Natural disturbances are important drivers of ecosystem change in the boreal forest and new approaches to sustainable forest management draw on natural disturbance patterns as a template for harvesting. The main premise for such approach is that species have evolved and adapted to stand-replacing natural disturbances and thus are more likely to be maintained on landscapes managed to preserve spatial patterns of natural disturbance. I used spiders – one of the most important, diverse and ubiquitous groups of terrestrial predatory invertebrates – as a model for assessing the impacts of variable retention harvesting practices on biodiversity in the mixedwood boreal forest.Spiders were collected from the Ecosystem Management Emulating Natural Disturbance (EMEND) project landbase over a period of five years from different harvest treatments and forest-types. Results for 249 species in 18 families are included in this study. Changes in composition of spider assemblages clearly followed the disturbance gradient from clear-cut to uncut treatments and these changes were linked to habitat and forest structure. Ground-dwelling assemblages were mostly affected by harvesting, whereas canopy assemblages were less influenced when stand connectivity remained.Low tree retention (i.e., 10-20%, which is the range currently applied by the forestry industry) showed some ability to mitigate adverse effects of clear-cutting; yet, higher retention levels are needed to maintain forest specialist species, especially in late successional seres (e.g., conifer dominated). A “life-boating” effect of aggregated retention was evident, and was more effective when applied in combination with dispersed retention. The application of different harvesting practices alone is unlikely to entirely emulate the effects and processes caused by major disturbances on the landscape.Thus, keeping in mind that fire is an important component in the boreal forest, this type of disturbance cannot be excluded from management if the goal is to preserve a natural range of biodiversity. In conclusion, to sustain rich and diverse spider assemblages, management of the boreal mixedwood should aim towards maintaining landscape heterogeneity. Consequently, no single practice is effective to emulate natural post-disturbance patterns and to adapt harvesting to effectively imitate the processes of a disturbance driven system, a combination of prescriptions is recommended.

Carta en inglés enviada al Director del Fondo Monetario Internacional, Horst Köhler, con el fin de actualizar la información sobre el estado general de la economía colombiana a inicios del 2001. A lo largo del documento se describe la evolución positiva del sistema financiero colombiano, los cambios en las operaciones comerciales e infraestructura nacional y se lamenta el alto nivel de desempleo que persiste en el país; elementos que habían sido fijados como objetivos para cumplir entre 1999 y 2000, pero que continuaron teniendo importancia para la economía nacional en el 2001, pues al final del documento se hace un reajuste de las políticas económicas para desarrollar a lo largo de ese año. El documento aparece firmado por Juan Manuel Santos y Miguel Urrutia.