- Restoring Native Perennial Grasses by Changing Grazing Practices in Central Coastal California
The conversion of California grasslands from a system dominated by perennial bunchgrasses to one dominated by exotic annual grasses is recognized as an ecologically significant biological invasion (D’Antonio and Vitousek 1992). The invasion by exotic annual grasses has drastically altered ecological structure and functional processes through the competitive suppression of native grass seedlings and adults, altering community response to disturbance, and causing changes in soil carbon, other soil nutrients, soil microbes, and soil water profiles (Holmes and Rice 1996, D’Antonio et al. 2007, Koteen et al. 2011).
Given the impact that exotic annual grasses have on California biodiversity and ecosystem function, there is great interest in developing strategies for restoring native perennial grasses (Corbin and D’Antonio 2004). One opportunity for restoration is planned grazing designed to focus the timing and intensity of grazing pressure on exotic annuals and allow native perennial grasses to grow and seed as much as possible (Biswell 1956, Menke 1992, George et al. 2013). Despite the promise of this approach, the efficacy of using grazing to promote native perennial grasses remains unclear (Bartolome et al. 2004, Stahlheber and D’Antonio 2013).
To evaluate the efficacy of a planned grazing program to restore native perennial grasses, we have been conducting vegetation monitoring at TomKat Ranch in Pescadero, California. TomKat Ranch is a 728 hectare (1,800 acre) property located south of the San Francisco Bay Area in San Mateo County, 27 kilometers (km) south of Half Moon Bay and 2.6 km from the Pacific Ocean. The area is characterized by steep forested slopes, deep canyons, a fertile coastal valley, and grasslands and coastal scrub. The elevation ranges from 12 to 380 meters. Like other central California coastal locations, TomKat Ranch experiences a maritime, Mediterranean climate characterized by cool, wet winters and mild, mostly dry summers. Fog and low overcast skies are common throughout the year, particularly during the summer months. Average annual precipitation is 750 mm (29.5 in), mostly falling as rain and fog. The rain year (precipitation for July–June, what is often reported in California) for July 2010 to June 2011 was 757 mm (29.8 in), for July 2011 to June 2012 was 503 mm (19.8 in), and for July 2012 to June 2013 was 655 mm (25.8 in). TomKat’s grasslands comprise approximately [End Page 352] 324 hectares (800 acres) and are dominated by annual exotic grass species, with some native perennial grasses, mostly purple needlegrass (Stipa pulchra (synonym Nassella pulchra) and California oatgrass (Danthonia californica).
TomKat Ranch has a cow-calf operation (a permanent herd) with approximately 100–150 head grazing the grassland portions of the Ranch year-round that supplies beef to their grass-fed/grass-finished beef business. Beginning in 2008 and continuing until 2011, the Ranch employed season-long continuous grazing practices where cattle were often split into multiple herds and were left out over large portions of the ranch for several months at a time providing plants with little rest in between grazing periods. In 2011, the ranch adopted a planned grazing approach where they increased cattle density (112,085–168,128 kg/ha or 100,000–150,000 lbs/acre) by putting them in small blocks and moved them quickly through subdivided fields (20 permanent fields further subdivided into paddocks using temporary electric fence). Grazing periods typically ranged from one day to one week in each paddock, providing plants with 70–120 days of rest (no grazing) in between grazing periods. The amount of rest depended on time of year, the growth phase of grasses, and on field quality. Rest periods are typically longer during the non-growing season (approximately July until first fall rains, but depending on year) when plants experience little to no growth. In each field, the timing of grazing varied by year.
To monitor changes in grassland plant community, we measured vegetation composition across all grasslands each July from 2011 to 2013. For vegetation monitoring, we subdivided the 20 permanent fields into a total of 74 vegetation survey units (Figure 1) based on similar slope and...