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 (y and Nei's genetic distance) were estimated using 41 putative RAPD loci. All three species show high levels of genetic variation (H S ¼ 0.12-0.29, P ¼ 63.4-95.1), and low genetic differentiation between populations and species (y populations ¼ 0.1470.02 (SE); G st ¼ 0.1170.02). Accordingly, gene flow among populations was estimated as high by three different methods (N m ¼ 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

Reproductive ecology of the rare clonal cactus Stenocereus eruca in the Sonoran desert

Abstract. Stenocereus eruca is a clonal cactus with an extremely narrow distribution in Baja California, in which seedling recruitment has rarely been observed. Low seedling recruitment in clonal plants may be caused by low seed production as a consequence of pollinator limitation or if seed input is sufficient, by lack of favorable conditions or microsites for seedling establishment. In this paper, we study the reproductive ecology of S. eruca along four years in order to explore the proximate causes of the low seedling recruitment observed in its populations

Microenvironmental heterogeneity and space utilization by desert vines within their host trees

The three-dimensional biomass distribution and the microenvironments experienced by several desert vine species growing within the canopy of host trees were studied at the Centro Ecológico de Sonora in México. The light environment within the crown of the host tree Cercidium microphyllum showed a horizontal and vertical gradient from the base of the trunk to the edges of the canopy. Within this gradient total daily photosynthetically active radiation (PAR) varied from 47.8 mol m-2outside the crown to 4.6 mol m-2at the centre of the crown and close to the ground. Maximum air temperature was 3 °C lower beneath the crown than outside. Within the canopy, most vines experienced less than 50% of the daily available PAR outside the canopy. For most of the day, leaves of vines received 15% or less of the maximum available PAR. Our study shows that vines do not grow towards full sunlight but rather they exploit different habitat possibilities within their host tree crown. Leaves along the stems of vines experienced a wide range of light environments, showing coefficients of variation (CV) in total daily PAR from 36.4 to 94.6%. Daily courses of PAR also showed that leaves within the canopy experienced short-term temporal variation in the light environment. Differences in CV of daily PAR values and preferences in heterogeneous light microenvironments among species suggested that different vine species might be spatially separated in the canopy. We suggest that in desert habitats, conditions within the crown of host trees result in an important microhabitat that vines can exploit, allowing them to avoid the high light, temperature and water deficits found in the surrounding environment

Evidence of gynodioecy in Kallstroemia grandiflora (Zygophyllaceae): Microsporogenesis in hermaphrodite and female plants and lack of reproductive compensation

Understanding mechanisms involved in the maintenance of gynodioecy in natural populations requires evidence concerning the phenotypic expression and reproductive consequences of male sterility. We have recently discovered male sterility in populations of Kallstroemia grandiflora from northwestern Mexico. Here we describe the development of anther and pollen grains in hermaphrodite and female plants and explore whether females experience any compensatory advantage over hermaphrodites. The major difference in anther development between morphs was the early disintegration of tapetal cells in anthers of pistillate flowers. Abnormalities in developing pollen grains in pistillate flowers were seen in the microspore mother cells, microspore tetrads, and young and mature pollen grains. Biomass allocation to flowers, petals, and stamens were larger in hermaphrodites, while both sexes had the same number of ovules per flower. Hermaphrodite flowers were self-compatible. Pollination experiments revealed evidence of pollinator limitation for female plants but not for hermaphrodites. No evidence of reproductive compensation was detected, as flower and fruit production were similar for both sexes in two populations of K. grandiflora. We suggest alternative mechanisms that could maintain male sterility in natural populations of K. grandiflora

Distribution of heavy metals polluting the soil near an abandoned mine in Northwestern Mexico

Abandoned mine wastes pollute the soil in their vicinities and threaten the health of livestock and human beings. This is the situation around San Felipe de Jesús in northwestern Mexico. We surveyed 900 ha of agricultural land to assess and map the concentrations of toxic elements in the topsoil to discover where pollution is serious, what its source might be and to decide whether remediation is needed. The total concentrations of Pb, As, Zn, Cu and Mn plus Fe and Ca were analysed by X-ray fluorescence spectrometry. We found that all of the first five elements listed were concentrated near the tailings pile as a ‘hot spot’ and where the concentrations of Pb, As and Zn exceed national and international standards. Iron and Ca, in contrast, are evenly spread throughout the region. The elements Pb, Zn and Mn gradually decrease in concentration from the tailings pile toward the Sonora River, probably because they have spread in dust or by water during storms. Arsenic and Cu also decrease in concentration from the the tailings pile towards the centre of the region, but they increase again in the soil on the river flood plain, most likely as the result of spills in the catchment north of the river basin. These results will serve to assess the risks incurred in the use of the land for agriculture and to define policies for that use and possible remediation

how important is clonal recruitment for population maintenance in rare plant species? the case of the narrow endemic cactus, stenocereus eruca, in baja california, mexico

Stenocereus eruca is a postrate columnar cactus whose regeneration seems to occur mainly through clonal propagation. It is a narrow endemic species of the Sonoran desert in Baja California Sur and currently considered as threatened under Mexican legislation. In this paper we describe the demography of ramets in four populations along its distribution range and the demography of genets in one population during a 3-year-study period in order to evaluate its conservation status. We also analyze the relative contribution of sexual reproduction and clonal propagation to population maintenance and provide guidelines for the formulation of conservation programs. Elasticity analyses were used to explore the relative contribution of sexual and clonal recruitment to projected population growth rate (lambda). During the three years of study, regeneration occurred only through clonal propagation while sexually derived seedlings were not detected within or outside the permanent plots. Our demographic data showed that the four population of S. eruca are in equilibrium (lambda approximate to 1), and elasticity analyses showed that the relative contribution to of clonal recruitment was larger than sexual recruitment, at least during the analyzed ecological time scale. Simulations showed that removing sexual recruitment had a minor impact on lambda, but the absence of clonal propagation alone was sufficient to keep below unity. We propose the establishment of at least one reserve with adequate protection from human disturbance to conserve S. eruca. (c) 2005 Elsevier Ltd. All rights reserved

Buffelgrass (Pennisetum ciliare) land conversion and productivity in the plains of Sonora, Mexico

Bufflelgrass (Pennisetum ciliare syn. Cenchrus ciliaris) is an African grass that has been widely introduced in subtropical arid regions of the world to improve rangelands for cattle production. However, it can have a negative effect on the diversity of native plant communities. Buffelgrass was introduced to Sonora, Mexico in the 1970s as a means to bolster the cattle industry. "Desmonte," the process by which native desert vegetation is removed in preparation for buffelgrass seeding, alters the land surface such that buffelgrass plots are easily detectable from aerial and Landsat satellite images. We estimated the extent of conversion to buffelgrass in a 1,850,000 ha area centered on Hermosillo, from MSS and TM images from 1973, 1983, 1990 and 2000. We then compared the relative above-ground productivity of buffelgrass to native vegetation using Normalized Difference Vegetation Index values (NDVI) from Landsat and Moderate Resolution Imaging Spectrometer (MODIS) satellite sensor systems. Buffelgrass pastures have increased from just 7700 ha in 1973 to over 140,000 ha in 2000. Buffelgrass pastures now cover 8% of the land surface in the study area. Buffelgrass pastures have lower net primary productivity, estimated by MODIS NDVI values, than unconverted desert land. The desmonte process removes trees and shrubs, while the buffelgrass plantings are often sparse, leading to an apparent net loss in net primary production from land conversion. We recommend that the desmonte process be discontinued until its efficacy and safety for native ecosystems can be established, and that a comprehensive plan for preserving biodiversity while accomodating economic development be established for this region of the Sonoran Desert in Mexico. © 2005 Elsevier Ltd. All rights reserved