Response of four species of Sonoran Desert trees to buffel grass removal treatments

Plant Ecology - Buffel grass (Cenchrus ciliaris L.) is an African grass used as forage for livestock that is also invasive. Several methods have been evaluated to remove invasive buffel grass, but...     

On the mechanical properties of the rare endemic cactus Stenocereus eruca and the related species S. gummosus

We examined the hypothesis that the procumbent growth habit of the rare, columnar cactus Stenocereus eruca is in part the result of a diminution of the mechanical properties of stem tissues by comparing the properties of S. eruca plants with those of the putatively closely related semi-erect shrub S. gummosus. Intact stems and surgically removed anatomically comparable regions of the stems of both species were tested in bending and tension to determine their Young's modulus and breaking stress. A computer program was used to evaluate the contribution of each region to the capacity of entire stems to resist bending forces. Our analyses indicate that the principal stiffening agent in the stems of both species is a peripheral tissue complex (= epidermis and collenchyma in the primary plant body) that has a significantly higher tensile breaking stress and greater extensibility for S. gummosus than that of S. eruca. Computer simulations indicate that the wood of either species contributes little to bending stiffness, except in very old portions of S. gummosus stems, because of its small volume and central location in the stem. These and other observations are interpreted to support the hypothesis that S. eruca evolved a procumbent growth habit as the result of manifold developmental alterations some of which reduced the capacity of tissues to support the weight of stems.     

Genetic differentiation in the Agave deserti (Agavaceae) complex of the Sonoran desert

The Agave deserti complex, comprising A. deserti, A. cerulata and A. subsimplex, represents a group of species and subspecies with a near allopatric distribution and clear differences in morphology. Genetic differentiation and taxonomic status with respect to spatial distribution of 14 populations of the complex were analyzed in an effort to understand the evolution and speciation process within the genus. Allelic frequencies, levels of genetic variation, expected heterozygosity (H(S)), proportion of polymorphic loci (P), and genetic differentiation (theta and Nei's genetic distance) were estimated using 41 putative RAPD loci. All three species show high levels of genetic variation (H(S)=0.12-0.29, P=63.4-95.1), and low genetic differentiation between populations and species (theta populations=0.14+/-0.02 (SE); G(st)=0.11+/-0.02). Accordingly, gene flow among populations was estimated as high by three different methods (N(m)=2.91-6.14). Nei's genetic distances between the three species were low compared to the values obtained from other Agavaceae, and there was no clear correlation with taxonomic divisions. In a UPGMA analysis, A. subsimplex and A. cerulata formed exclusive monospecific clusters, whereas the A. deserti populations appear in more than one cluster together with other species. The results were consistent with a pattern of genetic isolation by distance.     

Patterns of abundance and population structure of Pachycereus pringlei (Cactaceae), a columnar cactus of the Sonoran Desert

Understanding the mechanisms that determine the distribution and abundance of plants is a major problem in ecology. However, very few studies have explored the factors controlling the abundance of columnar cacti throughout their range of distribution. In this paper, we describe the density and size structure of 26 populations of Pachycereus pringlei throughout its distribution range in the Sonoran Desert. Major differences in abundance were detected between island and mainland and peninsular areas, with islands sustaining significantly larger densities than mainland and peninsular populations. Within peninsular populations, the abundance was negatively associated with latitude and positively associated with annual and seasonal rainfall. In contrast, the abundance in mainland populations showed neither latitudinal trend nor an association with rainfall. In peninsular populations, mean height and basal diameter of branched plants showed a negative association with population density whereas mainland populations showed no significant association. None of the populations exhibited a population structure that fitted the log-normal distribution expected for young, growing populations with constant recruitment. Insular, peninsular and␣mainland populations showed a population structure with an uneven size distribution typical of populations experiencing regeneration pulses.     

Reproductive Consequences of Clonal Growth in Stenocereus eruca, a Rare Clonal Cactus of the Sonoran Desert

Stenocereus eruca is a prostrated, self-incompatible cactus endemic to the Sonoran Desert that regenerates primarily through clonal propagation. Clonal growth is expected to affect mate availability by influencing the number and spatial distribution of mating types. In this paper we examine the role of clonal growth on female fecundity through a series of pollination experiments in a population of S. eruca. We set up a pollen supplementation experiment using five distance treatments with pollen collected at 1, 10, 100, 1000 and 25000 m from receptor flowers during the years 2001 and 2002 and evaluated genetic sifmilarities between pairs of receptor-donor ramets through RAPD markers. Our data on fruit set, number of seeds/fruit, germination and overall fecundity revealed that S. eruca show a significant reduction in female fecundity when pollination occurs between ramets located at short distances (1 and 10 m), while genetic data showed high levels of similarity at those distances. The reduction in female fecundity is apparently a consequence of geitonogamy and inbreeding depression. Our data suggest that clonal growth and geitonogamy are likely to be partially responsible for the low levels of sexual reproduction and seedling recruitment observed in populations of S. eruca. Co-ordinating editor: H. Kudoh     

Reproductive ecology of <Emphasis Type="Italic">Agave colorata</Emphasis>: the importance of nectar-feeding bats and the germination consequences of self-pollination

Agave colorata is a paniculate agave distributed along the migratory route of the nectar-feeding bat Leptonycteris yerbabuenae. In this paper, we evaluate the importance of nectar-feeding bats in the reproduction of A. colorata in a population in Sonora, Mexico, and describe the germination consequences of self-pollination. We estimated abundance using five plots and set pollination treatments to evaluate the importance of bats. We recorded 14.8?±?6.8 plants/400 m², with a bimodal size distribution. Flowers are protandrous and visited mainly (>?20 visits/plant/30 min) by L. yerbabuenae. Pollination exclusion experiments showed that flowers excluded from diurnal visitors had maximum fruit set values (0.49?±?0.42), while the autonomous self-pollination treatment had the lowest value (0.03?±?0.06). Similarly, the greatest number of viable seeds per fruit was recorded in the diurnal exclusion treatment, while the greatest number of empty seeds was observed in the self-pollination treatment. Fruit set values among untreated plants varied from 32 to 54%, with a mean value of 41.8%. Seeds derived from self-pollination had a narrower window of opportunity for germination compared to seeds derived from nocturnal pollination. Self-pollinated seeds had lower germination, rate of germination or lag time in response to light, osmotic potential and heat shock treatments, compared to other pollination treatments, revealing an inbreeding cost. Overall, our results show that L. yerbabuenae is the likely major pollinator of the studied A. colorata population. However, under pollinator limitation A. colorata may produce seeds by autonomous self-pollination, at a cost expressed as lower germination.     

Resource availability, population dynamics and diet of the nectar-feeding bat Leptonycteris curasoae in Guaymas, Sonora, Mexico

Nectar-feeding bats play important roles in the pollination of columnar cacti. Migratory bats such as Leptonycteris curasoae are vulnerable to extinction due to their migratory behavior and cave-roosting habit. The conservation of migratory bats requires the identification and protection of key roosting sites and their floral resources. In this paper we describe the availability of resources, the population dynamics and the diet of L. curasoae in the Guaymas region, and its diet at three additional sites in coastal Sonora, Mexico. Our results indicate that L. curasoae is a seasonal resident in the Guaymas area, with local occupation of around 4 months. Seasonal occupation seems to be determined by the availability of flowers and fruits of columnar cacti as peak abundance and peak availability coincide in time. The analyses of fecal and fur samples revealed that columnar cacti constitute the major food source during their seasonal occupation of coastal Sonora. The frequency of pollen from Agave and Bombacaceae in fecal samples was generally low but increased in importance in lower latitudes. Our results suggest that the availability of resources provided by columnar cacti determine seasonal occupation of L. curasoae in Sonora. Given that we detected newborns in the studied colony, we suggest active cave protection for the conservation of the maternity roosting site in Guaymas, Sonora, Mexico.     

Local genetic differentiation among populations of the mass-flowering tropical shrub Erythroxylum havanense (Erythroxylaceae)

This study is aimed to understand the role of life-history traits in determining the genetic structure of populations. We used Erythroxylum havanense, a distylous shrub with synchronous and massive patterns of flowering and fruiting. We suggest that the high concentration of ephemeral resources produced by mass flowering satiates both pollinators and frugivores, restricting gene flow and leading to genetic differentiation among populations. Using random amplified polymorphic DNAs as genetic markers, we estimated genetic diversity and structure statistics to quantify the amount and distribution of genetic variation within and among five populations from the Pacific coast of Mexico. High levels of genetic variation within populations and significant differentiation among populations located very near to each other were found. Furthermore, spatial autocorrelation analysis indicated the presence of significant genetic structure at short spatial distances. We suggest that by influencing the foraging behavior of pollinators and frugivores, mass flowering may produce the observed patterns of genetic structure, while small differences in flowering or fruiting phenology could further reinforce the isolation of nearby populations.     

The biomechanics of Pachycereus pringlei root systems

We report on the root system of the large columnar cactus species Pachycereus pringlei to explore the hypothesis that increasing plant size decreases the ability to resist wind-throw but increases the capacity to absorb and store nutrients in roots (i.e., plant size limits the performance of these functions and may shift the performance of one function in favor of another as size increases). Based on 18 plants differing in size, the root system is characterized by a broad and deep bayonet-like root central to a shallow and extensive lateral system of root elements bearing sinker roots near the stem base. All root types have a living secondary cortex and contain wood with a large volume fraction of ray tissues that increases toward the stem base. Wood stiffness and tensile strength are correlated negatively with the ray tissue volume fraction and thus decrease toward the stem base in lateral and bayonet roots. Calculations show that the ability of the bayonet and proximal lateral root elements to resist wind-throw decreases with increasing plant size, whereas the nutrient absorption/storage capacity of the total root system increases with plant size (i.e., a size-dependent shift between these two root functions occurs).