Soil Water and Temperature Effects on Resilience Resistance of Sagebrush Steppe Plant Communities
Faculty Mentor: Bruce Roundy, Plant and Wildlife Science
Project Purpose-The purpose of this project was to evaluate the effects of environmental factors on overall resistance and resilience of the sagebrush steppe community. Resistance is defined as a system’s ability to avoid change after a disturbance has taken place and resilience is defined as the ability of a system to return to its natural state after a disturbance has taken place.
Project Importance-The sagebrush steppe ecosystem provides vital habitat for deer, big horned sheep, elk and other animals. Sage grouse populations, in particular, have declined due to loss of sagebrush habitat by large-scale wildfires. Two major threats to sagebrush steppe plant communities in the Great Basin of the western U.S. are invasion by the exotic annual grass, cheatgrass, and woodland invasion by pinyon and juniper trees. Both of these pose risks to the overall health of the ecosystem because they result in severe wildfires followed by weed dominance (Mclver and Brunson 2014, Miller et al. 2014).
Project Outline-There were three main objectives to this project. The first was to compare soil temperature regimes as classified by the Natural Resources Conservation Service (NRCS) with those based on actual field measurement of soil temperatures over the last five years. The second was to use Partition Analysis to examine the major soil temperature and soil moisture factors that affect resistance and resilience. Lastly, we sought to determine the effects of prescribed fire on resistance and resilience.
Soil Temperature Regimes- Because scientists have categorized resistance and resilience of sagebrush communities in relation to soil temperature regimes as classified by the NRCS, it is important to determine the accuracy of these classifications. I was able to write a formula that used average annual measured soil temperatures, as well as the difference between summer average temperature and winter temperature to classify the soil temperature according to NRCS criteria. Field measurements of soil moisture and temperature at a 50-cm depth were made on untreated and prescribed burned plots on 16 sagebrush steppe sites in the Great Basin, USA from 2011-2016. We found that soil temperature regimes designated from actual soil temperatures were generally warmer than those designated by the NRCS soil surveys. NRCS-designated classifications are estimations that take into account factors such as elevation, slope, orientation, and plant community composition, but are not necessarily based on measurement of soil temperatures. Estimation is often used because of costs associated with implementation of remote sensors as well as time needed to collect data. The number of differences can be seen in figure 1 in the appendix. This finding is extremely relevant because one of the key factors affecting resistance is soil temperature and so classification of soil temperature needs to be accurate in order to properly manage an area.
Factors affecting Resistance and Resilience- Using the statistical analysis program JMP, we
used Decision Tree partition analysis to determine which soil temperature and soil moisture
variables were most associated with plant community resistance and resilience on untreated and
burned plots. We used cheatgrass cover as an indicator of resistance and total perennial
herbaceous cover as an indicator of resilience. High cheatgrass cover indicates low resistance to
change because it can result in increased fire frequency and continued dominance by weeds.
High total perennial herbaceous cover indicates high resilience because it resists weed
dominance and protects soil resources from erosion, thus reinforcing ongoing water and nutrient
cycling. I found it extremely interesting that resilience seems to be influenced more by seasonal
timing of warm or cold temperatures and water availability than resistance. What I mean by this
is that we measured not only how long dry periods lasted for, or average temperatures but the
timing of these events and compared them. It appeared from our analysis that resistance to
invasion and change is most influenced by overall averages and lengths of factors (including
temperature, wet/dry period etc) while resilience seems to be affected heavily by the timing of
when dry/wet periods begin. Put in more specific terms cheatgrass cover is influenced more by
the overall length of dry/wet periods while the rest of the herbaceous cover is more influenced by
the timing in which those dry/wet periods begin.
Untreated Compared to Treated Areas-Overall Cheatgrass population density dramatically rose
in areas treated with controlled burns see figure 2. This indicates that resistance is often
diminished due to controlled burn. Total perennial herbaceous cover on the other hand
experienced a slight increase. This suggests that there are some instances in which controlled
burns can have a positive effect on resilience see figure 3.
McIver, J, and M. Brunson. 2014. Multidisciplinary, multisite evaluation of alternative sagebrush
steppe restoration treatments: The SageSTEP project. Rangeland Ecology and
Miller, R.F., J.C. Chambers, M. Pellant. 2014. A field guide for selecting the most appropriate
treatment in sagebrush and piñon-juniper ecosystems in the Great Basin. General
Technical Report RMRS-GTR 322-rev. Fort Collins, CO: U.S.Department of Agriculture,
Forest Service, Rocky Mountain Research. 68 p.