restoration, responded particularly well. This effect reduced community diversity in the plots receiving both global changes. Based on our findings, we suggest that without appropriate management, projected global changes may jeopardize tallgrass prairie restoration efforts.

References

Ipcc (2012) Managing the risks of extreme events and disasters to advance climate change adaptation, Cambridge, England, Cambridge University Press.

Ipcc (2013) Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Resport of the Intergovernmental Panel on Climate Change, New York, NY, Cambridge University Press.

Samson F. B., Knopf F. L., Ostlie W. R. (2004) Great Plains ecosystems: past, present, and future. Wildlife Society Bulletin, 32, 6-15.

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