Gelbard, Jonathan, and Susan Harrison. 2003. Roadless habitats as refuges for native grasslands: Interactions with soil, aspect, and grazing. Ecological Applications 13(2):404-415.
RELEVANT TO: GRASSLANDS/SHRUBLANDS
ROADS/OFF-ROAD VEHICLES
LIVESTOCK
INVASIVE SPECIES
DESCRIPTION
This study was undertaken to contribute understanding as to where remnant native grasslands occur and why, in California grasslands, which have been undergoing heavy invasion by exotics for 200 years.
Ninety-two sites in a 130,000-hectare inland California foothill grassland landscape were measured for numbers and cover of native and exotic grasses and forbs at distances of 10m, 100m, and >1000 m from roads. Soil type, aspect, and livestock grazing were recorded.
MAJOR FINDINGS
Results were different for non-serpentine and serpentine (i.e., low Ca ++:Mg ++ ratio, and low nitrogen and phosphorus) soils.
Non-serpentine grasslands:
- Cover and number of native species, and native grass diversity were greatest in sites >1000 m from roads and lowest in sites 10 m from roads, while exotic cover showed the reverse pattern
- Native cover was greater on cool and relatively flat slopes and on ungrazed compared to grazed cool slopes, but was not significantly affected by livestock grazing on warm or neutral slopes.
- Native cover was negatively correlated with litter cover
- Exotic cover was greater on warm slopes
"...[H]abitats distant from roads may provide a significant refuge for California's native grassland species on nonserpentine soils. (p. 412).
"Unfortunately, these results suggest that, on nonserpentine soils, exotic species are still continuing to spread, and natives are continuing to lose ground." (p. 412).
"We would have been unlikely to find significant effects if we had not stratified by soil type and aspect..." (p. 412).
Serpentine grasslands
- Native cover was greater on cool and warm slopes than on neutral, and greater on grazed sites.
- Exotic cover was greater in ungrazed sites.
- There was no effect of distance from roads on the numbers or cover of native or exotic species.
"In serpentine grasslands, grazing may benefit native species, especially forbs, by releasing them from competition with exotics [citations], but this effect may be less prevalent on steep slopes, where there are few exotic species to begin with." (p. 413). In other words, grazing may benefit native species on serpentine soils only because they have been invaded by exotics, and because exotics are less competitive on low-nutrient soils.
Both grasslands
- Two exotics, yellow starthistle (Centaurea solstitialis) and barbed goatgrass (Aegilops triuncialis) occurred more frequently in plots close to roads than in plots distant (100m and >1000 m), on both soils.
Only 15% of all grasslands were >1000 m from roads, and these grasslands composed only 1.5% of the landscape.
"Appropriate management...involves identifying a regime of livestock grazing that favors the persistence of natives over the continued spread of exotics [citations]." (p. 413).
"...[W]e speculate that the effects of distance from roads may be even more pronounced in less thoroughly invaded landscapes, such as remote grasslands and shrublands of the Colorado Plateau and Great Basin [citations]." Op. 413)
QUESTIONS RAISED FOR THE THREE FORESTS
- Have these Forests documented any soils or conditions on these Forests in which native grasses and/or forbs are favored by livestock grazing?
- Do these Forests have any reason/evidence to believe that the findings relating to exotic species and roads do not hold in these Forests?
- Do these Forests have a general model of where remnant native grasslands occur and why?
RELEVANCE TO FOREST MANAGEMENT
- Roads, off-road vehicle travel, and livestock grazing must be regarded as conduits for the introduction, establishment, and spread of exotic invasive species unless evidence is offered to the contrary, e.g., as in the case of exotic-invaded serpentine grasslands.