Phytoalexins are secondary metabolites of a diverse chemical nature, principally flavonoids of low molecular weight that are synthesized in plants after a microbial infection. The synthesis may be triggered by the action of factors such as elicitors or inducers, which may be produced exogenously by synthesis and accumulation of phytoalexins. Not only do they originate in the host plant, but also in the pathogen (fungi, bacteria and virus). They have been identified principally in dicots. There are few reports of their presence in monocots and gymnosperms. The technique of culture in vitro is an alternative for the production of phytoalexins and research on these metabolites can contribute to the control of certain pests.

In the Milpera region of Yucatan, México there are areas of traditional forest reserves, which have been established by farming communities. In these areas, the people of the region put into practice their own traditional conservation policies. This study shows the importance of traditional management, knowledge and use of the forest genetic reserves. The study was conducted using the following methodology: the areas were determined through interviews and satellite images; tree measurements were taken using the sweep method; and random samples in five communities were taken. With this information, the abundance of species, diversity index, vegetation structure and use of plant resources were determined. Statistically significant differences were observed between the communities studied with respect to tree composition, and a similar tendency was found among the areas on comparing the different ways the wood is used.

As population increases, along with its demands on forest products, the land available for production decreases. Thus, coordinated efforts are required to achieve the sustainability of forest production. Although the traditional systems of silviculture and genetic improvement of trees are still important in current forestry activities, the existing conventional programs of genetic improvement and production are limited by the long growth cycle of the trees and the constant difficulty in distinguishing between genotypical expression and environmental aspects. Biotechnology offers new techniques, which complement traditional methodologies of forest genetic improvement. The important advances in techniques of plant tissue culture and molecular biology that have taken place in the last two decades are the basis of the development of fields such as cryoconservation and plant regeneration (expression of cellular totipotency), DNA markers, genomics of trees and genetic transformation. In the realm of genetic resources, DNA markers are making it possible to characterize the nature, extension and distribution of the genetic variability in plant species, therefore facilitating decisions of what and how to preserve. Cryoconservation and in vitro plant regeneration are being employed to conserve and micropropagate specific plant material for carrying out ex situ conservation and for the development of clonal silviculture. In this study, applications of these fields to the forest species are reviewed. In this context, information is provided on forest biotechnological activity in the different areas of investigation.

This study evaluated the performance of Pinus patula Schl. et Cham. planted in two environments in Chignahuapan, Puebla, Mexico. The first environment was a low-density Pinus teocote Schl. & Cham. and P. patula stand. The second was the same stand after a prescribed low-intensity burn. Acclimatation of four height classes were tested one year after planting date, considering survival, growth in height and diameter, biomass and levels of foliar nitrogen and phosphorous. One year later (June 2001), survival of 92 and 94% for the burned and non-burned sites, respectively, was found. This variable was not affected by the factors treatment, height class, or their interaction. The small and middle size plants from both the burned and non-burned areas had the highest relative height growth rate, and the small plants in non-burned areas had the largest height increment. The largest plant in the non-burned area had the highest relative diameter growth rate, and also the highest lateral root biomass. Foliar nitrogen was lower in the burned sites, probably due to volatilization. No effects were found for phosphorous.

A set of yield estimation equations was generated, for Pinus durangensis in the state of Durango, Mexico, for over-stocked natural stands.

A review of the importance of carbon absorption, fixation and measurement on natural forest populations (boreal, temperate and tropical forest) as well as forestry plantations was done. Finally, the importance of forest masses in the sequester of carbon and optimization of the use of this resource are emphasized. It is concluded that forest masses play and important role in the fixation and retention of carbon emitted by man into the atmosphere. Thus, reduction of emissions is a major factor toward which government efforts can be directed, together with a greater sensitivity of the population in the reduction of these emissions and the use of sustainable methods of forest management.

To carry out economic valuation, biomass density of tropical forest species was calculated by means of an indirect methodology based on existing data of total volume. The carbon content of 11 species was also determined as representing 66 % of the total volume. The result of both processes indicated that the stock of carbon was 353.341 tC/ha. Using volumes to be harvested in the 2003 felling area and sale prices as well as extraction costs per m3 in 2002, income was determined by forest use estimated at $ 6,021.850.44, while the income from the sale of the environmental service of carbon storage for the same area was estimated at $21,200.442.00. The opportunity cost is 3.5:1, and therefore, it is worthwhile for the ejido to sell environmental services to increase its revenues.

A model is presented of the change in landuse of Mexico for the period 1980-1990 and 1990-2000. Based on multiple regression equations, the change that eight classes of use of soil undergo is modeled, in function of the change measured in 24 socio-demographic variables. The unit of measurement was each state of the Mexican Republic, which was stratified into three regions to diminish variability in the analysis. Of 24 observations (8 classes in three regions), 15 continue their trend of change, whereas 9 reverse the trend from one period to another. The most stable classes are grasslands and urban areas, while the most unstable are forests and bodies of water. Although the relationships between socio-demographic dynamics and territorial dynamics are highly significant, these relationships change over time. This indicates that on the national scale, both territorial dynamics and population dynamics are parallel phenomena that respond in a similar manner to transformations that are occurring in other sectors, principally the economic and technological sector.

This paper offers some considerations on the efficiency of the process of mechanical transformation in sawmills based on the use of mathematics as a tool to improve sawing lumber. By reducing the width of the cut, efficiency of the mechanical conversion of lumber in the sawmills can increase up to 4.7 and 8.3%, making better use of the raw material possible. Also, a mathematical procedure is developed to define the width of the cut in the log to facilitate use of more of the wood next to the bark.

Exposure to abiotic aspects such as sunlight and water plays an important role in deterioration of wood, giving it an “old” appearance. The action of the weathering transforms the outer layers into rough zones, with changes in color. This kind of change affects only the exposed wood surface (from 0,05 to 2,5 mm). Density and the proportion of early and late wood are factors that determine the rate of degradation. The exposure angles of 0° and 45° result in the highest erosion index. Also described is the effect of weathering and the anatomical structure of wood.