Marshall Johnston some 30 years ago identified B. trifoliolata X B. swaseyi hybrids on our small (c. 50-acre) tract in the Hays County hill country west of Austin, Texas (30° 17' 20" N, 98° 09' 57" W; elevation c. 280–335 m). Our land consists of a valley of bottomland, largely of oaks and grasses, with a creek down the middle and arid slopes on the east and west, dominated by ashe juniper and grasses plus scattered oak groves. It is 2 miles upstream from the Pedernales River, where B. swaseyi was first discovered by S. B. Buckley in 1866.
B. swaseyi is only rarely found on the steeper areas with thin calcareous soil, and even then, only in the immediate vicinity of oaks. It is uncommon in the juniper thickets. Its distribution is essentially complementary to that of our Arbutus xalapensis, which volunteers abundantly under the junipers. B. trifoliolata thrives in all areas.
In our area of the hill country B. swaseyi is not typically a plant of
"limestone ridges and canyons" (Whittemore) - following Correll & Johnston, Manual of the Vascular Plants of Texas, 1970).
A census of B. swaseyi bushes over 18 inches tall counted some 540 plants. I have never attempted a census of the somewhat more common B. trifoliolata. The number of assumed hybrids at the start of the current study (January 2004) was 7. As it progressed, and a clearer set of hybrid characteristics was determined, the number of certain hybrids grew to 48, with two of the original 7 no longer satisfying these criteria. In addition, as more detailed morphological features were investigated, a number of plants that would otherwise key out to one of the two species appeared to provide evidence of possible introgressive hybridization (such as T23 pictured above).
My impression was that Johnston had based his hybrid identification primarily on the large, juicy fruits of a few of my multileaflet Berberis bushes - normally keyed to B. swaseyi. But my study of ripe fruits of all Berberis plants in 2002-2003 found only a continuum of size, fleshiness, shape, and color, with no clear boundaries that could be used to identify an individual plant.
In 2004 I decided to undertake a phenological survey of 20 plants of each species plus a half dozen or so plants that - on other grounds (primarily leaf characteristics) I had identified as potential hybrids. All but one survey plant (a hybrid) were evenly distributed along a half-mile route of bottomland, most commonly along the edges of oak groves, with a wide variation in exposure. Each potential hybrid had more than three leaflets; most had predominantly unstalked, sessile terminal leaflets. In the case of hybrids both B. trifoliolata and B. swaseyi plants in their immediate vicinity were selected for the survey.
The survey began the last week of January, when only a few B. trifoliolata had just begun to bud out, and twice a week stages of development for the 50 some plants of the original list were recorded. In the course of the spring I added plants to each category as I discovered individual bushes that seemed to have interesting properties, as well as another half dozen potential hybrids. The biweekly survey was continued until the beginning of July, when the last fruits of B. swaseyi had disappeared. I continued the survey with reduced frequency to record changes in leaf and stem development through January 2005, and then once again for the bloom and fruiting periods of spring 2005 and 2006.
Roughly a half week of overlap was found in the bloom period of the two species, the last week of March (recorded as March 25 & 27) and this was extended to almost three weeks in 2006 (March 12 — March 30).
Details of the blooming and fructescence periods from February to the end of June (2004 & 2005) are:
Given that my earlier examination of Berberis fruits did not match details in the literature, I decided to conduct a careful examination of every aspect of Berberis morphology that might possibly provide support for the status of my assumed hybrid plants as well as introgressive hybridization.
In studying a number of morphological features I included specimens and species from outside Hays County and even outside our Central Texas area, in part through visits to other locales and by examination of specimens in the U. Texas Herbarium. To understand the basis for some of the statements about Berberis in the taxonomic literature I consulted earlier taxonomic sources, from Linnaeus to Fedde, as well as recent taxonomies from other regions of the U.S.