Tendencias minimalistas en la gestión de los bosques tropicales mexicanos: motivaciones y experiencias

Tropical timber regions across the world share common problems such as degradation and poor regeneration after timber harvesting. Traditional Mayan land management through slash and burn is now recognized as an effective way of renewing forest stands in multispecies tropical forests. The practice of slash and burn for forest management in Mexico has led to area regulation, which has made land value a convenient means of assessing alternative forest plans. The use of expected land value as a performance indicator shifts the manager's attention from managing a species mix to balancing financial tradeoffs between liquidation or retention of the standing biomass. Since the forest-wide residual stock is so large, land value overrides the importance of revenue from timber sales. Several forest management methods along these lines have appeared in tropical regions of Mexico over a thirty-year time span and represent a patrimonial system of forest management (PS). The gradual innovation generated by PS is described here, as well as examples of PS practices. PS methods today provide stewardship for a total of 155,814 ha in different parts of Mexico. PS performance will become evident in the long run; in the meantime, the embrace of PS by private landowners and regulatory institutions is equated with a positive, independent opinion about PS design. The Mexican experience suggests pathways for rational management of all types of forests. PS features that are worth replicating are, for instance, the inclusion of disturbance patterns as factors in decision making, as well as the use of specific silvicultural regimes for roads, woodlands, closed forests, hilltops, swamps, riparian zones, clearings and forest edges.

Influence of Tool Wear on Cutting Characteristics in Ultra-Precision Cutting

Recently, increasingly high efficiency and high performance have become to be required of information equipment. As a result, optical scanning parts that reduce optical aberrations, scatter, and diffraction are required in laser printers. It is therefore necessary to improve the geometric surface roughness achieved in mirror cutting of Al alloys and eliminate tear-out marks and scratch marks that can be created during the cutting process. In this study, we investigated the effect of tool wear on the occurrence of surface discontinuities in ultra-precision cutting of Al alloys. In our previous studies, a crystal orientation of {110} plane was adopted in cutting an Al-Si alloy (AHS material, 11wt% Si) and Al-Mg alloy (A5186 material, 4.5wt% Mg) using a straight diamond tool. The cutting edge recession that occurs when cutting AHS material has been reported to be approximately 5 times greater than that which occurs when cutting A5186 material. Therefore, we cut the AHS material for accelerated wear and investigated the cutting edge recession, the surface roughness and the cutting force. We found that the cutting edge recession decreases as the tool wear angle γ increase. For example, at a tool wear angle γ = 40°, the cutting edge recession is approximately 7 times greater than that which occurs at a tool wear angle γ = 12°. As the tool wear angle increases, the cutting distance increase, which produces a mirror like surface. In addition, we were able to obtain a good machined surface using a positive tool setting angle because side cutting edge produces residual stock of removal 0.1 μm when the cutting edge recession is 0.3 μm or more and when it is cut by following end cutting edge.

Effect of Contact Stiffness of Grinding Wheel on Ground Surface Roughness and Residual Stock Removal of Workpiece

In the grinding operation, grinding wheels are deformed by grinding forces, so that residual stock removal of the workpiece takes place. Since this residual stock removal of the workpiece causes low machining efficiency and deterioration of machining accuracy, high hardness grinding wheels may be selected in order to obtain high machining efficiency and/or high quality machining accuracy. On the other hand, when grinding operations used by low hardness grinding wheels are carried out, it is well known that ground surface roughness is smaller than in case of higher hardness grinding wheels. From such a viewpoint, this study aims to investigate experimentally the effect of the contact stiffness of grinding wheel on the ground surface roughness and the residual stock removal of the workpiece. Grinding operations were carried out using three grinding wheels which are different hardness type, and ground surface roughness and residual stock removal of the workpiece were measured. The contact stiffness of grinding wheel was calculated by a support stiffness of single abrasive grain and a contact area between grinding wheel and workpiece. Comparing the contact stiffness of grinding wheel with the ground surface roughness and the residual stock removal of the workpiece, it was known that ground surface roughness increases and residual stock removal of workpiece decreases with increaseing the contact stiffness of grinding wheel. From these results, since elastic deformation of the grinding wheel changed depending on the suppot stiffness of single abrasive grain, it was clarified that the ground surface roughness and the residual stock removal of the workpiece were changed by the contact stiffness of grinding wheel.

Estimation of the Grinding Time by Means of the Grinding Process Model

In grinding operation, elastic deformations of the grinding machine and the grinding wheel induce a residual stock removal of workpiece. On the other hand, thermal expansions of the workpiece and the grinding wheel increase the depth of cut. Therefore, calculation of a ground depth of cut and/or the grinding time has to be considered by the elastic deformations and the thermal expansions. From such a viewpoint, in this study, grinding process model taking into account the elastic deformations and the thermal expansions was proposed. This paper aims to estimate the grinding time by means of the proposed grinding process model.

Contact Stiffness of Grinding Wheels due to the Difference of Table Feed Rate

Usually, the contact stiffness between a grinding wheel and a workpiece has been measured in a stationary state. So, in this study, the contact stiffness under the grinding operation is measured under different table feed rate of the workpiece. From this result, it is known that, while the contact stiffness in the stationary state increases with the increase of the contact force, the contact stiffness under the grinding operation decreases with the increase of the normal grinding force relating the table feed rate. In this paper, since the number of contacting abrasive grain with workpiece is constant irrespective of the table feed rate, and the residual stock removal of workpiece is varied by the table feed rate, it is clarified that the contact stiffness under the grinding operation differs from the contact stiffness measured by the stationary state.

Infections of Diphyllobothrium dendriticum, D. ditremum (Cestoda), and Cystidicola farionis (Nematoda) in a north Norwegian population of Arctic charr (Salvelinus alpinus) during winter

Infections of the parasites Diphyllobothrium dendriticum, Diphyllobothrium ditremum, and Cystidicola farionis in Arctic charr (Salvelinus alpinus) from Takvatn, northern Norway, were followed throughout a 9-month overwintering period. The relative density of all three parasite species increased with the age of the host in response to ontogenetic shifts in habitat and food of Arctic charr in the lake. Our results indicate that some recruitment of the three parasite species, particularly D. ditremum, takes place during the winter. However, winter parasite recruitment was lower than in the ice-free season. In spite of high parasite infection in some individuals, no strong indications of parasite-induced host winter mortality were found for any of the three species. Low overwintering mortality of heavily parasitized charr may be connected with a stock-reduction program in the lake that has improved the conditions of the fish, and the residual stock is probably less affected by parasites.

Prevalence and abundance of helminth parasites in an intensively fished population of brook trout (Salvelinus fontinalis) at a small subarctic lake

Brook trout from a 5-ha lake near Schefferville, Quebec, were intensively sampled by small-mesh gill nets during 2 successive years to study patterns of helminth parasite occurrence in relation to changes in the age and size class structure of the fish population. Nearly 1800 brook trout were removed from the lake, of which all were measured and weighed, 568 were aged by otolith readings, and 361 were examined for parasites. The digeneans Crepidostomum farionis, Phyllodistomum umblae, and Diplostomum sp. and the cestode Eubothrium salvelini were present in more than 50% of the necropsied fish, and the prevalence and (or) abundance of all species except C. farionis increased from the first to the second year of the study. The fish catch in the first year was dominated by the 3+ year class and older fish, whereas by the second year 1+ and 2+ fish were proportionately more abundant, and the importance of older cohorts declined. This change in brook trout population structure, resulting from gill net selectivity, appears to have had the effect of improving per capita food availability for members of the residual stock. After the initial year of intensive fishing, the fish that remained fed more upon both benthic and planktonic invertebrates and their acquisition of parasites was intensified.