Berberis Shrub Forms in Hays County
| B. swaseyi | B. trifoliolata | |
|---|---|---|
| Cheatham 2000 | like ... B. haematocarpa: much branched | rounded shrubs usually rise only 5-15 dm |
| Correll & Johnston 1970 | small intricate shrub | with rigid ascending or spreading branches |
| Vines 1960 | with erect or spreading branches | evergreen shrub to 10 ft [30 dm] |
| Buckley 1870 | much branched | grows in dense clumps ... forming an impenetrable mass when fully grown |
Comparing shrub forms and structural details of our species is difficult without knowledge of each bush's age and history, especially the impact of wildfires, browsing by both cattle and deer, as well as insect depredation (the larvae of the barberry webworm, Omphalocera dentosa). Some differences are clearly the result of microenvironmental conditions; e.g., sun, soil and moisture. I have limited my comparisons below to plants in nearly full sun along the bottomland of our valley, with moisture solely from rain.
Although I found no categorically distinctive characteristics, a number of general tendencies seem valid, with individual exceptions.
Shrub Form
B. trifoliolata tends to form a dense, rounded shrub, wider than tall, rounded by virtue of branches radiating out from a subterranean base (lignotuber) in all directions, lowermost at an angle close tothe ground. B. swaseyi is generally more open, less dense, rarely rounded, and often taller than wide, lowermost branches generally not arching to the ground. Both species are branched from the base, but B. trifoliolata is typically more strongly branched, contributing to its dense clump form. In contrast B. swaseyi tend to have numerous well developed lateral (long shoot) branches (see discussion of stem dimorphism). Typical shape and branching is illustrated in the pictures below. As with other features the hybrid plants vary a great deal. These sometimes share the dense foliage of B. trifoliolata, but are taller than wide. (For hybrid shrub pictures.)
| Three perspectives of the branch structure of one larger B. swaseyi bush. | ||
|---|---|---|
Viewed from directly above. | 
Viewed from the side. |
Side view of the base. |
| Three perspectives of the branch structure of one larger B. trifoliolata bush. | ||
Viewed from directly above. | 
Viewed from the side. |
Side view of the base. |
Two different bushes, with full leaf cover:
B. swaseyi |
B. trifoliolata |
Durand (1972) suggests that browsing by deer may play a role in the size and shape of B. swaseyi (p. 322):
Browsing by deer may account for a dwarfing effect, and the larger bushes may escape pruning of terminals and thereby attain a much greater height.This view is not without merit, but numerous other factors need to be considered. The dense growth habit of B. trifoliolata together with its stiff spiny leaflets creates an impenetrable barrier to browsing during periods of primary long shoot growth following rainy periods. Cattle as well as deer do eat the succulent young shoots, but much less so than with B. swaseyi, with its sparser, smaller and less rigid armament. In a sense B. trifoliolata is perfectly adapted for protection from herbivores.
North leaning B. swaseyi.
(Asymmetrical growth via a larger oak, now gone?)
In contrast, B. swaseyi's primary period of vegetative growth is seasonally triggered, occurring at roughly the same time as flowering and leaf fall of older leaves – making the plant more vulnerable to browsing. This difference might seem to account for the symmetrical rounded habit of B. trifoliolata as opposed to B. swaseyi. But it wouldn't account for height differences, why one species is more subject to dwarfing than the other (if this is in fact true), or what enables B. swaseyi to grow large enough to escape terminal pruning in the first place. Perhaps the answer is to be found in the fact that while B. trifoliolata is ubiquitous in our area, B. swaseyi is mostly found in close association with oaks (and other hardwoods), more often than not on the north side of a live oak motte. (Note the above asymmetrical, north-leaning habit of the large bush in the above picture.) The drip line of large oaks is well mulched and tends to abound in prickly young Ashe juniper, planted by migratory birds, and other brush, some of which is less palatable — an environment both protective and conducive to relatively faster vertical growth.
