Winter Berries

In between the rainstorms of January and the arctic fronts of February, Lee and I have been exploring the hiking trails in the Bull Creek Preserve.  Bull Creek Preserve, one of the City of Austin's sections of the larger Balcones Canyonlands Preserve, is nesting habitat for the Golden-Cheeked Warbler, an endangered songbird that nests exclusively in Edwards Plateau region of Texas.  To minimize disturbance of the birds during their mating season, the preserve is closed (except to entry-permi holders) from March 1 through July 31.  The preserve, which is located within the Bull Creek watershed between St. Edwards Park and the Bull Creek Greenbelt, includes 150 acres of juniper and oak woodland.  Because several intermittent creeks, tributaries of Bull Creek, originate in and cross through the preserve, the terrain is hilly, with the limestone slopes and steep, narrow canyons characteristic of the Hill Country.

Bull Creek Preserve in winter

The preserve has six miles of trails, including a longer loop and a few shorter loops.  From the Bull Creek Greenbelt, the lower trail (go to the left at the entrance map), is the most scenic, with a short, steep climb up to an viewpoint overlooking the falls on Bull Creek below.  That trail drops back down to the creek and follows the greenbelt trail back to the entrance point.  The rest of the trails (go to the right of the entrance map) wind up and down on a limestone-gravel path that is mostly shaded but periodically offers good views of the juniper-forested hills of the preserve.  The only disappointing section of the trail is the northern-most part of the main loop, where the trail follows an access road used by the Austin Water utility trucks.

The dominant tree in the preserve is Ashe Juniper (Juniperus ashei), which is usually called "cedar" by Austinites.  The densely-branched juniper trees provide nesting sites for the Golden-Cheeked Warblers.  I also saw many Plateau Live Oak (Quercus fusiformis) trees and a mix of deciduous trees without their leaves, mostly Cedar Elm (Ulmus crassifolia) and, especially on slopes and hilltops, Spanish Oak (Quercus buckleyi) trees.

Three evergreen, fruit-bearing shrubs were the noticeable part of the understory.  In sunny clearings, Evergreen Sumac (Rhus virens) branches held many fuzzy, red-orange fruits.  The Evergreen Sumac is dioecious, which means that male and female flowers are produced on separate plants, and the fruit of the shrub provides food for birds and small mammals.  The fruit are high in vitamin C and, if soaked in water, can be used to make a tart-tasting tea.

Evergreen sumac (Rhus virens)

The evergreen shrub common along the shady to part-sun sections of the trail was Yaupon (Ilex vomitoria), a native holly that produces bright red, shiny berries if it receives enough sunlight.  The densely-branched shrub provides nesting and hiding spots for birds, and its berries are food for birds and small mammals.  Like the Evergreen Sumac, Yaupon shrubs are also dioecious, with separate male and female plants.

Yaupon (Ilex vomitoria)

A third, evergreen shrub holding many blue-black fruits grew in sunny woodland openings.  I had seen this shrub along the Greenbelt trail earlier in the fall, so I knew that it was common in our area, but despite its evergreen leaves and distinct blue-black fruits, I had not been able to identify the plant.  This time around I noticed that the branching pattern on this mystery shrub was familiar to me and looked just like the branching pattern of Viburnums or dogwood trees, with strictly opposite branches and V-shaped nodes.  But the dogwoods, which are usually restricted to streamsides in our area and have distinct, parallel-veined leaves that I know by sight, and the Viburnum common to central Texas, Rusty Blackhaw (Viburnum rufidulum), have deciduous leaves.  Eventually, by searching through shrubs related to dogwoods and Virburnums, I found my plant, Lindheimer Silktassel.

Lindheimer Silktassel (Garrya ovata spp. lindheimeri) used to be classified in the Dogwood Family but was reclassified into its own family, the Garryaceae, or Silktassel Family.  Lindheimer Silktassel is a subspecies of Silktassel that is endemic to the Edwards Plateau, where it is common on rocky slopes and limestone ledges, or as an understory shrub at the edge of wooded areas.  The blooms of Lindheimer Silktassel appear in the spring, followed by round fruits that are blue with a whitish coating.  By winter, when much of the whitish coating is gone, the fruit appear blue-black in color.  Lindheimer Silktassel grows to be a large shrub or small tree and provides shelter and food for birds.

Lindheimer Silktassel (Garrya ovata spp. lindheimeri)

Like the Evergreen Sumac and Yaupon, the Lindheimer Silktassel is dioecious, with separate male and female plants.  Overall, being dioecious is unusual in plants – most are monoecious, with male and female flowers on the same plant, or, more commonly, male and female parts in the same flower – yet all three of these evergreen shrubs are dioecious.  Maybe it's just good strategy in a juniper forest, where the dominant tree itself, Ashe Juniper, is also dioecious.

I would like to think that all these winter berries will be food for the Golden-Cheeked Warblers in March, when they return from their wintering grounds in Mexico and Central America and begin establishing territories and nesting in the hills of the preserve.  But warblers eat insects and spiders found in the leaves and bark of the trees, not berries.  Still, healthy shrubs provide shelter for the birds and food for other birds and mammals in the preserve, which promotes biodiversity in the preserve and in the larger Bull Creek watershed, which ensures that the Golden-Cheeked Warblers will continue to have a place to nest in central Texas.