The effects of gap formation on solar radiation, soil and air temperature, and soil moisture were studied in mature coniferous forests of the Pacific Northwest, U.S.A. Measurements were taken over a 6-year period in closedcanopy areas and recently created gaps in four stands of mature (90-140 years) and old-growth (>400 years) Douglasfir (Pseudotsuga menziesii (Mirb.) Franco) forest in the western Cascade Range of central Oregon and southern Washington. Gap sizes ranged from 40 to 2000 m 2 . Summer solar radiation levels and soil temperatures differed significantly among gap sizes and positions within gaps and were driven primarily by patterns of direct radiation. Nevertheless, effects on air temperature were slight. Soil moisture was more abundant in gaps than in controls, was most abundant in intermediate gap sizes, and tended to decline during the growing season in single-tree gaps and on the north edges of large gaps. However, there was substantial variation in moisture availability within individual gaps, primarily related to the variety of organic substrates present. Moisture in gaps declined over multiple years, likely caused by encroachment of vegetation within and around gaps. Low light levels probably limit filling of natural gaps in these forests, but the variety of microenvironments in large gaps may facilitate diverse plant communities.Résumé : Les effets de la formation des trouées sur le rayonnement solaire, la température du sol et de l'air ainsi que l'humidité du sol ont été étudiés dans des forêts matures de conifères du Pacifique Nord-Ouest, aux États-Unis. Les mesures ont été prises pendant une période de 6 ans dans des endroits à couvert fermé et dans des trouées récemment créées dans quatre peuplements de forêt mature (90 à 140 ans) ou ancienne (>400 ans) de douglas de Menzies (Pseudotsuga menziesii (Mirb.) Franco), située dans la partie ouest de la chaîne des Cascades du centre de l'Oregon et du sud de l'état de Washington. La taille des trouées variait de 40 à 2000 m 2 . Le niveau de rayonnement solaire estivale et la température du sol variaient significativement selon la taille des trouées et selon la position dans les trouées. Le rayonnement au sol était surtout fonction du rayonnement solaire direct. Les effets sur la température de l'air étaient toutefois minimes. L'humidité du sol était plus élevée dans les trouées que dans les zones témoins; elle était surtout importante dans les trouées de taille moyenne et tendait à diminuer pendant la saison de croissance dans les petites trouées causées par la chute d'un arbre et à la bordure nord des grandes trouées. Il y avait toutefois une variation substantielle de la disponibilité en eau à l'intérieur d'une même trouée, essentiellement en fonction de la variété de substrats organiques présents. L'humidité dans les trouées déclinait au cours des années, probablement à cause de l'empiètement de la végétation à l'intérieur et autour des trouées. Les faibles niveaux de lumière limitent probablement le repeuplement des trouées naturelles dans ce...

Process-based model analyses are often used to estimate changes in soil organic carbon (SOC), particularly at regional to continental scales. However, uncertainties are rarely evaluated, and so it is difficult to determine how much confidence can be placed in the results. Our objective was to quantify uncertainties across multiple scales in a processbased model analysis, and provide 95% confidence intervals for the estimates. Specifically, we used the Century ecosystem model to estimate changes in SOC stocks for US croplands during the 1990s, addressing uncertainties in model inputs, structure and scaling of results from point locations to regions and the entire country. Overall, SOC stocks increased in US croplands by 14.6 Tg C yr À1 from 1990 to 1995 and 17.5 Tg C yr À1 during 1995 to 2000, and uncertainties were AE 22% and AE 16% for the two time periods, respectively. Uncertainties were inversely related to spatial scale, with median uncertainties at the regional scale estimated at AE 118% and AE 114% during the early and latter part of 1990s, and even higher at the site scale with estimates at AE 739% and AE 674% for the time periods, respectively. This relationship appeared to be driven by the amount of the SOC stock change; changes in stocks that exceeded 200 Gg C yr À1 represented a threshold where uncertainties were always lower than AE 100%. Consequently, the amount of uncertainty in estimates derived from process-based models will partly depend on the level of SOC accumulation or loss. In general, the majority of uncertainty was associated with model structure in this application, and so attaining higher levels of precision in the estimates will largely depend on improving the model algorithms and parameterization, as well as increasing the number of measurement sites used to evaluate the structural uncertainty.

No abstract

Agriculture is considered to be among the economic sectors having the greatest greenhouse gas mitigation potential, largely via soil organic carbon (SOC) sequestration. However, it remains a challenge to accurately quantify SOC stock changes at regional to national scales. SOC stock changes resulting from SOC inventory systems are only available for a few countries and the trends vary widely between studies. Process-based models can provide insight in the drivers of SOC changes, but accurate input data are currently not available at these spatial scales. Here we use measurements from a soil inventory dating from the 1960s and resampled in 2006 covering the major soil types and agricultural regions in Belgium together with region-specific land use and management data and a process-based model. The largest decreases in SOC stocks occurred in poorly drained grassland soils (clays and floodplain soils), consistent with drainage improvements since 1960. Large increases in SOC in well drained grassland soils appear to be a legacy effect of widespread conversion of cropland to grassland before 1960. SOC in cropland increased only in sandy lowland soils, driven by increasing manure additions. Modeled land use and management impacts accounted for more than 70% of the variation in observed SOC changes, and no bias could be demonstrated. There was no significant effect of climate trends since 1960 on observed SOC changes. SOC monitoring networks are being established in many countries. Our results demonstrate that detailed and long-term land management data are crucial to explain the observed SOC changes for such networks.regional inventories | soil organic carbon dynamics modeling | land use history

No abstract