Stage and Size Structure of Three Species of Oaks In Central Coastal California

2014·10.6 MB·English

by  Ian S Pearse

Madrono, Vol. 61, No. 1, pp. 1-8, 2014
STAGE AND SIZE STRUCTURE OF THREE SPECIES OF OAKS IN
CENTRAL COASTAL CALIFORNIA
Ian S. Pearse
Cornell Lab of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850 and
Illinois Natural History Survey, 1816 S Oak St, Champaign, IL 61820
ianspearse@gmaiLcom
Sophie Griswold and Desirree Pizarro
Cornell Lab of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850
Walter D. Koenig
Cornell Lab of Ornithology, 159 Sapsucker Woods Road, Ithaca, NY 14850 and
NY
Department of Neurobiology and Behavior, Cornell University, Ithaca, 14853
Abstract
OaksarefoundationalspeciesinmuchofCalifornia, andmanyoakpopulationsinthestatemaybe
injeopardydue to a lack ofrecruitment ofyoungtrees. Despiteconsiderable interestin this problem,
therehavebeenfewcomprehensivesurveysofallstagesofoakdevelopment. Wesurveyedallstagesof
threeoaks: Quercuslobata, Q. douglasii, and Q. agrifoUainaforestplotwithmixedland-useincentral
coastal California. We found abundant seedlings ofall oak species, but an apparent paucity of Q.
lobata and Q. douglasiisaplings. First year seedlings ofall species were less abundant in parts ofthe
study site with cattle grazing, but later-stage seedlings of Q. lobata and Q. douglasii were equally
abundant across land-use types. Quercus agrifolia seedlings were associated with non-grazed areas;
Quercus agrifolia late-stage seedlings in the grazed area were smaller and less abundant than in non-
grazedareas. Quercusagrifoliaseedlingsofallstagestendedtobeclusteredaroundconspecificmature
trees. Quercus lobata late-stage seedlings, and to a lesser degree those of Q. douglasii, were often
distant from any potential parent tree. These data indicate that young stages ofthe three species of
oak have different spatial distributions and occur in different abundances at two sites with different
grazing regimes. They are also consistent with a relative lack of regeneration in Q. lobata and Q.
douglasii, although it remains to be determined that the small number of saplings of these species
observed is insufficient to replace mortality of mature trees. This survey provides a baseline from
which future resampling can assess the long-term demographic success of three Californian oak
species.
Key Words: regeneration, demography, Quercus.
Californian oaks are a dominant component Callaway and Davis 1998; Davis et al. 2011), and
of at least a third of the forest and savanna increased drought stress (Mahall et al. 2009).
ecosystems of the state (Pavlik et al. 1991). Each of these factors has undergone widespread
However, there has been recent concern for oak anthropogenic change in the last two centuries,
populations, as there is a perceived lack of thus potentially explaining the apparently poor
regeneration of oak forests and a lack of young recruitment currently suffered by oak popula-
tree stages (Griffin 1976; Tyler et al. 2006). tions (reviewed by Tyler et al. 2006). However,
Studies have attributed the lack of oak regener- the relative importance of each of these factors
ation to various causal factors (Griffin 1971; may be site specific. For example, while grazing
Borchert et al. 1989; Callaway and Davis 1998; may decrease seedling abundance at some sites
Gordon and Rice 2000), while others have (Reiner and Craig 2011), even sites without
questioned the severity of regeneration problem livestock grazing may experience a lack of
altogether (Tyler et al. 2006). seedling recruitment (Griffin 1971). The lack of
The apparent lack of recruitment of two a single causal factor in altering oak populations
common California oaks, Quercus lobata Nee calls for a more integrative approach in assessing
and Q. douglasii Hook. & Arn, (Fagaceae), have oak population dynamics.
prompted numerous causal explanations for A recent conceptual review (Tyler et al. 2006)
seedling mortality and lack of regeneration in and meta-analysis (Zavaleta et al. 2007) of
these species, including cattle grazing (Hall et al. California oak recruitment both suggested that
1992; Tyler et al. 2008; Reiner and Craig 2011), two Californian oaks, valley oak {Q. lobata) and
competition from non-native grasses (Griffin blue oak {Q. douglasii), suffer from declining
1971; Gordon and Rice 2000), altered herbivore populations. Both studies concluded that site-
populations (Griffin 1971; Borchert et al. 1989; specific causes may affect recruitment of these
MADRONO
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species. Moreover, they indicated that detailed Methods
demographic models are needed to address the
question of whether low observed recruitment Study Area and Natural History
levels are actually insufficient to replace stands of
long-lived adults. Likewise, a detailed demo- The survey was conducted on Haystack Hill
graphic analysis of oak populations could ask andsurroundingareas(36.385114N, —121.561906
specifically which life stages most limit popula- W) on Hastings Natural History Reservation
tion growth (Tyler et al. 2006; Zavaleta et al. (HNHR) and the adjacent Oak Ridge Ranch
2007; Davis et al. 2011). Meanwhile, studies have (ORR) (Fig. 1). The study area is typical ofcoast
suggested that a third species ofCalifornian oak, range oak habitat, is the site of several studies of
Q. agrifolia Nee, may not be undergoing similar oak populations (White 1966; Griffin 1971, 1976;
cloonwgenreergse,nereavteinonthroautgehs iatsmiatsy tbweoafsfyemctpeadtrbiyc K70oeknmigofa1n/d3 oKfntohpesst2u0d0y7)s,iteas)ndtoitshecesnittresalus(ewditihnian
some of the same recruitment challenges (Call- recent meta-analysis of Californian oak popula-
away and Davis 1998; Mahall et al. 2009). tions (Zavaleta et al. 2007). The study area has a
While studies have successfully documented MediterraneanclimatetypicalofCalifornia’scoast
multiple site specific factors likely affecting range. Intheyearofthesurvey,theaveragerainfall
rpeolpautliavetiloancsk ooffCtahleifexoprlniicaint doeakmso,grtahpehreicismostdilell-a w30asye4a7r5avmemr,agewhfoirchthweassitesl(i5g4h0tlymmless- tHhaasntintghse
Weather Station http://www.hastingsreserve.org/
ing of oak populations needed to quantify the
weather/Weather.html). Thestudyareawasrough-
sritsakgetsooofakoapkospualraetioofnsthaendhitgoheasstsescsonwsheirvcahtiliofne lydelimitedbythecontourofthehill. Weexcluded
a large patch ofdense chaparral that would have
value. The only current study that explicitly
been within the study, as the area was too densely
reconstructs Q. lobata demography uses an vegetated to use the same survey techniques as in
experimental approach, which is very valuable other areas and contained few oak trees, with the
in determining causal factors that limit Q. lobata exception of a small number of Q. agrifolia
success (Davis et al. 2011). Observational studies individuals at the bottom of dry washes that
that thoroughly survey oak populations that are descended the hillside through chaparral.
both geographically referenced and include all life The study area was divided into three habitat
stages of the oaks complement experimental types based on tree cover and groundcover type:
studies. Such surveys will be valuable because forest, savanna, and grassland (Griffin 1990).
they can be easily resampled in order to directly ‘Forest’ had a complete canopy cover. Forest
assess vital rates and stage transitions. At the understory was dominated by poison oak {Tox-
same time, comprehensive site surveys observe icodendron diversilobum [Torr. & A. Gray]
oak populations across all microhabitats at a site, Greene) and had little to no grass cover. One
which is difficult in manipulative studies. forest region had a substantial stand ofmadrone
Here we present a baseline survey of the oaks {Arbutus menziesii Pursh), whereas oaks domi-
on a portion ofHastings Natural History Reserve nated all others. ‘Savanna’ had a partial canopy
(HNHR) and the adjacent Oak Ridge Ranch coverand an understory ofnative and non-native
(ORR) property in Monterey County of central grasses including Avena L. sp., Bromus hordea-
coastal California. We surveyed the size, stage, ceus L., R diandrus Roth, Hordeum L. sp., Stipa
and geographic location of 19,755 individuals of pulchra Hitchc., and Aira caryophyllea L., as well
the three dominant oak species {Q. lobata, Q. as forbs such as Madiagracilis (Sm.) D. D. Keck,
douglasii, and Q. agrifolia) in a 52.6 hectare area Asclepias eriocarpa Benth., Plagiobothrys Fisch.
encompassing forest, grassland, and savannah & C.A. Mey. sp., and Amsinckia menziesii
habitats. This survey also encompassed areas (Lehm.) A. Nelson & J.F. Macbr. ‘Grasslands’
with strikingly different recent land use histo- had sparse to no tree cover and were dominated
HNHR
ries, as has been protected from cattle by the same mix of grasses and forbs. Small
ORR
grazing since 1937, but continues to remnants of perennial grasslands (sensu Griffin
experience significant regular cattle grazing. 1990) dominated by Stipa pulchra were included
We use this survey primarily to establish a in this category. There wasconsiderable variation
baseline for parameterizing future demographic in both slope and aspect of habitat types within
models of oak populations, and we make this the study area, but both study sites (HNHR and
baseline data available to the general scientific ORR) had both steep and relatively flat forests,
community. We analyzed the size structure and grasslands, and savannahs. One difference be-
spatial distribution of early and late seedling tween the two sites was the presence of a small
HNHR
stages of these oaks in order to ask to what creek bed at the site that had a high
degree habitat type and land use affect the size abundance of Q. agrifoUa seedlings.
of late-stage seedlings and the proximity of The study site was divided approximately
HNHR
seedlings to a potential parent. down the middle by a fence separating
2014] PEARSE ET AL.: SURVEY OF CENTRAL CALIFORNIAN OAK STAGES 3
Fig. 1. A map of the survey area (highlighted in light dashes and outlined with a dotted line). The survey is
divided roughly in halfby a property line (central E~W line), where the northern property (Oak Ridge Ranch) is
grazed andthesouthernproperty (HastingsNatural History Reserve (HNHR) isungrazed. In May-June 2012, we
surveyed and recorded thelocation ofallvalley oak (Q. lobata, white dots), blue oak (Q. douglasii, greydots), and
coast live oak (Q. agrifolia, black dots) first year seedlings, late-stage seedlings, saplings, and trees within the
demarcated study area.
from ORR. The land use ofthe two areas can be transects across the study area and used a GPS to
summarized as follows. Prior to 1930, both sites recordthelocalityofeachoakwithintheareatoan
had a history of mixed land use including accuracy of5 m. For each individual, we recorded
ranching, timber harvest, and small-scale agricul- one offourgrowthstages. Seedlingswerefirstyear
HNHR
ture (Griffin 1990). was donated to the germinants from acorns. All had unbranched
UniversityofCaliforniain 1937 atwhichpointall stems, and most were still connected to a partially
grazing and agriculture was stopped. At ORR, buried acorn. Late-stage seedlings were over one
moderate year-round cattle grazing continues to year old, less than 1.5 meters in height, had
present day, including during the active survey branched ormultiple stems, and lacked a connect-
period, although no tillage or significant timber ed acorn. Saplings were greater than 1.5 meters in
harvest has been recorded within the study area height, less than 10 cm DBH, and had not yet
within recent history. While oak plantings and beguntoproducefruitorflowers. Adulttreeswere
restoration efforts have been made in other over 10cmDBH.Weconsideredtheseclassestobe
portions ofHNHR, none ofthese activities have biologically useful, as demographic parameters
occurred within the study area. includinggrowthrate, survivorship, andreproduc-
The year preceding our survey had a moderate tion likely vary between stages (e.g., Griffin 1971;
togoodacorncropforallthreeoakspecies. Based Koenig and Knops 2007). For all late-stage
on the Koenigvisual countmethod (Koenigetal. seedlings, we also recorded height and canopy
1994a), the 2011 Q. lobata acorn crop was 15% width and calculated seedling volume as height X
greater than the 30-year mean Q. lobata crop widtffi. For all first-year seedlings, late-stage
(Pearse et al. 2014). Likewise, the 2011 Q. seedlings, and saplings, we also calculated Euclid-
douglasii acorn crop was 63% greater than its ean distance to the nearest conspecific adult tree.
30-year mean, and the 2011 Q. agrifolia acorn Wetaggedeachadulttreeandrecordedwhichfirst
crop was 29% greater than its 30-year mean. year seedlings, late-stage seedlings, and saplings
m
were within 20 ofits trunk.
Survey Methods
Statistical Analysis
We surveyed allvalley oak, blue oak, andcoast
live oak individuals within the study area in May While the two landscape level factors - habitat
through early July 2012. We marked 10 m wide and property, the latter corresponding to differ-
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Table 1. The Density of Three Oak (Quercus) Species (Trees/Hectare) in Three Habitat Types
Spanningthe Ungrazed Hastings NaturalHistory Preserve(HNHR)andGrazed OakRidgeRanch
(ORR). All trees (>10 cm DBH), saplings (>1.5 m tall), late-stage seedlings (multiple stems, no attached acorn),
and first-year seedlings (single stem, often acorn still attached) within the total 52.57 ha survey areawere recorded
resulting in 19,755 records ofoak individuals.
Forest Savannah Grassland
HNHR ORR HNHR ORR HNHR ORR
Area (ha) 8.21 8.89 8.23 7.73 10.83 8.65
Quercus lobata 1st year seedling 161.75 7.2 49.09 5.3 16.34 0.55
Late stage seedling 47.14 17.55 24.79 54.59 15.51 18.47
Sapling 0.24 0 0 0 0.09 0.09
Tree 17.54 10.01 13.49 6.47 2.4 0.37
Quercus douglasii 1st year seedling 18.39 176.04 84.93 34.67 3.23 0.09
Late stage seedling 29.6 194.94 94.9 60.28 5.26 2.4
Sapling 0.49 0 0 0 0.09 0
Tree 11.33 69.74 22.72 6.21 0.09 0.83
Quercus agrifolia 1st year seedling 455.18 23.17 57.84 1.29 61.68 0.28
Late stage seedling 209.38 76.83 24.18 8.8 6.74 3.42
Sapling 15.23 0.45 0 0 0.55 0
Tree 55.05 27.67 6.44 0.91 0.92 0
ent land use - are spatially non-independent, we with savannah, although the distribution of
used these data to build a case study of oak overstory trees only partially reflected this trend.
populations at a single, large site. Specifically, our All growth stages of Q. douglasii were uncom-
goal was to explore variation in 1) the size oflate- mon ingrasslands at all sites (Table 1). Therewas
stage seedling oaks, as this size class may escape a greater abundance ofearly growth stages of Q.
ORR HNHR
grazing at large sizes (either by being tall or very douglasii at than at (4059 versus
wide), 2) the distance of seedlings and late-stage 1966), reflecting the distribution ofadult trees at
seedlingoaksfromapotential (conspecific)parent the two sites (677 versus 281). There was a full-
tree, and 3)thecompositionofoakseedlingsinthe canopied Q. douglasii forest on ORR, and all
understory of individual adult trees. All statistics growth stages of Q. douglasii at ORR were more
were calculated in R using package car (R Core closely associated with forest than at HNHR,
Development Team 2008; Fox and Weisberg where Q. douglasii was more abundant in open
2011). Our georeferenced dataset is made avail- savannah habitat. Quercus agrifolia was associ-
able at the Dryad data depository http://dx.doi. ated with full-canopied forest habitats at both
HNHR
org/10.5061/dryad.467g5 and on the authors’ sites and was particularly abundant in
websites http://www.nbb.cornell.edu/wkoenig; forests. Quercusagrifoliawas also more abundant
HNHR
http://ianpearse.wordpress.com. in grasslands and savannas than the
same habitat types at ORR. There were strikingly
Results fewer Q. agOriRfoRlia first yeaHrNHseRedlings in all
habitats on than on (219 versus
Habitat and Land-use Associations 4881), which would be consistent with an impact
of grazing on Q. agrifolia seedling abundance.
We surveyed 52.6 ha and gathered data on a For Q. agrifolia, this trend persisted into late-
total 19,755 individuals of the three oak species stage seedlings, where there were fewer Q.
acrossboth properties (HNHR andORR) and all aHgrNifHolRia late-stage seedlings at ORR than at
habitats (Fig. 1, Table 1). The surveyed area of (788 versus 1991).
each habitat type at each property was similar, There were few saplings ofboth Q. lobata and
andwereportthedensitiesofoaksateachstagein Q. douglasiiinallhabitatsatbothproperties(four
each habitattype on the two properties (Table 1). Q. lobata saplings and five Q. douglasiiseedlings),
Quercus lobata seedlings were more abundant consistent with perceived recruitment problems
at HNHR, the ungrazed site, than at ORR, the with these species. In contrast, Q. agrifolia
grazed site - 1909 seedlings versus 111 (Table 1). saplings were reasonably common (n = 135),
However, this difference did not carry into the almost all of which wHerNeHaRssociated with forest
late-stage seedling individuals, and there were sites on the ungrazed property (Table 1).
slightly fewer late-stage seedling Q. lobata at
HNHR than at ORR (759 versus 778). At Size of Late Stage Seedlings
HNHR
both Q. lobata seedlings and late-stage
seedling stages tended to be located in forested We measured the volume of late-stage seed-
habitats, whereas at ORR, they were associated lings (excluding first-year seedlings) (Fig, 2).
2014] PEARSE ET AL.: SURVEY OF CENTRAL CALIFORNIAN OAK STAGES 5
stage seedlings in grasslands than in otherhabitat
types (Fig. 2). Unlike Q. lobata, they were
roughly the same size across land-use types in
all habitats.
Quercus agrifolia late-stage seedlings showed
the strongest size association with land use type
of the three species. In both grasslands and
HNHR
forests, Q. agrifolia late-stage seedlings at
were 300-400% larger than those in the same
ORR
habitats on (Fig. 2). Quercus agrifolia late-
C0/55 forest savannah grassland stage seedlings in savannah habitat were smaller
g than in other habitats (Fig. 2). The variation in
size of seedlings was greatest in grassland
0
0 habitats.
(/)
0
Spatial Distribution of Seedlings
05
S
to We measured the distance from each first year
BI seedling and late stage seedling to its nearest
mature conspecific tree (i.e., potential parent)
o (Fig. 3). Quercus lobata first-year seedlings aver-
aged 12.4 meters from the nearest potential
parent. Quercus lobata late stage seedlings were
on average further from the nearest potential
forest savannah grassland parentthan seedlings ofthe otherspecies, andthis
ORR
distance was even greater on than on
HNHR m m
(15.6 and 14.3 respectively, Fig. 3).
Quercus douglasii seedlings were generally closer
to a potential parent than Q. lobata seedlings,
Q. agrifolia likely reflecting the density ofstands ofthese tree
species (Fig. 3). Both Q. douglasii first-year and
m
late-stage seedlings averaged eight from the
nearest potential parent on HNHR (Fig. 3). On
ORR, first-year Q. douglasii seedlings averaged
m
4.7 from the nearest potential parent and late-
stage seedlings averaged seven m. Quercus agrifo-
m
lia seedlings averaged 7.5 from the nearest
potential parent, a value that did not vary by
forest savannah grassland either seedlings class (Fi 3855 =2.0, P = 0.16) or
property (Fi^gss =0.002, P = 0.96, Fig. 3).
Fig. 2. The volume of late-stage seedlings of Q. We also measured the composition of the
tloybpaetsa,onQ.udnogurglaazseidi,(aHnNdHQR.,agdraifrokliagrieny)tharnede hgarbaizteadt seedling oak community underneath mature trees
(ORR, light grey) properties. Our designation of“late- of each of the three oak species and found that
stage seedling” included oaks that were less than 1.5 m the understories of both Q. douglasii and Q.
in height, had multiple stems, and no attached acorn. agrifolia were dominated by their conspecific
This size ofthis stage may be particularly important to seedlings (Fig. 4). In contrast, the understories of
oak regeneration, as late-stage seedlings persist over Q. lobata trees had a roughly even distribution of
many years and are susceptible to grazing, especially seedlings from all three oak species. Similarly, Q.
when small. Bars are means ± SE. lobata seedlings were the least abundant compo-
nent ofthe understories of either Q. douglasii or
Quercus lobata late-stage seedlings tended to be Q. agrifolia.
larger in more open habitats such as grasslands
irrespective ofwhether the site was grazed or not Discussion
(Fig. 2). Quercus lobata seedlings in savannahs
were four times larger on ORR than on HNFIR The size and age distributions of the three
(Fig. 2) and similar in size in other habitat types, species of oaks surveyed in this study varied
a finding that is not consistent with cattle grazing between properties with different land-use histo-
being a factor limiting seedling growth of this ry, but these differences were fairly subtle. There
species. were fewer valley oak {Q. lobata) first-year
Quercus douglasii late-stage seedlings showed seedlings per adult tree on the grazed site
similar size trends to Q. lobata, with larger late- (ORR) than on the ungrazed site (HNHR).
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Fig. 3. ThedistributionofthedistanceofQ. lobata, Q. douglasii, and Q. agrifoliafirstyearseedlings(dashedline)
and late-stage seedlings (solid line) to their nearest conspecific (i.e., potential parent) tree at the ungrazed site
(HNHR) and grazed site (Oak Ridge Ranch). Valley oak late-stage seedlings and to a lesser degree first year
seedlings are located at a greater distance from a potential parent than the other two species.
However, the abundance of late-stage seedlings Interestingly, our survey found fewer (Table 1)
was similar at both sites (Table 1). Moreover, Q. and smaller (Fig. 2) early-stage Q. agrifolia
lobata late-stage seedlings were on average larger individuals on the grazed property (ORR) than
at the grazed site, further suggesting that cattle on the ungrazed property (HNHR). Unlike in Q.
grazing may affect the survival of first-year douglasii, Q. agrifolia later-stage seedlings were
seedlings, but may have little or no effect on the both smaller and less abundant (with reference to
ORR HNHR
long-term survival ofestablished seedlings. Quer- mature trees) on than on (Fig. 2,
cus douglasii populations had generally the same Table 1). Moreover, there were far more Q.
HNHR
associations as Q. lobata with the two major land agrifoUa saplings per mature tree on
use patterns (Table 1, Fig. 2), except that there than on ORR (Table 1). As noHn-NadHuRlt stages
was a large number offirst-year and late-stage Q. appear to be more represented at than at
douglasii seedlings in a dense forest patch on ORR, this suggests that Q. agrifolia population
ORR (Table 1). The larger size of late-stage growth on HNHR may be higher than on the
seedlings at the grazed site suggests that cattle grazed adjacent property. Quercus agrifoUa pop-
grazing does not limit (and may even promote) ulations are thought to be stable or even
the growth ofthese seedlings at our study area. increasing in many regions with high forest or
The land-use differences in this survey are shrub cover, but are unable to colonize open
consistent with effects of cattle grazing on Q. habitat, presumably because shrubs provide a
douglasii and Q. lobata observed in other studies. refuge from wildlife or livestock grazing (Call-
For example, experimental manipulation of away and Davis 1998). In the current survey, Q.
grazing did not affect the survivorship of late- agrifoUa was strongly associated with habitats
stage (2-year old) seedlings in a Sierra Nevada with greater shrub and tree cover, and livestock
foothills population of Q. douglasii. (Hall et al. grazing may have imposed a sufficient herbivore
1992), and grazing decreased the total abundance pressure to partially negate the protective effect
of naturally recruiting Q. douglasii seedlings ofhighvegetation coverfor Q. agrifolia seedlings.
(likely including a large portion of first-year The seedlings of each of the three oak species
seedlings) in only one out of four years (Reiner had unique spatial distributions with reference to
and Craig 2011). Livestock grazing may still be a their potential parental trees. The distribution
significant mortality factor for Q. lobata and Q. of first-year seedlings likely reflects patterns of
douglasiv, however, our results suggest that other acorn fall and immediate dispersal. As acorn
mortality factors are potentially more important production is pulsed (i.e., varies dramatically
even in ungrazed sites. from yearto year) and synchronous between trees
2014] PEARSE ET AL.: SURVEY OF CENTRAL CALIFORNIAN OAK STAGES 7
Q. douglasii some sites, resulted in an underrepresentation of
Q. lobata seedlings in the understory ofQ. lobata
mature trees (Fig. 4). Quercus douglasii seedlings
were more abundant under their parent tree
(Fig. 4), although late-stage seedlings tended to
be located further from a potential parent than
first-year seedlings (Fig. 3). Both first year and
late-stage Q. agrifolia seedlings were strongly
clustered around potential parental trees (Fig. 3),
and Q. agrifolia seedlings were highly represented
in the understories of Q. agrifolia trees (Fig. 4).
These results suggest a hierarchy in the ability
of oak seedlings to colonize open habitat.
Quercus lobata was often found far from its
Q. agrifolia parental species, consistent with previous studies
showing that Q. lobata seedlings better tolerate
dry habitats (Mahall et al. 2009) and deer
browsing (Tyler et al. 2008) better than Q.
agrifolia. Quercus douglasii has been shown to
tolerate dry, exposed habitats (Griffin 1971), but
it may be negatively affected by direct competi-
tion from non-native grasses in open areas
(Gordon and Rice 2000) and is associated with
shrub cover (Callaway 1992). Quercus agrifolia
has been shown to be strongly associated with
shrubby or forested habitat, which provides
protection from herbivores (Callaway and Davis
1998).
The factors that limit oak populations are
thought to be somewhat site-specific. This has led
to both studies that attempt to find those factors
that are important across many site (Zavaleta et
al. 2007) and detailed studies that attempt to
assess the causal factors limiting oak seedling
success at individual sites (Davis et al. 2011). Our
study will be useful in both of these goals. By
establishing a baseline survey at a site, where the
historyoffactors thatcouldpotentiallyaffect oak
populations (e.g., grazing, habitat affiliation,
wildlife density) is well documented, our study
yFeiag.r 4a.ndTlhaete-sspteacgieessiedeednltiintgys)ofwialtlhiinmm2a0tumreoofaaksva(flilresyt can easily be combined with other surveys at
oak(Q. lobata = Q. lo),blueoak{Q. douglasii= Q. do), different geographic locations in order to assess
and coast live oak {Q. agrifolia = Q. ag). The the generality of these factors. Likewise, by
understories of both blue oak and live oak were having a population of oaks where all stages are
dominated by conspecifics (likely acorn fall from the surveyed and georeferenced, this surveyopens the
focal tree), however the understory ofvalley oak had a possibility of resampling to determine the key
roughly even representation ofall three species. demographic limitations at this representative
site.
(Koenig et al. 1994b), the distribution of first- Explicit demographic models of Californian
year seedlings may vary substantially from year oak populations are important to understand the
to year. On the other hand, the late-stage sustainability of the state’s oak populations
seedlings of oaks in this habitat may be very (Davis 2011). This study provides a thorough
long-lived (Koenig and Knops 2007), so their baseline estimate ofpopulations of Q. lobata, Q.
distribution is unlikely affected by temporal douglasii, and Q. agrifolia that can be used by
variation in acorn crop. Quercus lobata first year future resampling and modeling efforts to esti-
seedlings were clustered around potential paren- mate demographic transitions between life-histo-
tal trees, likely reflecting patterns of acorn fall. ry stages and thus the long-term dynamics ofoak
Late-stage Q. lobata seedlings, however, were on populations. For example, at the same site, a
average further from a potential parent, especial- marked population of blue oak late-stage seed-
ly on the grazed property (Fig. 3), This, in lings has persisted with little growth, little
combination with few seedlings altogether at mortality, and no transitions to saplings within
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8 [Vol. 61
the past 60 years (Koenig and Knops 2007). blue oak seedling damage and survival. Journal of
Moreover, there arecurrentlyno robust estimates Range Management 45:503-506.
of age-biased mortality for adult valley oaks, Koenig, W. D. andJ. M. H. Knops. 2007. Long-term
which is a critical parameter in estimating the growth and persistence of blue oak {Quercus
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