María C. Mandujano

Comparative demographic analysis of three Neobuxbaumia species (Cactaceae) with differing degree of rarity

In this study we use a demographic approach to analyse the differing abundance of three congeneric columnar cacti: Neobuxbaumia macrocephala (the rarest), Neobuxbaumia tetetzo (intermediate), and Neobuxbaumia mezcalaensis (the most common). Populations of these species were studied in the Tehuacan Valley (Central Mexico) over a 3-year period. We employed traditional models and life table response experiments (LTRE) to explore the association between particular demographic traits and the degree of rarity of each species. Most matrices showed population growth rate (λ) values close to unity; the only exception was N. mezcalaensis in 2001–2002 (λ=1.091±0.088). In the three species the highest elasticity values corresponded to entries referring to the stasis of pre-reproductive plant and small adults. However, the LTRE indicated that most important differences in the λ values between years and species were associated with variation in fruit production and seedling recruitment. N. mezcalaensis yielded the highest mean λ due to its high fecundity in 2001–2002. This result suggests that the variation in demographic behaviour observed between species is sufficient to explain the higher abundance of N. mezcalaensis compared to the rare N. macrocephala. The high fecundity values and recruitment success observed in the former, even if they occur only seldom, may result in higher average λ values accounting for its high population densities.

Effect of reproductive modes and environmental heterogeneity in the population dynamics of a geographically widespread clonal desert cactus

The dynamics of plant populations in arid environments are largely affected by the unpredictable environmental conditions and are fine-tuned by biotic factors, such as modes of recruitment. A single species must cope with both spatial and temporal heterogeneity that trigger pulses of sexual and clonal establishment throughout its distributional range. We studied two populations of the clonal, purple prickly pear cactus, Opuntia macrocentra, in order to contrast the factors responsible for the population dynamics of a common, widely distributed species. The study sites were located in protected areas that correspond to extreme latitudinal locations for this species within the Chihuahuan Desert. We studied both populations for four consecutive years and determined the demographic consequences of environmental variability and the mode of reproduction using matrix population models, life table response experiments (LTREs), and loop and perturbation analyses. Although both populations seemed fairly stable (population growth rate, λ∼1), different demographic parameters and different life cycle routes were responsible for this stability in each population. In the southernmost population (MBR) LTRE and loop and elasticity analyses showed that stasis is the demographic process with the highest contributions to λ, followed by sexual reproduction, and clonal propagation contributed the least. The northern population (CR) had both higher elasticities and larger contributions of stasis, followed by clonal propagation and sexual recruitment. Loop analysis also showed that individuals in CR have more paths to complete a life cycle than those in MBR. As a consequence, each population differed in life history traits (e.g., size class structure, size at sexual maturity, and reproductive value). Numerical perturbation analyses showed a small effect of the seed bank on the λ of both populations, while the transition from seeds to seedlings had an important effect mainly in the northern population. Clonal propagation (higher survival and higher contributions to vital rates) seems to be more important for maintaining populations over long time periods than sexual reproduction.

Why be a honeyless honey mesquite? Reproduction and mating system of nectarful and nectarless individuals

Populations of Prosopis glandulosa var. torreyana in the Chihuahuan desert have a fixed dimorphic system of nectar production in which half the individuals produce nectar (are nectarful) and the other half are nectarless. We analyzed the impact of nectar production on different estimates of fitness, comparing nectarful against nectarless individuals in size, mating system, seed traits, and fruit set in a 1-ha scrubland. Of the reproductive individuals (358), 46% were nectarful and 54% were nectarless. Neither tree size nor flowering phenology differed between nectar morphs. Fixation indices (F) for both progeny (F = -0.2) and adults (F = -0.45) were negative, and high heterozygosities were found in adults and progeny (H = 0.45). No differences were found between nectar morphs for F, H, and single (t(s) = 1.1) and multilocus (t(m) = 1.03) outcrossing rates. Controlled pollinations showed differences between selfing and control treatments with no differences between nectar morphs. Nectarless individuals produced significantly more pollen grains than did nectar producers, but all other measured floral traits showed no differences. Nectarful trees were visited by pollinators 21 times more often and had a significantly higher overall fruit set than did nectarless trees. No differences between nectar morphs in seed mass or in percentage seed germination were found, but heavier seeds tended to have higher heterozygosities. Both morphs had similar success as females, but nectarless trees had ∼7% higher male function. We discuss three possible scenarios for the evolution of the fixed dimorphism in nectar production, two involving unstable phases (substitution of one morph by the other, and evolution towards dioecy) and one stable scenario (maintenance of the dimorphic system).

Reproductive Biology of Cactaceae

Floral biology in the Cactaceae represents a new field of research, with only 2% of approximately 2,000 species having been studied. Studies on breeding systems cover functional and morphological floral traits of sexual expression, while research on mating (hybridization) systems includes experiments on pollination, morphological and functional floral traits vis-a-vis estimation of out-crossing rates, and inbreeding depression. Most cacti are hermaphroditic with some exceptions of dioecy. Herkogamy and dichogamy seem to be common, and are coupled with self-incompatibility and inbreeding depression as mechanisms to avoid selfing; these traits are important in guiding the evolution of mating (fertilization) systems from mixed to mainly either out-crossing or selfing in all three subfamilies. We found no clear pattern between mating systems and genetic diversity. The impressive variety of fertilization and breeding systems, as well as the genetic diversity within Cactaceae highlights the complex evolution of this family and the plasticity of their reproductive response to the spatially and temporally unpredictable habitats in which they occur. This chapter reviews information on the floral biology, pollinators and genetics of Cactaceae, covering about 70 references: 36% on genetics, 43% on different aspects of pollination ecology, and 21% on diverse subjects with limited descriptions.

Spatial distribution of three globose cacti in relation to different nurse-plant canopies and bare areas

We report the frequency of 3 globose cacti (Mammillaria carnea, M. haageana, and Coryphantha pallida) associated with nurse plants or bare areas and assess the size of plants in each site. We found 3 times more cacti established beneath nurse plants than in bare areas, and all cacti species were the same size when associated with shrub cover or bare areas. Under a plant canopy, the size structure was pyramidal, whereas in bare areas, individuals were found isolated with an even size structure, and few small plants were recorded. There was a positive relationship between the type of shrub cover and the associated globose cacti. We found more individuals of all 3 species of cacti associated with Prosopis laevigata than with Mimosa luisana, but size structure was similar between them. We discuss 2 possible scenarios that may result from the nurse-protege interaction.

Fruit abortion in the Chihuahuan-Desert endemic cactus Opuntia microdasys

Resource and pollen limitation, as well as pollen/ovule incompatibility, have been proposed as causes to explain fruit abortion. To assess whether abortion in Opuntia microdasys was due to resource and/or pollen limitation and could therefore be reversed fruit set and seed set were studied using controlled pollination experiments on 60 plants that had been randomly assigned a combination of watering and fertilization treatments. On the other hand, to test whether fruit abortion was irreversible, due to pollen/ovule incompatibility, we examined the reproductive biology of the species. This included observations on floral phenology, nectar production, flower visitors, numbers of pollen grains and ovules, and self-pollination experiments. Results showed that O. microdasys is a fully self-incompatible species and its floral biology and the activity of the main pollinator allow constant deposition of incompatible pollen onto stigmas, which may contribute to fruit abortion. Reproductive success was limited by nutrients and pollen, but the fruit set increased only by 58%, compared to 47% of the control, after the experimental addition of pollen, nutrients and water. The magnitude of pollen and resource limitation suggests that similar levels of abortion will be present in good as well as in bad years. Selfing as well as incompatibility between ramets from the same clone and between closely related plants seem plausible candidates to explain the large proportion of fruit abortion, and experimental cross pollination between genotypes identified through molecular markers are necessary to fully understand the considerable abortion rate that remains unexplained after pollen and resource addition. Interestingly, the possible reason why the abortion of energetically expensive fruits has not been eliminated by natural selection is that the aborted fruits are propagules able to root and produce new plants with the same genotype of the mother. Abortion would have a dramatic effect on cross-fertilized genotypes because they result in zero fitness, but it would have a positive effect on the fitness of the maternal genotype because a clonal offspring is produced. Evidently, the exact fitness consequences to the maternal plant will depend on the differences in survival and reproduction of these different offspring types.

Population Dynamics of Ariocarpus scaphirostris Bödeker (Cactaceae): Evaluating the Status of a Threatened Species

Our aim was to assess the conservation status of the cactus Ariocarpus scaphirostris by describing its population dynamics and spatial distribution in the only known large population left. Population dynamics were analyzed with the use of matrix projection models from a census period that encompassed 2 yr (2005 and 2006). Density of the individuals (0.25 individuals/m2) changed when compared with previous studies, and similar to other dense globose cacti, the pattern of spatial distribution was aggregated. Our results indicate that the original population has decreased considerably during a 20‐yr period ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Seed size and photoblastism in species belonging to tribe Cacteae (Cactaceae)

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