Summary A theoretical basis for the forest transition (the change from contraction to expansion of national forest area) is suggested in terms of increasing agricultural adjustment to land quality. This adjustment, operating through a process of learning by farmers, results in the concentration of agricultural production in smaller areas of better land, and the agricultural abandonment of larger areas of poorer land, which are then available for reforestation through natural regeneration or planting.

Decreasing body size has been proposed as a universal response to increasing temperatures. The physiology behind the response is well established for ectotherms inhabiting aquatic environments: as higher temperatures decrease the aerobic capacity, individuals with smaller body sizes have a reduced risk of oxygen deprivation. However, empirical evidence of this response at the scale of communities and ecosystems is lacking for marine fish species. Here, we show that over a 40-year period six of eight commercial fish species in the North Sea examined underwent concomitant reductions in asymptotic body size with the synchronous component of the total variability coinciding with a 1-2 °C increase in water temperature. Smaller body sizes decreased the yield-per-recruit of these stocks by an average of 23%. Although it is not possible to ascribe these phenotypic changes unequivocally to temperature, four aspects support this interpretation: (i) the synchronous trend was detected across species varying in their life history and life style; (ii) the decrease coincided with the period of increasing temperature; (iii) the direction of the phenotypic change is consistent with physiological knowledge; and (iv) no cross-species synchrony was detected in other species-specific factors potentially impacting growth. Our findings support a recent model-derived prediction that fish size will shrink in response to climate-induced changes in temperature and oxygen. The smaller body sizes being projected for the future are already detectable in the North Sea.

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Stock-recruit relationships that use spawning stock biomass (SSB) to represent reproductive potential assume that the proportion of SSB composed of females and the relative fecundity (number of eggs produced per unit mass) are both constant over time. To test these two assumptions, female-only spawner biomass (FSB) and total egg production (TEP) were estimated for the Northeast Arctic stock of Atlantic cod (Gadus morhua) over a 56-year time period. The proportion of females (FSB/SSB) varied between 24% and 68%, and the variation was systematic with length such that SSB became more female-biased as the mean length of spawners increased. Relative fecundity of the stock (TEP/SSB) varied between 115 and 355 eggs·g -1 and was significantly, positively correlated with mean length of spawners. Both FSB and TEP gave a different interpretation of the recruitment response to reductions in stock size (overcompensatory) compared with that obtained using SSB (either compensatory or depensatory). There was no difference between SSB and FSB in the assessment of stock status; however, in recent years (1980-2001) TEP fell below the threshold level at which recruitment becomes impaired more frequently than did SSB. This suggests that using SSB as a measure of stock reproductive potential could lead to overly optimistic assessments of stock status.Résumé : Les relations stock-recrues qui utilisent la biomasse du stock reproducteur (SSB) pour représenter le potentiel reproductif présupposent que la proportion de SSB représentée par les femelles et que la fécondité relative (nombre d'oeufs produits par unité de masse) sont toutes deux invariables dans le temps. Afin d'évaluer ces deux présupposi-tions, nous avons estimé la biomasse des reproducteurs femelles seuls (FSB) et la production totale d'oeufs (TEP) chez un stock de morues franches (Gadus morhua) de l'Arctique sur une période de 56 ans. La proportion de femelles (FSB/SSB) varie de 24 à 68 % et elle change systématiquement en fonction de la longueur de telle manière que SSB favorise de plus en plus les femelles à mesure que la longueur moyenne des reproducteurs augmente. La fécondité relative du stock (TEP/SSB) varie de 115 à 355 oeufs·g -1 et elle est en corrélation positive significative avec la longueur moyenne des reproducteurs. FSB et TEP fournissent toutes deux une interprétation différente de la réaction du recrutement à la réduction de la taille du stock (surcompensation) par comparaison à la réaction du recrutement obtenue à partir de SSB (compensation ou bien effet d'Allee). Il n'y a pas de différence entre SSB et FSB pour ce qui est de l'évaluation du statut du stock; cependant, ces dernières années , TEP est tombée sous le seuil sous lequel le recrutement se détériore plus fréquemment que SSB. Cela laisse croire que l'utilisation de SSB comme mesure du potentiel reproductif du stock pourrait mener à des évaluations trop optimistes du statut du stock.[Traduit par la Rédaction] Marshall et al. 994

The relationship between national trends in forest area and population is reviewed at the global scale. Evidence of an inverse relationship is confirmed. The relationship, however, may have weakened in recent decades, and it has clearly undergone a reversal in some countries during the nineteenth to twentieth centuries. The theme of a changing relationship through time is thus developed, as is that of an asymmetrical relationship in the sense that the forest area is likely to stabilize before population. On the basis of modelling from the current demographic relationship, the global forest area should stabilize before the middle of the twenty-first century.