Long-term Residual Dry Matter Mapping for Monitoring California Hardwood Rangelands

Long-term residual dry matter mapping on the San Joaquin Experimental Range provides a working example of this monitoring technique for grazing management and research. Residual dry matter (RDM) is the amount of old plant material left on the ground at the beginning of a new growing season. RDM indicates the previous season’s use and can be used to describe the health or condition of annual rangelands. An RDM evaluation is made before the first effective fall rains, usually in late September or early October. Direct clipping and weighing, comparative yields, or photo standards are used to obtain RDM estimates. Mapping of RDM provides a means of recording the total amount of herbage remaining, as well as its distribution on the landscape. RDM is mapped on three different range sites at low, moderate, and high amounts: 1) flat slopes and swales: <400, 600 and >800 lbs/acre; 2) gentle rolling slopes: <600, 800 and >1,000 lbs/acre; and 3) steep slopes: <800, 1,000, and >1,200 lbs/acre. Tracking this information assists management in adjusting stocking rates, selecting locations for livestock supplements and evaluating grazing systems. Researchers can evaluate different grazing impacts on sites, evaluate grazing models, and have a clear understanding of ambient grazing prior to experimentation. Introduction Many ranchers and range managers do not fully understand the importance of residual dry matter (RDM) on the California annual grasslands. RDM, sometimes referred to as mulch or plant litter, is the dry plant material on the ground left from previous seasons of growth (Clawson and others 1982, Frost and others 1988, Hedrick 1948). The exact terminology may vary among workers, but RDM consists of three classes or forms: 1) ungrazed mature vegetation still attached to the root system (forage residue), 2) vegetation residues detached from the roots laying on the ground (plant litter), and 3) decomposing residues partially or completely 1 An abbreviated version of this paper was presented at the Fifth Symposium on Oak Woodlands: Oaks in California's Changing Landscape, October 22-25, 2001, San Diego, California. 2 Faculty Research Associate, Oregon State University, Dept. of Rangeland Resources, 202 Strand Hall, Corvallis OR 97331-2218 (e-mail: [email protected]) 3 Natural Resources Advisor, University of California Cooperative Extension, Placerville, CA 95667 (email: [email protected]) 4 Farm Advisor, University of California Cooperative Extension, Madera, CA 93637 (e-mail: [email protected]) 5 Extension Range Specialist, University of California, Agronomy and Range Science, Davis. CA 95616-8515 (e-mail: [email protected]) 6 NRCS District Conservationist, USDA Service Center, Madera, CA 93637 (e-mail: [email protected]) USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. 2002. 87 Long-term Residual Dry Matter Mapping for Monitoring California Hardwood Rangelands—Harris, Frost, McDougald, George, and Nielsen incorporated into the soil (humus) (Hedrick 1948, Molinar and others 2001). RDM estimation and mapping for management purposes deals primarily with the first two classes. The rancher’s main tool for influencing soil surface conditions is by managing RDM (George and Menke 1996). Annual plants respond each year to conditions that influence seed germination and early establishment, with fall weather having the greatest impact (Clawson and others 1982). RDM provides favorable microenvironments for early seedling growth, soil protection from erosion, soil organic matter, and it increases water infiltration and soil water holding capacity (Clawson and others 1982, George and Menke 1996, Molinar and others 2001, Rauzi 1960). Plant species composition on annual rangelands is also heavily influenced by RDM. High amounts of RDM favor the development of grass-dominated communities, while low amounts of RDM favor broadleaf-dominated communities (George and Menke 1996). Under normal climatic conditions, adequate amounts of RDM increase forage production on most sites (Bartolome and others 1980, Schwan and others 1949). Furthermore, RDM provides low-moisture fall forage for livestock feed (Clawson and others 1982). Bentley and Talbot (1951) developed a system of site classification for rangelands typified by the San Joaquin Experimental Range. For the purposes of RDM estimation and mapping, the system consisting of ten site classes can be simplified by grouping sites into three slope categories: swales, open rolling, and steep brushy (Frost and others 1988). For range management, a rapid survey of rangelands can be done using visual estimation and comparative photos. RDM for the sites can be classed into light, medium and high categories using the class descriptions and comparative photos in the U.C. Cooperative Extension Leaflet 21327 (Clawson and others 1982). RDM classes can then be mapped using manual techniques or entered into a geographic information systems (GIS) database. Mapping RDM periodically through the dry forage period can enhance livestock and rangeland management. RDM maps provide a record of residual dry forage amount and its distribution on a pasture, ranch or allotment. These records assist land managers in adjusting stocking rates (Clawson and others 1982) and selecting locations for water and supplemental feed to better utilize forage (Frost and others 1988). Long-term mapping and livestock use records provide accurate estimates of livestock carrying capacity for rangeland (Frost and others 1988). Researchers can use RDM records to evaluate grazing impacts on sites, evaluate grazing models, and evaluate treatment effects on grazing distribution. Methods Long-term mapping of RDM was implemented on the San Joaquin Experimental Range (SJER) in 1985 and is still continuing today. Only the RDM maps from 1985 to 1999 are presented here. The SJER (UTM zone 11N, Easting 257600, Northing 4109300) is located in the Sierra Nevada foothills 28 miles north of Fresno along state highway 41. The SJER (fig. 1) encompasses 4,539 acres and has been a USDA Forest Service research facility since 1934. The SJER is within the oak savanna region of the foothills and is characterized by grassy rolling hills with scattered trees and shrubs. Occasionally the stands of trees and shrubs become quite dense. The climate is Mediterranean with mild, rainy winters and hot, dry summers. Long-term average precipitation (65 years) is 19 inches. However, annual precipitation varies between 9 and 37 inches with most occurring as rain. Snowfall is rare at this location. USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. 2002. 88 Long-term Residual Dry Matter Mapping for Monitoring California Hardwood Rangelands—Harris, Frost, McDougald, George, and Nielsen Rainfall usually occurs between the months of October and March, but may occur earlier or later. Forage production varies considerably with the amount and timing of storm events. Monthly mean air temperature ranges from 43F in January to 81F in July. The study site is better described in Bentley and Talbot (1951) or Frost and others (1988). Figure 1—Fences on the San Joaquin Experimental Range showing the locations of range units used for the cattle distribution study. Livestock management prior to 1995 consisted of rotational management of small groups of cattle through the range units. In 1995, a system of high density-short duration grazing was implemented on the station. Cattle are moved as a large herd unit into each range unit for a short time, one to two weeks, to graze available forage. RDM evaluation and mapping is made before the first effective fall rains, usually in late September or early October. A double sampling method is used for determining RDM amounts. The double sampling consists of both visual estimation USDA Forest Service Gen. Tech. Rep. PSW-GTR-184. 2002. 89 Long-term Residual Dry Matter Mapping for Monitoring California Hardwood Rangelands—Harris, Frost, McDougald, George, and Nielsen and weight determination. Clipping and weighing a few plots periodically calibrates the visual estimation in the field. Weights are estimated in pounds per acre. The three comparative photos shown in U.C. Cooperative Extension Leaflet 21327 (Clawson and others 1982) are used to provide a consistent reference for delineating RDM classes. The entire station is surveyed in a single day. RDM is mapped on the three different slope classes at low, medium, and high amounts: 1) flat slopes and swales: <400, 400-800, and >800 lbs/acre, 2) gentle slopes: <600, 600-1,000, and >1,000 lbs/acre, and 3) steep slopes: <800, 800-1,200, and >1,200 lbs/acre. An additional class is used to delineate areas burned by wildfires. RDM classes are delineated on a topographical base map using color-codes. The RDM map is then converted to digital format by digitizing it into a GIS database (fig. 2).