Carlos Vázquez-Yanes

Effects of Nutrient Limitation on Aboveground Carbon Dynamics during Tropical Dry Forest Regeneration in Yucatán, Mexico

Tree growth (as diameter increment), litterfall production, and litter biomass were studied in two secondary tropical dry forests of the Yucatán Peninsula under four treatments of nutrient addition. The study’s objective was to assess how variations in the nutrient supply affect aboveground net primary production and carbon (C) accumulation on the floor of two forests in different stages of regeneration. The study included an area of young forest (10 years old) with phosphorus (P)-poor soils and an area of old forest (around 60 years old) where soil P was comparatively less limiting. Four replicate plots (12 × 12 m) at each forest were either left intact (controls) or fertilized with nitrogen (N), P, or N plus P during 3 consecutive years. After 3 years of fertilization, relaxation of the constraints on nutrient limitation resulted in increased trunk growth rates at both the young and old forests. This effect was more pronounced with the addition of P or N plus P (trunk growth doubled with respect to controls), whereas N addition increased tree growth by 60% in comparison to trees in plots without nutrient supplements. In both forests, there were no significant differences in litterfall production among treatments during the first 2 years after fertilization. In the 3rd year of nutrient addition, litterfall production was significantly higher in plots fertilized with N plus P compared to control plots at both forest sites; however, changes in litterfall were not accompanied by litter accumulation in the floor of the two forests. The results of this study support the hypothesis that there is nutrient limitation during tropical dry forest regeneration. They further show that it may be maintained in the long term during secondary succession.

Seed response to temperature of Mexican cacti species from two life forms: an ecophysiological interpretation

The effect of seven different constant temperatures and five ranges of alternating temperatures on seed germination of seven species of cacti from Puebla, México was analyzed. Six cacti species germinated in a wide range of temperature. Columnar cacti were more tolerant to low temperatures and germinated in a wider range of temperature than barrel cacti. One of the barrel cacti studied (Ferocactus recurvus) only reached full germination at 25 °C . Temperature fluctuations did not produce significant effects on germination compared to the results obtained at constant temperatures. This may reveal differing ecophysiological adaptations with respect to temperature requirements during the establishment conditions for each life form. Columnar cacti may become established mainly under the shade of desert shrubs, whereas barrel cacti maybe can also become established in open areas, beneath the shade of small rocks or soil irregularities. In both cases, temperature fluctuations are attenuated by the shade, but mean temperatures may be higher in the second condition than beneath the shade of plants.

Natural priming of Wigandia urens seeds during burial: effects on germination, growth and protein expression

To determine whether seeds of the weedy shrub Wigandia urens, from the Valley of Mexico, undergo natural priming when buried in soil, comparative experiments were performed with seeds: (1) harvested directly from the plants; (2) buried in three natural habitat conditions; and (3) laboratory primed with polyethylene glycol. Seeds were sown in a growth chamber and in a shade house. Final germination percentages, emergence, germination and emergence rates, survival and initial growth were determined. Burial and priming enhanced the germination and emergence parameters evaluated in the laboratory and in the shade house. Effects of treatments on survival were not significantly different. Nevertheless, burial improved emergence and mean survival, and induced differences in specific leaf area of seedlings that could have ecological significance. Heat-stable proteins were extracted and electrophoresed. Proteins formed in W. urens seeds during burial had molecular weights (14‐21 kDa) similar to those reported for late embryogenesis abundant (LEA) proteins induced by priming in other species. Nevertheless, the presence and abundance of proteins expressed (14‐23, 36 and more than 45 kDa) differed among control, primed and buried seeds. During soil burial, molecular and physiological responses were induced that were similar to the effects of priming.

Differentiation in seed germination among populations of Capsicum annuum along a latitudinal gradient in Mexico

Toinvestigate variation in germination capability of wild Capsicumannuum seeds, the effects of light, fluctuating temperature,gibberellic acid (GA), soaking and sulfuric acid were analyzed in samples from14 populations from northwest Mexico. Germination was inhibited by darkness inall populations studied. Darkness inhibition of germination was overcome byfluctuating temperature and GA, factors which also increased germinationpercentage but not germination rate. Soaking treatments did not significantlyincrease germination of C. annuum seeds, while sulfuricacid treatments had negative effects on the process. Populations showed highvariation in germination response in all treatments, except for continuousdarkness. Principal component analysis differentiated some populations withhighgermination percentage and slow germination rate. Such differences did notcorrelate with climatic variables from collection sites. Given the variation ingermination patterns observed, some of these totally contrasting, we have to becautious when considering the results of studies based on a single wildpopulation.

Seed germination of woody legumes from deciduous tropical forest of southern Mexico

Abstract Seed germination experiments were conducted on six woody leguminous species which are widely used by rural inhabitants of a mountainous tropical subhumid region in southern Mexico. Conditions for optimum germination were sought in nursery conditions by applying various hot water treatments (1 min, 3 min, 5 min and 10 min in boiling water) and scarification to seeds of differing storage durations. Lysiloma divaricata germinated promptly without any treatment. Leucaena esculenta and L. macrophylla seeds germinated more readily after boiling (1 min), regardless of storage duration. Acacia species (A. cochliacantha, A. farnesiana and A. pennatula) had optimum germination with scarification treatments for all storage durations, while short-time boiling (1 min, 3 min) of their seeds increased germination only moderately. Seeds of species typical of primary vegetation lost viability sooner than those of species more frequent in disturbed areas. From these results, seed handling routines and germination pre-treatments practices can be derived for these species, all of which can potentially be used in reforestation on degraded soils.