Six woodland communities have been identified within the Reserve
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.
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.
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.
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
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.