Jorge López-Portillo

SURVEY OF ROOT PRESSURE IN TROPICAL VINES AND WOODY SPECIES

Observations on root pressure were made for 109 species in Costa Rica, Mexico, and Florida during the rainy season. Native and naturalized species, as well as a few outdoor cultivated specimens, were used. Predawn guttation was observed in 15 species. Predawn root pressure, as evidenced by xylem sap exudation from cut stems, was observed in 61 species. Root pressures, measured with a bubble manometer, ranged from 2 kPa to a maximum of 148 kPa in Cissus (Vitaceae). A broad range of climbing monocotyledons, dicotyledons, and one fern (Lygodium) displayed root pressure. Root pressures of selected shoots were followed over consecutive days and were highest in the nighttime, especially at sunrise. The maximum observed root pressure can raise a capillary water column, assuming no resistance, to a height of 14.8 m. Although root pressure alone could not refill embolized xylem vessels in tall trees and vines, the reduction of xylem sap tension would facilitate the dissolving of gas bubbles and the reestablishment of vessel function, at least in the lower parts of the plant or throughout shorter plants.

Water Flows and the Dynamics of Desert Vegetation Stripes

Around 1950, the development of aerial surveys showed the presence, in the arid and semiarid zones of Africa, of particular vegetation patterns, corresponding to plant communities established in parallel stripes, alternating with unvegetated zones. Such landscapes consist of a mosaic of bare areas and vegetated stripes, the major axes of which are always perpendicular to the slope. These striped vegetation patterns have subsequently been reported in many parts of the arid and semiarid regions of the world.

Litter fall of Avicennia germinans L. in a one-year cycle in a mudflat at the Laguna de Mecoacan, Tabasco, Mexico

The fall of four morphological components of Avicennia germinans L. was measured at monthly intervals along a transect in a monospecific mangrove near the inlet of the Laguna de MecoacAn, Tabasco, Mexico. Estimated mean total annual litter fall was 614.4 g.m-2 yr-1. Seasonal fluctuations of mean water level, insolation, temperature and evaporation were highly intercorrelated and were summarized in a principal component axis that explains 82 percent of their variability. The results suggest that litter fall in A. germinans responds to the environmental variables already mentioned but is statistically independent from local rainfall. Each litter component (leaves, flowers and reproductive parts, branches and woody parts, and propagules) shows a distinctive sequence, and significant correlations were obtained when comparing the principal component axis against leaf and propagule fall. Phenologic variation in litter fall as an adaptive response to environmental changes is discussed. DESCRIPTIONS OF THE PHENOLOGY OF MANGAL WOODY SPECIES have been done in Florida (Gill and Tomlinson 1971, Pool et al. 1975); Thailand (Christensen 1978, WiumAndersen and Christensen 1978); Malaysia (Gong et al. 1980); Veracruz, Mexico (Rico 1979); and northeastern Australia (Williams et al. 1981). Emphasis, however, has been given in most cases to the whole community, or to species of Rhizophora. Recent reports have made clear that there are different species responses to the mangal environment and dimate (e.g., Wium-Andersen 1981, Williams et al. 1981, Snedaker and Brown 1982). This paper describes the annual phenological variations in litter fall of Avicennia germinans L. in a mudflat environment and analyzes their relation to environmental parameters, such as temperature, insolation, water level, and rainfall.

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).

The desert vegetation of El Pinacate, Sonora, Mexico

The Pinacate region is part of the Gran Desierto, one of the driest deserts in North America. The presence-absence of perennial plant species, together with soil and landform characteristics, were registered in 110 sampling sites within this region. A classification and ordination of plant communities showed soil and landform units to be good predictors of plant variation. Plant distribution and species richness were also strongly related to altitude and rockiness. A Generalised Linear Model was used to fit the response curves of individual species. The environmental factors related with plant distribution are indicators of the water regime within a given site. Plant communities repeat along topographic gradients the large-scale biogeographic variation of the Sonoran Desert. Microphyllous shrubs colonise the drier bajada slopes, while more diverse communities, dominated by cacti and drought-deciduous trees, grow on the wetter pediments and in pockets within rocky soils.

Artificial modifications of the coast in response to the Deepwater Horizon oil spill: Quick solutions or long-term liabilities?

The Deepwater Horizon oil spill threatened many coastal ecosystems in the Gulf of Mexico during the spring and summer of 2010. Mitigation strategies included the construction of barrier sand berms, the restriction or blocking of inlets, and the diversion of freshwater from rivers to the coastal marshes and into the ocean, in order to flush away the oil, on the premise that these measures could reduce the quantity of oil reaching sensitive coastal environments such as wetlands or estuaries. These projects result in changes to the ecosystems that they were intended to protect. Long-term effects include alterations of the hydrological and ecological characteristics of estuaries, changes in sediment transport along the coastal barrier islands, the loss of sand resources, and adverse impacts to benthic and pelagic organisms. Although there are no easy solutions for minimizing the impacts of the Deepwater Horizon disaster on coastal ecosystems, we recommend that federal, state, and local agencies return to the ...

Allometry of Prosopis glandulosa var. torreyana along a topographic gradient in the Chihuahuan desert

Abstract The allometric relationships of trees in temperate and tropical forests are relatively well known, but not those of woody shrubs or transitional (shrub/tree) life forms. We explored the transition of Prosopis glandulosa var. torreyana from tree to shrub along a semi-arid topographic sequence comprising of six landforms (hillslope, footslope, upper and lower bajada, playa and dune) with varying soil texture and water availability. In each landform, we measured P. glandulosa shoot pre-dawn water potentials (Ψ) in one ‘dry’ and one ‘wet’ year. We also measured plant height, widest basal stem diameter, crown area and number of basal branches. Total basal stem area was calculated. We used simple (Model II linear regression) and expanded (incorporating an asymptote to height or crown area) allometry models to compare height with widest basal stem diameter and crown area with total basal stem area. There were significant correlations between maximum plant size and inter-year Ψ means suggesting that soil water availability decreased down the topographical sequence. The height asymptote was statistically significant in all landforms and lower toward finer-textured soils. On the other hand, crown area was a linear function of total basal stem area and was also site specific. Our results suggest that more basal branches are produced as plant height decreases in more stressful environments, effectively increasing crown area with a minimum investment in supporting tissues. The polymorphic characteristics of Prosopis may partly explain their occurrence in many arid and semi-arid environments. Nomenclature: Correll & Johnston (1970); for families: Anon. (1998).

Nectarless honey mesquites.

We present a case of intrapopulational dimorphism in nectar production (nectarless/nectar producers) in the honey mesquite, Prosopis glandulosa var. torreyana (Leguminosae, Mimosoidae), and use this dimorphism as a natural experiment to explore the effects of nectar production on pollinator numbers and on pod production. In our study sites, roughly half the honey mesquites produce no nectar. Controlled pollinations indicate that there were no significant differences between morphs in pod production per raceme and compatibilities, and that fruit set is significantly higher in open, cross-pollinated inflorescences. There is a high abortion rate in both naturally and hand-pollinated inflorescences, regardless of nectar presence (...)

The demographic costs of nectar production in the desert perennial Prosopis glandulosa (Mimosoideae): a modular approach

Nectar production in angiosperms is considered to represent a reproductive cost, and has been associated with a decrease in fruit set or an overall decrease in the energetic budget of the plant. Populations of Prosopis glandulosa var. torreyana (honey mesquite) are a suitable system to evaluate the demographic costs of nectar production, as populations are composed of a 1:1 proportion of nectarful to nectarless individuals. The study was carried out in a population of 404 individuals of Prosopis g1andulosa var. torreyana found in an area with differing water availability in the Southern Chihuahuan Desert. The possible costs of nectar production were assessed on 1212 shoots of the honey mesquite that were tagged in 1994 and followed until 1998. We used two methods of analysis to describe the effect of nectar production on modular population dynamics: matrix analysis and log-xlinear models. Water availability and the varying environmental conditions affected plant growth, but nectar production did not have an effect on the demographic parameters we measured. The values of λ did not differ between nectar morphs and the only important effects we detected were the year to year variation in precipitation and microclimate differences at each site. Furthermore, the elasticity of each demographic process (growth, fecundity, retrogression and stasis) between nectar morphs did not differ. The log-linear models suggested a similar pattern but could discriminate the importance of each factor (nectar morph, year and site) on module fate. We were not able to detect a demographic cost of nectar production in the honey mesquite. The absence of a demographic response could be due to the negligible cost of producing nectar for this species or that the resources allocated for growth are different from those allocated for reproduction. Our results suggest that the modular fates of mesquites are mainly determined by environmental factors.

Avicennia germinans (black mangrove) vessel architecture is linked to chilling and salinity tolerance in the Gulf of Mexico.

Over the last several decades, the distribution of the black mangrove Avicennia germinans in the Gulf of Mexico has expanded, in part because it can survive the occasional freeze events and high soil salinities characteristic of the area. Vessel architecture may influence mangrove chilling and salinity tolerance. We surveyed populations of A. germinans throughout the Gulf to determine if vessel architecture was linked to field environmental conditions. We measured vessel density, hydraulically weighted vessel diameter, potential conductance capacity, and maximum tensile fracture stress. At each sampling site we recorded mangrove canopy height and soil salinity, and determined average minimum winter temperature from archived weather records. At a subset of sites, we measured carbon fixation rates using a LI-COR 6400XT Portable Photosynthesis System. Populations of A. germinans from cooler areas (Texas and Louisiana) had narrower vessels, likely reducing the risk of freeze-induced embolisms but also decreasing water conductance capacity. Vessels were also narrower in regions with high soil salinity, including Texas, USA and tidal flats in Veracruz, Mexico. Vessel density did not consistently vary with temperature or soil salinity. In abiotically stressful areas, A. germinans had a safe hydraulic architecture with narrower vessels that may increase local survival. This safe architecture appears to come at a substantial physiological cost in terms of reduction in conductance capacity and carbon fixation potential, likely contributing to lower canopy heights. The current distribution of A. germinans in the Gulf is influenced by the complex interplay between temperature, salinity, and vessel architecture. Given the plasticity of A. germinans vessel characters, it is likely that this mangrove species will be able to adapt to a wide range of potential future environmental conditions, and continue its expansion in the Gulf of Mexico in response to near-term climate change.