Restoration of California sage scrub

Introduction

The purpose of this symposium is to bring together the results of work done by CSULB biogeographers on California sage scrub in Southern California. The team has included three summers of field and lab work done by the NSF-funded Geoscience Diversity Enhancement Program (or GDEP), led by Paul Laris and myself, several master's theses, a biogeography graduate seminar, and several sections of courses in biogeography, landscape restoration, and field methods.

My remarks will provide an overview and context. [ Viewgraph 2 ] California sage scrub is a scrub vegetation dominated by smaller shrubs and subshrubs that form a more open canopy than does chaparral. Its adaptations to summer drought include a greater emphasis on pubescent leaves and shallower root networks than typical in chaparral, and several species have developed facultative summer deciduousness. [ Viewgraph 3 ] The result is a visually distinct vegetation that in summer is predominantly grey to tan. [ Viewgraph 4 ] The bulk of it is found near the coast in Southern California, where fog drip can be an important moisture source during the summer. [ Viewgraph 5 ] A significant amount of it is also found inland away from the coast in such locations as the San Fernando Valley, San Gabriel Valley, and western San Bernardino and Riverside counties. It is commonly called coastal sage scrub along the coast and interior sage scrub inland. We'll usually refer to the whole complex as California sage scrub or CSS.

[ Viewgraph 6 ] CSS has markedly declined in Southern California in the face of grazing and associated clearing, plowing, construction, residential landscaping practices, changes in fire regimes, and air pollution. Dwindling to an estimated 10-15% of its previous range, the loss of CSS has threatened a number of endangered animal species with loss of habitat. [ Viewgraph 7 ], [ Viewgraph 8 ], [ Viewgraph 9 ], [ Viewgraph 10 ]. Among these are the California gnatcatcher (Polioptila californica), Palos Verdes blue butterfly (Glaucopsyche lygdamus palosverdesensis), the coastal cactus wren (Campylorhynchus brunneicapillus), and several others. The belated recognition of the vital habitat functions played by CSS has led to attempts to conserve and restore it and to understand the forces converting it to other land cover types.

[ Viewgraph 11 ] A common type conversion is from CSS to exotic-dominated annual grassland, which is often strikingly able to maintain its dominance long past the disturbance that originally led to its replacement of CSS. Several hypotheses have been put forward to explain the persistence of the grassland conversion.

CSS Suppression

[ Viewgraph 12 ] Fire regimes are altered by the new light fuel that can grow rapidly in winter and spring, move quickly to reproduction, and then die off by summer, encouraging the rapid spread of fire. More frequent fire favors grasses, which have already completed their life cycles when summer and fall fires strike and consume their dead biomass. More frequent fires can exceed the ability of CSS to stump sprout and maintain seed banks dense enough to regenerate the CSS species mix.

[ Viewgraph 13 ] Nitrogen fertilization has shown an intriguing connection to CSS suppression and grassland persistence. Annual grasses and exotic forbs seem better able to utilize nitrogen than native shrubs and forbs in CSS, and native gardening nurseries strongly recommend not using nitrogen fertilizer. Nitrogen fertilization occurs due to the deposition of nitrous oxides in air pollution, and research on interior sage scrub has shown marked spatial correlation between CSS degradation and grassland type conversion in San Bernardino and Riverside counties. Our own work on Palos Verdes and the Santa Monicas has not shown any such effect, perhaps because of the cleaner air by the coast.

[ Viewgraph 14 ] Allelopathy has been posited to explain the persistence of Brassica nigra and Hirschfeldia incana mustards in annual grassland and the inability of CSS to recolonize mustard-infested grasslands. Our own work evaluating the effect of mulched dead mustard stalks on the germination and growth of California poppies proved inconclusive.

[ Viewgraph 15 ] Grazing has often been cited in the decline of CSS and the persistence of grasslands. CSS was actively cleared to create rangeland and pastures, and the combination of soil compaction, accelerated erosion, uprooting by livestock, and active planting of forage species prevented recolonization by CSS. The question was whether the damage would lead to permanent type-conversion. Hearteningly, several studies, including those done by the CSULB group, show that CSS can reclaim territory once grazing ends.

[ Viewgraph 16 ] Less hearteningly are findings about the permanence of mechanical disturbance of the soil, as by plowing. Our own work indicates that, if CSS is cleared and the soil plowed even once, grasses and associated exotic forbs, notably black mustard and fennel, will not relinquish a site to CSS. Paul Laris found a site high on a steep slope near Del Cerro Park in Palos Verdes that had been plowed once around 1928 and, though never used again, the field is still discernible by the black mustard dominating it.

[ Viewgraph 17 ] Soil factors may have something to do with it, but there's no unambiguous signal. Soils occupied by grasslands tend to be softer and have a higher clay and silt content than contemporary CSS soils. Whether this has anything to do with CSS preference for edaphic conditions is moot: It is more likely that farmers selected the softer and more fertile soils for their fields and avoided rockier and more gravelly soils and outcrops to avoid damaging their equipment. pH does not seem to vary much between CSS and grassland, and nitrogen fertilization does not distinguish recovering from stable boundaries.

[ Viewgraph 18 ] It is possible that the extreme persistence of grassland in once plowed soils may reflect the disruption of subsurface symbioses between native plant roots and various mycorrhizal fungus species. Weedy exotics and many grasses get along fine without mycorrhizal infections and, so, may establish and maintain dominance over a plowed site. Many native shrubs and forbs strike up symbioses with particular mycorrhizal species and may be unable to compete with exotics without their symbiotic partners. The mechanical disturbance of plowing may destroy these subsurface relationships. One of the papers in this session gives a progress report on evaluating this idea.

CSS Restoration

[ Viewgraph 19 ] Concerted efforts to restore CSS have often been disappointing, as grasses and associated exotic forbs reclaim restoration sites. Conservancies struggle with trying to find the right way to re-establish CSS, trying different soil treatments, different palettes of plant species, various irrigation systems, and a lot of weeding. There have been successes, such as the Trump Golf Course, which is de facto an intensively gardened landscaping project, way past the resources of most volunteer organizations. There have been a lot of failed projects or projects that are only partially successful, including one in the Tijuana watershed documented in a thesis by Samantha Antcliffe. Active restoration can be a frustrating slog.

That said, there are a number of places where CSS has been able to encroach back onto grasslands on its own. CSULB Geography became involved in this through Scott Eckardt's thesis, which used air photos and remote sensing to track CSS decline in Calabasas. He found patches of CSS that were actually expanding in the face of overall decline. Kyra Engelberg worked in the Serrano and La Jolla valleys in the westernmost Santa Monica Mountains, identifying sites through air photos and satellite imagery where CSS and grass had maintained an unchanging boundary for decades and others where CSS was pushing into grassland. The GDEP team and subsequent classes have gone out there transecting these border types at sites she selected, in order to characterize their flora and succession processes and the relevance of slope, soil factors, underlying geology, and subsurface communities. You'll be hearing progress reports on much of this today.

Applications for CSS Active and Passive Restoration

[ Viewgraph 20 ] Understanding the reasons for CSS self-restoration is critical for the success of landscape restoration programs. If we can identify conditions correlated with CSS self-restoration, conservancies can identify sites in their holdings or potential acquisitions where passive restoration might occur with little demand on their scarce labor and material supplies.

There may be significant differences in the CSS species mix found behind a stagnant boundary and behind an expanding boundary. Also, we have been censusing CSS species found out in front of the expanding boundaries, CSS species that can tolerate the conditions in the grasslands and perhaps serve as nurses fostering other CSS species. It is possible that active restoration efforts could be designed to mimic a succession process modeled on the expanding CSS boundaries, planting only vanguard species at first, later adding species found disproportionately behind expanding CSS boundaries, and then adding the rest of the local CSS species palette.

A cautionary note emerges from the findings about the permanent type conversion created by plowing and other forms of mechanical disturbance and soil overturn. Some active restoration sites are prepared first by soil scraping, which removes topsoil and weed seed banks, presumably to cut down on exotic weed recruitment back to the site to give native plantings a fighting chance. To the extent that this mechanically disturbs soil, it may lead to enduringly poor outcomes. Soil scraping and plowing may have this unintended consequence by disrupting mycorrhizal communities below ground.

If mycorrhizæ prove to be suppressed in grassland and ubiquitous in CSS, or, alternatively, if mycorrhiza are disrupted in once plowed or scraped sites, active restoration needs to inoculate new sites with appropriate species of symbiotic fungus. Mycorrhizæ: are available for purchase from suppliers Back East, but there is some evidence from a variety of ecosystems that there are particular mutualisms involved and a generic inoculum may not have the intended effect.

Rest of the Symposium

Kyra Engelberg and Paul Laris will summarize transects taken in Serrano and La Jolla valleys and present results about slope, soil texture, and species mixes in the stable and shifting boundary zones (.ppt).

Paul Nesbit and Scott Winslow will summarize a project they did to analyze the association between geological substrate and stable CSS-grassland boundary zones (.ppt).

Mystyn Mills and Paola Ducoing Chaho will present work she's done on the presence of mycorrhiza on rootballs and soil samples taken in La Jolla Valley (.pptx).

Sean Brennan, Chelsea Vaughan, and Carin Tabag will summarize work they did in the westernmost La Jolla Valley to assess the rôle of herbivory on the expansion of CSS (.pptx).

Sean Brennan will also present work he's done evaluating the rôle of Baccharis pilularis as a vanguard species and CSS facilitator (.pptx).

Jade Dean will outline the work she did on the differences in species mixes with distances from the CSS-grassland boundary in both expanding and stable boundaries (.pdf).

Paul Laris will wrap up, summarizing the overall findings from the symposium papers and from another paper given a week ago at another conference and drawing out conclusions for land managers and restoration ecologists (.ppt).

Note: The other paper discussed by Laris was presented at the CSULB College of Liberal Arts Student Research Showcase by Abel Santana, Lourdes Johanna Avelar-Portillo, and Joseph Diminutto and evaluated gravitational bias in CSS self-restoration (.odp).

Selected References

Allen, Edith B.; Padgett, Pamela E.; Bytnerowicz, Andrzej; and Minnich, Richard. 1998. Nitrogen deposition effects on coastal sage vegetation of Southern California. USDA Forest Service General Technical Report PSW-GTR-166: 131-139. Available at http://gis.fs.fed.us/psw/publications/documents/psw_gtr166/psw_gtr166_002_allen.pdf. .

Allen, Edith B.; Eliason, Scott A.; Marquez, Viviane J.; Schultz, Gillian P.; Storms, Nancy K.; Stylinski, Cathlyn Davis; Zink, Thomas A.; and Allen, Michael F. 2000. What are the limits to restoration of coastal sage scrub in Southern California? In 2nd Interface between Ecology and Land Development in California, ed. Jon E. Keeley, Melanie Baer-Keeley, and C.J. Fotheringham. U.S. Geological Survey Open-File Report 00-62: 253-262. Article available at http://www.faculty.ucr.edu/~eallen/allenetal_2000.html (full anthology available as a 118 meg PDF at http://pubs.usgs.gov/of/2000/of00-062/of00-062.pdf).

Bell, David T., and Muller, Cornelius H. 1973. Dominance of California annual grasslands by Brassica nigra. American Midland Naturalist 90, 2: 277- 299. JSTOR URL: http://www.jstor.org/stable/2424453.

Cox, Robert D., and Allen, Edith B. 2008. Stability of exotic annual grasses following restoration efforts in southern California coastal sage scrub. Journal of Applied Ecology 45, 2: 495-504. doi: 10.1111/j.1365-2664.2007.01437.x.

Fleming, Genie M.; Diffendorfer, James E.; Zedler, Paul H. 2009. The relative importance of disturbance and exotic-plant abundance in California coastal sage scrub. Ecological Applications 19, 8: 2210-2227.

Hobbs, E.R. 1986. Characterizing the boundary between California annual grassland and coastal sage scrub with differential profiles. Vegetatio 65: 115-126.

Keeley, Jon E.; Fotheringham, C.J.: and Baer-Keeley, Melanie. 2005. Determinants of postfire recovery and succession in Mediterranean-climate shrublands of California. Ecological Applications 15, 5: 1515-1534.

Stinson, Kristina A.; Campbell, Stuart A.; Powell, Jeff R.; Wolfe, Benjamin E.; Callaway, Ragan R.; Thelen, Giles C.; Hallett, Steven G.; Prati, Daniel; and Klironomos, John N. 2006. Invasive Plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms. PLOS Biology 4, 5: e140. doi: 10.1371/journal.pbio.0040140. Available at http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040140.

Talluto, Matt V.; Suding, Katherine Nash; and Bowler, Peter A. 2006. Factors affecting understory establishment in coastal sage restoration. Madroño 53, 1: 55-59.

Vogelsang, Keith M., and Bever, James D. 2009. Mycorrhizal densities decline in association with nonnative plants and contribute to plant invasion. Ecology 90, 2: 399-407.

Yoshida, Lidia C., and Allen, Edith B. 2001. Response to ammonium and nitrate by a mycorrhizal annual invasive grass and native shrub in Southern California. American Journal of Botany 88, 8: 1430-1436.