Oaks of Southern California Key

2016

Influence of climatic niche suitability and geographical overlap on hybridization patterns among southern

Madroño, 2014

Oaks are foundational species in much of California, and many oak populations in the state may be in jeopardy due to a lack of recruitment of young trees. Despite considerable interest in this problem, there have been few comprehensive surveys of all stages of oak development. We surveyed all stages of three oaks: Quercus lobata, Q. douglasii, and Q. agrifolia in a forest plot with mixed land-use in central coastal California. We found abundant seedlings of all oak species, but an apparent paucity of Q. lobata and Q. douglasii saplings. First year seedlings of all species were less abundant in parts of the 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 nongrazed areas. Quercus agrifolia seedlings of all stages tended to be clustered around conspecific mature 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 of the 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.

The status and prospects of oaks—those native to California and the many elsewhere—are insufficiently known, despite historical volumes of work done in the past and more ongoing today. That globally there is a blistering diversity of oaks in different environments, and put to distinct uses, is beyond dispute. Less agreed upon, though, is their complex history and the variation in the nature of the oak woodland. Because oak landscapes are so varied, and yet so ubiquitous, perhaps their one single shared feature worldwide is a multiplicity of uses. If individual oaks are cherished, and sometimes known by name, the larger woodlands that they comprise are ecosystems of formidable complexity. That is known; what is not is how best to value oak woodlands—and California's in particular. Different attempts have been made; what is crucial to the continued well-being of California's oak woodlands is coming up with some scheme that makes valuation credible, shareable, inclusive, and ...

Madroño, 2014

Although prolonged seed retention, or serotiny, is believed to be an adaptation to highly variable environments such as the Mediterranean regions of California, no prior study has systematically investigated the prevalence of seed retention among California oaks (family Fagaceae), the dominant woody taxon in California foothill woodlands. We quantified the extent to which acorns were retained into and through the winter and spring within the canopy of five species of California oaks at Hastings Reservation, Monterey County. Significant serotiny was found in coast live oak (Quercus agrifolia) and, to a lesser extent, valley oak (Q. lobata), but was absent in blue oak (Q. douglasii), canyon live oak (Q. chrysolepis), and California black oak (Q. kelloggii). In both species where serotiny was observed, seed retention was primarily predicted by the size of the focal tree's acorn crop. In addition, serotiny in coast live oaks was more prevalent in dry years and when the overall acorn crop of coast live oaks was large. We found no evidence that acorn fall in these species is triggered by a specific environmental event. Prolonged seed retention in California oaks renders acorns available in the canopy to wildlife throughout the winter in some years with potentially significant effects extending beyond those of acorn abundance per se.

In this paper an overview of the range of variations that occur in the genus Quercus (oaks) of the family Fagaceae is given. Oaks are one of the most important groups of flowering plants and dominate large regions of the Northern Hemisphere. We considered oaks from all regions of the world, while focusing on Himalayan oaks. These have not been studied as much by Western workers or by Indian botanists and foresters. With more than 600 species, Quercus is possibly the greatest natural ecosystem-forming genus of the world. Oaks are known for their great variety of growth forms, leaf shape and size, and for their nutrient-rich acorns. Many other trees might exceed the commercial value of oaks, but with regard to biodiversity, they have few parallels in the living world. Outside tropical rainforests, oaks are the greatest biodiversity-centric plants on the planet.

2010

Historical aerial photographs were examined for changes in canopy cover of Quercus pacifica to test the hypothesis that this endemic oak tree has decreased in extent on Catalina Island within the last 60+ years. Photographs from each decade starting in the 1940's were systematically searched for the entire area west of the island's isthmus and changes were mapped and quantified. Since the 1940's, 159 hectares (31%) of oak habitat have been lost in this area, and no recruitment into the canopy layer was noted to offset this loss. The majority of the loss was due to a gradual dieback, with its peak initiation in the 1970's; a small portion was also due to human clearing. Browsing and acorn predation by introduced animals is likely an important factor in the lack of regeneration of these declining woodlands. If this decline was consistent with the rest of the island, oaks would have covered 33% of Catalina in the 1940's, a loss of 1,988 ha. over the past 60+ years. ...

The black oaks of California include 4 tree species (California black oak, coast live oak, Shreve oak, interior live oak) that are known to hybridize. Complex patterns of population variation within each species are likely to result from these hybrid combinations and from subsequent introgressions. We have been studying population variation using biochemical and molecular markers and report results from the former here. Diversity is much greater in interior live oak and in Shreve oak than in either of the other two species, and is least in coast live oak. Shreve oak has not received complete acceptance as a valid species, and is considered as synonymous with interior live oak by many ecologists. However, our biochemical data provided a marker that was present in all populations identified as Shreve oak from the central coast, and was extremely rare in populations of interior live oak from the Cascade and Sierra Nevada mountains. This marker occurred at intermediate frequencies in ma...

Professional Paper

The Southern and Central California Chaparral and Oak Woodlands Ecoregion, which covers approximately 102,110 km 2 (39,425 mi 2), is characterized by a Mediterranean climate with cool, moist winters and hot, dry summers (Omernik, 1987; U.S. Environmental Protection Agency, 1997). Natural vegetation includes chaparral (for example, manzanita, Arctostaphylos spp.) and oak (Quercus spp.) woodlands with extensive grassland and shrubland cover. The low mountains and foothills of the ecoregion border or parallel the Pacific Ocean from Mexico to Point Reyes, California, and continue inland surrounding the Central California Valley Ecoregion (fig. 1). These mountains and hills are interrupted by limited areas of flat land generally used for development or agriculture. The largest developed area in the ecoregion is the Los Angeles Basin, followed by the San Francisco Bay area and the San Diego metropolitan area (fig. 1). The largest agricultural area

Proceedings of the National Academy of Sciences, 2015

Significance Declines in the number of large trees in temperate and tropical forests have attracted attention, given their disproportionate importance to forest structure, function, and carbon storage. Yet, factors responsible for these declines are unclear. By comparing historic (1930s) and contemporary (2000s) surveys of California forests, we document that across 120,000 km 2 , large trees have declined by up to 50%, corresponding to a 19% decline in average basal area and associated biomass, despite large increases in small tree density. Contemporary forests also exhibit increased dominance by oaks over pines. Both large tree declines and increased oak dominance were associated with increases in climatic water deficit, suggesting that water stress may be contributing to changes in forest structure and function across large areas.

2016

The taxonomy of oaks (Quercus) is always a challenge because many species exhibit variable phenotypes that overlap with other species. The scrub White Oaks of California are no exception. In California, Quercus section Quercus (i.e., White Oaks) includes six species of scrub oaks plus four tree oak species. Field identification utilizes leaf traits and acorns, when available, as well as geographic location, but often botanists – with the exception of specialists – are not confident of their assignments. Complicating our understanding of scrub oaks further is the historical and ongoing introgression among taxa. Fortunately, new research using nuclear microsatellite genetic markers and RADseq-based sequences are clarifying their evolutionary relationships. Based on these genetic markers, we describe the phylogenetic relationships among the California scrub and tree White Oaks. Given the impact of hybridization in oaks, we then present a specific example involving three Southern Califo...