Prosopis Laevigata Research Articles

Carbon‐nitrogen interactions in fertility island soil from a tropical semi‐arid ecosystem

Summary 1. Biological nitrogen (N) fixation by symbiotic and free‐living organisms is considered the main pathway for N soil enrichment in desert and semi‐desert ecosystems. This fact is more noticeable in tropical ecosystems where legume species have a high relative abundance. However, this biological fixation pathway does not guarantee the maintenance of soil N pools, and N conservation pathways are important in understanding controls over soil N cycling. 2. In dryland ecosystems, desert plants can form a ‘fertility island’ (FI) as soils beneath plants show higher concentrations of N and organic matter. 3. Here we assess how carbon (C) and N may interact to conserve soil N within the FI soil of two legume species (Prosopis laevigata and Parkinsonia praecox), one a known N‐fixer and the other believed not to fix N, as well as within adjacent bare ground soil. In a semi‐arid tropical ecosystem in central Mexico, we examined spatial patterns in C and N pools and transformation rates, and we investigated seasonal variations in these relationships. 4. Results show that FI soil C and N could be linked to total N storage through net C and N immobilization in microbial biomass and heterotrophic microbial activity. Soil under P. laevigata canopy had greater total N as well as N accumulated in microbial biomass than soil under P. praecox and bare ground soil. Nevertheless, inorganic N and potential net N mineralization rates were similar under soils of both species, although we expected higher inorganic N and N‐mineralization values in N‐fixer species to explain the greater total N. Higher total N concentrations under P. laevigata probably result from greater inputs of organic C and a higher potential net C mineralization rate in comparison to P. praecox and bare ground soil. 5. Even though N input and output values were not measured, the results highlight the importance of assessing the role of organic C, heterotrophic microbial activity, and N storage in microbial biomass in order to understand controls over N retention in soil N cycling. Thus, soil C‐N interactions could be a control factor of N soil conservation in this tropical semi‐arid ecosystem.

Soil erosion and runoff in different vegetation patches from semiarid Central Mexico

article i nfo Vegetation patches in arid and semiarid areas are important in the regulation of surface hydrological processes. Canopy and ground covers developed in these fertility islands are a natural cushion against the impact energy of rainfall. Also, greater levels of organic matter improve the soil physicochemical properties, promoting infiltration and reducing runoff and soil erosion in comparison with the open spaces between them. During the 2006 rainy season, four USLE-type plots were installed around representative vegetation patches with predominant individual species of Huisache (Acacia sp), Mesquite (Prosopis sp), Prickly Pear or Nopal (Opuntia sp) and Cardon (Opuntia imbricata), to evaluate soil erosion and runoff, in semiarid Central Mexico. A comparative bare surface condition (Control) was also evaluated. Vegetative canopy and ground cover were computed using digital images. Selected soil parameters were determined. Soil erosion was different for the studied vegetation conditions, decreasing as canopy and ground cover increased. There were not significant differences in runoff and soil erosion between the Control and O. imbricata surfaces. Runoff was reduced by 87%, 87% and 98% and soil loss by 97%, 93%, and 99% for Acacia farnesiana, Prosopis laevigata and Opuntia sp, respectively, as compared to the Control. Soil surface physical conditions were different between the low vegetation cover conditions (Control and O. imbricata surfaces) and the greater vegetation cover conditions (A. farnesiana, P. laevigata and Opuntia sp), indicating a positive effect of vegetation patches on the regulation of surface hydrological processes.

Effect of gut passage and dung on seed germination and seedling growth: donkeys and a multipurpose mesquite from a Mexican inter-tropical desert

We address the idea that donkey's gut passage and dung could increase seed germination and seedling growth of Prosopis laevigata. This species is a useful plant in a Mexican inter-tropical desert, where land use change and habitat fragmentation are currently affecting P. laevigata shrublands. We conducted greenhouse experiments in which seeds ingested by donkeys and non-ingested seeds were placed in dung pats or soil. Results suggested that gut passage and dung did not have significant effects on seed germination. On the other hand, dung significantly increased the height and dry weight of P. laevigata seedlings. Information about the effects of donkeys on seedling establishment could be used to promote the natural regeneration of this plant species.

Vertical distribution of microbial communities under the canopy of two legume bushes in the Tehuacán Desert, Mexico

Prosopis laevigata and Parkinsonia praecox are the most abundant perennial shrubs in the Tehuacán Desert, forming ’islands of fertility’ that dominate the alluvial terraces. Both species exhibit very similar phenology, with the timing of litter foliage being the only difference between them. P. praecox litter occurs shortly after the rains, while P. laevigata maintains its leaves until the next wet season. As degradable organic matter (OM) is one of the leading factors determining soil biota composition and activity, because of the OM provided by littering, we expected that the vertical distribution of the microbial community in the vicinity of the root zone of P. praecox would be higher in comparison to P. laevigata. One soil sampling was performed; during the rainy season in August, soil samples were collected from a 0–50-cm depth at 10-cm intervals, in the vicinity of the root canopy of four individual plants of each species and the interspaces between them. Soil moisture, organic matter, and counts of bacteria and fungi under shrubs were found to decrease from the upper to deeper layers. Respiratory activity was higher in the deeper layers ( p < 0.01) in all three sampling sites. Total bacterial, fungal, and heterotrophic diazotrophs were found to be significantly ( p < 0.001) more numerous under shrubs than in the interspace soil. No nitrogen-fixing bacteria were isolated from interplant soils in comparison to the soil samples collected beneath the shrubs. Heterotrophic diazotrophs significantly ( p < 0.01) reduced more acetylene under P. praecox (29.0 nmol/g soil) than under P. laevigata (20.1 nmol/g soil). Although the microbial numbers were unaffected by differences in plant phenology, greater nitrogenase activity under P. praecox may influence nitrogen distribution in this arid environment. Due to the fact that only one sampling was undertaken, this study elucidates the differences in the microbial community between the two shrubs, but the dynamics in the above community could not be shown.

Arsenic distribution in mesquite ( Prosopis laevigata ) and huizache ( Acacia farnesiana ) in the Zimapán mining area, México

Arsenic concentrations have been determined in mesquite ( Prosopis laevigata ) and huizache ( Acacia farnesiana ) growing on As-rich soils in the semi-arid mining area of Zimapán, México. The elevated As contents in the substrates (up to 32 000 mg/kg) were reflected in the dry weight concentrations of twigs (up to 82.7 mg/kg in mesquite and 225 mg/kg in huizache), and leaves (up to 78.2 mg/kg in mesquite and 67.0 mg/kg in huizache). Arsenic concentrations in twigs and leaves collected in less impacted soils (containing up to 110 mg/kg of As) reached a maximum of 20.8 mg/kg in mesquite twigs and 27.8 mg/kg in mesquite leaves. Differences of several orders of magnitude in surface substrate As concentrations were not reflected in plant As contents. A low proportion of available As in substrates, as well as deep plant roots which may reach less contaminated sediments, explains this behaviour. The pod As concentrations from tailings-affected sites were not significantly different from background samples. Nearly all mesquites and huizaches from the high-As substrates were above standards for foodstuffs. The As concentration in vegetation growing on soils affected by tailings and smelter/slags points to a potential contamination of the food chain, mainly through goats, the most common mammals in the area. Mesquites and huizaches are not hyperaccumulators but are As-tolerant plants and can therefore be considered as an option for a remediation programme to stabilize eroding tailings.

In vitronematicidal effects of medicinal plants from the Sierra de Huautla, Biosphere Reserve, Morelos, Mexico againstHaemonchus contortusinfective larvae

Twenty extracts from plants from Sierra de Huautla Biosphere Reserve, Morelos, Mexico were evaluated against Haemonchus contortus infective larvae in an in vitro assay. The plant species evaluated were Bursera copallifera, B. grandifolia, Lippia graveolens, Passiflora mexicana, Prosopis laevigata, Randia echinocarpa and Urtica dioica. The plants were separated into their parts and macerated with different solvents (n-hexane, acetone, ethanol and methanol). An in vitro assay was used to evaluate the anthelmintic activity against unsheathed third stage H. contortus infective larvae. The experiment was carried out in 24-well cell culture plates at room temperature with three replicates per treatment and using a concentration of 20 mg ml- 1. Ten 5 microl aliquots were taken from the corresponding wells and deposited on a slide for microscopical observation at 24, 48, 72 and 96 h post-exposure. The evaluation criteria were based on the average numbers of live and/or dead larvae in the different treatments. Alive and dead larval numbers were statistically analysed through the ANOVA test (P>0.01). The Tukey test was used as a complementary tool to determine which treatment was different from the other treatments (P>0.05). The highest mortality was observed with P. laevigata hexanic extract from stem and leaves combined, which produced 51%, 81% and 86% larval mortality at 24, 48 and 72 h post-exposure, respectively. On the other hand, B. copallifera stem acetonic extract exhibited 18%, 59% and 66% nematicidal activity after 24, 48 and 72 h of exposure, respectively.

The effect ofCercidium praecoxandProsopis laevigataon vertical distribution of soil free‐living nematode communities in the Tehuacán Desert, Mexico

AbstractVegetation cover is known to act as an abiotic mediator influencing the structure of soil fauna communities in arid and semi‐arid ecosystems. The aim of the current research was to determine the spatial dispersion of the soil free‐living nematode community under the canopy ofCercidium praecoxandProsopis laevigataduring the rainy season. These shrubs are the dominant plant associations in the western part of the Tehuacán‐Cuicatlán Valley in Mexico. Soil samples were taken from each 10‐cm depth between 0 and 50 cm in August 2004. Our results demonstrated that the abundance and structure of the soil free‐living nematode communities in the study area were strongly dependent on plant effects, specified by limited factors such as soil moisture and organic matter availability. The greatest degree of abundance of soil‐free‐living nematodes (88%) was found in the upper (0–10 cm) soil layer. Plant parasites were the most abundant trophic group under the two plants (58 and 36% underParkinsonia(Cercidium) praecoxandProsopis laevigata, respectively), whereas omnivore‐predators were the most dominant (96%) in inter‐plant spaces. The fungivore/bacterivore (F/B) ratio was found to be the most useful tool of the ecological indices tested in the present study, reflecting the vertical distribution of the free‐living nematode communities beneath different plant species in the different soil layers. The soil free‐living nematode communities and their vertical distribution were found to be affected by plant ecophysiological adaptation, soil moisture, and the interaction between them.

Fertility islands around Prosopis laevigata and Pachycereus hollianus in the drylands of Zapotitlán Salinas, México

The abundance of microorganisms in desert soil environments is closely related to the accumulation of resources around individual plants, called “islands of fertility”. This research was undertaken (1) to study the formation of islands of fertility in soils under Prosopis laevigata and Pachycereus hollianus in a preserved terrace and a degraded terrace in Zapotitlán Salinas, México and (2) to probe the relationships between the insularity under the plants and cultivable bacteria. Islands of fertility were evaluated for total organic carbon accumulation, water content, and acetylene reduction activity. Insularity was only evident in degraded soil, while resources were more evenly distributed in preserved soil. Acetylene reduction activity showed an undefined pattern of seasonal variation in both terraces. All parameters had maximum values in the preserved terrace, including colony-forming units of cultivable bacteria. The bacteria belonged to genus Rhizobium (one strain) and Streptomyces (six strains) according to 16S rDNA sequencing, revealing an unsuspected dominance of actinobacterias. Fertility islands formed under P. laevigata are the most important nutrient sources, enhancing microbial activity in degrading soils. This plant–microbe interaction delays badlands transformation and could be an important strategy for avoiding the degradation of cultivated lands.

Seasonal Trends of Chlorophylls a and b and Carotenoids in Native Trees and Shrubs of Northeastern Mexico

During two consecutive years, the content of chlorophylls a and b and carotenoids(x + c) was determined, seasonally, in foliar tissue of native trees (T) and shrubs (S) from northeastern Mexico such as Acacia rigidula (S), Bumelia celastrina (T), Castela texana (S), Celtis pallida (S), Croton cortesianus (S), Forestiera angustifolia (S), Karwinskia humboldtiana (S), Lantana macropoda (S), Leucophyllum frutescens (S), Prosopis laevigata (T), Zanthoxylum fagara (T). Pigment determinations were carried out in a region of the state of Nuevo Leon, Mexico at three county (Los Ramones, China, Linares) sites, which are grouped under a similar climatic pattern. Measurements were quantified spectrophotometrically. With exception of the interaction year*plant of carotenoids content at Los Ramones site, all pigments were significantly different between years, seasons and between plants within years and seasons. All plants had marginal higher chlorophyll a content at Linares (overall mean = 0.79 mg g -1 fw) than China (0.71) or Los Ramones (0.66) site. Chlorophyll b content followed a similar trend as chlorophyll a (0.29, 0.25 and 0.23, respectively). Marginal differences in carotenoids content, in all plants, were found among sites being the overall mean of 0.2 mg carotenoids g -1 fw. Yearly and seasonal variations in plant pigments might have been related to seasonal water deficits, excessive irradiance levels during summer and extreme low temperatures in winter that could have affected leaf development and senescence.

Wood Anatomical Characteristics and Chemical Composition Of Prosopis Laevigata Grown in the Northeast of Mexico

Structural heartwood characteristics for Prosopis laevigata (Humb. &amp; Bonpl. ex Willd.) M.C. Johnst., including a histometrical evaluation, were obtained by light microscopy coupled with a digitised image analysis system. The growth ring boundaries of the semi-ring-porous or diffuseporous wood are often marked by a marginal parenchyma band. Average fibre length is 975 μm, the fibres are thick-walled with a single cell wall thickness of 13 μm on average. Average diameter of the vessels which are arranged in non-specific patterns differs significantly between earlywood (116 μm) and latewood (44 μm). The topochemical distribution of lignin and phenolic deposits in the tissue was investigated by means of scanning UV microspectrophotometry (UMSP). Thereby, in heartwood tissue the deposition of extractives in vessels, pit canals, parenchyma cells, fibre lumina and partly also in the S2 layers of the fibres was detected. Monosaccharides were qualitatively and quantitatively determined by borate complex anion exchange chromatography. Holocellulose content is between 61.5 and 64.7% and Klason lignin content between 29.8 and 31.4%. Subsequent extraction of the soluble compounds was performed with petrolether, acetone/water and methanol/water by accelerated solvent extraction (ASE). Total extractives content in heartwood ranges between 14 to 16% on a dry weight basis. Major compounds in acetone/water extracts were identified as (-)-epicatechin, (+)-catechin and taxifolin, and quantitatively determined by liquid chromatography (RP-HPLC-UV).

Modificaciones en la corteza de Prosopis laevigata por el establecimiento de Tillandsia recurvata

El crecimiento de Tillandsia recurvata sobre las ramas de Prosopis laevigata parece causar varios grados de daño. El objetivo de este trabajo fue estudiar y comparar los posibles cambios anatómicos en la corteza del forofito provocados por la interacción con la epifita. En la corteza carente de la epifita se distinguen floema no colapsado y floema colapsado, córtex y peridermis, semejante a lo descrito para otras especies de leguminosas. La fijación de T. recurvata en la corteza de P. laevigata provoca diferentes respuestas en su estructura anatómica, afectando diferentes tejidos. Los cambios se manifiestan por la formación de súber de reacción y de células con contenido obscuro, probablemente de naturaleza fenólica y como barrera química al establecimiento de enfermedades. Cuando la fijación de la epifita es mayor, se observan diferentes grados de invaginación que pueden llegar a la madera y alterar su patrón típico, es decir, causar una reducción en el número y diámetro de los vasos. El impacto que la epifita causa sobre el forofito puede favorecer el ataque de patógenos en los tejidos de los hospederos.

Establecimiento de plantas mediante el uso de micorrizas y de islas de recursos en un matorral xerófilo deteriorado

En este trabajo se evaluó la supervivencia de plantas de mezquite (Prosopis laevigata) y de huizache (Acacia farne -siana), previamente inoculadas con hongos micorrizógenos arbusculares (HMA), en condiciones de campo, durante un año. Las plantas fueron cultivadas durante nueve meses en invernadero y dos meses en vivero, antes de ser trasplantadas a un matorral xerófilo deteriorado localizado en el Valle de Actopan, Hidalgo, México. Cada individuo, micorrizado o no micorrizado, fue transplantado bajo la cobertura de una planta nodriza perteneciente a una de ocho especies generadoras de islas de recursos, con el fin de evaluar el efecto de la micorrización y del nodrizaje vegetal sobre el establecimiento y la supervivencia de las plantas, durante el periodo de sequía (octubre-abril). Los resultados muestran que la micorrización de plantas de mezquite y de huizache aumenta de manera significativa la supervivencia, incrementándose el porcentaje de 19 a 54% para P. laevigata, y de 18 a 48% para A. farnesiana. Respecto a la especie de planta nodriza, no se encontraron diferencias en el porcentaje de supervivencia de los individuos trasplantados debidas al tipo de isla de recursos. El experimento también mostró que las plantas micorrizadas desarrollan mayor número de pinnas con relación a sus testigos. Se concluye que es recomendable la inoculación de plántulas con HMA nativos, en programas de restauración ecológica de ecosistemas semiáridos deteriorados

Clonal propagation of mesquite tree (Prosopis laevigata Humb. &amp; Bonpl. ex Willd. M.C. Johnston). I. Via cotyledonary nodes

Plantlet regeneration in Prosopis laevigata (Humb. & Bonpl. ex Willd.) Johnston (Fabaceae), a multipurpose tree, has been achieved from cotyledonary nodes excised from in vitro grown seedlings. The explants were cultured on MS media containing different concentrations of N-6 benzyladenine (BA) and 2,4-dichlorophenoxyacetic acid (2,4-d) and a mixture of organic components. The highest number (3.37 + 0.51) of multiple shoots was observed in MS media containing 2,4-d (9.05 μM) + BA (6.62 μM). The regenerated shoots were then transferred onto half-strength MS medium containing a plant growth regulator that was either: indole-3-butyric acid, 1-naphthaleneacetic, indole-3-acetic acid, or 2,4-d as well as phytagel or vermiculite for adventitious root initiation. Best rooting efficiency of 44.0% was obtained when NAA (16.11 μM) and vermiculite were used. After rooting, the cloned plantlets were successfully hardened to ex vitro conditions. This work may help to reduce the devastation caused by the overexploitation of this species.

Microbial biomass in a semi arid soil of the central highlands of Mexico cultivated with maize or under natural vegetation

Microbial biomass (MB) is the key factor in nutrient dynamics in soil, but no information exists how clearing of vegetation to cultivate maize in the central highlands of Mexico might affect it. Soil MB was measured with the chloroform fumigation incubation (CFI) and fumigation extraction (CFE) techniques and the substrate-induced respiration (SIR) method in soil sampled under or outside the canopy of mesquite ( Prosopis laevigata) and huisache ( Acacia tortuoso), N 2 fixing shrubs, and from fields cultivated with maize. Microbial biomass C as measured with the CFI technique ranged from 122 mg C kg −1 in agricultural soil to 373 mg C kg −1 in soil sampled under mesquite shrubs. Microbial biomass N as measured with the CFI technique ranged from 11 mg N kg −1 in agricultural soil to 116 mg N kg −1 in soil sampled under mesquite shrub. The ratio of microbial biomass C as measured with CFI related to the ninhydrin-positive compounds (NPC) was 12.23 after 1 day and 8.43 after 10 days while the relationship with extractable C was 3.15 and 2.96, respectively. The metabolic quotient ( qCO 2) decreased in the order OUTSIDE > MESQUITE > HUIZACHE > AGRICULTURE, and the microbial biomass:soil organic C ratio decreased in the order MESQUITE > HUIZACHE > OUTSIDE > AGRICULTURE using SIR to determine the microbial biomass. It was found that converting soil under natural vegetation to arable soil was not only detrimental for soil quality, but might be unsustainable as organic matter input is limited.

Seed Removal Rates Under Isolated Trees and Continuous Vegetation in Semiarid Thornscrub

Abstract Clearing native vegetation to increase the amount of land available for agriculture in northeastern Mexico has left remnants ranging in size from fragments of continuous vegetation to isolated individual trees. These provide valuable opportunities for restoring larger areas of native vegetation. We explored whether fragmentation of Tamaulipan thornscrub affects the removal of seeds from 12 woody species that encompass a range of sizes and dispersal mechanisms. We tested whether (1) seed removal rates under isolated trees were higher than under continuous vegetation; (2) dispersal structures, such as fleshy pericarps, made some seeds more attractive to seed removers; and (3) microenvironmental variation affected seed removal rates. Seeds were placed under canopies of Texas ebony (Ebenopsis ebano) and Mesquite trees (Prosopis laevigata). Seed removal trials were conducted three times, each trial lasting 30 days. Most seeds were removed in all trials by the end of one month. Seed removal rate was slower under isolated trees. In general, fleshy fruits were removed faster than other fruits; whole fruits and fleshy tissue were removed faster than depulped seeds. In species with fleshy pericarps, acid washing of seeds, to simulate seeds processed in the digestive tract of dispersers, reduced the seed removal rates, suggesting that it would be a good pre‐treatment for restoration efforts.

Processing, Nutritional Evaluation, and Utilization of Whole Mesquite Flour (Prosopis laevigata)

ABSTRACT: Basic crops such as wheat and maize are not cultivated easily in the central highlands of Mexico, but pods of mesquite (Prosopis laevigata), a N2−fixing plant that dominates the vegetation, could be used as an additional food source. The physicochemical and functional properties and nutritional value of whole mesquite pod flour dried at 60 °C and 70 °C and muffins prepared with the mesquite flour were investigated. Drying temperature did not change the true protein concentration of the whole mesquite pod flour, that is, 97%. The digestibility for pod toasted at 70 °C was 78% compared to 77% at 60 °C, tannin content was 58% and 48% while trypsin inhibitor was 347 and 495 unit of inhibition (UI)/g sample, respectively. Functional characteristics of whole mesquite pod flour were similar to those of bean flour (Phaseolus vulgaris), and superior to those of whole wheat flour (Triticum sp). Mesquite proteins were high in tryptophan and histidine. Sensorial analysis scores for products made with whole mesquite flour were high. The results of this work suggested that whole mesquite pod flour could be used as a supplement for human consumption.

Relationship between soil nutrient availability and plant species richness in a tropical semi‐arid environment

AbstractQuestion:What is the relationship between soil fertility and plant species richness in the ‘fertile islands’ occurring beneath two species of legume (Cercidium praecoxandProsopis laevigata)?Location:Tehuacán‐Cuicatlán region, central Mexico.Methods:Plant richness was measured in three micro‐environments (below canopies ofC. praecox, below canopies ofP. laevigataand in areas without canopies). The concentration of soil nutrients (C, N and P), C and N in the microbiota, and processes of ecosystem functioning (net C mineralization rate and N mineralization) were measured. The relationship between soil variables and plant richness were assessed with ANCOVAs.Results:Soil nutrients and species richness increases markedly under fertility islands. There were higher concentrations of C and N in the soil, faster rates of C mineralization, and higher species richness underP. laevigatacanopies. The relationship between soil fertility and species richness was always positive except for total N, ammonium and net C mineralization rate underC. praecox, and for available P underP. laevigata.Conclusions:The sign of the relationship between soil fertility and species richness varies according to the nutrient and the micro‐environment. Positive relationships could result from between species complementarity and facilitation. Negative relationships could be explained by a specific limitation threshold for some soil resources (P and N for plants and C for the soil microbiota) which eliminate the possibilities of between species complementarity and facilitation above that threshold. As in all observational studies, these relationships should be considered only correlational.

Relationship between soil nutrient availability and plant species richness in a tropical semi-arid environment

Abstract Question: What is the relationship between soil fertility and plant species richness in the ‘fertile islands’ occurring beneath two species of legume (Cercidium praecox and Prosopis laevigata)? Location: Tehuacan-Cuicatlan region, central Mexico. Methods: Plant richness was measured in three micro-environments (below canopies of C. praecox, below canopies of P. laevigata and in areas without canopies). The concentration of soil nutrients (C, N and P), C and N in the microbiota, and processes of ecosystem functioning (net C mineralization rate and N mineralization) were measured. The relationship between soil variables and plant richness were assessed with ANCOVAs. Results: Soil nutrients and species richness increases markedly under fertility islands. There were higher concentrations of C and N in the soil, faster rates of C mineralization, and higher species richness under P. laevigata canopies. The relationship between soil fertility and species richness was always positive except for total N, ...

Leguminous seedling establishment in Tamaulipan thornscrub of northeastern Mexico

Due to extensive land clearing in northeastern Mexico, there is an increasing need for restoration for which knowledge on plant establishment biology becomes a priority, for restoration practices. In here we tested the influence of current environmental variation on the establishment biology of common woody species from Tamaulipan thornscrub. Seedling establishment was monitored; four native species ( Acacia berlandieri Benth., Ebenopsis ebano (Benth.) Coult, Havardia pallens (Benth.) Brintton and Rose, Prosopis laevigata (Humb & Bonpl. ex. Wild.) M.C. Johston), and one exotic species ( Leucaena leucocephala (Lam.) de Wit). Seedling emergence, seedling survivorship, length of stems and number of leaves were evaluated over 1 year in environments with different light regimes: (i) dense thornscrub; (ii) thornscrub edge; (iii) cleared thornscrub (direct sunlight). Ten plots of 4 m 2 (2 m × 2 m) were sampled in each environment. Seedling emergence occurred on spring and late summer and was greater on dense thornscrub for all species. Native species had higher survivorship and growth and had more leaves in dense thornscrub than in other environments. Exotic L. leucocephala had similar survivorship, shoot length and number of leaves across environments. All seedlings from all species died by late spring, possibly due to environmental stress. Seedling survival was longer for all native plants, as there were surviving seedlings in early spring for all species but not of exotic L. leucocephala. Perhaps mainly as a result of high temperature and low humidity. Additional watering, shading and moisture retaining gels should thus be considered when rehabilitation programs are made with seeds and seedlings in the region.

Habitat preference of the epiphyteTillandsia recurvata(Bromeliaceae) in a semi-desert environment in Central Mexico

Tillandsia recurvata L. is an atmospheric epiphyte that occupies tree canopies in many parts of tropical America. We investigated the host preferences of this species by analyzing its occurrence on trees in a 1-ha plot in a semi-desert environment in Mexico. Additionally, we carried out germination experiments and recorded seedling survival and growth in different microsites on the three preferred host species. Our results indicate that T. recurvata occupies the crowns of certain host species (Prosopis laevigata (Humb. &amp; Bonpl. ex Willd.) M.C. Harms, Acacia bilimekii Macbr., and Cercidium praecox (Ruiz &amp; Pav.) Harms) with a higher frequency than expected by chance. In addition to species identity, tree size affected the probability of being occupied: small trees were occupied with lower frequencies and with lower densities than larger ones. On the other hand, distance to the nearest colonized tree was not related to the probability of a tree being occupied. Seed germination ranged from 0% to 7.5% and did not differ among the three host species tested. However, both seed germination and seedling growth were higher in the outer and upper parts of tree crowns. Given the patchy distribution of this epiphyte, we propose this system may be viewed as a metapopulation in which patch suitability varies according to host identity and size.