Slide 1

Differences in California sage scrub composition
behind stable and recovering boundaries with annual grassland

Association of Pacific Coast Geographers
Palm Springs, CA, 24 October 2015

Christine M. Rodrigue

Department of Geography
Environmental Science and Policy Program
California State University
Long Beach, CA 90840-1101
rodrigue@csulb.edu

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Introduction

While California is recognized as a biodiversity hotspot, many of the vegetation formations here have experienced massive losses to agricultural and urban, suburban, and exurban development, air pollution, and anthropogenic climate change. These include the focus of this paper: California sage scrub. Slide 2

California sage scrub or CSS is a distinctive complex of shrubs, subshrubs, and herbaceous plants, generally forming a rather open scrub, dominated by shrubs and subshrubs about half a meter to 2 meters in height. It contrasts visually with chaparral scrub, not only in terms of density and height, but in color, especially during summer months. CSS plants are often greyish or a beige, tan, or taupe color. This brownish appearance results from facultative summer deciduousness. Here are a few images of CSS Slide 3 Slide 4 Slide 5

CSS occurs in a few subtypes depending on coastal or interior locations, extending from the Bay Area down into Baja. Slide 6 The entire complex has seen very substantial territorial losses, Slide 7 declining by half since the 1930s (Talluto and Suding 2008) and estimated to occupy only about 10-33% of its original extent (Westman 1981; Allen et al. 2000). Human pressures effecting this loss include early ranching and grazing activities, plowing and cultivation, residential development, alterations in fire regimes, and soil nitrogen deposition due to air pollution. Slide 8 Very commonly, patches of CSS will be type-converted to exotic annual- dominated grassland, itself subject to further invasions by very aggressive tall forbs. Slide 9 Once type-converted, exotic annual grassland is very persistent and successfully resists re-encroachment by CSS species or native perennial grass species (Vann-Foster 2015).

This reduction in area has cost several animal species critical habitat, putting several on endangered and threatened species lists. Slide 10 These include the California gnatcatcher (Polioptila californica), Slide 11 the coastal cactus wren (Campylorhynchus brunneicapillus), Slide 12 Stephen's kangaroo mouse (Dipodomys stephensi), Slide 13 Belding's orange-throated whiptail lizard (Aspidoscelis hyperythra beldingii), and many others.

Realization of the habitat function of CSS has spurred efforts to identify and protect intact CSS and to restore sites. Restoration projects have generally been disappointing, with the planted CSS overwhelmed with exotic grasses and forbs and dying off (Cox and Allen 2008; Antcliffe 2009). As Allen et al. (2000) put it, "CSS . is becoming one of the most intractable vegetation types to restore."

Slide 14 Work at CSULB has uncovered a few heartening exceptions to the general gloom surrounding CSS restoration efforts. In 2006, Scott Eckardt utilized decades of air photos and remote sensing imagery to plot the decline of CSS in Calabasas. He documented the decline of CSS there but also unexpectedly identified small sections in his study site where CSS had actually expanded on its own. Slide 15 These small areas of recovering CSS have since been spotted in several other areas in Southern California, as seen in this image of Charmlee Park in Malibu. Note the area on the north side of the CSS patch that expands aggressively while nearby to the south the patch holds stable throughout the timeline.

The idea that CSS could self-restore became the focus of several projects. Slide 16 The Geoscience Diversity Enhancement Project (NSF Award # 0703798) there organized research one summer to identify differences between long-stable boundaries between CSS and grasslands and boundaries where CSS was recovering, this in the Serrano and La Jolla valleys of the westernmost Santa Monicas. These data were collected on behalf of Kyra Engelberg, who selected the sites using air photos and remote sensing imagery to classify boundaries as stable or recovering. Her thesis (2011) linked the different boundaries to disturbance histories. Stable boundaries were more often associated with sites having a history of plowing, while areas having a grazing history were likelier to host recovering boundaries. Her work and one of the GDEP projects (Lough et al. 2010) found differences in plant communities between recovering and stable boundaries.

Slide 17 My graduate seminar in 2012 returned to La Jolla Valley to collect more transects and quadrats and test hypotheses about other contrasts between stable and recovering boundaries, including facilitating species, hard rock geology, soil classifications, slope and aspect, herbivory, and mycorrhizal communities (Rodrigue et al. 2013). Several classes in the Department of Geography and the Environmental Science and Policy Program have since continued to collect transect and quadrat data at several sites throughout Southern California.

Data and Methods

I have assembled the seven years of data into a common database. Slide 18 The database so far includes 1,093 transect sampling points from 49 transects, with 56 species identified. Of these 1,093 sampling points, 496 are from recovering boundaries and 597 are from stable boundaries. Sites include La Jolla Valley, Serrano Valley (both in Pt. Mugu State Park in the westernmost Santa Monicas), Charmlee Park (Malibu), Stoney Point Park (Chatsworth), Sepulveda Dam (San Fernando Valley), Palos Verdes Peninsula, and Bolsa Chica Wetlands in Huntington Beach.

Slide 19 There may be shortcomings: The data were collected by teams of students, so there may be errors in identification. There are different types of sampling systems used, and there were different goals in getting students into the field.

Slide 20 The sites in the western Santa Monica Mountains have been the ones most explicitly concerned with the stable/recovering boundary divergence,so, for this case study, I extracted these three sites: La Jolla Valley, Serrano Valley, and Charmlee Park. This yielded 201 identifications from recovering boundaries and 126 from stable boundaries. Gamma diversity for this cluster was 26; alpha diversity for La Jolla Valley was 20, for Serrano Valley it was 12, and for Charmlee Park it was 9.

Slide 21 The species richness, however, is not produced by an even relative abundance of species. There are, rather, 9 CSS shrub, subshrub, or bunchgrass species that are quite common (with more than 13 individuals counted in each), all 9 found in La Jolla Valley, 5 in Serrano Valley, and 6 in Charmlee Park. The Shannon Equitability index is highest for La Jolla Valley at 0.83, declining to 0.78 for Serrano Valley, and to 0.64 for Charmlee Park.

Slide 22 Here are the 9 dominant species. Given that these are less subject to small sample effects, I used these 9 to analyze differences in the native plant community in the 5 following contexts: Slide 23

  • community behind the recovering boundary or ecotone
  • community behind the stable boundary
  • individual CSS species in the transition area, up to 25 m out from recovering boundaries
  • individual CSS species in the transition area, up to 25 m out from stable boundaries
  • CSS individuals found isolated in the middle of grassland (these were not transected but individually hunted and GPSed)

Results

Slide 24 The first comparison was of the frequencies with which each of the 9 species occurred behind recovering and stable boundaries and the distribution of frequencies among all native species found behind the two boundary types. Three species significantly favored recovering CSS boundary zones: Baccharis pilularis, Artemisia californica, and Eriogononum fasciculatum. Another 3 occurred significantly more frequently behind stable boundaries: Salvia mellifera, Eriogonum cinereum, and Malosma laurina. The remaining 3 show no significant bias: Stipa pulchra, Salvia leucophylla, and Mimulus aurantiacus. So, each boundary type has a triad of species that markedly favor it.

Slide 25 The second comparison examined those CSS species encountered on just those transects that were placed half in CSS and half in grassland (25 m out from the boundary in either direction). In front of the recovering boundary, 34 CSS individuals of 9 species were encountered, of which 27 were of 4 of the dominant species: Baccharis pilularis, Eriogonum cinereum, Artemisia californica, and Stipa pulchra. In front of the stable boundary, there were only 12 individuals of 6 species found among the 7 transects. Of these 12, only 4 were of the dominant species: 1 Artemisia californica and 3 Stipa pulchra. Baccharis pilularis is significantly more prevalent in the grasslands in front of recovering boundaries and is, in fact, absent from the grassland in front of stable boundaries, as is Eriogonum cinereum . There is no preference in the case of Stipa pulchra or Artemisia californica. So, the grassland in front of recovering boundaries shows a number of juveniles trying to establish themselves, and these are dominated by Baccharis pilularis and Eriogonum cinereum.

Slide 26 The third comparison focusses on the CSS individuals found out in the middle of grassland far from CSS communities. Thirty-three native plant individuals were censused, identified, and GPSed, belonging to 6 species. Of these, 14 or 42% were Baccharis pilularis and an additional 12 or 36% were Artemisia californica. These 2 species, belonging to the 9 dominant species in the western Santa Monica Mountains, make up nearly 80% of the vanguard species making a go of it out in the grassland environment.

Slide 27 This slide represents the relative abundances of each of the 9 species in each of the 5 contexts.

Discussion

These results have implications to improve active restoration efforts. The CSULB group has established that, while CSS is dwindling throughout Southern California in the face of development and type-conversion, it has shown an ability to self-restore in at least a few places. It is apparent that beachheads of CSS juveniles are getting well out into the grasslands far from CSS patches.

Slide 28 This is not a random process: There are definite vanguard species out in the grassland, species that lead out from expanding boundaries, and species that are disproportionately represented in the CSS communities behind the recovering boundaries. Slide 29 The alpha diversity of the communities behind recovering and stable boundaries is identical at 17, but the recovering communities show more dominance among their 17 species than the stable communities do with their 17 (Shannon's Equitability index is 0.71 for the community behind recovering boundaries, which indicates greater primacy among fewer species than seen in the communities behind the stable boundaries, which have an index of 0.86). Moving out into the near grassland, the areas in front of the recovering boundaries has an alpha diversity of 9, while those in front of the stable boundaries have only 6. The areas in front of recovering boundaries show the same tendency toward primacy, with a score of 0.84, compared with the more even distribution of species seen in the 0.91 score for the areas in front of stable boundaries. Out in the grassland itself, the process of winnowing continues, with an alpha diversity of 6 but a Shannon Equitability score of 0.74.

What this suggests is that there are a few CSS species that can tolerate the conditions out in the grasslands. These are the vanguard species: Baccharis pilularis at 42% and Artemisia californica at 36%. Moving in from the edges of the grassland, Baccharis pilularis dominates the grassland near the recovering boundaries at 32%, but now joined by Eriogonum cinereum at 24% and, in minor roles, Artemisia californica with 12% and Stipa pulchra with 12%. Back in the CSS behind recovering lines, Artemisia californica rises to dominance at 40%, while Baccharis pilularis subsides to 12% and Stipa pulchra at 8%, while Eriogonum californica, Salvia leucophylla, and Mimulus aurantiacus, become noticeable between 6 and 8%. While the CSS community behind stable boundaries may reflect a kind of "ailing" CSS unable to get out and mix it up with the exotic annual grassland, another way of looking at it is that the flora back there may represent "old growth" CSS. In that spirit, its mix of species shares 3 dominant species with the community behind recovering boundaries Salvia leucophylla, Mimulus aurantiacus, and Stipa pulchra, but it disproportionately includes some different species: Eriogonum cinereum and Salvia mellifera and Malosma laurina, both the latter of which are also often seen in chaparral communities. Meanwhile, this community is conspicuous for the absence of the vanguard species, Baccharis pilularis and Artemisia californica, as though their job in recovery is done and they eventually fade away.

A recent CSULB thesis is apposite here. Sean Brennan (2015, 2013) tested the idea that Baccharis pilularis is a nursemaid species, facilitating the establishment of other CSS species. He found that young Baccharis pilularis plants easily get into grassland through wind dispersal and establish themselves. Their initial growth habit is dense and upright at first, with only grass around them. They are exploited as cover by rabbits, who make quick forays from them to forage in grass and then bolt back into them to avoid predation. As the plants age, however, their growth habit becomes more recumbent and open with much deadwood in the center. This stage is characterized by a number of other CSS seedlings and many Stipa pulchra bunchgrasses establishing under and around them, but these do not include juvenile Baccharis pilularis. After 30 - 50 years, the Baccharis pilularis individuals die out, leaving a diverse community of maturing CSS where they once stood. This phenomenon, in fact, is regarded as undesirable in conservancies trying to salvage the very few instances of native California grassland, especially in Northern California where Baccharis pilularis is regarded as a pest (Vann-Foster 2015). Brennan's findings, however, do offer evidence that Baccharis pilularis also acts as nursemaid to native California grasses, at least for Stipa pulchra. Given certain parallels with Artemisia californica - ability to establish in grassland, growth habit and change with age, it is possible that it, too, may be a nursemaid species.

Conclusions and Recommendations

Slide 30 For conservancies trying to conserve their shoestring budgets and volunteer labor, these findings suggest a few recommendations. First and most hearteningly, there is a possibility of passive, unattended self-restoration by CSS in certain circumstancpes. Figuring out what those circumstances are could identify areas where conservancy labor is not needed, freeing it up for more challenging situations requiring active restoration.

Second, the CSULB group has tried to evaluate geological (Nesbit and Winslow 2013), soil nitrogen (Patterson et al. 2010), and slope steepness (Chea et al. 2010) with recovering and stable boundaries, with no significant differences in underlying geology, soil nitrogen levels, or slope. One of our studies found no significant difference between the boundary types and soild grain size (Ming et al. 2010), though another did (Engelberg 2013). There is some evidence that CSS may restore more efficiently downhill ("the gravity theory")(Santana et al. 2013; Laris 2013), though slope steepness was dismissed in another study using a different methodology (Chea et al. 2010). Native animal herbivory may produce sharp boundaries with nearly bare transition zones, which are more common in stable than recovering boundaries (Vaughan et al. 2013). We have not found that too-frequent fire or a history of intense grazing will prevent CSS self-restoration or recovery after disturbance (Engelberg 2011; Eckardt 2006; Ko et al. 2010; Dean et al. 2009). What we have found, however, is that mechanical disturbance of soil, as in plowing, is a show-stopper (Laris 2013; Engelberg 2011; Fenderson et al.2009). Areas plowed, even once, are persistently dominated by exotic annuals. Those grazed or burned, even four times in one decade, come back. A promising line of work, followed by Mills and Ducoing Chaho (2013) is the idea that mechanical disturbance may work its persistent effects through disruption of subsurface mycorrhizæ.

Third, an important implication is that conservancies might want to stop using soil scraping or plowing to suppress the exotic seed bank: That is, ironically, the one thing that might ensure a poor outcome for an active restoration project! So, rather than preparing a site for active restoration by plowing or scraping it, this to reduce the weed seed bed, perhaps they could experiment with planting CSS species that have shown the ability to survive in grassland, notably Baccharis pilularis and, probably, Artemisia californica. These plants may provide encouragement for certain fauna to venture out to exploit grasses, such as brush rabbits (Silvilagus bachmani) and black-tailed jackrabbits (Lepus californica).

Any groups of surviving Baccharis pilularis or Artemisia californica could be augmented with a wider palette of CSS species in a few years' time, such as Eriogonum cinereum and Stipa pulchra. If the small patches of CSS species persist and coalesce, other species could be added in, selected from the species typical of the area behind a recovering boundary: Eriogonum fasciculatum, Salvia leucophylla, and Mimulus aurantiacus. When this is established and when the original Artemisia californica and Baccharis pilularis pioneers are dying off, a wider palette could be added in, including Salvia mellifera and Malosma laurina. Basically, rather than try to restore with soil preparation and planting of a wide palette, conservancies could try imitating the gradient of species going from the grassland vanguard through the species just in front of the recovering boundary to the mix of species found behind the recovering boundary and ultimately including the species found behind the stable boundaries.

Scraping a site and planting most of the CSS species in an active restoration site often fails as exotics overwhelm the restoration site. Trying to copy natural processes of post-disturbance succession here, notably the post-fire succession of fire-follower annual forbs, then perennial subshrubs, and then the larger shrubs may not work with CSS, either, since exotic annuals will swamp the native forbs, too. What is worth a try is a kind of space-time substitution or faux succession: starting with the shrubs and subshrubs that can establish in grasslands, moving on to include more species that venture into the edges of the grassland in front of self-recovering boundaries, and then adding a fuller palette of CSS species seen behind the self-recovering boundaries.

For the CSULB group, we will continue augmenting this database, having students explore the themes of restoring and stable boundaries in other study sites. As the database grows, we will acquire the statistical power to evaluate distance from the boundary as a driver of species mix. We've become interested in the vanguard species and will census for them more systematically and in a wider set of field sites. The database will eventually be made public.

Last but not least would be an experimental approach, working with a local conservancy to devise test plots in which the graduated approach suggested here could be tested and contrasted with other restoration efforts. This would move this from an inductive approach to a fully experimental, deductive approach and enable outcomes assessment for future active restoration projects.

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References

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This document is maintained by: Christine M. Rodrigue
First placed on web: 10/24/15
Last Updated: 10/25/15