Publikasjoner
NIBIOs ansatte publiserer flere hundre vitenskapelige artikler og forskningsrapporter hvert år. Her finner du referanser og lenker til publikasjoner og andre forsknings- og formidlingsaktiviteter. Samlingen oppdateres løpende med både nytt og historisk materiale. For mer informasjon om NIBIOs publikasjoner, besøk NIBIOs bibliotek.
2018
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Sammendrag
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Forfattere
Anne Kjersti BakkenSammendrag
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Forfattere
Arne Stensvand Aruppillai Suthaparan Belachew Asalf Tadesse Rodrigo B. Onofre Pål Johan From Natalia A. Peres W. Turechek Andrew Bierman Mark Rea David M. GadourySammendrag
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Sammendrag
Horizontal Visibility Graphs (HVGs) are a recently developed method to construct networks from time series. The values of the time series are considered as the nodes of the network and are linked to each other if there is no larger value between them, such as they can “see” each other. The network properties reflect the nonlinear dynamics of the time series. For some classes of stochastic processes and for periodic time series, analytical results can be obtained for network-derived quantities such as the degree distribution, the local clustering coefficient distribution, the mean path length, and others. HVGs have the potential to discern between deterministic-chaotic and correlated-stochastic time series. Here, we investigate the sensitivity of the HVG methodology to properties and pre-processing of real-world data, i.e., time series length, the presence of ties, and deseasonalization, using a set of around 150 runoff time series from managed rivers at daily resolution from Brazil with an average length of 65 years. We show that an application of HVGs on real-world time series requires a careful consideration of data pre-processing steps and analysis methodology before robust results and interpretations can be obtained. For example, one recent analysis of the degree distribution of runoff records reported pronounced sub-exponential “long-tailed” behavior of North American rivers, whereas another study of South American rivers showed hyper-exponential “short-tailed” behavior resembling correlated noise.We demonstrate, using the dataset of Brazilian rivers, that these apparently contradictory results can be reconciled by minor differences in data-preprocessing (here: small differences in subtracting the seasonal cycle). Hence, data-preprocessing that is conventional in hydrology (“deseasonalization”) changes long-term correlations and the overall runoff dynamics substantially, and we present empirical consequences and extensive simulations to investigate these issues from a HVG methodological perspective. After carefully accounting for these methodological aspects, the HVG analysis reveals that the river runoff dataset shows indeed complex behavior that appears to stem from a superposition of short-term correlated noise and “long-tailed behaviour,” i.e., highly connected nodes. Moreover, the construction of a dam along a river tends to increase short-term correlations in runoff series. In summary, the present study illustrates the (often substantial) effects of methodological and data-preprocessing choices for the interpretation of river runoff dynamics in the HVG framework and its general applicability for real-world time series.
Sammendrag
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Forfattere
Kjersti Bakkebø FjellstadSammendrag
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Forfattere
Inger Sundheim FløistadSammendrag
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Sammendrag
Numerous species of wild berries are abundant in the Nordic forests, mountains and peat lands. They ripen throughout the early summer until late autumn. Both lingonberry (Vaccinium vitis-idaea) and bilberry (Vaccinium myrtillus), that are among the most picked wild berries, are characteristic field layer species in boreal forests. Other species that have potential of being better exploited are cloudberry (Rubus chamaemorus), crowberry (Empeterum nigrum), bog blueberry (Vaccinium uliginosum), arctic bramble (Rubus arcticus), wild strawberries/woodland strawberries (Fragaria vesca) and wild raspberries (Rubus idaeus). Wild berries have always been an important part of the Nordic cuisine. However, only about 5–10 per cent of the annual wild berry crop of approximately a billion kilograms are currently picked for private or commercial consumption. There are several challenges towards an increased utilization as year-to-year variation in crop, topography, logistics of berry picking including traceability, fragmented sector structure and the high share of unprocessed raw material in export. The scientific interest for these berries have in the recent years focused on their value concerning human health benefits. Nevertheless, commercialization and innovation of wild berries should focus on multiple use of the whole raw material into many different products. The Nordic wild berries are perfectly adapted to their environment and are well suited to studies of environmental effects on growth, development and quality. Additionally, they represent a valuable genepool for future breeding.
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De nordnorske plantene er bedre til å lese høsttegnene og forbereder seg tidligere til vinteren enn de som er utviklet i sør.