Eastern Mojave Vegetation
Tom Schweich
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Eastern Mojave Vegetation
| Autecology of Desert Elkweed Frasera albomarginata S. Watson (Syn: Swertia a.) Gentianaceae in the American Southwest |
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Tom Schweich |
Topics in this Article: Introduction Literature Review Field Work and Methods Results Taxonomy Distribution Growth Habits Relationships to Soils and Other Plants Life History Discussion Summary Literature Cited Appendix A -- Field Data Communications Received. |
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IntroductionDesert Elkweed (Frasera albomarginata) is a pretty little plant which is found occasionally in Pinyon-Juniper woodlands of southwestern USA deserts. It grows low to the ground with a rosette of green leaves that have white edges. The white flowers with purple spots are borne on 3-4 dm (12-16 in) stalks. | ||
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Desert Elkweed grows for more than one year which makes it a perennial. However, it does not form wood like a tree does, so it is called an herb. The only permanent part of the plant is the root, and the top of the root, where the leaves and flower stem grows, is called a caudex. | ||
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The leaves of F. albomarginata grow in a circular pattern around the top of the caudex, in what is called a rosette. You can see the rosette in the photograph at left. The leaves have a small white edge, or margin, around the green leaf and that is how the plant got its name: albomarginata. Albo- means "white" and -marginata means "margined" so the scientific name Frasera albomarginata could mean "White-margined Frasera." The genus name of Frasera is named for J. Fraser, an English collector, whereas the genus name of Swertia is named for E. Sweert, a Dutch herbalist, who was born in 1552. | ||
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When F. albomarginata flowers it sends up a stem which is 8 to 24 inches (2-6 dm) tall and has very thin leaves aranged around the stem in a whorl. | ||
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The stem and all the leaves are smooth and shiny, and do not have hairs, prickles (like roses), or glands (like sundews) on them, and so the plant is said to be glabrous. There are some F. albomarginata in the mountains near Las Vegas, Nevada, that have a thin covering of sticky cobwebby hairs, but these are thought be a a separate subspecies. | ||
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The flowers are about 1 inch (2.5 cm) in diameter, with greenish white petals that have purple dots. Nectar, to attract pollinating insects, is secreted in pits which are located well out on the petals, as can be seen in the photograph at left. F. albomarginata flowers only once in its life history and then dies, and is therefore called semelparous, or monocarpic. | ||
| A typical description of Desert Elkweed F. albomarginata in a scientific publication would say: | |||
Desert Elkweed (F. albomarginata) is a semelparous perennial 2-6 dm high, a single-stemmed, glabrous herb with a basal rosette of leaves having a distinctive white margin, and very thin, 1 cm. wide whorled stem leaves. The flowers are greenish-white with purple dots and are borne on slender branches. Each petal has an oblong nectary pit which is fringed and wider or two-lobed at the tip. | |||
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The species (i.e., Frasera albomarginata; it is common in botanical literature to refer to a species being described by simply saying, "the species." This avoids having to spell out Frasera albomarginata every time the species is referred to.) is known from the California desert mountains through southern Nevada, northern Arizona and southern Utah to Colorado (Hickman, 1993). Its Colorado range is quite limited, being known only from Montezuma County in extreme southwestern Colorado. There are several other species of Frasera in southwestern American deserts, but it is unlikely that the species will be confused. | ||
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Common names for the species are "Mojave gentian" (DeDecker, 1984), "White-margined Swertia" (Welsh et al., 1987), "White-margined Frasera" (Albee, Shultz and Goodrich, 1988) and "Desert Elkweed" (Kartesz, 1988). | ||
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Locations: Pinto Mountain. Wild Horse Mesa. |
In the Mid Hills of the eastern Mojave Desert, Frasera albomarginata is found on the north face of Wild Horse Mesa and on the south face of Pinto Mountain. | ||
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Wild Horse Mesa.
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Pinto Mountain.
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The location of the Pinto Mountain site is shown in the photograph at left. It is on the south side of Pinto Mountain at 1660 m elevation, north of Cedar Canyon Road, associated with Juniperus osteosperma. Like the Wild Horse Mesa site, the soil is white, being derived from the Winkler Formation, and quite loose. By GPS receiver, the location is N 35° 10.549' W 115° 23.237'. | ||
 Location of one experimental plots in Frasera albomarginata.
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The species has also been reported at the Quail Rock Ranch near the Pinto Mountain location (Adrienne Knute, personal communication, 1995) and in Caruthers and Keystone Canyons of the New York Mountains, as well as the marine limestones of Clark Mountain (A. Romspert, personal communication, 1995). | ||
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Literature ReviewThere is very little published about Frasera albomarginata, other than descriptions in the various floras. | ||
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Ecology of Frasera | ||
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Post's (1956) dissertation on the Frasera and Frasera of North America contains perhaps the most data of any kind available on the species. There is a lengthy description of the species and a review of synonymy. | ||
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Post's (1958) paper on the nodal anatomy of Frasera and Swertia perennis is a further elaboration of work begun in his dissertation. In this study he examined three F. albomarginata. Post's (1958) comments add little to understanding of the ecology of F. albomarginata. He does note, however, that "... F. gypsicola (unilacunar) and F. albomarginata (trilacunar) were found together on "gypsum" dunes along the White River in an arid region of central Nevada." Post (1958) also suggests some interspecies relationships in his comments, "... the phyllotaxy in F. gypsicola is opposite, wheras F. tubulosa has whorled leaves, as does on of the trilacunar species, F. albomarginata, with which it appears to have its closest affinity." | ||
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Pringle's (1990) taxonomic notes on western American Gentianaceae are intended to explain the treatment of Frasera and Swertia in The Jepson Manual (Hickman, 1993) of the flora of California. Unfortunately, Pringle (1990) makes no comments about F. albomarginata. | ||
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Beattle, Breedlove and Ehrlich (1973) reported their results with the better-studied and well-known Frasera speciosa (Syn: Swertia radiata), commonly known as Monument Plant. | ||
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Taylor and Inouye (1985) followed populations of Frasera speciosa for 9 years in a study much like the present one. Since the flowers of F. speciosa and F. albomarginata are very similar, there may be some similarity in life history and pollination biology. However, any similarity would be validated through field studies. | ||
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Phylogeny of Frasera | ||
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Phylogeny is the evolutionary history of a group or lineage, or the description and explanation of the temporal sequence of morphological, ecological and biogeographic changes of a taxon. | ||
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Chassot et al. (2001) studied of the phylogeny of Swertia, and I use his results to support my use of the genus name Frasera rather than Swertia as has been used in California. | ||
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Swertia barunensis. P. Chassot sp. Nov. from Nepal is described and illustrated. It was collected in 1997 in an alpine meadow in the Makalu Barun National Park at an elevation of 4200 m. It belongs to Swertia section Macranthos T.-N. Ho & S.-W. Liu and resembles S. pseudohookeri H. Smith, from which it differs mainly by the shape of the nectary and the exomorphic seed structure. A key to all the species of sect. Macranthos is provided. The affinities of S. barunensis with some other taxa in the subtribe Swertiinae (Griseb.) Rchb are also briefly discussed. | ||
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The embryological features of three species of Swertia (s.l.)– S. erythrosticta, S. franchetiana, and S. tetraptera – were characterized, and the observations were used, together with previously gathered data on other species, to evaluate a recently proposed polyphyly, based on molecular data, of Swertia s.l. Comparisons of species within the genus showed that they have diversified embryologically, and there are significant between-species differences. Notable features that vary between species include the number of cell layers that form the anther locule wall, the construction of the wall of the mature anther, tapetum origin, the cell number in mature pollen grains, the structure of the fused margins of the two carpels, the ovule numbers in placental cross-sections, the shape of the mature embryo sac, the degree of ovule curvature, antipodal variation and the presence of a hypostase, and seed appendages. They share characters that are widely distributed in the tribe Gentianeae, such as a dicotyledonous type of anther wall formation, a glandular tapetum with uninucleate cells, simultaneous cytokinesis following the meiosis of the microsporocytes, tetrahedral microspore tetrads, superior, bicarpellary and unilocular ovaries, unitegmic and tenuinucellar ovules, Polygonum-type megagametophytes, progamous fertilization, nuclear endosperm, and Solanad-type embryogeny. The presence of variation in embryological characters amongst the species of Swertia s.l. strongly supports the view that Swertia s.l. is not a monophyletic group. Frasera is better separated from Swertia s.l. as an independent genus, and is only distantly related to Swertia s.s. judging from the numerous differences in embryology. Swertia tetraptera is very closely related to Halenia, as they show identical embryology. | ||
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Both chloroplast trnL (UAA) intron and nuclear ribosomal ITS sequences highly confirmed the monophyly of the tribes of the Gentianaceae defined by the recent classification, and revealed the tribe Exaceae as a basal clade just next to the basal-most lineage, the tribe Saccifolieae. Within the tribe Exaceae, Sebaea (except Sebaea madagascariensis) appeared as the most basal clade as the sister group to the rest of the tribe. The Madagascan endemic genera Gentianothamnus and Tachiadenus were very closely related to each other, together standing as sister to a clade comprising Sebaea madagascariensis, Ornichia, and Exacum. The saprophytic genus Cotylanthera nested deeply inside Exacum. Sebaea madagascariensis was shown closer to the Madagascan endemic genus Ornichia than to any other sampled Sebaea species. Exacum appeared as the most derived taxon within this tribe. The topology of the phylogenetic trees conform with the Gondwana vicariance hypothesis regarding the biogeography of Exaceae. However, no evidence for matching the older relationships within the family to the tectonic history could be corroborated with various divergence time analyses. Divergence dating estimated a post-Gondwana diverging of the Gentianaceae about 50 million years ago (MYA), and the tribe Exaceae as about 40 MYA. The Mozambique Channel land-bridge could have played an important role in the biogeographic history of the tribe Exaceae. | ||
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To understand flower morphological evolution in Gentianaceae-Swertiinae, we studied generic relationships using trnL intron, matK, and nuclear ITS sequences of a total of 13 genera and 59 species of the subtribe. The phylogenetic incongruencies between the chloroplast and nuclear genes are likely to be the result of long branch attraction. The East Asian Megacodon and Latouchea and the eastern North American Bartonia and Obolaria were determined as the most basal genera, and several well-supported subgroups were revealed. Swertia, Lomatogonium, and Gentianella s. l. were highly polyphyletic and the position of Veratrilla and several species was ambiguous. The main flower types found in Swertiinae can be transformed into each other by simple developmental variation in proportion. This apparently happened several times during the evolution of Swertiinae and, in conjunction with other homoplastic characters, explains the difficulty of recognizing generic limits and the mosaic pattern of character distribution. Phylogenetic relationships, extant distribution ranges, and a preliminary molecular clock approach led to the hypothesis that the last common ancestor of the Swertiinae lived approximately 15 mya, and that an exchange of lineages between East Asia and North America happened frequently from the time of origin until only recently. | ||
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Ecology of Semelparity | ||
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One of the more dramatic life histories in the natural world is that characterized by a single, massive, fatal reproductive episode (‘semelparity’). A wealth of increasingly sophisticated theoretical models on differential life history evolution have been produced over the last two decades. In recent years, empirical studies of the ecology of semelparous plants (and their iteroparous relatives) have begun to address many aspects of the biology of these species, and to test the assumptions and predictions of theoretical models. Semelparity in long-lived plants is one of the few natural phenomena that has yielded specific quantitative tests of mathematical evolutionary theory. | ||
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In the life cycle of Frasera caroliniensis Walt., seed germination, seedling establishment, bud dormancy break, rosette expansion and bolting occur before the canopy closes in early May. Flowering occurs from early May to mid-June, and seeds are mature by early to mid-August; most of the seeds are dispersed during late autumn and winter. Senescence of the flowering stalk and/or rosette occurs from mid-June to late August. Temperature and precipitation can cause variations in the timing of phonological events. Plants of F. caroliniensis are monocarpic, but they live for many years before flowering. Factors which stimulate flowering are unknown. Size alone does not appear to be the sole determinant of whether or not a plant will flower in a given year. Although plants must grow to a certain minimum size before they can flower, not all plants flower when they reach this minimum size. Thus, there is an overlap in sizes of flowering and nonflowering plants. | ||
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Some semelparous desert plants are capable of clonal reproduction. Tissue and Nobel (1990) studied Agave desertii, an agave whose northern range overlaps slightly with Frasera albomarginata, a semelparous species that exhibits clonal reproduction. | ||
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LeBuhn (2004) … studies Ipomopsis longiflora (Torr.) V. Grant subsp. australis, a Chihuahuan desert annual that has two discrete flowering seasons. Seasonal variation in reproductive traits is commonly documented in flowering plants. This variation is critical because it is the material for evolution by natural selection. Understanding the mechanisms that maintain that variation is important, because it can tell us about the ecological and evolutionary forces acting on populations. Using Ipomopsis longiflora (Torr.) V. Grant subsp. australis, a Chihuahuan desert annual that has two discrete flowering seasons, I studied the relative influence of seasonality, variation in the individuals present in the population, and prior reproduction on reproductive traits. I found that traits that represent the allocation of resources within a plant (ovule number, flower number, and flower size) were influenced by seasonality and the individuals present in the population, whereas traits that represent the efficiency of reproduction (seeds/ovule, fruits/flower, and seeds/fruit) were influenced only by seasonality.Key words: iteroparity, semelparity, efficiency, phenology. | ||
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Huxman and Loik (1997) … studied two varieties of Yucca whipplei … one which is primarily semelparous and one primarily iteroparous … Flower, fruit, and seed production were compared for two varieties of Yucca whipplei, Y. whipplei whipplei (which reproduces semelparously) and Y. whipplei caespitosa (which is iteroparous), to determine differences in fecundity with respect to leaf surface area. Vegetative characteristics such as leaf surface area and leaf area index, which are related to the potential for gas exchange and photosynthate production in these plants, and reproductive characteristics such as inflorescence size, total flowers, mature fruit, and seed production as well as seed viability were determined for individuals of each variety. There was no significant difference in leaf surface area between varieties, but the number of viable seeds produced per unit leaf surface area was greater for Y. w. whipplei than for Y. w. caespitosa. There was a significant difference in seed quality between varieties; Y. w. whipplei had seeds that were 1.5 times more viable and germinated twice as fast as Y. w. caespitosa. The total number of viable seeds per plant increased with leaf surface area for Y. w. whipplei, but for Y. w. caespitosa the total number of viable seeds per plant increased with increasing rosette number per individual. For Y. w. whipplei, the percentage of viable seeds per plant decreased as inflorescence size increased. However, the total number of viable seeds produced was positively correlated with inflorescence size, indicating that for a greater investment in flowers and ultimately fruit, the plant received diminishing returns in terms of seed number. There was no such relationship for Y. w. caespitosa, indicating that attached rosettes may provide resources for producing viable seeds. These reproductive patterns are consistent with observed patterns of resource use in semelparous and iteroparous plants but allow new insight on size/fecundity patterns in closely related plants. | ||
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Metcalf, Jessica C., Karen E. Rose, and Mark Rees (2003) … Monocarpic plants, which flower once then die, are ideal systems for testing evolutionary ideas because the cost of reproduction is easily quantified and the timing of flowering is a key determinant of darwinian fitness. | ||
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Kamenetsky (1994) … Analyses of life cycle, morphogenesis of the monocarpic shoot, and propagation have been carried out on Allium rothii, a desert geophyte from the Western Irano-Turanian region. Analyses of life cycle, morphogenesis of the monocarpic shoot, and propagation have been carried out on Allium rothii, a desert geophyte from the Western Irano-Turanian region. Plants from a natural population in the Negev Desert highlands were examined during a 2-yr study. The bulb of A. rothii is a temporary organ that is completely replaced by a renewal bulb within 2 yr. The ephemeroid root system is annual, superficial, and adapted for effective absorption during short periods of moist soil. Seeds germinate best in January and February at temperatures between 5?C and 15?C; germination is not affected by light. The germination process and stages of the life cycle are characterized. The synaptospermic mechanism assures protection of the mature seeds against insects and germination under more favorable conditions in depressions and near shrubs. Intrabulb development was studied by SEM. Five leaf primordia form in apical meristem in June-September; in October the apical meristem becomes reproductive. Anthers and perianth lobes appear in January, while the gynoecium is formed before flowering at the end of February. | ||
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Hegazy (2000) … Verbascum sinaiticum Benth. (Scrophulariaceae) is a biennial rosette plant. The species is rare in Egypt and its distribution is restricted to the Sinai peninsula. The investigated population was located in Saint Catherine, Southern Sinai. Verbascum sinaiticum Benth. (Scrophulariaceae) is a biennial rosette plant. The species is rare in Egypt and its distribution is restricted to the Sinai peninsula. The investigated population was located in Saint Catherine, Southern Sinai. To assess the reproductive ecology of the species the population dynamics, age-specific survival, resource allocation, energy content, and seed longevity of a natural population were studied. The germinable seed bank represents an average of 0·08% of the seeds in the seed rain. The plants are juvenile during 33% to 67% of their lives. Total seed rain output comes from four different cohorts of adult plants. Seeds produced during the summer constituted 83·5% of the total seed rain. Adult plants 15–18 months in age attained a maximum rosette leafiness of 2·85 m2m-2and a maximum root crown diameter of 2·3 cm. Highly reproductive individuals allocated the least dry mass to their roots. The root/shoot ratio was 0·25. Caloric content reached up to 6·37 kcal g-1dry mass in roots of juvenile plants and 7·5 kcal g-1dry mass in the summer seeds. Seed viability was higher in seeds produced during summer and autumn (9 years) as compared to seeds produced during spring (6 years). Life span ranges between 1 and 2 years, which results in an intra-population variation of survival, resource allocation, energy content, and seed longevity and viability. These variations act as an evolutionary filter, an important asset to increase the genetic diversity and stability of populations in areas with low or periodical rainfall. | ||
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Hegzy [Hegazy?] … Autecology of the desert monocarpic Rumex cyprius as influenced by water treatment | ||
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Gross (1984) … Effects of Seed Size and Growth Form on Seedling Establishment of Six Monocarpic Perennial Plants … … in non-competitive cover types (litter and bare soil), seedling weight was independent of initial seed weight … relative growth rates of seedlings in non-competitive cover types were inversely related to seed size … in bare soil and litter, the small-seeded species had relative growth rates twice those of the large-seeded species … | ||
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Burd, et al. (2006) … AGE–SIZE PLASTICITY FOR REPRODUCTION IN MONOCARPIC PLANTS … tropical trees … Cerberiopsis candelabra … | ||
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Hegazy and Kabiel (2007) … microhabitat heterogeneity in the spatial pattern and size-class structure of Anastatica hierochuntica L. Field data verified by green house experiment were used to evaluate the response of Anastatica hierochuntica L. to the amount of rainfall. Field study of the populations was carried out in the runnel and depression microhabitats of gravel and sand sites. Four water treatments, equivalent to 100, 200, 500 and 1000 mm rainfall, were used to simulate different levels of water availability. Under 500 and 1000 mm rainfall, the size-class structure of A. hierochuntica populations consists of a high proportion of large size-class individuals, while a higher proportion of small size-class individuals was obtained under 100 and 200 mm rainfall. The dry skeletons of A. hierochuntica can be used as a “rain gauge” to predict the amount of rain or water received. The dominance of small size-classes (from <1 to 8 cm3) gives a prediction of less than 200 mm rainfall received. Intermediate size-classes (8–64 cm3) characterize habitats with 200–500 mm rainfall, while habitats with >500 mm rainfall produce large size-classes (>64 cm3). Small size-class individuals produced under low amounts of rainfall allocated up to 60% of their phytomass to the reproductive organs. Allocation to reproductive organs decreased with the increase in the amount of rainfall, while allocation to the stem increased in large size-class individuals produced under the highest amount of rainfall (1000 mm) reaching 54%. Increased allocation to stem in large-sized individuals favours the hygrochastic seed dispersal role in the plant. The root/shoot ratio decreased with the increase of the individual size-class, i.e. under high rainfall treatments. Higher values of relative growth rate, net assimilation rate and leaf area index were obtained under high water treatments. Conversely, less expanded leaves, i.e. lower specific leaf area, were manifested in the lowest water treatments. | ||
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Date and time this article was prepared: 5/10/2024 11:10:09 AM |
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