Six woodland communities have been identified within the Reserve (See Vegetation Map): the Interior Live Oak, Interior Live Oak - Blue Oak, Mixed Oak, Blue Oak, and Valley Oak Alliances, and the Foothill Pine/Mesic Non-serpentine Chaparral Association. The first five of these are dominated by five species of oaks including two evergreen and three deciduous species. The evergreen oaks are interior live oak (Quercus wizlizeni), with grayish, furrowed bark and hard, spiny, shiny green leaves, and the rare oracle oak (Q. xmorehus), a hybrid between interior live oak and black oak. The deciduous oaks include black oak (Q. kelloggii), with deeply furrowed dark bark and bright green, deeply lobed leaves with bristle-tipped teeth, blue oak (Q. douglasii) with scaly, grayish bark and small, rounded, drought-resistant, bluish green leaves, and valley oak (Q. lobata) with thick, furrowed bark and rounded, lobed, dark green leaves. Iris; Photo by Ellen Dean

Relatively mesic, north-facing slopes, particularly in the south portion of the Reserve, support stands of the Interior Live Oak Alliance, co-dominated by interior live and black oaks with a closed canopy. They occupy about 32.5% of the Reserve. The shady understory in these areas contains delicate spring wild flowers such as milk maids (Cardamine californica), varied-leaf nemophila (Nemophila heterophylla), hound’s tongue (Cynoglossum grande), shooting stars (Dodecatheon hendersonii), and iris (Iris fernaldii). Also in the understory are luxuriant native bunchgrasses including California and Idaho fescues (Festuca californica and F. idahoensis) and California melic (Melica californica). The woody component of the Interior Live Oak community includes many of the shrubs found throughout the peninsula, such as toyon (Heteromeles arbutifolia), manzanita (Arctostaphylos manzanita), and ceanothus (Ceanothus oliganthus).

The Interior Live Oak Alliance intergrades in many areas with the newly defined Interior Live Oak - Blue Oak Association (20.2% of the Reserve). In general, this community occupies areas that are slightly drier than the live oak-black oak dominated areas, and slightly steeper than the Blue Oak Alliance areas.
Big-leaf Maple; NRS Archive
The most mesic area of the Reserve, Decker Canyon and its tributaries, supports the only patch of Mixed Oak Alliance (2.1%), in this instance dominated by valley oaks in the riparian areas. Mixed with the stately valley oaks are redbuds (Cercis occidentalis), California buckeyes (Aesculus californica), occasional big-leaf maples (Acer macrophyllum), all entwined in wild grape (Vitis californica) and pipevine (Aristolochia californica). Blue and black oaks are mixed with the valley oaks in this area.

In some of the wettest areas of Mixed Oak, the understory is lush with maidenhair fern (Adiantum jordanii), bedstraw (Galium boreale), and hedge nettle (Stachys ajugoides). In the more open areas, large individuals of both poison oak (Toxicodendron diversilobum) and skunkbrush (Rhus trilobata) mix with native and exotic grasses. Snowberry (Symphoricarpus spp.) is also common.Blue Oaks; Photo by Virginia Boucher

Compared with the forest-like, closed canopies of the Live Oak and Mixed Oak areas, the Blue Oak Alliance (14.9%) is more of a woodland, with widely-spaced, generally fairly small blue oaks and few other woody species. Patches of Blue Oak Alliance occur primarily in relatively flat areas or ridge tops and often have beautiful relictual stands of purple needlegrass (Nassella pulchra), blue wildrye (Elymus glaucus), and small fescue (Vulpia microstachys), in addition to the ubiquitous wild oats (Avena fatua) and bromes (Bromus hordeaceus and B. diandrus), as an understory. Many lovely spring forbs grace the slopes as well – these include baby stars (Linanthus spp.), yarrow (Achillea millefolium), climbing bedstraw (Galium porrigens), and woodland stars (Lithophragma affine and L. parviflorum). In many areas the Blue Oak community grades into Interior Live Oak - Blue Oak. This community covers large areas near the north tip of the peninsula.

A small patch of another woodland community is found in the northeastern part of the Reserve – the Foothill Pine/Mesic Non-serpentine Chaparral Association (0.4%). This is a unit that was defined during the vegetation mapping of Napa County and is characterized by gray pines (Pinus sabiniana) as emergents from non-serpentine chaparral.

Nearby, in the canyon bottoms both east and west of the Reserve, there are patches of habitat that map as Valley Oak. The bottomland, riparian habitat in which the alliance is found is characteristic. Valley oak is commonly mixed with blue or black oak in this alliance.

Photo Credits: Title, Ribes malvaceum (Dan Tolson), Iris (Ellen Dean), Big-leaf maple (NRS Archive), Blue Oaks (Virginia Boucher). For more pictures see:

This page last updated: March 15, 2016  

Contact: Dr. Virginia Boucher
John Muir Institute of the Environment
109 The Barn, University of California, Davis, CA 95616
Phone: 530-752-6949; email:

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