Soil carbon stock change following afforestation in Northern Europe: a meta-analysis.

Northern Europe supports large soil organic carbon (SOC) pools and has been subjected to high frequency of land-use changes during the past decades. However, this region has not been well represented in previous large-scale syntheses of land-use change effects on SOC, especially regarding effects of afforestation. Therefore, we conducted a meta-analysis of SOC stock change following afforestation in Northern Europe. Response ratios were calculated for forest floors and mineral soils (0-10 cm and 0-20/30 cm layers) based on paired control (former land use) and afforested plots. We analyzed the influence of forest age, former land-use, forest type, and soil textural class. Three major improvements were incorporated in the meta-analysis: analysis of major interaction groups, evaluation of the influence of nonindependence between samples according to study design, and mass correction. Former land use was a major factor contributing to changes in SOC after afforestation. In former croplands, SOC change differed between soil layers and was significantly positive (20%) in the 0-10 cm layer. Afforestation of former grasslands had a small negative (nonsignificant) effect indicating limited SOC change following this land-use change within the region. Forest floors enhanced the positive effects of afforestation on SOC, especially with conifers. Meta-estimates calculated for the periods <30 years and >30 years since afforestation revealed a shift from initial loss to later gain of SOC. The interaction group analysis indicated that meta-estimates in former land-use, forest type, and soil textural class alone were either offset or enhanced when confounding effects among variable classes were considered. Furthermore, effect sizes were slightly overestimated if sample dependence was not accounted for and if no mass correction was performed. We conclude that significant SOC sequestration in Northern Europe occurs after afforestation of croplands and not grasslands, and changes are small within a 30-year perspective.

The temporal development in a hybridizing population of wild and cultivated chicory (Cichorium intybus L.).

Hybridization and its possible impacts is a subject of increased attention in connection with the risk of unintended gene flow from cultivated (including genetically modified) plants to wild relatives. Whether such gene flow by hybridization is likely to take place depends among other things on the persistence of the hybrids in a natural environment over time. To evaluate this, we studied an experimental hybridizing population of wild and cultivated chicories (Cichorium intybus) relative to a previous study on the same population 2 years earlier. We compared the genetic composition, morphology and fitness traits of plants from 2004 to the plants in the same plot in 2002. The majority of the plants in 2004 was more morphologically and genetically intermediate than in 2002. This indicates that no selection towards being wild-like or cultivar-like was present over the period of 2 years. Furthermore, no distinct fitness differences existed between the plants of 2004, probably due to most of the plants being intermediate. No hybridization barriers appeared to be present between wild and cultivated chicories beyond the F1 generation, since F2 hybrids and backcrosses were in abundance; in fact, hybrids of probably fourth or fifth generation were present. In conclusion, all results indicate that no barriers exist to the temporal persistence of chicory hybrids in a natural environment.

Genealogy, morphology and fitness of spontaneous hybrids between wild and cultivated chicory (Cichorium intybus).

Crop species are known to hybridize spontaneously with wild relatives, but few studies have characterized the performance of hybrids at various genealogies, life stages and environments. A group of cultivar-like individuals and potential hybrids were observed in a roadside population of wild chicory plants in Denmark. Seeds were collected from all reproductive plants and grown in a common garden experiment, and their morphological and genetic compositions were analysed. Intermediate plants were identified as hybrids and comprised various backcross and F(n) combinations. A genotypic hybrid index (HI), spanning from wild-like to cultivar-like, was highly correlated to a morphological index. Plant survival, growth and reproduction were evaluated and compared to the genotypic HI. Overall, cultivar-like and intermediate plants grew larger than wild-like plants, flowered longer, and produced more flowers and seeds. The common garden included a nutrient gradient. At higher nutrient levels, intermediate and cultivar-like plants produced more flowers and seeds than wild-like plants, whereas this effect was less pronounced at lower nutrient levels. During winter, small rodents consumed roots of cultivar-like and intermediate plants preferentially. Thus, cultivated and wild chicory are able to hybridize spontaneously, producing hybrid offspring of several generations that may reproduce more effectively than their wild parent, but herbivory and poor environmental conditions may negatively affect their fitness.