A Half Century of Change to the Flora of a Hardwood Rangeland in Northwest California
Oaks ’n’ Folks – Volume 15, Issue 1 – March 2000
The 2,168 ha University of California Hopland Research and Extension Center (HREC) in southeastern Mendocino County, California, was established as a range experiment station in 1951. During the following year, a botanical survey was initiated to document all the vascular plants on the property. By 1954, nearly 450 species were recorded on the field station, and by 1996 the number had grown to 611. Alfred H. Murphy, the station’s first superintendent, and Harold F. Heady, a UC Berkeley professor of range management, initiated the inventory and together contributed 1,177 specimens to the HREC herbarium, about 92% of the collection. They published a list of the vascular plants for HREC in 1983.
Between the spring of 1996 and the summer of 1999, an extensive inventory of vascular plants was conducted throughout HREC to compare the flora documented in the herbarium to that present today. Visits were made to all known collection sites or areas and timed to coincide with historic collection dates as described on the original herbarium labels. In addition, we were able to obtain comparable abundance estimates from three time periods: the early 1950’s, early 1980’s, and late 1990’s.
As a result of our inventory, 51 species (37 native and 14 exotic) in 28 families were added to the flora (Table 1), bringing the total number of species and infra-specific taxa at HREC to 662. The aster family was the most represented with 6 native and 4 introduced species, followed by the celery, sedge, and figwort families, with 4 species each. Of the 37 native species, 21 were found in ungrazed areas, 5 in lightly grazed, 7 in moderately grazed, and 4 in heavily grazed. Of the 14 non-native species, 2 were found in ungrazed areas, 3 in lightly grazed, 2 in moderately grazed, 6 in heavily grazed, and 1 found throughout all grazing regimes.
Of the original 611 species documented at HREC before this study, 41 native and 2 exotic species could not be relocated. Roughly half of these were in the grass, aster, and sedge families. The grass family lost the greatest number of species, with 9 native and 1 exotic species not relocated. In general, species loss occurred across a wide variety of habitat types and grazing regimes, and was mostly confined to species of low abundance. Eleven of the missing species occurred on sites that receive heavy grazing pressure, 6 were lost in annual grassland, and 5 in wetland habitats.
Herbarium records indicate that in 1954, 90 introduced species were present at the field station. Of these, 22 were considered “numerous” at their collection site. Thirty years later, 136 introduced species were reported and by 1999 the number had grown to 154 (Fig. 1). A third of the introduced species are annual grasses, followed by annual forbs primarily of the aster and pea families. University staff and researchers intentionally introduced at least 19 eotic plant species to the field station. Some of these include smilo grass (Piptatherum miliaceum), Harding grass (Phalaris aquatica), orchard grass (Dactylis glomerata), rose clover (Trifolium hirtum), and sub-terranean clover (T. subterraneum). Many were seeded into pastures following fire or after clearing to improve range forage and have since expanded their ranges well beyond the initial introduction sites. Today, introduced plant species at HREC comprise 23.3% of the total flora compared to 18.8% in the early 1950s.
Many introduced species that were either absent or sparse during the early 1950s have since undergone dramatic increases in abundance. First collected at HREC in 1969, pennyroyal (Mentha pulegium), an exotic herbaceous perennial, now dominates the margins of several permanent ponds and seeps in grazed and ungrazed sites. Barbed goat-grass (Aegilops triuncialis), another extremely invasive plant, was first collected at HREC in 1961 and now is the dominant species on approximately 125 ha of serpentine grassland. The barbed spikelets readily attach to sheep wool, facilitating its spread to disturbed sites and areas with infertile soils. Japanese hedge-parsley (Torilis arvensis) was recently collected in 1996, which suggests a rapid increase in abundance over the past 10 years. This plant is common and widespread in Mendocino County and is rapidly spreading throughout California, aided by its copious production of prickly fruits. Big quaking grass (Briza maxima) and hedgehog dogtail (Cynosurus echinatus) were recorded as “sparse” and “occasional”, respectively, in 1983, and now dominate the understory of oak woodlands on several south and southwest-facing slopes at HREC.
Research has shown that the combination of heavy livestock grazing and the establishment of invasive species within native plant communities can result in the loss of species diversity, alter successional patterns, and change physical characteristics of ecosystems and ecosystem processes. The potential effects of these threats may play a role in the observed species loss at HREC. This is supported by the fact that 11 of the 43 species that were not relocated occurred on sites that were heavily grazed. Loss of these species was associated with concomitant increases in big quaking grass, hedgehog dogtail, and barbed goatgrass in grasslands and increases in velvet grass (Holcus lanatus) and pennyroyal at wetland sites.
An additional 5 species are believed extirpated from HREC due to vegetation type conversions at the three sites where they originally occurred. Pine grass
(Calamagrostis rubescens) and an uncommon California endemic, serpentine collomia (Collomia diversifolia), were last collected in the spring of 1957. In the summer of 1956, the woody vegetation of the 26 ha watershed where these plants occurred was removed, burned, and later seeded with a mixture of non-native clovers and grasses. Semaphore grass (Pleuropogon californicus) and popcorn flower (Plagiobothrys stipitatus var. micranthus) were last collected in 1952 and 1953 along a seasonal watercourse. Since then, this pasture has been tilled and seeded numerous times with non-native grasses and clovers. Reed grass (Calamagrostis koelerioides) likely occurred on a chaparral site that has since been cleared and seeded with non-native grasses and forbs.
The remaining 27 species were not relocated were originally recorded as “rare” or “sparse” and may have been overlooked during our inventory. These species were found in upland areas with little or no grazing; in addition, there was nothing particularly atypical about the species composition at the original collection sites that would suggest species replacement.
This work represents the first step in documenting the floristic changes that have occurred at HREC since the 1950s. To better demonstrate the spread of invasive species, a monitoring program should be established that includes mapping the distribution of exotic species through time using the existing geographic information system for HREC. Continued surveys of the site will help verify local extinction and document new losses, as well as new additions, to the flora. While we implicate shifts in land-use as a likely cause for the observed changes to the flora, long-term experiments on the effects of livestock grazing and species introductions are essential to test the direct contribution that grazing and exotic species have on native flora. Also, a quantitative study comparing vegetation composition and abundance in similar plant communities on comparable soil types between grazed and ungrazed sites would be useful. Studies of this sort would complement this one and provide a better understanding of species turnover and the associated causes in north coast hardwood rangelands.
TABLE 1. RESULTS OF A 1996-99 PLANT INVENTORY AT THE
U.C. HOPLAND RESEARCH AND EXTENSION CENTER.
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prepared and edited by Richard B. Standiford, Justin Vreeland, and Bill Tietje