[en] 20 Scots pine (Pinus sylvestris L.) commercial early thinnings, fully mechanised, in Central and North Spain are studied. Harvesting system is shortwood, while one grip harvester and forwarder (1 + 1 system) is used in 13 of the 20 operations. The analysis has been carried out from a double point of view. On one hand, productivity and cost equations are developed from stopwatching time study data (13 for harvesters, 17 for forwarders). Both mechanised felling and hauling off by forwarders are analysed. Results are different for heavy forwarders –more than 15 t of unloaded weight– than for light or medium-sized forwarders. Regarding cost circumstances, ratio "Forest harvester to forest operator hourly wages" is nowadays close to 6.0 in Spain. At this stage, mechanised thinning cost proves to be always higher than motor-manual options. Anyway, mechanisation is necessary due to increasing difficulties to find skilled forest operators and the analisys shows that a slight increment in the above mentioned ratio would make mechanised thinnings more profitable than motor-manual. Commercial margin is also estimated for both thinning systems, concluding that silvicultural prescriptions must be often overpassed when possitive margins are one of the operational targets, particularly in mechanised thinnings. On the other hand, inmediate environmental effects are also studied. Soil surface disturbation and compaction –by means of penetrometer cone index– are quantified, as well as tree damages, shrubs cover reduction and slash accumulations. Although these effects can not be considered as severe, damages are greater than those fixed by foreign legal regulations in most developed countries. Besides, some factors conditioning the damages are identified, as a basis to upgrade forest harvesting practices.

[es] Se estudian 20 primeras claras, 15 de ellas totalmente mecanizadas, sobre repoblaciones de Pinus sylvestris L. en el centro y norte de Espa&ntilde;a. Su sistema de aprovechamiento fue de madera corta y la combinaci&oacute;n de medios m&aacute;s usual (en 13 de las 20 claras) consisti&oacute; en cosechadora de cabezal en punta de gr&uacute;a y autocargador (esquema 1 + 1), aunque en dos de ellas se ape&oacute; y elabor&oacute; con cosechadora ligera y en las cinco restantes con motosierra. Por un lado, se desarrollan (mediante t&eacute;cnicas de estudio de tiempos por cronometraje discontinuo) ecuaciones predictivas de sus rendimientos y costes, compar&aacute;ndose &eacute;stos con los de los trabajos semimecanizados y estudi&aacute;ndose su rentabilidad directa en funci&oacute;n del peso de la clara. Los rendimientos del apeo y procesado dependen b&aacute;sicamente del volumen unitario medio del &aacute;rbol extra&iacute;do y de la fisiograf&iacute;a (pendiente), y los de la saca con autocargador de la distancia de desembosque y la pendiente. En este caso, se obtienen modelos distintos para los autocargadores pesados (peso en vac&iacute;o mayor de 15 t) y los ligeros/medios. En cuanto a los costes, los m&aacute;rgenes comerciales son escasos o negativos, excepto para vol&uacute;menes unitarios o pesos que exceden los prescritos por la selvicultura. Por otro lado, se estudian los efectos ambientales inmediatos de la mecanizaci&oacute;n sobre los suelos (superficies alteradas, compactaci&oacute;n evaluada mediante penetrometr&iacute;a) y sobre la masa remanente y la vegetaci&oacute;n arbustiva, as&iacute; como las acumulaciones de restos. Aunque dichos efectos no pueden considerarse graves, se detectan niveles de da&ntilde;os mayores que los admitidos por las legislaciones extranjeras m&aacute;s avanzadas. Se identifican algunos factores de los que dependen los da&ntilde;os, base de posibles mejoras en las pr&aacute;cticas de aprovechamiento.