Small-flowered sand-verbena COSEWIC assessment and status report: chapter 6

Biology

General

Normally closed perennial plant communities of the desert become open to invasion by annuals during brief periods of excess soil moisture. To be successful in the desert, an annual plant must promptly respond to favorable growing conditions, reproduce itself abundantly, and return to a dormant state (seed) awaiting the next opportunity. Warm spring temperatures, coupled with favorable soil moisture conditions resulting from snow melt and seasonal precipitation, bring on rapid phenological development to maturity. Death of the mature plant ensues when summer temperatures and surface soil moisture conditions become critical (Evans and Thames 1981).

Reproduction

Ephemerals such as small-flowered sand-verbena usually germinate in the spring. They grow quickly, flower and produce seeds before dying and scattering their progeny to the desert floor. This is a very successful adaptation to desert conditions as the plants survive the torrid heat, and often drought, of midsummer as dormant seeds. These seeds are extremely hardy. The plants survive in this way until the following spring, sometimes two or three springs, when they repeat the cycle (Danin 1996).

With all annuals there is an ever-present danger that germination may be triggered by precipitation from a storm of short duration and the resulting highly vulnerable seedling crop killed in a subsequent hot, dry period. Germination of seeds of most annuals is inhibited by biochemical controls so they are delayed in germination until this danger period has passed (Evans and Thames 1981).

The fruit of small-flowered sand-verbena is composed of an achene (seed) which is enclosed by a persistent winged structure. Moisture is required to leach substances from these wings or promote fungal growth and hence mechanical softening. Therefore, germination will also depend not only on the total rainfall but also on the duration. A slow rain brings on more germination through the leaching effect on seeds than does a cloudburst yielding the same precipitation total. Temperatures also interact strongly with precipitation (Evans and Thames 1981). Exact effects of temperature and precipitation on small-flowered sand-verbena requires further study.

Flowering occurs from May to July over the range of the species but at Canadian sites occurs mostly in middle June with fruit set in middle July.

Survival

Adaptation of annuals in arid conditions requires prompt response to irregularly occurring favorable growth periods, rapid completion of the reproductive process, and production of massive numbers of durable seeds that can survive long periods of unfavorable environment (Evans and Thames 1981). Small-flowered sand-verbena is superbly adapted for survival within its sand dune habitat. Most arid plant adaptations are geared around increasing the ability of the plant to obtain and maintain moisture levels so necessary for survival. The low, shrub-like form of the plant's canopy and silhouette is typical of species which have been adapted to trap fine silts and clays as a result of lower wind velocities within the canopy and around the base of the plant (Danin 1996). Silt and clay have a higher water holding capability than find to coarse sand, and this silt/clay can absorb early morning dews more efficiently (Danin 1996).

The stems, leaves, and roots of small-flowered sand-verbena are also adapted in a variety of ways to ensure the survival of the plant in arid conditions. The rigid stems of small-flowered sand-verbena offers protection from dessication. The stems of this plant may be viscid (with sand adhering to the surface) which insulates them from abrasive hazards such as airborne sand. Lower surfaces of leaves may have a similar coating. Leaves also are fairly thick and simple in form offering a reduction in leaf area and decreasing potential water loss (Danin 1996). Also, the smooth upper surface and erect form of the leaf may direct condensation from early morning dews to the root system (Figure 1). As the plant is an annual the root system is a simple taproot (Welsh 1987).

Annual plants of the desert generally resemble mesic rather than xeric plants in morphology and function (Evans and Thames 1981). A perennial desert or xeric plant would have more extensive adaptations to their stem, leaf, and root systems given the necessity of multi-year survival in difficult circumstances.

The formation of a viable seed bank is vital for the future survival of the species within its sandy habitat. Directed wind dispersal of the fruit of desert dune species enable them to remain in their most appropriate sites and avoid being carried off the dunes to unsafe sites. The wind causes the fruit to roll or glide eventually coming to rest on leeward slopes of dunes where wind velocity approaches zero. The fruit is covered by sand during future wind depositions. As the leeward slope slowly moves forward, new fruits are deposited in new leeward slopes. Consequently, the entire dune area becomes a seed bank at all depths. Seeds remain in the system of mobile sand protected from animals. Exposed fruit may move with strong winds to nearby areas where correct moisture and temperature regimes result in germination (Danin 1996).

Physiology

Annuals commonly exhibit rapid germination of seeds, rapid extension of primary root systems, and high vigor in seedlings. High seed germination percentage is the usual response in annuals. The growth of desert plants is directly related to the availability of moisture. The timing of the availability of moisture is extremely important (Evans and Thames 1981).

Annuals have very low structural carbon requirements for maintaining leaves, roots, and other plant parts when compared to perennials. Annual plant structures are normally functional but once, and then they die and disintegrate. It is not necessary that annual plants be heavily protected as perennial plants against drought and excessive temperature because they grow only in relatively favorable seasons (Evans and Thames).

Movements/dispersal

Since small-flowered sand-verbena is an annual species it would not be likely to be found in exactly the same location from year to year. The fruit, an achene, is enveloped by the persistent calyx in the form of three thin wings that facilitate dissemination. The fruit, seeds and seed dispersal system are all fully adapted for survival within arid habitats. The winged fruit may roll on the ground or float well on water. Hairs are absent on fruits as this would impede wind dispersal during rolling or gliding on sandy surfaces (Danin 1996). The wings on these fruits aid dispersal by the gusty winds of sand dune fields (Alberta Native Plant Council 2001). It is probable to expect establishment of the seeds downwind or downstream from the original sites as a result of wind or water dissemination of the seeds. Two past investigators (Hudson 1982, Johnson and Hallworth 1975) located additional populations by following this logic.

Hudson (1982) made several assumptions in searching for the Saskatchewan localities. He assumed small-flowered sand-verbena would be found in generally sandy country, deep down in a hot microclimate, and on the inner or convex side of a bend in the river, and therefore, on the slip-off slope where the substrate would be alluvially deposited sand. Hudson's assumptions were proven to be correct during field examination of the rediscovered site.

Nutrition and interspecific interactions

There was little data found on the specific nutrition and interspecific interactions of small-flowered sand-verbena.

Dense canopies of annuals prevent further germination by some interference pattern. Germination has been shown to occur in soil removed from below the canopy suggesting that germination inhibition under the canopy may be caused by the reduced red-to-far-red radiation ratio (Evans and Thames 1981).

Behaviour/adaptability

Small-flowered sand-verbena is apparently unable to adapt to habitat that does not contain sand deposition, therefore, the species is restricted to habitat containing blowing sand.

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