San Antonio Natural Area Parks
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Photo by Bill Lindemann, 2007

It flowers pink to violet purple, the leaves change their appearance as they go up the stem, and its fruit bears the enchanting name “silique.” This rare, annual treasure, commonly known as the bracted twistflower and the bracted jewelflower (Streptanthus bracteatus), remains in many ways a mystery. For example, one has to wonder what adaptive value is locked in the mystery of having leaves that have a long petiole (leaf stalk) and are deeply lobed at the base of the stem, but at the top of the stem are clasping and shaped like an arrowhead (Enquist, 1987).

One adaptation the plant does appear to demonstrate is a “sporadic” or “bet-hedging” germination pattern (Pepper, 2007). S. bracteatus seems to hedge its bets by having seeds that germinate gradually over a period of time, rather than all within a few days of each other. To try to understand the potential benefits of a bet-hedging strategy for the jewelflower, investigators considered lab data and field observations.

Researchers studied optimal conditions for germination in the lab. Using a growth chamber, investigators determined that “about 1/3 of the seeds germinate under optimal conditions, and if the soil is allowed to dry out and then watered again, another third of the remaining seeds germinates each time.” (Price, 2007).

Next, came observations of plants in the wild. In the past, field workers usually waited until Spring to begin monitoring known locations of S. bracteatus populations. This was at least partly because by Spring the plant is typically developed enough to positively identify. However, San Antonio Parks and Recreation (SAPAR) staff helped change the timing of field searches for Streptanthus. After SAPAR staff identified newly germinated bracted twistflowers in November 2006, they alerted Texas Parks and Wildlife Department staff, who in turn asked others to begin checking their sites in the late Fall and Winter (Pepper, 2007). Moreover, by monitoring over the longer period (from late Fall into Summer), the SAPAR staff collected evidence that bracted twistflower seeds continue to germinate over a 5 month period.

To answer the question of how this kind of bet-hedging might be adaptive for S. bracteatus, a Streptanthus work group suggested that perhaps “repeated rain events produce repeated bouts of germination.” The work group also hypothesized that “this ‘bet-hedging’ strategy is adaptive because of our variable climate” (Price, 2007). For example, suppose one Winter there are frequent good rains followed by a prolonged dry period. Twistflower seeds that germinate following the rains might not be able to sustain life during the long dry spell. However, by hedging their bets, another group of seeds could wait, germinate once wet conditions returned, and in this way insure the survival of the population. Another interesting “twist” is that bracted twistflower seeds can “remain dormant in the soil for years” (Fowler, 2007).

There are other questions about Streptanthus bracteatus that add to its mystery. For example, almost nothing is known of its habitat requirements. The plant is found “in a number of varied habitats on thin clay soils in the semi-arid to mesic woodland habitat of the Balcones Canyonlands region of the Edwards Plateau” (Center for Plant Conservation, 2007). Since jewelflower appears to be adapted to both dry as well as wet areas, it is hard to say what its water requirements are.

Equally mysterious are the twistflower’s sunlight requirements. Where we see the plant in the Natural Areas, the plant grows in open, sunny places as well as in the shade.

One University of Texas researcher hypothesizes that “the difference for wild plants growing under juniper or within openings of different sizes is not so much the degree of shading, but how many hours of sunlight the plants receive daily” (Price, 2007). How shading and hours of sunlight may be related, and how this may affect bracted twistflower survival are as yet unknown.

Many believe the bracted twistflower should be federally listed, and its reduced numbers may in part be due to picking (Enquist, 1987), herbivory (Purvis, 2007), and habitat loss from development. The colony surviving in a San Antonio Natural Areas preserve is the only known population in Bexar County, although, this year SAPAR staff found additional plants on the adjoining landowner’s property. Since at least 1998, SAPAR staff and volunteers have monitored the colony, recording data such as number of plants, their heights, evidence of herbivory, and presence of fruit (siliques). Over the years, the population size has fluctuated and has even gone as low as zero.

In the Natural Areas preserve, we have taken management action to help our Streptanthus. We have fenced the colony to protect it from rabbits and deer. Hoping to positively impact the colony, we have also experimented with removing selected junipers.

While much about Streptanthus remains a mystery, mystery is not necessarily a bad thing. Mystery stimulates questions and engenders wonder. Mystery, like Streptanthus bracteatus, is a jewel to be treasured.

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