Table Of ContentMadrono, Vol. 53, No. 1, pp. 55-59, 2006
FACTORS AFFECTING UNDERSTORY ESTABLISHMENT IN COASTAL SAGE
SCRUB RESTORATION
Matt V. Talluto, Katharine Nash Suding, and Peter A. Bowler
Department of Ecology and Evolutionary Biology, University of California, Irvine,
CA
92697-2525
[email protected]
Abstract
Coastalsagescrub(CSS)isatarget forrestorationbecauseitprovideshabitat fornumerousspecial-
status species and it has been impacted by urbanization, agriculture and invasion by non-native
species. Many restoration designs have neglected the herbaceous understory component of CSS,
although itmay comprise themajority ofvascular plant species in a natural CSS stand. The omission
ofan understory may promote invasion by non-native plants and reduce overall success. This study
investigated the role ofnative seed addition, non-native species removal, gaps in the shrub canopy,
and soil moisture, upon establishment ofa native understory. Native biomass increased significantly
with seed addition, and the abundance of experimentally seeded native species was positively
correlated with soil moisture. Natives were not affected by competition with non-natives or the
presence of gaps. Although all seeded native species germinated, only two of seven established
successfully, perhaps due to very low rainfall. Non-native species were negatively affected by the
addition ofnative seeds and had greatergrowth in gaps. Weconclude that planting shrubs in a dense
configuration to reduce gap size may reduce non-native species abundance in the understory while
having little effect on the native understory. Seeding may be all that is required to establish a native
understory, and may also be an effective method ofsuppressing non-native species.
Key Words: coastal sage scrub, restoration, competition, invasive species, seed limitation.
Coastal sage scrub (CSS) is one of the most 1997) and compete with native understoi*y herbs.
endangered habitats in southern California. Although many restoration designs target only
Estimates of CSS loss vary, however it is likely the native shrub canopy, restoration of the
that CSS currently occupies less than half of its understory as well is a more logical approach
historical distribution (Westman 1981; O'Leary (Bowler 2000). Thus, understanding the ecolog-
1995). Primary causes ofCSS loss are urbaniza- ical factors controlling understory structure and
tion, agriculture, and the degradation and re- composition is important.
placement of natural stands by the invasion of Previous studies have suggested numerous
non-native species, inostly annual grasses from mechanisms involved in competition between
the Mediterranean region (Freudenberger et al. non-native and native plant species. Competition
1987; O'Leary and Westman 1988; O'Leary 1995; with non-native annual grasses for soil moisture
Minnich and Dezzani 1998). These ecosystem often limits the success of native perennials
stressors, as well as the importance of CSS as (Melgoza et al. 1990; Eliason and Allen 1997;
habitat for numerous rare, threatened, or endan- Humphreyand Schupp2004; but see Seabloom et
gered plants and animals, make the community al. 2003a). Competition with non-natives for soil
a priority for conservation (Davis et al. 1994). moisture could also have an impact upon native
Regional multi species habitat conservation plans understory annuals, although this has not been
throughout southern California anticipate addi- directly tested in CSS. Reduced light availability
tional loss of CSS and require both preservation beneath shrubs iTiay also reduce understory
and restoration of CSS to mitigate for this loss. growth, resulting in a relatively higher density
Thus, development is resulting in increasing of understory plants in gaps, and shading from
numbers of CSS restoration sites, making suc- non-native grasses growing in gaps may affect
cessful restoration strategies essential for effective these native plants (Thompson and Harper 1988;
CSS conservation. Dyer and Rice 1999).
Typical CSS has a dense shrub canopy 0.5 Coastal sage scrub understories share many
m
1.5 in height and a sparse herbaceous un- species with Cahfornia's coastal grasslands. In
derstory ofpriinarily annual species concentrated two coastal grassland experiments in California,
in gaps between shrubs. Invasive non-native Seabloom and colleagues found that native
species are increasingly common in CSS unders- perennial grasses (2003a) and annual forbs
tories (O'Leary 1995). Non-natives are undesir- (2003b) were strongly seed-limited. Following
able in restoration projects because they reduce seed addition, natives successfully established
the success ofplanted shrubs (Eliason and Allen despite competition from non-natives. These
MADRONO
56 [Vol. 53
i
Table Seedingdensity,%emergence,andrelativesuccessofnativeannualplantsadded. Biomass
1.
values (±1 SE) werecalculated fromseed addition plots only. Seed emergencedata wereobtained from S&S Seeds
(Carpinteria, CA).
Mean end ofseason
Species N seeds added/plot % Seed Emergence biomass (g m"-)
Anisinckia nienziesii (Boraginaceae) 280 54 0.38 ± 0.15
Cryptautlui niuricata (Boraginaceae) 990 26 0.00
Lasthenia califoruica (Asteraceae) 6240 80 8.03 ± 2.31
Lcpidiiiiu uitidwn (Brassicaceae) 970 51 0.00
Plantago erecta (Plantaginaceae) 360 88 0.00
Liipiuus hicolor (Fabaceae) 190 85 0.00
Liipmiis truucatus (Fabaceae) 60 91 0.00
results suggest that seed limitation also could Experimental Design
explain the failure of many restored CSS
communities to develop a native understory. This experiment was conducted during Febru-
This experiment's objective was to identify the ary-May 2004, 1.5 years after the A. californica ;
primary factors limiting establishment of native clusters were planted. Native propagule abun- ,,
understory herbs during CSS restoration. Limi- dance was manipulated by adding native seeds of
tation likely results from several interacting five native annual forbs and two annual legumes
factors. Through an experimental restoration, (Table 1). Presence of non-native species was 'j
we addressed the following four questions: 1. Is manipulated by clipping all non-native species at \
seed addition alone sufficient to restore a CSS ground level. Seed addition and non-native
native understory? 2. Do non-native and native removal were combined in all possible combina- .
understory plants compete with one another? 3. tions, resulting in the following four plot types: 1. i
Native seeds added, non-native species removed;
mDplaoatenustrse?thse4h.rurbDesdouercseedduscoeliilgthhtmeoiegsnrtvouiwrrteohnmoaeffnfetucntdbeernnsaettaoitrvhye sa2.dedeNidtaiatodindv,ietnisooenne-dnasantdaidvndeoedsr,peecmniooevsarlre.emmoovvale;d;3.anNdo4.sNeeod
understory establishment and competition with
Experimental plots were situated a minimum
non-natives? of 25 cm from the edge of each A. californica
\
cluster and from other plots. The plots were
Methods 50 cm X 50 cm, with treatments extending an
additional 10 cm beyond the plot boundary. Plot
i
Site Description locations were assigned randomly within eight
replicate blocks, each ofwhich was located within |
This experiment was conducted on an existing a single cluster of shrubs and included all four i
CSS restoration site adjacent to the University of treatment combinations. Within blocks, plot j
CaHfornia, Irvine Arboretum and UC Natural orientations were assigned non-randomly to
Reserve System's San Joaquin Freshwater include a single shrub immediately outside the
Marsh Reserve. Prior to restoration in 2002, plot at oneend and a gap in the shrubcanopy (no
this site was an abandoned agricultural field that shrub canopy directly overhead) at the other end.
had no resident native taxa and was dominated This plot orientation allowed investigation ofthe
!
by non-native species including Brassica nigra importance of gaps in the distribution of un-
(Brassicaceae), Foenicuhun vu/gare (Apiaceae), derstory herbs beneath a CSS canopy. Each plot
Cynara cardmuulus (Asteraceae), and annual was divided into two 25 cm X 50 cm subplots: j
grasses, primarily ofthe genus Bronius. (Nomen- one under the canopy and one in the adjacent j
clature follows Hickman 1993) These same gap. j
species dominated the area surrounding the site. j
Annumalm rainfall in the area is approximately Experimental Treatments I
300 yr''. {
During October November 2002, Artemisia Seed addition. We amended plots with seed
califarnica (Asteraceae) shrubs were planted in from five native annual forb and two annual
circular clusters approximately 3 m in diameter. legume species at a density of4.5 g m"- pertaxon j
Shrubs were planted at an average density of2.5 (Table 1). All seeds were obtained commercially
plantings m To minimize mortality during from S&S Seeds (Carpinteria, CA). Plots were
establishment, plantings were watered weekly seeded on 19 February 2004. Prior to seed
until February 2003, and were hand-weeded addition, the soil was disturbed by hand-raking !|
during the first spring (2003). No herbicides were to a depth of 2 cm. Control plots were similarly
applied at any time during restoration. disturbed. After seed addition, the loosened soil |
|
2006] TALLUTO ET AL.: COASTAL SAGE SCRUB UNDERSTORY RESTORATION 57
Control S SR R Canopy Gap
Fig. 1. Biomass ofnative (shaded bars) and non-native (open bars) species in (A): plots with native seed addition
only(S), seed additionandnon-nativespecies removal (SR), non-nativespeciesremovalonly(R), and no treatment
(control), and(B): subplotsbeneath theshrubcanopyand ingaps. Incontroland R plots, Artemisiacalifornicawas
the only native present. In S and SR plots, most ofthe native biomass was from Lastheuia califomica. Bars are
means ± 1 SE.
was spread overthe seeds to minimize losses from were averaged prior to analysis. Soil moisture
wind, runoff, and predation. Because rainfall was was recorded on 1 1 May, one day prior to
below average during the experiment, supple- harvesting all aboveground biomass in the plots.
mental water was provided in late March and
early April to prevent excessive mortality. Statistical Procedures
Seeded species were harvested on 12 May 2004,
at the end ofthe growing season. To determine if To meet assumptions of normality and homo-
seed addition had an effect on any preexisting geneity ofvariance, a natural log transformation
native understory, all volunteer native species on all biomass measurements was performed, and
werecollected as well. Aboveground biomass was soil moisture measurements were rank trans-
dried for 48 hr at 60 C and weighed. formed. Because light intensity measurements
were normally distributed, they were not trans-
Non-native species retuoval. All non-native
formed.
cspleicpipeisngweartegrreomuonvdedlevfelr.omEawreleydegdermpliontastibnyg To determine how shrubs influenced light and
species were removed on 9 March, as soon as soil moisture, a paired t-test was conducted to
plants were identifiable. A second removal was compare light intensity between canopy and gap
pspeercfieosr.meNdoonno1n-Anpartilivteosrpeemcoievsewleatreergoebrsmeirnvaetdinign ususbepdlottos,tesatnfdoraaWsoiillcmooxiosnturseigdniefdferraenncketbeesttwweaesn
subplot types.
any weeded plots after the second removal. Non- We used analysis of covariance to examine
natives from unweeded plots were harvested,
effects of seed addition, non-native removal,
dried, and weighed with native species at the end
light, and soil moisture on the biomass ofnative
ofthe growing season. herbs. A similar analysis was used to test effects
Soil moisture and light availability. Photosyn- of native seed addition, light, and soil moisture
thetically active radiation (PAR) was measured in on non-native biomass within non-removal plots.
each subplot within 2 hr of solar noon on 29
April. Values were recorded as the ratio of light Results and Discussion
measured below the canopy to incident light
measured directly above the canopy. Soil mois- Native Seed Addition
ture in the top 12 cm of soil was also recorded
using a Hydrosense TDR probe (Campbell Native seed addition significantly increased
Scientific, Logan, UT). Deeper readings were native understory biomass. Native biomass in
not taken to avoid excessive disturbance within seed addition plots averaged 8.41 ± 2.45 g m
the plots and because the rocky soil made probe compared to 0.03 ± 0.02 g m - in control plots
insertion difficult. Three soil moisture readings (F,,5o = 310, P < 0.001; Fig. la). Natives
were taken from each subplot, and the results averaged 2.6% of total herbaceous biomass in
MADRONO
58 [Vol. 53
unseeded, unweeded plots and 88.5% in seeded, shrub canopied and gap plots, suggesting that
unweeded plots. any competitive effect of shrubs on the natives
Although all seven seeded species were ob- may have been balanced by a facilitative effect.
served germinating, only Lasthenia californica Non-native biomass was significantly less be-
(Asteraceae) and Amsinkia luenziesii (Boragina- neaththecanopy(Fj 20 = 5.80, P = 0.03; Fig. lb).
ceae) survived long enough to produce flowers This was likely due to the large reduction in light
(Table 1). Lasthenia californica produced 98% of intensity beneath the canopy, although it may
native biomass, while A. menziesii produced the also be due to belowground effects not measured
remaining 2%. The only volunteer natives ob- in this study.
served were A. californica seedlings.
No substantial populations of native CSS
Soil Moisture
herbs were observed within several hundred
meters ofthe study site, and it is likely that seed There was a significant positive relationship
dispersal beyond this distance is very low (Van between soil moisture and native biomass (Fi 50
Dorp et al. 1996; Jongejans and Schippers 1999). = 5.54, P = 0.02), but not between soil moisture
Furthermore, the absence of any native herba- and non-native biomass. Although soil moisture
ceous growth in unseeded plots suggests that no was limiting for both natives, the importance of
native seed bank remained on the site. Past soil moisture may have been exaggerated by the
disturbance and several decades of non-native very low precipitation throughout the growing
species dominance has likely eliminated any season. Low soil moisture may also explain the
native seed bank that may have been present. high mortality observed for most seeded native
Because an impoverished native seed bank and species. These species were observed germinating,
few local seed sources may be typical of many but died during mid-season when rainfall also
potential CSS restoration sites, seed addition of declined sharply below weekly averages. Finally,
understory herbaceous plants is likely to be the lack ofa difference in native growth between
a necessary component of many restoration low light (canopied) and high light (gap) subplots
projects. would be expected if soil moisture was more
limiting than light, and shading by shrubs
Competition reduced water stress for the native understory.
In unweeded plots, native seed addition caused
Conclusions
a small but significant decline in non-native
biomass, from 1.68 gm - to 1.19 gm~- (F120 This study suggests that in restored dense-
= 4.76, P = 0.04; Fig la). The small magnitude of canopy CSS, soil moisture is an important
this change is likely due to low non-native limiting factor for native understory species,
biomass throughout the planted area. The high while light is more limiting for non-natives.
biomass ofL. californica in seeded plots suggests Non-natives were also more abundant in gaps
that this species, if seeded at high densities, may than they were beneath the shrub canopy,
be effective at controlling non-native species. suggesting a competitive effect of the shrubs.
There were no significant effects ofnon-native Thus, the presence ofa mature, dense shrub layer
removal on native biomass, regardless ofseeding may effectively exclude a large fraction of non-
treatment. However, due to low non-native native species without adversely affecting some
biomass and low survival of most seeded native native species.
species, it is difficult to make any conclusions Competition with the restored native under-
about a competitive effect of non-native species story (which consisted primarily ofL. californica)
on natives. Strong competitive effects of non- significantly reduced non-native biomass. Thus,
native species in CSS and similar systems have the addition of seeded native understory herbs
been demonstrated in the past, and may have may be an effective secondary restoration strat-
been observed in this system had non-natives egy, particularly given that long distance dispers-
been more abundant (D'Antonio and Vitousek
al ofnative understory herbs is likely to be a rare
1992; Eliason and Allen 1997; but see Seabloom event. Seeding may be accomplished at a fairly
et al. 2003b). low cost and can successfully establish some
native herbs and reduce the abundance of non-
Shrub Effects native species.
As expected, light intensity was significantly Acknowledgements
lower beneath the canopy (mean 47 ± 3.0%) than We thank the University of California Natural
it was in gaps (74 ± 3.0%). There was no Reserve System's San Joaquin Freshwater Marsh Re- I
significant difference in soil moisture between serve for permitting us to conduct research on lands it
canopy and gap subplots. Native understory manages, and for letting us use its equipment. The UCI
biomass did not differ significantly between the Arboretum also provided equipment used in this study.
TALLUTO ET AL.: COASTAL SAGE SCRUB UNDERSTORY RESTORATION 59
Comments and field assistance from Bradford Haw- JONGE.IANS, E. and P. SCHIPPERS. 1999. Modelingseed
kins, Erin Hayes, Emily Grman, and Robert Arkle, and dispersal by wind in herbaceous species. Oikos
comments from two anonymous reviewers, greatly 87:362-372.
improved this manuscript. This study was supported Melgoza, G., R. S. Nowak, and R. J. Tausch. 1990.
in part by Grant 01-130 from the California Coastal Soil water exploitation after fire: competition
Conservancy. between Browns tectorwn (cheatgrass) and two
native species. Oecologia 83:7-13.
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