Monographs of the Western North American Naturalist

Volume 7 8th California Islands Symposium Article 15

12-4-2014 Stand structure and acorn production of the island scrub (Quercus pacifica) Mario B. Pesendorfer School of Biological Sciences, University of Nebraska, Lincoln, NE, [email protected]

Kathryn M. Langin Department of Biology, Colorado State University, Fort Collins, CO, [email protected]

Brian Cohen The Nature Conservancy, San Francisco, CA, [email protected]

Zachary Principe The Nature Conservancy, San Francisco, CA, [email protected]

Scott A. Morrison The Nature Conservancy, San Francisco, CA, [email protected]

See next page for additional authors

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Recommended Citation Pesendorfer, Mario B.; Langin, Kathryn M.; Cohen, Brian; Principe, Zachary; Morrison, Scott A.; and Sillett, T. Scott (2014) S" tand structure and acorn production of the island scrub oak (Quercus pacifica)," Monographs of the Western North American Naturalist: Vol. 7 , Article 15. Available at: https://scholarsarchive.byu.edu/mwnan/vol7/iss1/15

This Monograph is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Monographs of the Western North American Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Stand structure and acorn production of the island scrub oak (Quercus pacifica)

Authors Mario B. Pesendorfer, Kathryn M. Langin, Brian Cohen, Zachary Principe, Scott A. Morrison, and T. Scott Sillett

This monograph is available in Monographs of the Western North American Naturalist: https://scholarsarchive.byu.edu/mwnan/ vol7/iss1/15 Monographs of the Western North American Naturalist 7, © 2014, pp. 246–259

STAND STRUCTURE AND ACORN PRODUCTION OF THE ISLAND SCRUB OAK (QUERCUS PACIFICA)

Mario B. Pesendorfer1,2,5, Kathryn M. Langin3, Brian Cohen4, Zachary Principe4, Scott A. Morrison4, and T. Scott Sillett2

ABSTRACT.—Island scrub oak (Quercus pacifica), a keystone chaparral species on Santa Rosa, Santa Cruz, and Santa Catalina islands, provides habitat for a diverse assemblage of and animal species. The restoration of oak habitat is a management priority, but little is known about Q. pacifica stand structure and acorn production, 2 parameters that are important in the species’ recovery. To investigate whether species interactions and abiotic conditions have an effect on stand structure and acorn production, we sampled within-stand densities, sizes, and acorns in the 3 island popula- tions that have been exposed to different herbivores, seed predators, and climate conditions. Stand densities varied more within than between islands; but Santa Rosa, the coldest of the 3 islands, had smaller with smaller acorns than the other 2 islands. To quantify the temporal and spatial variation in acorn production on , we conducted acorn counts at 2 spatial scales: (1) an island-wide survey of 200 trees along the east–west axis of the island (2008–2012) and (2) small-scale surveys within three 100-ha study plots (150 trees; 2009–2012). Acorn production varied strongly, both temporally and spatially, with little temporal synchrony and spatial autocorrelation. Trees at higher eleva- tions produced more acorns, but the roles of temperature and precipitation were unclear in this relatively short study. To increase our understanding of the drivers of Q. pacifica acorn production, we propose that annual oak surveys be incor- porated into a long-term monitoring program across the California Channel Islands.

RESUMEN.—El roble de la isla (Quercus pacifica), una de las especies clave en el chaparral de las islas de Santa Rosa, Santa Cruz y Santa Catalina, proporciona un hábitat adecuado para una gran variedad de especies de plantas y animales. La restauración del hábitat del roble es una prioridad, pero se conoce poco sobre su estructura y la producción de bel- lotas de Q. pacifica, 2 parámetros muy importantes para la recuperación de la especie. Con el fin de investigar si la inter- acción de las especies y las condiciones abióticas tienen algún efecto sobre la estructura forestal y la producción de bel- lotas examinamos la densidad de pies, el tamaño de los árboles y el tamaño y producción del bellotas en las poblaciones de las 3 islas, las cuales han estado expuestas a diferentes herbívoros, depredadores de semillas y condiciones climáticas. Las densidades variaron más dentro de la misma isla que entre islas, pero Santa Rosa, la más fría de las 3 islas, tenía árboles más pequeños con bellotas más pequeñas que las otras 2 islas. Para cuantificar la variación temporal y espacial en la producción de bellotas en la isla de Santa Cruz, estimamos la producción de bellotas en 2 escalas espaciales: (1) mediciones de 200 árboles en el eje este-oeste de la isla (2008–2012) y (2) en el interior de 3 parcelas de 100 ha (150 árboles; 2009–2012). La producción de bellotas varió considerablemente, tanto temporal como espacialmente, con poca sincronización temporal y autocorrelación espacial. Los árboles situados a mayor altura produjeron más bellotas, pero el papel de la temperatura y las precipitaciones no quedó claro en este estudio relativamente corto. Proponemos que se incorporen controles anuales de los robles en un programa de monitoreo de larga duración a lo largo de las Islas del Canal de California (o Archipiélago del Norte) con el fin de aumentar nuestro conocimiento sobre la producción de bel- lotas de Q. pacifica.

Oak (Quercus spp.) woodlands and chaparral Endemic island scrub (Quercus pacifica form the majority of woody vegetation on the Nixon & C.H. Muller) are the dominant species California Channel Islands and represent an of chaparral on Santa Cruz, Santa Rosa, and ecosystem of conservation concern (Schoen- Santa Catalina islands (Junak et al. 1995), yet herr et al. 1999, Knapp 2010a). Restoring native little is known about their reproductive strate- oak habitat is a management priority because gies and interactions with other species (Knapp these keystone organisms facilitate establish- 2010a, Stratton 2010). We surveyed the 3 ment of other native and herbs, as well island populations for systematic differences as harbor an array of vertebrates and inverte- in stand structure potentially caused by inter- brates (Borchert et al. 1991, Knapp 2010a). actions with herbivores and seed dispersers.

1School of Biological Sciences, University of Nebraska–Lincoln, Lincoln, NE 68588-0118. 2Migratory Bird Center, Smithsonian Conservation Biology Institute, National Zoological Park, Washington, DC 20013. 3Department of Biology, Colorado State University, Fort Collins, CO 80523. 4The Nature Conservancy, San Francisco, CA 94105. 5Present address: Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY 14850. E-mail: [email protected]

246 2014] QUERCUS PACIFICA STANDS AND ACORN CROPS 247

We also present the first 5 years of data from acorn crops varies tremendously in most oaks longitudinal surveys we established to monitor and is often synchronized over relatively large the spatial and temporal variation in acorn geographical areas—a reproductive pattern production of Q. pacifica on Santa Cruz Island. called masting (Koenig and Knops 2000, Kelly Oaks have long-lasting effects as ecosystem and Sork 2002). Masting occurs in response to engineers by changing the biotic and abiotic environmental cues and may be driven by limi- environment (Jones et al. 1994, Johnson et al. tations in plants’ abilities to pollinate (Kelly 2009). Carbon and nitrogen deposition enhance and Sork 2002). The temporal variation in available nutrients in the soil (Dahlgren et al. seed crops may be an evolved strategy with 1997, Fralish 2004). Deep roots and the ability endogenous cyclical patterns, rather than only to tolerate water stress allow oaks to establish a response to environmental cues, but precipi- in seasonally dry areas intolerable to other tation and temperature also appear to affect chaparral species (Davis and Mooney 1986). pollination and fertilization (Sork et al. 1993, Acorns provide an important food resource for Koenig and Knops 2013). Acorn crop sizes in many animals, including endemic subspecies drought-stressed areas of Florida, for example, of the deer mouse (Peromyscus maniculatus) appear to be strongly dependent on precipita- and the Island Scrub-Jay (Aphelocoma insu- tion (Abrahamson and Layne 2003). laris), which occurs only on Santa Cruz Island Temperature and seed disperser selectivity (Schoenherr et al. 1999). Oak habitat through- have both been suggested as determinants of out the California Channel Islands is recover- acorn size (Aizen and Woodcock 1996, Koenig ing from livestock overgrazing and woodcut- et al. 2009). Plants that depend on few or even ting during the 19th and 20th centuries a single animal species for dispersal often (Schoenherr et al. 1999). Once-common oak experience selective pressures created by the woodland and chaparral on Santa Rosa Island disperser’s behavior and selectivity (Gómez were reduced to remnant populations (Lom- 2004). Jays are perhaps the most important bardo and Faulkner 2000). On Santa Catalina avian dispersers of acorns, and their local pref- Island, continued herbivory by nonnative un - erences can result in selection pressure on gulates coupled with adult dieback of stem acorn size (Gómez 2004). Acorn germination tips have reduced the scrub oak distribution and survival data suggest that larger acorns by 31% since 1943 (Knapp 2010b), and a vari- are more likely to survive drought and have a ety of restoration strategies have produced greater chance of reaching the seedling stage mixed results (Stratton 2010). On the other (Aizen and Woodcock 1996, Bonfil 1998). hand, management actions on Santa Cruz and Acorn sizes decrease with increasing latitude Santa Rosa islands show promising results. within (Aizen and Woodcock 1992, Koenig et Native vegetation on Santa Cruz Island has al. 2009) and between species (Aizen and Pat- experienced a strong recovery following the terson 1990), mirroring a global pattern across removal of feral sheep (Ovis aries) by the late a majority of seed-producing plants (Moles 1990s and pigs (Sus scrofa) by 2007 (Schuyler et al. 2007). The latitudinal gradient in acorn 1993, Faulkner and Kessler 2011, Morrison sizes across North America could have arisen 2011). Similarly, since the removal of intro- from differential dispersal of small seeds duced mule deer (Odocoileus hemionus) and northward as hardwood populations expanded Roosevelt elk (Cervus canadensis) from Santa after glacial maxima (Aizen and Patterson 1990, Rosa Island in 2011, seedlings of woody species Aizen and Woodcock 1992). More recent work, are slowly reappearing in areas where they however, failed to support this hypothesis, and were long absent (K. McEachern, USGS, per- abiotic parameters such as temperature and sonal communication). Nonnative mule deer precipitation appear to explain the pattern, at are still common on Santa Catalina (Schoen- least in some oaks (Koenig et al. 2009). herr et al. 1999, Manuwal and Sweitzer 2010). Herbivory, seed dispersal, and seed preda- Acorn production and seed dispersal will tion can also affect stand density of oaks. In likely play an important role in habitat re - Wisconsin, ungulate herbivory has been shown covery (Stratton 2010), but we have no data to reduce the number of red oak (Q. rubra) on the spatial and temporal patterns of acorn seedlings. This reduction resulted in lower production in Q. pacifica and other oak adult densities and thus affected the structure species on the islands. The annual size of of whole hardwood forest stands (Rooney and 248 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST [Volume 7

Waller 2003). Similarly, herbivores negatively Catalina islands (15.90 +– 0.66 °C; WRCC 2013). affect stand densities of Mediterranean oaks Comparing the 3 populations of Q. pacifica in Spain because many eat oak seedlings thus allows us to gain an initial overview of (Plieninger et al. 2003). The presence of seed differences between populations and to evalu- predators can also affect within-stand densi- ate the potential role of biotic (dispersal and ties by reducing the number of seeds dis- predation) and abiotic conditions (tempera- persed by birds (Siepielski and Benkman ture) in shaping acorn production and stand 2008). Santa Cruz Island has no remaining structure. If temperature drives acorn size, we introduced ungulates that are acorn or seed - would expect smaller acorns on Santa Rosa ling predators (Bowen and VanVuren 1997, Island, the colder island. In contrast, if seed Schoenherr et al. 1999). Herbivory can cause dispersal affects acorn size of Q. pacifica, we resprouting in oaks, both in seedlings and in would expect Santa Cruz Island acorns to dif- adults (Del Tredici 2001). Therefore, the 3 Q. fer systematically from the other 2 popula- pacifica populations on the Channel Islands tions. We tested our predictions by comparing may have been equally shaped by browsing, in volumes and size categories of Q. pacifica addition to seed dispersal and seed predation. acorns from all 3 islands. We hypothesized that within-stand densi- To investigate whether Q. pacifica acorn ties (the number of trees per area of oak habi- production follows a masting pattern, we deter- tat) would be greater on islands with prolific mined the spatial and temporal fruiting syn- long-distance seed dispersers (Santa Cruz) chrony of individual trees. We also modeled than on the islands with only seed predators the relationships between these data and tree and herbivores (Santa Catalina, Santa Rosa). characteristics, such as crown size and eleva- Island Scrub-Jays occur in high densities on tion. This effort establishes a baseline for future Santa Cruz Island, and individual birds cache studies of Q. pacifica acorn production and more than 3500 acorns a year, many of which can inform oak management on the Channel remain in the ground (Sillett et al. 2012, Islands. We also provide data on Q. agrifolia Pesendorfer 2014). In contrast, other birds for comparison and discussion purposes. that handle acorns occur in low densities on all 3 islands. Northern Ravens (Corvus corax) METHODS tend to cache carrion, not seeds (Goodwin 1983); whereas Acorn Woodpeckers (Melaner- Island Comparison pes formicivorus) store acorns in granaries We measured stem density and distribution (specialized storage areas in trees), a dead end of crown sizes on Santa Cruz, Santa Rosa, and in terms of seed dispersal (Koenig 1987). If Santa Catalina islands by using point-centered herbivory, not seed dispersal, is the driving quarter transects (PCQT; see below) through factor behind stand densities of Q. pacifica, intact Quercus stands (Mitchell and Evans we would expect to find little difference 2006, Siepielski and Benkman 2008; Table 1, between the islands. Fig. 1). On Santa Cruz Island, we selected 2 The distribution of Q. pacifica lends itself stands (one north facing, one south facing) on to addressing hypotheses about the drivers of each of 3 study plots used for a long-term seed size variation and stand structure on a avian study (Caldwell et al. 2013; Fig. 1). small scale because the islands vary in abiotic These stands were near but not overlapping parameters, such as temperature and precipi- the areas selected for annual acorn surveys. tation, as well as in the presence or absence of On Santa Catalina and Santa Rosa islands, we seed predators, dispersers, and herbivores. In selected stands accessible from roads. For terms of temperature and precipitation, Santa PCQT, points were marked every 50 m Catalina and Santa Cruz islands experience (depending on stand size) along straight similar temperatures, whereas Santa Rosa 300–500-m transects through continuous stands; Island is colder, mainly due to localized and at each point, the distance to the nearest oceanic currents and fog input (Spalding et al. oak (taller than 30 cm) in each quadrant (NW, 2007, Fischer et al. 2009). The annual average NE, SW, SE) was measured (Mitchell and temperature from 2002 to 2012 was lower on Evans 2006, Siepielski and Benkman 2008). In Santa Rosa (13.02 +– 0.36 °C; mean +– SD) than September and October 2010 and 2011, we on Santa Cruz (15.38 +– 0.62 °C) and Santa conducted 12 PCQT on Santa Cruz (446 trees) 2014] QUERCUS PACIFICA STANDS AND ACORN CROPS 249

Fig. 1. Map of transect and survey locations in the 3 Quercus pacifica populations: A, Santa Rosa and Santa Cruz islands; B, Santa Catalina Island; C, plot-wide survey trees on Santa Cruz Island. Inset shows location of study area in relation to California. and 5 on Santa Rosa (147 trees) islands. We (0–4 scale; Koenig et al. 1994), level of insect conducted 3 PCQT on Santa Catalina Island seed predation (proportion of a tree’s acorns (96 trees) in September 2011. For each tree, we with oviposition holes; Espelta et al. 2009), determined species, measured longest crown and the presence or absence of oak galls. Galls diameter, and made visual estimates of are induced by oak gall wasps (Hymenoptera: crown cover (0–9 scale), brown leaves (in 5% Cynipidae) that lay their eggs into meristematic increments), acorn size (0–9 scale, 9 represent- tissue of branches; the effect of these galls ing extremely large acorns), acorn abundance on host trees is unclear (Stone et al. 2002). 250 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST [Volume 7

TABLE 1. Temperatures and precipitation on Santa Cruz weeks of October: the island-wide survey Island from 2008 to 2012a. starting in 2008 and the plot-wide survey in b c d Precip Temp dry Temp wet 2009. Here, we report results of data collected Year (mm) (°C) (°C) up to fall 2012: 5 years of island-wide and 4 2008 511 17.7 (2.7) 13.3 (2.6) years of plot-wide surveys. 2009 254 18.4 (2.0) 14.1 (3.0) Acorn productivity was quantified annually 2010 479 17.0 (2.4) 12.5 (2.2) using the canopy survey method (Koenig et al. 2011 651 15.6 (1.8) 12.9 (1.9) 2012 319 16.1 (1.8) 12.7 (2.3) 1994). Two researchers counted acorns simul- taneously for a duration of 15 s on separate aData from WRCC (2013) bTotal precipitation June–May preceeding annual acorn survey areas of the crown. The 2 counts were then cMean (SD) monthly temperature for April–September preceeding annual summed, providing the 30-s acorn count. acorn survey dMean (SD) monthly temperature for October–March preceeding annual Koenig et al. (1994) found this method to yield acorn survey results equivalent to trap methods (i.e., meth- In 2011, we collected 10 acorns from trees ods that collect acorns after they have fallen with sufficient crop sizes on all 3 islands (total from the tree) and found that the counts pro- n = 1090), measured their width and length vide a reliable measure of relative acorn abun- using calipers, and weighed them using a dance, independent of tree size. The location Mettler Toledo digital scale. We assessed of each sampled tree in both surveys was insect predation by searching the acorns for recorded with a GPS unit, and all trees were oviposition holes (Espelta et al. 2009). marked with a numbered identification tag to facilitate locating them in subsequent years. Acorn Crop Surveys on Santa Cruz Island Statistical Analysis We conducted both island-wide and plot- wide surveys of acorn productivity on Santa One goal of this study was to describe how Cruz in an effort to examine spatial and tem- species interactions and abiotic factors shape poral variation. The island-wide survey that oak populations by comparing Q. pacifica started in 2008 ranges from the Montañon stands, trees, and acorns among all 3 islands. range to the western end of the island’s south To estimate absolute stand density and the rela- ridge, near Sauces Canyon (Fig. 1A). Oak tive density of Q. pacifica and Q. agrifolia, we stands were selected within 100 m of a road used PCQT methods (Mitchell and Evans while maximizing the variation of potential 2006, Siepielski and Benkman 2008). Estimates predictors such as distance from shore, eleva- were corrected when no tree was within 25 m tion, geology, soil, and presence of fog (Cohen in a quadrant (Warde and Petranka 1981). Due et al. 2009). Multiple trees were chosen on to low sample sizes on Santa Catalina and arrival at each location by using randomly Santa Rosa islands, we did not use statistical selected compass bearings. Of the 200 selected tests to compare density estimates, and we trees, 140 were Q. pacifica and 58 were Q. summarized the results of the density tran- agrifolia. We augmented the island-wide sur- sects in Table 2. To compare crown size distri- vey in 2009 with additional surveys in 3 study butions, we pooled the data from trees in the plots (Coches Prietos Canyon, Portezuela, and stand-density transects with the data from our the area around the University of California 2 longitudinal acorn surveys on Santa Cruz Santa Barbara Field Station in the central val- Island (total n = 1190 trees) and used the ley of the island; Fig. 1C) used since 2008 to Kolmogorov–Smirnoff test with a Bonferroni monitor Island Scrub-Jays (see Caldwell et al. correction for multiple testing to determine 2013). We selected 150 trees throughout the whether the distributions differed significantly total area of the Island Scrub-Jay research between the islands. plots (~300 ha) by generating random GPS To compare acorn sizes across islands, we points a minimum distance of 20 m apart using calculated acorn volume from length and Hawth’s tools for ArcGIS 9.2 (Beyer 2004, width by using the formula for the volume ESRI 2009). From each point, we located the of an ellipsoid. Volume predicted wet mass of nearest oak tree, unless it was in a hazardous weevil-free acorns well (R2 = 0.93), which location. Of the 150 trees, 131 were Q. paci- allowed us to include acorns with weevil dam- fica and 19 were Q. agrifolia. Both surveys age in the comparison. We constructed linear were conducted annually within the last 2 mixed models (LMMs) in the lme4 package 2014] QUERCUS PACIFICA STANDS AND ACORN CROPS 251

TABLE 2. Stand-density transects. Location (general aspect), number of sampling points, overall and partial density for Quercus pacifica and Quercus agrifolia (trees · ha–1), and percentage of Q. pacifica trees with acorns. Density ______Q. pacifica Island Transect (aspect) Points Total pacifica agrifolia w/acorns Santa Rosa Cherry Canyon (W) 9 307.8 307.8 — 36% Cherry Canyon (E) 11 566.9 276.9 158.2 77% Windmill Canyon (S) 11 252.0 76.2 — 62% Lobo Canyon (S) 8 87.9 10.3 43.9 95% Lobo Canyon (N) 7 164.4 — 127.8 50% Santa Catalina Bulrush Canyon (S) 8 217.0 217.0 — 75% Bulrush Canyon (N) 8 84.4 84.4 — 88% Airport (N) 8 182.3 182.3 — 41% Santa Cruz Field Station (S) 8 170.4 165.1 5.3 41% Field Station (N) 10 816.3 816.3 — 10% Portezuela (S) 10 97.8 97.8 — 80% Portezuela (N) 10 398.4 309.0 16.3 53% Coches Prietos (S) 10 58.3 58.3 — 68% Coches Prietos (N) 9 76.0 76.0 — 15% Isthmus (S) 10 46.4 46.4 — 23% Isthmus (N) 10 465.5 465.5 — 45% Christy Canyon (N) 10 468.5 468.5 — 49% Sauces (W) 10 1164.8 1164.8 — 26% Willows Canyon (S) 10 218.8 131.3 49.2 33% Valley Peaks 10 103.3 103.3 — 28% in R (Bates et al. 2012, R Core Team 2012) matrix and does not report the degrees of free- with Island as fixed effect, and Tree nested in dom (Bolker et al. 2009). Transect as a random effect to account for We analyzed acorn counts from our annual sampling multiple acorns from the same tree surveys to test (1) whether year, elevation, and and to control for variation within transects. canopy size predicted acorn production and For all models with trees as replicates, rather (2) whether acorn production showed spatial than within-tree counts, we used Transect as a or temporal synchrony. The 2 surveys were random effect to control for local effects (Zuur each analyzed separately, with 5 years (2008– et al. 2009, Bates et al. 2012). 2012) of data for the island-wide survey and 4 We used generalized linear mixed models years (2009–2012) for the plot-wide survey. (GLMMs) for comparison of data that could We constructed GLMMs with Poisson error not be transformed to meet model assump- distributions—the fixed effects being Crown tions of LMMs. To compare the proportions of Diameter, Elevation, and Year—while control- parasitized acorns, we used a beta error distri- ling for repeated sampling by using Tree bution; for the presence or absence of oak nested within Point (island-wide survey) or galls, we used a binomial distribution. Acorn Plot as random effects (Zuur et al. 2009). The size scores and abundance scores are ordinal comparison of the summed squared residuals scales, best compared using GLMMs with to the residual degrees of freedom indicated Poisson error distributions (Zuur et al. 2009, that the data were overdispersed (c2 = 4335.8, Hox 2010). All models contained the same P < 0.0001). Therefore, we included observa- random effect, Transect. If the overall island tion number (Obs_nmbr) as a random effect comparison indicated significant differences, for each unit of replication, thereby effectively we used the Tukey HSD post hoc comparison eliminating overdispersion (Elston et al. 2001, in the glht function of the R package mult- Bolker et al. 2009). comp (Hothorn et al. 2008). Significance To test for spatial autocorrelation of acorn tests (i.e., c2 for GLMMs, F values for LMMs, counts, we performed a Mantel test in the R and P values associated with fixed effects) package ncf (Bjornstad 2012). In this proce- were obtained from Type II Wald tests in the dure, annual acorn counts for each possible car package (Fox and Weisberg 2011). This pairwise combination of trees are compared procedure is based solely on the estimated with a linear regression, and the Pearson’s parameter coefficients and their covariance regression coefficients are then correlated to 252 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST [Volume 7

Cruz (n=704) Rosa (n=147) Catalina (n=96) Proportion of Trees Proportion of Trees Proportion of Trees 0.00 0.10 0.20 0.30 0.00 0.10 0.20 0.30 0.00 0.10 0.20 0.30 0 5 10 15 20 0 5 10 15 20 0 5 10 15 20

Crown Diameter (m) Crown Diameter (m) Crown Diameter (m)

Fig. 2. Frequency distribution of crown diameters for Quercus pacifica trees in the 3 island populations. the distance between the trees. We investi- Catalina vs Rosa: D = 0.4775, P > 0.0001). gated temporal synchrony in acorn production Acorn abundance index scores on the in- with Kendall’s concordance coefficient W in dividual trees in 2011 were similar for all the R package vegan (Oksanen et al. 2012), islands (c2 = 2.697, P = 0.26), and trees on and we tested whether the synchrony was Catalina had significantly fewer oak galls stronger than expected by chance by permu- than trees on Santa Cruz (c2 = 19.49, P > tating the acorn counts for each tree and year 0.0001; Tukey HSD: z = 4.2, P > 0.001) and 999 times to generate a null distribution of Santa Rosa islands (z = 2.7, P = 0.02). W values. Acorns varied in size, but not in weevil infestation, across the islands. The volumes RESULTS and size scores of acorns were significantly smaller on Santa Rosa Island than on the other Comparison of Island Populations 2 islands (volume: c2 = 7.20, P = 0.03; size Stand densities of Q. pacifica varied broadly scores: c2 = 18.13, P < 0.001; all post hoc within islands but did not reveal a systematic tests with Rosa: P < 0.05). Quercus pacifica difference between islands. Overall, stand acorns on Santa Cruz (mean +– SE: 1913.0 +– densities spanned 3 orders of magnitude, 35.8 mm3) and Santa Catalina islands (1991.6 ranging from 10 to 1164 trees per hectare +– 57.2 mm3) had, respectively, 18.9% and (Table 2). On Santa Cruz and Santa Rosa 23.8% larger mean volumes than acorns from islands, stands on south-facing slopes generally Santa Rosa Island (1608.2 +– 41.8 mm3). Weevils occurred in lower densities and had higher infested between 36% and 40% of acorns, and proportions of trees with acorns than their infestation rate did not differ significantly north-facing counterparts (Table 2). Crown between islands (c2 = 0.24, df = 2, P = 0.887) diameters differed significantly between islands and was not affected by acorn volume (c2 = (GLMM: F2, 957 = 7.91, P = 0.02, Fig. 2), as 0.04, P = 0.841). trees on Santa Rosa Island (mean +– SE: 3.38 +– Acorn Production on Santa Cruz Island 0.19 m, n = 147) were significantly smaller than on Catalina (6.49 +– 0.34 m, n = 96) and Acorn production varied widely between Santa Cruz (5.07 +– 0.13 m, n = 704). The years. Quercus agrifolia trees in the island- distributions of crown diameters also varied wide survey had only one year of high pro- significantly among all 3 islands (Kolmogorov– ductivity (2009), with only a few acorns Smirnoff test with Bonferroni-adjusted P produced in the other 4 years (Fig. 3). Year values: Catalina vs. Cruz: D = 0.2274, P = explained the most variance of Q. pacifica 0.00124; Rosa vs Cruz: D = 0.282, P > 0.0001; acorn counts in both the island-wide (Fig. 3, 2014] QUERCUS PACIFICA STANDS AND ACORN CROPS 253

Fig. 3. Acorn productivity of all trees (n = 198), Quercus pacifica (n = 141), and Quercus agrifolia (n = 57) in the island-wide survey on Santa Cruz Island, 2008–2012. Black horizontal bars indicate the mean; boxes, the interquartile range; and whiskers extend to 1.5 times that range.

TABLE 3. Island-wide acorn counts on Santa Cruz TABLE 4. Plot-wide acorn counts on Santa Cruz Island. Island. The role of year, elevation, crown diameter, and The role of year, elevation, plot, crown diameter, and their their interactions in predicting acorn counts on specific interactions in predicting acorn counts on specific trees. trees. Type II ANOVA for island-wide acorn count GLMM Type II ANOVA for island-wide acorn count GLMM with with Poisson error distribution (n = 123 trees; 5 years). Poisson error distribution (n = 150 trees; 4 years). Fixed effect df c2 P Fixed effect df c2 P Year 4 134.8 <0.001 Year 3 111.4 <0.001 Elevation 1 15.6 <0.001 Plot 2 57.3 <0.001 Crown 1 7.3 0.007 Crown 1 7.6 0.006 Crown : elevation 1 0.1 0.820 Elevation 1 13.0 <0.001 Crown : year 4 5.8 0.214 Year : plot 6 24.9 <0.001 Elevation : year 4 21.9 <0.001 Year : crown 3 6.7 0.083 Crown : elevation : year 4 3.7 0.446 Year : elevation 3 11.6 0.009 RESIDUALS 528 Plot : crown 2 2.3 0.332 Plot : elevation 2 9.6 0.008 Crown : elevation 1 2.0 0.153 Year : crown : plot 6 4.0 0.680 Table 3) and plot-wide (Fig. 4, Table 4) sur- Year : elevation : plot 6 8.7 0.193 veys. In the island-wide survey, acorn counts Year : crown : elevation 3 1.8 0.618 declined from 23.8 +– 2.8 (mean +– SE) acorns Plot : crown : elevation 2 1.8 0.415 per tree in 2008 to 4.9 +– 1.1 acorns per tree in RESIDUALS 451 2012 (Fig. 3). Throughout the 5-year study period, 135 of 140 Q. pacifica trees (96.4%) produced acorns at least once, but only 87 enced higher mean temperatures in both the (63.1%) did so in 2012 (Fig. 5). Plot-wide wet (October–March) and dry (April–Sep- counts declined from 20.3 +– 2.7 acorns per tember) seasons compared to low productivity tree in 2009 to 5.1 +– 1.0 acorns in 2012 (Fig. years. The amount of precipitation, however, 4). Of the 131 Q. pacifica trees in the plot- was the highest in 2008 and only half that wide survey, 111 (84.7%) produced acorns at amount in 2009 (Table 1). some point throughout the study period. In Elevation above sea level also explained 2009 and 2010, 73% of trees produced acorns, significant variation in acorn counts in both whereas <60% of trees carried any acorns in surveys. Trees at higher elevations produced 2011 and 2012. The Coches Prietos plot had more acorns, especially during high productivity the lowest productivity: 30% of trees never years (Figs. 3, 4). In the plot-wide survey, ele- produced acorns and only 12% had acorns in vations were not equally distributed between 2012. The years preceding the 2 years (2008 plots, but the plot at lowest elevation always and 2009) of higher acorn productivity experi- recorded the lowest acorn production. The 254 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST [Volume 7

Fig. 4. Acorn productivity of Quercus pacifica trees (n = 131) in the plot-wide survey on Santa Cruz Island from 2009 to 2012, overall and by plot. Black horizontal bars indicate the mean; boxes, the interquartile range; and whiskers extend to 1.5 times that range. effect of crown diameter was significant in the (n = 123, W = 0.13, P < 0.001) in the island- plot-wide survey but explained little variation wide counts. Plot-wide counts, in contrast, (Table 3) and was not significant in the island- had a moderate degree of temporal synchrony wide data (Table 2). Visual inspection of the over 4 years (n = 131, W = 0.49, P < 0.001). data suggested that the smallest trees drove However, the Mantel test for spatial autocor- the effect of crown diameter. Therefore, we relation indicated no signifi cant effect of dis- reran models on the 2 datasets but excluded tance on the synchrony of acorn counts the lowest 10% of values. The effect of crown between tree pairs in the island-wide survey was no longer significant for the island-wide (Mantel statistic r = –0.050, P = 0.963) or the survey (n = 110 trees, 5 years, c2 = 1.67, P = plot-wide survey (r = –0.008, P = 0.334). 0.196) and was much weaker for the plot-wide survey (n = 132 trees, c2 = 5.58, P = 0.018). DISCUSSION Acorn counts showed weak yet signifi cant Island Comparison signatures of temporal and spatial synchrony. Kendall’s concordance coefficient for temporal Our comparison of Q. pacifica stands pro- synchrony of counts over 5 years was very low vides new information on a keystone chaparral 2014] QUERCUS PACIFICA STANDS AND ACORN CROPS 255

Fig. 5. The influence of elevation on acorn counts of Quercus pacifica trees (n = 140) in the island-wide survey on Santa Cruz Island, 2008–2012, by year and for all years combined. Filled circles indicate zero counts. species on the Channel Islands. Stand struc- on trees is poorly understood, but gall wasps ture, characterized by crown diameter distribu - can be important indicators of ecosystem tions, acorn size, and gall infestation, all differ condition and diversity (Stone et al. 2002, between islands. The trees we sampled on Hayward and Stone 2005). We will thus con- Santa Rosa Island were generally smaller in tinue recording their presence but hesitate crown diameter. Despite similar acorn abun- to speculate about the observed patterns at dances in all 3 populations, acorns on Santa this point. Rosa Island had up to 25% less volume. The smaller crown sizes on Santa Rosa Finally, we observed that the 3 stands we Island could lead to less acorn availability than visited on Santa Catalina Island had a low on Santa Catalina and Santa Cruz Islands but occurrence of oak galls. Only 3% of trees had reveal little about different stand structures. galls, compared to 35% on Santa Cruz and Acorn counts and abundance index scores 19% on Santa Rosa Islands. The effect of galls describe the relative density of acorns on the 256 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST [Volume 7 canopy, and less canopy area would therefore acorns on trees at high elevations (1400 m; result in fewer available acorns overall. Crown Phillips 1992). A similar pattern was found for diameters tell us little about different age blue oaks: acorns at 1000–1300 m were 35% structures across populations. Quercus paci- smaller than acorns at 400–650 m (Phillips fica occurs in 2 main growth forms, a shrubby 1992). On a broader scale, reduced tempera- form (<2 m height) and an arborescent form tures during acorn development are the driv - (<10 m), which makes age determination ing factor for the latitudinal decrease in acorn using trunk diameter difficult (de Gouvenain size across 32 North American oak species and Ansary 2010). It is also unclear how (Aizen and Woodcock 1992, Koenig and Knops ungulate herbivory and subsequent sprouting 2013). Additional years of acorn counts on Santa contributed to this effect. Nevertheless, crown Cruz Island will allow us to address tempera- diameter tends to correlate strongly with ture-related hypotheses in more detail, as the tree biomass or volume and is increasingly western part of that island is cooler and ex- applied in remote sensing studies (Dubayah periences more wind and fog than the eastern and Drake 2000, Popescu et al. 2011). Further- and interior parts (Fischer et al. 2009). more, recent work shows that crown area, esti- Acorn Production on Santa Cruz Island mated from crown diameter, is a reliable predictor of overall acorn production in 5 We observed large variation in acorn pro- common oak species of the eastern United duction throughout our study period, but our States (Rose et al. 2012). inferences about environmental correlates of Reduced tree sizes on Santa Rosa Island annual variation were limited with only 5 years could be due both to species interactions and of data. Our study covered one year of high to abiotic factors. Cooler and windier condi- acorn production in 2008, one year of interme- tions may have contributed to reduced tree diate production (2009), and 3 years of low growth. Our transects on Santa Rosa Island counts (2010–2012). Quercus agrifolia masted also were within the vicinity of the main in 2009, improving island-wide acorn avail- ranch and could have been affected by human ability; but thereafter, relatively few acorns activity over the past 150 years. Basal sprout- have been available to wildlife on Santa Cruz ing in response to ungulate herbivory likely (Fig. 2). The 2 seasons with increased acorn played a major role in shaping the crowns of productivity were both preceded by years with the existing stands, making inference about the relatively higher temperatures. Precipitation role of acorn availability in shaping stand showed no pattern of association with our structure difficult (Knapp 2010a). The presence count data, but we found a positive correlation of ungulates could have exacerbated any between elevation and acorn abundance (Fig. anthropogenic effects because both elk and 5, Tables 3, 4). The annual pattern of the deer feed preferentially on recently emerged island-wide productivity was mirrored by leaves and target seedlings and sprouts, the overall results of the smaller-scale plot- thereby stifling regeneration and growth of wide survey. However, the variation between oak stands (Manuwal and Sweitzer 2010). We our plots underscores the important role of would expect to observe such effects mostly elevation and local abiotic environment in Q. on Santa Catalina Island, which is still home pacifica acorn production: even in good years, to a sizable deer population (Knapp 2010a). trees at low elevation carried few acorns or Temperature differences may also be a failed to produce seed crops. The difference cause of reduced acorn sizes on Santa Rosa between plots also suggests other variables not Island. Acorns of Q. pacifica mature by early captured in our study, such as soil composi- October after pollination in early spring. The tion, moisture, and exposure to solar radiation. maturation process of acorns thus coincides The spatial and temporal synchrony of with the span of the largest temperature dif- acorn production on Santa Cruz was low, de - ference between Santa Rosa and the other spite the strong island-wide fluctuation of acorn islands (WRCC 2013; Table 1). A study of acorn production. The fact that the temporal syn- size of valley oaks in the Sierra Nevada of Cali- chrony was stronger in our small-scale survey fornia also found that trees at low elevations than in the island-wide survey suggests that (400–600 m) and warmer temperatures had local conditions, in addition to island-wide acorns that were almost twice as large as the influences, affect acorn production. Large 2014] QUERCUS PACIFICA STANDS AND ACORN CROPS 257

California oaks, such as Q. lobata, Q. douglasii, ACKNOWLEDGMENTS and Q. agrifolia, exhibit spatial concordance over distances as large as 300 km (Koenig et This research was funded by The Nature al. 1996). Interestingly, the temporal masting Conservancy (TNC), the U.S. National Park patterns of these species arise in response to Service, and the Smithsonian Institution. The variation in environmental factors such as pre- work was performed (in part) at the University cipitation and temperature and appear to have of California Natural Reserve System’s Santa an evolved cycling mechanism (Koenig and Cruz Island Reserve on property owned and Knops 2013). This combination of cyclical managed by TNC. Comments by A. Kamil, R. behavior and a partial dependence on envi- Perea, S. Russo, K. Funk, and 2 anonymous ronmental conditions has also been observed reviewers greatly improved this manuscript. in 5 Florida scrub oak species (Abrahamson We thank Julie King and the Catalina Island and Layne 2003). We recorded only one Conservancy, Christie Boser, Linda Dye, and period of high productivity (2008) and are thus Lyndal Laughrin for logistical support. Luke likely underestimating the degree of spatial Caldwell, Michelle Desrosiers, Elizabeth and temporal reproductive synchrony for Q. Donadio, Laura Duval, Claire Giuliano, Cas- sidy Grattan, Chance Hines, Justin Houck, pacifica. Alexi Kimiatek, Juan Klavins, Katrina Mur- Resprouting will likely affect the ability of böck, Jessica Piispanen, Brandt Ryder, and existing trees to persist, but the dispersal Collin Wooley assisted with data collection. of acorns may be necessary to convert non - native grassland into chaparral. Quercus LITERATURE CITED pacifica readily resprouts, and in the ab - sence of ungulate herbivory, such sprouts ABRAHAMSON, W.G., AND J.N. LAYNE. 2003. Long-term may develop into full-grown trees (Knapp patterns of acorn production for five oak species in 2010a). However, a combination of sprouting xeric Florida uplands. Ecology 84:2476–2492. AIZEN, M.A., AND W. A . P ATTERSON. 1990. Acorn size and and seeding is likely adaptive in regularly geographical range in the North American oaks disturbed vegetation communities (Belling- (Quercus L). Journal of Biogeography 17:327–332. ham and Sparrow 2000). Restoration trials AIZEN, M.A., AND H. WOODCOCK. 1992. Latitudinal trends in acorn size in eastern North American species of on Santa Catalina Island suggest that acorns Quercus. Canadian Journal of Botany 70:1218–1222. planted in a way similar to scatter-hoarding ______. 1996. Effects of acorn size on seedling survival and show high rates of survival, especially during growth in Quercus rubra following simulated spring wet years and if protected from herbivory freeze. Canadian Journal of Botany 74:308–314. BATES, D., M. MAECHLER, AND B. BOLKER. 2012. lme4: (Stratton 2010). Additional years of acorn linear mixed-effects models using S4 classes. R pack- counts and oak stand surveys that include age version 0.999999-0. seedling and sprout counts will provide a BELLINGHAM, P.J., AND A.D. SPARROW. 2000. Resprouting as a life history strategy in woody plant communi- clearer understanding of recruitment in Q. ties. Oikos 89:409–416. pacifica. BEYER, H.L. 2004. Hawth’s Analysis Tools for ArcGIS. The complexity of our results underscores Available from: http://www.spatialecology.com/htool the important role of long-term data collec - BJORNSTAD, O.N. 2012. ncf: spatial nonparametric covariance functions. R package version 1.1–4. tion in understanding the dynamics of oak BOLKER, B.M., M.E. BROOKS, C.J. CLARK, S.W. GEANGE, seed production and stand structure. These J.R. POULSEN, M.H.H. STEVENS, AND J.S.S. WHITE. data establish a baseline for future moni- 2009. Generalized linear mixed models: a practical toring of the Channel Islands’ oak popula- guide for ecology and evolution. Trends in Ecology and Evolution 24:127–135. tions. Future work should be extended to BONFIL, C. 1998. The effects of seed size, cotyledon include detailed stem mapping of oaks at reserves, and herbivory on seedling survival and multiple sites per island and quantifying growth in Quercus rugosa and Q. laurina (). patterns of flowering within and between American Journal of Botany 85:79–87. BORCHERT, M., F.W. DAVIS, AND B. ALLEN-DIAZ. 1991. islands. We also need systematic studies of Environmental relationships of herbs in blue oak how endemic animal populations, such as the (Quercus douglasii) woodlands of central coastal Catalina California ground squirrels (Sper- California. Madroño 38:249–266. BOWEN, L., AND D. VANVUREN. 1997. Insular endemic mophilus beecheyi nesioticus), interact with plants lack defenses against herbivores. Conserva- island oaks. tion Biology 11:1249–1254. 258 MONOGRAPHS OF THE WESTERN NORTH AMERICAN NATURALIST [Volume 7

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