Odilón Sánchez-Sánchez

Tropical forest communities in southeastern Mexico

A phytosociological study was carried out along a 450 kmtransect in tropical southeastern Mexico in order to identify plantcommunities.Five major plant communities were distinguished and described:Those communities reflect preference of different environmentalfactorssuch as soils, precipitation and local relief. Further, the major threats(hurricanes, fires, agriculture) to the vegetation of the study areaare discussed.

Jatropha curcas L. root structure and growth in diverse soils.

Unlike most biofuel species, Jatropha curcas has promise for use in marginal lands, but it may serve an additional role by stabilizing soils. We evaluated the growth and structural responsiveness of young J. curcas plants to diverse soil conditions. Soils included a sand, a sandy-loam, and a clay-loam from eastern Mexico. Growth and structural parameters were analyzed for shoots and roots, although the focus was the plasticity of the primary root system architecture (the taproot and four lateral roots). The sandy soil reduced the growth of both shoot and root systems significantly more than sandy-loam or clay-loam soils; there was particularly high plasticity in root and shoot thickness, as well as shoot length. However, the architecture of the primary root system did not vary with soil type; the departure of the primary root system from an index of perfect symmetry was 14 ± 5% (mean ± standard deviation). Although J. curcas developed more extensively in the sandy-loam and clay-loam soils than in sandy soil, it maintained a consistent root to shoot ratio and root system architecture across all types of soil. This strong genetic determination would make the species useful for soil stabilization purposes, even while being cultivated primarily for seed oil.

Distribution of Vegetation Types

This chapter presents an overview of the vegetation types of the Yucatan Peninsula. Mean annual precipitation, terrain and soil characteristics explain the large-scale distribution of forest types in the Yucatan Peninsula. Tropical (high) forest is found in areas with >800 mm of mean annual precipitation, while dry (low) tropical forest is distributed in areas with <800 mm of mean annual precipitation. Tropical forest has canopy heights of more than 30 m and mostly presents three well-defined vegetative strata. The distribution of tropical forest is mainly in the central, eastern and southern parts of the Yucatan Peninsula. Dry tropical forest has canopy heights up to 20 m, and is widely distributed in the Yucatan Peninsula in different successional stages. Other woody vegetation types include mangroves, petenes and pine savannas. Open vegetation types include coastal dunes, marsh, and savanna vegetation. Disturbed tropical and dry tropical forest of all successional stages covers more than 7.4 million ha at present, and requires detailed management plans to maintain future ecosystems benefits.

Alometría de semillas de "Jatropha curcas" L. mexicanas

This work presents a comparative morphological study of four Mexican sources. Three non-toxic, non-commercial sources from the state of Veracruz (Papantla, Medellin and Alvarado) and one toxic provenance belonging to a commercial plantation in the state of Colima (Tecoman), were analyzed over two years. An average of 480 seeds per provenance, per year were characterized by estimating their phorbol-ester contents, determining their sizes and weights, correlating data between kernel weight versus total seed weight, documenting their agronomic conditions and comparing them with other Mexican sources. Significant differences were found in seed morphology per provenance (p 0.86) and seed volume had the lowest coefficient of variation (CV< 4.5%) for all sources. Apart from genetic factors, weight and size could also be related to crop management practices and soil fertility, since the largest and heaviest seeds came from the more fertile soils or fertilized and irrigated plantations, while the smallest and lighter seeds came from semi-wild sources located in sandy soils.

Textural, chemical and sensorial properties of maize tortillas fortified with nontoxic Jatropha curcas L. flour

ABSTRACT Hunger and malnutrition still affecting part of the Mexican population, for whom the tortilla is a staple food. However, maize tortillas are high in calories but deficient in proteins; therefore, studies have been conducted to fortify tortillas, with protein-rich nonconventional sources. This study examines the physicochemical and rheological characteristics of dough and maize tortillas fortified with nontoxic Jatropha curcas flour. Fortified maize doughs were prepared with 0%, 5%, 10%, 15% and 20% J. curcas nontoxic flour, which contains 55% protein. Tortillas were made and their chemical profile and tortilla quality were quantified and subjected to sensorial analysis. Rheological characteristics of dough were slightly modified but the protein in the tortillas increased 10.8% with treatment T20. They did not change in color, were soft, and consumer acceptance was not affected. It is concluded that nontoxic J. curcas flour is an excellent option for increasing the protein value of tortillas.

Natural and Human Induced Disturbance in Vegetation

The present chapter analyses natural and human disturbance which impacted the vegetation of the Yucatan Peninsula. The relevance of the slash-and-burn system is outlined, a system used for many centuries, and the relevance of fire in this practice. Slash-and-burn and fires provide information about the present structure and composition of the tropical forests of the Yucatan Peninsula. Several seral stages are identified, which agree well with seral stages recognized by modern Maya farmers. In relation to natural disturbance, we analyze the influence of hurricanes and their immediate and general effects on the vegetation and related fauna. Hurricanes and droughts are relevant agents of disturbance, as hurricanes cause considerable structural damage to forests and contribute to the accumulation of large quantities of dry biomass. This biomass can act as fuel for large forest fires. We discuss the natural dynamics that have characterized the vegetation of the Yucatan Peninsula through time, showing that it is a resilient ecosystem.

Distribution and Abundance of Big-Leaf Mahogany (Swietenia macrophylla) on the Yucatan Peninsula, Mexico

Big-leaf mahogany (Swietenia macrophylla King) is an economically important timber species in the Neotropics. For over three centuries, it has been selectively extracted from tropical forests, threatening its populations. We investigate the actual and potential distribution of big-leaf mahogany and assess its abundance on the Yucatan Peninsula based on the National Forest and Soils Inventory database. Furthermore, we evaluate environmental factors associated with its distribution, abundance, and tree size. The actual and potential distribution models show the presence of mahogany in a wide geographic area covering the southern and eastern portions of the Yucatan Peninsula. Abundance of mahogany in the landscape varies and in general is low. The spatial potential distribution model was best explained by the environmental variables of vegetation cover (medium- and high-stature semievergreen tropical forest) and elevation (upland areas). Results also indicate that mahogany remains relatively abundant and contain larger size classes in localities where the species has been harvested and managed for decades under community forest management. Furthermore, statistical analyses show greater tree density of mahogany mostly associated with low-stature semievergreen tropical forest having deep soils (gleysols and vertisols), while larger tree size (diameter at breast height) was associated with medium-stature semievergreen tropical forests in upland areas with moderately deep or shallow soils (mostly rendzinas or leptosols). Despite deforestation, land-use change and forestry activities on the Yucatan Peninsula, particularly in the past 20 years, the distribution and abundance of mahogany do not appear to be as drastically reduced as described in other neotropical regions.