Plant remains are derived from archaeological contexts and therefore they provide an interesting source material for investigating the role of economic plants and environments in the past. Plant remains can reflect past activities such as cultivation, consumption, trade and migration. Plant remains provide data and information for a number of other disciplines, and methods from other fields such as 14C radiocarbon dating, genetics and isotope analysis can be used to obtain additional data from macrofossil plant materials. In this thesis, I combined macrofossil, pollen, radiocarbon and genetic data in order to study what kind of new information can be achieved from archaeological contexts. I studied how cultivation, consumption, trade and barley migration are reflected in the archaeobotanical and historical data from ca. AD 1000 to the 20th century in southern Finland. Primary data used in this study consists of carbonised, waterlogged and mineralised archaeobotanical plant remains i.e. macrofossils recovered from archaeological excavations, and dried plant material derived from historical collections. Besides archaeobotanical analyses, palynology and genetics as well as radiocarbon dating are also involved in reconstruction and interpretation of past activities. 

I studied early stages of agriculture and economy of a medieval rural village in Espoo at Mankby, and I found that rye (Secale cereale L.) cultivation started in the 12th century and continued until the abandonment of the village in 1556. The reason for the dominance of rye may be that it was considered a safer choice of crop as it is quite well suited to Finnish climatic conditions and it thrives on poor soils. Based on the study, it seems that the medieval cultivation of rye was increasing, but concentrated in some villages in southern Finland, and barley (Hordeum vulgare L.) was still important in most other parts of the country. I also studied what kind of information can be acquired from graves by combining macrofossil and pollen data. I found that plant material in graves is preserved only occasionally where the high proportion of metal oxides or calcium have preserved the plant remains. However, interesting results were gained from a 16th century grave at the Kappelinmäki cemetery in Kauskila, Lappeenranta. There, the grave gave evidence of the deceased's last meal, since a large number of mineralised raspberry (Rubus idaeus L.) seeds and some fish bones were discovered from the pelvis and stomach area. Pollen analysis of the same grave gave evidence of past environment, funeral practices and ritual usage of plants. 

I also discuss the usage and origin of the plants, as I found that the plant material derived from the 1790s latrine at the sea fortress of Ruotsinsalmi in Kotka was mainly influenced by Russian food traditions. For instance, millet (Panicum miliaceum L.) and buckwheat (Fagopyrum esculentum L.) played an important role in the diet in the sea fortress. Finally, I tested the utility of archaelogical and historical samples of barley as sources of genetic data. I show that DNA concentrations obtained from charred archaeological barley grains are too low for successful KASP genotyping, while dried grains from herbariums and seed collections had in general a sufficient DNA quality for genetic analysis. I further show that genotypic variation in the historical samples of Finnish six-row barley is geographically clustered, while in the two-row barley genetic structuring is not linked to geography. Genotyping of functional markers revealed that the majority of barley cultivated in Finland in the late 19th and early 20th century was late-flowering under increasing day-length. 

In general, the results of this thesis show that the selection of used plant taxa got more versatile from the Iron Age to the medieval era, however, from medieval to 1790s the number of used plant taxa did not grow remarkably as the archaeobotanical data is rather similar in 1790s as in the Middle Ages. From the medieval onwards, the number of used plants is growing due to the intensification of trading networks overseas. Significant agricultural change did not occur in Finland in 1000 AD to 1900 AD, as barley was the most common cultivated plant. However, local differences may occur, as e.g. rye was cultivated in coastal areas. As six-rowed barley has a long cultivation history in Finland as a main crop, it has adapted to the climate and the genetic structure of barley landraces is geographically clustered.