Together, these rapidly expanding species cover more than 15 million acres throughout California . The phenological development of medusahead and yellow starthistle is in part what makes these invaders so successful. Medusahead, and particularly yellow starthistle, mature late in the annual growing season . These species germinate after the first fall rains, with smaller germination events sometimes occurring later in the wet season . Although germination timing is similar to that of the surrounding grassland community, medusahead does not produce seed heads until late April or May, after most naturalized annuals have completed their life cycle . Yellow starthistle commonly produces seed heads in May and June; it begins flowering in June and can continue beyond October . In fact, many of yellow starthistle’s developmental stages generally extend well into the summer dormant period distinctive to Mediterranean climates . The later development periods enable medusahead and yellow starthistle to take advantage of late spring and early summer rains when they occur. When late-season moisture is present, medusahead and yellow starthistle will continue to grow after potential competitors have stopped, allowing them to dominate and dramatically alter the vegetation structure .Prescribed livestock grazing is commonly proposed as a low-cost, if not profitable, option to manage weedy species on rangelands. Prescribed grazing is the controlled implementation of the timing, vertical grow room design frequency and intensity of grazing to achieve specific goal, such as weed control. Small-scale grazing studies have examined the effects of livestock type , grazing intensity and grazing season on individual weed species .
These studies have consistently demonstrated that properly timed and intensive grazing can reduce medusahead cover by 30% to 100% and yellow starthistle flower heads by 75% to 90% . Experimentally manipulated livestock grazing has also been shown to enhance herbaceous diversity and native plant richness in vernal pools, interior annual grasslands and coastal grassland sites . However, there is little published work examining pasture-scale implementation of prescribed grazing to manage invasive weeds.Across California, rangeland managers have reported that livestock grazing can be managed to control medusahead and yellow starthistle . These findings are experiential rather than experimental — that is, based on direct implementation, observation and site-specific fine-tuning of intensity, season and frequency of livestock grazing to achieve specific goals. A recent scientific review of conservation effectiveness of rangeland management practices highlighted a critical need for the monitoring and reporting of practice effectiveness at the pasture scale . Collaborative, on-theground management implementation and monitoring will enable managers and researchers to better assess effectiveness and practicality of conservation practices such as prescribed grazing to control invasive weeds. Our objective was to assess the effect of a “real” prescribed grazing regime implemented by ranch personnel on medusahead and yellow starthistle populations on a Bureau of Land Management grazing allotment known as the Bear Creek Unit of the Cache Creek Natural Area.The Bear Creek Unit, located in Northern California’s interior coast range in Colusa County, is an 11,090-acre BLM-managed land that consists of a patch-mosaic of annual grasslands, blue oak woodlands and serpentine chaparral plant communities. The climate is Mediterranean, with hot, dry summers and mild, wet winters. Mean annual precipitation is 24 inches, and mean annual air temperature is 61°F .
Sites examined in this study ranged from approximately 1,200 to 1,600 feet in elevation. For this study, we targeted the annual grassland and blue oak woodland plant communities, as they provided the majority of forage on the management unit, and were dominated by the target weeds. In the study area, soils were largely formed from residuum of sandstone and shale , with a small inclusion of soils formed from alluvium . Common nonnative annual grasses include soft chess , slender oat and ripgut brome . This area also supports various native forbs, including miniature lupine , Ithuriel’s spear , owl’s clover Chuang & Heckard, mariposa lily and tidytips . Native grasses are widely scattered in the area, with purple needle grass being the most prominent native perennial grass. Medusahead and yellow starthistle are common across the landscape, with an emerging population of barb goatgrass also present.Until August of 2001, the Bear Creek Unit was continuously grazed throughout the growing season under grazing leases. The BLM, which acquired the Bear Creek Unit in 1999, terminated grazing in 2001 in an attempt to enhance native plant cover. In the 4 years following cessation of grazing, BLM monitoring teams reported increased invasive weed cover and high accumulations and persistence of vegetative litter, or thatch . In fall of 2006, average residual dry matter across the unit was estimated to be 4,200 pounds per acre. In working toward invasive weed control — one of BLM’s top management priorities — the BLM collaborated with local stakeholders to reintroduce grazing on the Bear Creek Unit in 2006. To target medusahead and yellow starthistle, we implemented a moderately stocked, rotational cattle grazing system across 11 paddocks, ranging from 80 to 600 acres in size. Paddocks were generally grazed January through May using cows calving between January and March — cattle on and off dates, stocking densities and paddock rotations were made at the discretion of the site manager based on factors such as drinking water availability, forage availability and cattle conditions . From 2006 to 2011, cattle numbers ranged from 318 to 520, averaging 392 total cows during the study period.
Grazing event duration ranged from several days up to 2 weeks, with two grazing events per paddock: one grazing from late November to February to reduce weed thatch, and allow alternative species to establish ; and one grazing event from March to June to target late-flowering invasives . By October of 2009, we estimated average RDM across the unit to be 1,400 pounds per acre, or approximately one-third the RDM observed under initial ungrazed conditions.Prior to reintroduction of cattle grazing, we established permanent paired plots in each of the 11 paddocks. Permanent plots were chosen in a random stratified manner to ensure sample sites were representative for each pasture. Exclosure plots measured 8 feet by 8 feet and were livestock proof. To examine shifts in plant species cover and abundance over the course of the study, we began monitoring plant community composition in June of 2006. At each set of permanent grazed and ungrazed paired plots, we estimated percent basal cover by species within a 10-ft2 hoop. Ocular estimates of herbaceous composition were collected after peak standing crop for both grazed and ungrazed plots in June of 2006, 2009 and 2011. This resulted in a total of 22 observations for each year, and 66 total observations for the study period. To determine if grazing management at the Bear Creek Unit significantly impacted medusahead and yellow starthistle over the course of the study, we used linear mixed effects regression to examine trends in cover of these species between grazed and ungrazed treatments. The dependent variables observed were percent medusahead and yellow starthistle cover, and the independent variables were treatment , year and the interaction between treatment and year. Within each treatment, we also examined changes in cover between the baseline and final evaluations for the most commonly occurring species: medusahead, yellow starthistle, soft chess, filaree , red brome , ripgut brome, slender oat and a composite functional group composed of several thatch-loving species including red brome, ripgut brome and slender oat. We used linear and generalized linear mixed effects regression models to test for differences in percent observed species cover between 2006 and 2011. For all analyses, site identity was included as a random term to account for repeated measurements . Standard diagnostic tests were used to check assumptions of linearity, normality and constant variance. Analyses were performed using STATA/SE 13.0 statistical software . To examine changes in overall plant community composition, clone rack we used non-metric multidimensional scaling . NMDS is an ordination technique widely used to examine patterns in multidimensional data and, unlike other ordination methods, makes few assumptions about the data. Species cover values were log-transformed, and NMDS scores were calculated based on a Bray-Curtis dissimilarity matrix . Analysis was conducted in the R software environment using the metaMDS routine from the vegan package . The metaMDS function selects several random start positions to find a global solution, so that it does not become trapped at local optima. The final configuration is rotated via principal components so that the first dimension explains the greatest amount of variance. NMDS was run for 2 through 6 dimensions, with the optimal number of dimensions selected via examination of a scree plot, which displays stress versus dimensionality for each solution .
To examine whether overall plant community composition significantly differed between grazed and ungrazed treatments, we used blocked multiresponse permutation procedures . MRBP provides a non-parametric test of multivariate differences between pre-defined groups, such as “grazed” and “ungrazed” plots . Observations were blocked by plot pair identification number, and species cover data were log-transformed. Our analyses showed that prescribed grazing applied to Bear Creek Unit did not impact yellow starthistle cover. Trends in basal cover of yellow starthistle did not significantly differ between grazed and ungrazed treatments , with no significant changes in yellow starthistle cover for either treatment between baseline and final evaluations . The lack of response to grazing may be due to a mismatch in the timing of grazing and the post-bolting/pre-flowering phenological stages of yellow starthistle. Since yellow starthistle matures and produces seeds later than other species, including medusahead, grazing late in the annual growing season is particularly important for effective suppression . In addition, yellow starthistle populations commonly exhibit multiple life forms simultaneously . This diversity creates an additional obstacle to suppression, because individual plants are not all susceptible to grazing at the same time. During this study, timing of cattle removal was dictated by real management considerations such as availability of water and desirable forage for livestock, which were both limited by May in most years. As a result, cattle were likely not present during the post-bolting/pre-flowering phenological stages when grazing can reduce yellow starthistle cover and seed production .Following baseline botanical evaluations, medusahead cover within grazed treatment plots fell by roughly half in 2009. Additionally, in 2009, medusahead cover in the grazed treatment was significantly lower than that observed in the ungrazed treatment . However, by the final evaluation , medusahead cover for both grazed and ungrazed treatments converged to similar levels. As with yellow starthistle, research has shown that grazing late in the growing season is critical to successful medusahead control . However, medusahead develops earlier in the spring than yellow starthistle and does not exhibit yellow starthistle’s diversity of life forms. Medusahead’s earlier maturing phenology narrows the window for grazing to achieve suppression. Although managerial constraints in this study made it impossible to graze late enough into the season to impact yellow starthistle, medusahead populations were impacted in several years. The differential reduction of medusahead cover in the grazedtreatment between the periods 2006 through 2009 and 2009 through 2011 is potentially explained by three interacting factors: timing and amount of rainfall; timing of cattle removal each spring; and ability of medusahead to recover from grazing and produce seed. During the period 2006 through 2009, when medusahead cover was significantly reduced in the grazed treatment , late spring and/or total annual rainfall were substantially lower than reported long-term averages in 2007 and 2008. Lack of late season precipitation created dry soil moisture conditions at the end of the grazing season, which potentially diminished the ability of medusahead to recover from grazing and produce new seed heads , which is why the plant reduction created in 2008 is apparent in 2009. With the exception of 2006 and 2010, cattle were removed from the management unit between May 22 and 27. The lower late season rainfall, and resulting depleted soil moisture levels, may have created a multi-year window of opportunity in which the timing of grazing overlapped with the most susceptible phenological stages of medusahead development. In contrast, late spring rainfall during the period 2009 through 2011 was well above the reported long-term average, which potentially enhanced the ability of medusahead to respond to postgrazing conditions. Although the timing of cattle removal was similar to that of the 2006–2009 period , this late season rainfall enabled medusahead plants to recover from grazing disturbances and produce new inflorescences.