My graduate research was on grassland restoration and invasive species management
The first study was a three-year competition study between purple needlegrass (Nassella pulchra) and yellow starthistle (Centaurea soltitialis) on simulated deep and shallow soils. It started in 1998 and ran through the summer of 2001. I measured size, reproductive output, water potential measurements after the spring rains had ceased, and also the date that the needlegrass broke dormancy in the fall. I also recorded mortality. It will soon be submitted to Ecological Applications. What I found was that Nassella pulchra survival, growth, and reproduction respond negatively to growth in shallow soils, and the grasses are also more water-stressed and break dormancy later in the fall when growing in shallow soils. Unfortunately, many of the relict populations of Nassella pulchra are growing on steep slopes in shallow, eroded soil. On the bright side, Centaurea solstitialis also does poorly in shallow soils, and in low rainfall years the addition of competition from Centaurea solstitialis to the shallow soils didn't have any impact on Nassella pulchra growth or reproduction. The Centaurea solstitialis just didn't get large enough in the shallow soils to have a competitive impact. However, in a higher rainfall year, the Centaurea solstitialis in the shallow soil tubes grew significantly larger and did have competitively suppress Nassella pulchra growth and reproduction. A different pattern was seen in deep soils. Centaurea solstitialis always had a significant competitive effect on Nassella pulchra, but this effect declined over the three years of the study. By the third year of the study, growth of Centaurea solstitialis was much lower in the deep soil tubes than it was in the first year, and competitive suppression of Nassella pulchra growth and reproduction was decreased. Thus, more mature (and larger) Nassella pulchra plants appear more able to resist competitive suppression by Centaurea solstitialis and may suppress Centaurea solstitialis growth in return.
The results of the first study suggested that it would be interesting to see how size of Nassella pulchra impacts invasion by Centaurea solstitialis. So, I used the same Nassella pulchra plants from the first study to conduct a second study from the fall of 2001 through the summer of 2003. I found a significant negative linear relationship between the size of Nassella pulchra and the number, weight, and flower production of Centaurea solstitialis. The same amount of relative competitive suppression of invasion was seen during a low rainfall year and during a high rainfall year, but during the high rainfall year there was much higher invasion by Centaurea solstitialis and negative impact of Nassella pulchra occured at more of the life stages of Centaurea solstitialis. This chapter was submitted to Restoration Ecology.
The Nature Conservancy awarded me an Oren Pollack grassland research grant to conduct a study of the ecology of two C4 grasses, an annual, Aristida oligantha, and a perennial, Aristida ternipes var. hamulosa. These are two of the few C4 species that were likely a part of the native Central Valley grasslands. I used a combination of surveys of natural populations and field experiments to describe where Aristida sp. grow, how they will potentially respond to fire and grazing, and what sorts of sites they can be planted in. Some of what I found: the perennial, Aristida ternipes var. hamulosa, is typically found on steep, dry, low-productivity slopes, overpasses, and roadcuts, and the annual, Aristida oligantha, likes even lower-productivity roadsides, roadcuts, trails, stream and canal edges, and other disturbed sites. They appear to have low competitive ability, probably as a trade off for high stress toleration ability. I also ran three competition experiments (two of which worked) and found that Aristida oligantha does not respond well to shading. In the first study I found that growth with Centaurea solstitialis has little effect on Centaurea solstitialis size or reproduction, though it did appear to increase water stress of Centaurea solstitialis. However, growth with large Centaurea solstitialis reduces size of Aristida oligantha. In another study I tested how Aristida oligantha responded to low and high densities of Bromus hordeaceus (a nonnative, cool-season grass) in two different soil textures, and found that Aristida oligantha grew more slowly and had a lower final size in plots with competition. Water potential of Aristida oligantha did not respond to any of the Bromus hordeaceus competition treatments, but Aristida oligantha has a much deeper root system than Bromus hordeaceus. So, planting Aristida oligantha in areas with a dense vegetation cover will limit it's success, and it is better used in relatively bare soil along trails, roads, etc. This one was published in Grasslands in April of 2004.
Beth Leger and I conducted a study at Jepson Prairie on the effects of the starthistle control herbicide Transline on about 52 plant species in the grassland community. We found that Transline is extremely effective at controlling starthistle, but will also very likely change community composition. This effect will impact exotic species and natives alike. So, here we have another example of a situation where land managers will have to weigh the benefits and the costs before using a weed control mechanism. I personally suggest collecting seeds of the natives that will be impacted from the local area and reseeding after treating the starthistle, and the manuscript was published in Weed Science in the summer of 2003.
I presented a poster at ESA in Portland on August 2nd, 2004. It showed how the size of Nassella pulchra influenced its ability to resist invasion by Centaurea solstitialis. All of my Davis research has been submitted and published and can be found on my CV.