The effects of grazing intensity on annual vegetation.

Abstract

Pastures grazed by sheep at moderate and 1%, 2-, and 2% times the moderate stocking rate from 1969-1973 were analyzed for relative changes in cover, herbage productivity, and botanical composition. All four pastures were less productive in 1973 than in 1969, but exhibited similar trends in cover and botanical composition regardless of grazing intensity. Only grazing at 2 ‘/z times the moderate stocking rate produced a residual decline in productivity following 1 year of rest from the grazing treatment. However, this decline in productivity was managerially negligible compared to other stocking rates, and would probably disappear within 2-3 years in response to the overriding influence of annual weather, especially precipitation, patterns. Hormay (1944) suggested that moderate stocking rates provided the best grazing technique in the annual type. Old vegetation left on the ground at the end of each grazing season would enhance soil fertility, provide progressively improved range condition, and ultimately promote high livestock weight gains. Grazing either greater or less than this moderate level would produce changes in both total herbage productivity and relative botanical composition. Heady ( 1958) postulated. that grazing occurred first on taller plant species, thereby increasing the relative proportion of shorter plant species. Alternatively, complete elimination of grazing animals encouraged taller annual plants relative to short plant species. Talbot et al. ( 1939), Talbot and Biswell ( 1942), and Jones and Evans (1960) all found that exclusion of grazing animals quickly led to grass dominance, particularly taller species, such as ripgut (Bromus rigidus). Biswell (1956) held the opinion that this increase in ripgut occurred at the expense of true clovers (Trifolium spp.) and bur-clover (Medicago hispida), with plant succession proceeding from forbs to soft chess (Bromus n&is), to wild oats (Avena spp.) to ripgut. Biswell (1956) hypothesized that the impact of grazing on botanical composition in the annual type operated primarily by altering the accumulation of mulch. Heady (1958, 1961) supported Biswell’s hypothesis by demonstrating that mulch exceeding 700 pounds per acre in Northern California encouraged taller grasses such as soft chess and ripgut. With no mulch accumulation small, unpalatable forbs such as goldfields (Bueriu chrysostomu), smooth cat’s ear (Hypochoeris glabra), and owl’s clover (Orthocurpus eriunthus) proliferated. With small amounts of mulch, diminutive, low-forage-value grasses such as little quaking grass (Briza minor) and silver hairgrass (Aira caryophylfea) abounded. Annual fescues (Festuca spp.) Authors are assistant professor of’ range ecology. University of British Columbia, Department of Piant Science, Vancouver, Canada V6T IW5; and professor of range management, University of California, Berkeley 94720. Manuscript received February 10, 1979. and nitgrass (Gastridium ventricosum) peaked in percent botanical composition with intermediate amounts of mulch, while filaree (Erodiurn spp.) was not significantly influenced by varying amounts of mulch. The amount of mulch present during germination exerts a major influence on total herbage productivity as well as botanican1 composition. Hooper and Heady (1970), at the Hopland Field Station in Northern California, discovered that removing all mulch prior to germination drastically reduced subsequent forage production compared to sites where mulch was not removed. Importantly, a single manipulation stimulated this response, and a moderate amount of mulch produced maximum herbage productivity. Although mulch influences botanical composition and herbage productivity of the annual type within a particular growing season, these differences are temporary and tend to persist only so long as the grazing and/or clipping treatments are maintained. At the San Joaquin Experimental Range, both Talbot and Biswell (1942) and Bentley and Talbot (1951) noted no pronounced, long-term effects in total yield as a result of grazing at both light and heavy stocking rates. However, these results cannot be readily applied to other areas in the annual type where forage production potentials are confounded by local seasonal variability and/or microsite differences (Biswell 1956; Heady 196 1; Smith 1970). Rossiter (1966) concluded that such microsite variabilities accounted for the lack of quantitative data describing the long-term impact of stocking rates on productivity and botanical composition throughout much of the annual type. This paper provides data describing the residual impact of gmzing on annual vegetation occurring in the coastal mountain ranges of Northern California. Location and Methods of Study The study was conducted at the Hopland Field Station located in Mendocino County in the central portion of the coast mountain ranges. The climate of the area is subhumid to humid mesothermal. The winters are mild with occasional frost in valley bottoms, and infrequent, light snow at higher elevations. The average annual rainfall is 89 cm with virtually no rain falling between June and September. Fog occurs frequently in valley bottoms during late fall, winter, and early spring, and occasionally during the summer. Average summer and fall temperatures range from 20-25°C (Pitt 1975). The pastures used in this study were initially fenced in 1953 to investigate the impact of continuous versus rotational grazing on both animal performance and vegetational response (Pitt 1975). Beginning in 196 1, these pastures enclosing approximately 11 ha each were calibrated to equalize grazing intensity. Equal grazing intensity among pastures Sl, S2, S3, and S7 occurred when: (1) lamb weights at 120 JOURNAL OF RANGE MANAGEMENT32(2), March 1979 days were similar in all pastures, and (2) equal ewe weight loss at the end of the dry period occurred in all pastures. The following grazing treatments existed in the summer of 1962: S 1, S3, and S7 contained 13 sheep, while S2 supported 20 sheep. These pastures operated on a yearlong basis throughout the calibration period during which time numbers of animals were adjusted in each pasture to obtain as near equal gains and losses as possible per individual among all pastures. This grazing intensity was on the heavy side of stocking so that animals were under stress and their weights reflected pasture effects rather than effects due to inherent properties of individual animals. The number of sheep in each pasture at the end of the calibration period in the summer of 1964 constituted a standard stocking intensity for all future experiments. The study investigating effects of different grazing intensities on annual vegetation began in the 1968-1969 grazing season. The calibrated stocking rate continued in pasture S3, while 1X-, 2-, and 2’/2-times the previous calibrated stocking rates were introduced into pastures S 1, S7, and S2, respectively. All pastures were grazed on a continuous yearlong basis with the same animals, insofar as possible. Vegetation was sampled twice each year: once at the beginning of the growing season in the fall of each year, and again at the end of the growing season in June. Each pasture contained 24 sampling locations consisting of a 1 -m diameter wire mesh exclosure and an outside stake. Each exclosure occurred in a homogeneous sampling unit approximately 8 m in diameter. Exclosure and associated outside stakes were moved prior to each fall sampling date to a different but vegetationally similar spot within this sampling unit. Changes noted in botanical composition or herbage productivity throughout the course of the study were therefore not the result of repeated sampling of the same location. Herbage productivity of each pasture was estimated by clipping a square plot 0.099 m” in size (1 ft’) to ground level at each sampling location, while botanical composition, cover, and height of annual vegetation were estimated with 30 points taken with a IO-point frame. All of these parameters, both inside and outside the exclosures, were estimated at the June sampling date, while only outside herbage productivity was measured at the fall sampling date. Vegetational Responses Tables 1,2,3, and 4 summarize data collected in pastures S3, Sl , S7, and S2, respectively. Only those values for ungrazed June vegetation are listed. Vegetational parameters measured in June, the end of the growing season, characterize the yearly response of annual vegetation to each associated animal stocking rate. Measurements in ungrazed vegetation characterize the carryover effect of stocking rate from one year to the next. Since the cages are moved each year, ungrazed vegetation reflects the residual impact of stocking rate 1 year following removal of gxazing. Alternatively, measurements in grazed vegetation primarily reflect vegetational variability within a single growing or grazmg season and may not indicate the impact of stocking rate over time. Ungrazed June vegetation in 1969 represents baseline values and not the result of any stocking rate treatments. Ungrazed June vegetation in 1973 represents values resulting from 4 years of stocking rate treatment followed by 1 year of rest. Table 5 summarizes results from a two-way analysis of variance analyzing the impacts of stocking rate and yearly variability on herbage productivity, cover, and botanical composition. Ungrazed June vegetation in 1969 was not subjected to any grazing treatment. Therefore, only 4 years of data, ungrazed June vegetation for the years 1970 through 1973 inclusive, were used in this analysis. Many researchers have described the dramatic impact of yearly weather patterns on annual vegetation (Talbot et al. 1939; Bentley and Talbot 195 1; Heady 1956; Heady 1958; Naveh 1967; McNaughton 1968; Hooper and Heady 1970; Murphy 1970; Duncan and Woodmansee 1975; Pitt 1975). These studies all indicate that observed trends in standing crop, cover, and botanical composition in