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.
2006
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Forfattere
Torstein KvammeSammendrag
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Forfattere
Torstein KvammeSammendrag
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Detailed analyses of thresholded ecological interactions can improve our understanding of the transition from aperiodic to periodic dynamics. We develop a threshold model of the population dynamics of outbreaking bark beetle populations that alternate between non-epidemic and epidemic behavior. The model involves accumulation of resources during low-density periods and depletion during outbreaks. The transition between the two regimes is caused by disturbance events in the form of major tree felling by wind. The model is analyzed with particular reference to the population dynamics of the spruce bark beetle (Ips typographus) in Scandinavia for which a comprehensive literature allows full parameterization. The fairly constant outbreak lengths and the highly variable waiting time between outbreaks that are seen in the historical records of this species agree well with the predictions of the model. The thresholded resource-depletion dynamics result in substantial variation in the degree of periodicity between stochastic realizations. The completely aperiodic tree colonizations are partly predictable when the timing of the irregular windfall events are known. However, the predictability of inter-outbreak periods is low due to the large variation of cases falling most frequently in the middle between the extremes of purely nonperiodic (erratic) and periodic (cyclic) fluctuations.
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In this study, forest treated with different strengths of selective cutting (2570 % of volume removed) was remeasured after twelve years in 2005. At each of the sixteen 0.2 ha plots, including four repeats of each treatment, all trees larger than dbh 2.5 cm were calipered. We investigated growth, changes in stand structure, tree age, tree damage and crown condition.The diameter distribution displayed a reverse J-curve at all plots both before and after the cuttings. Twelve years later the curve is only slightly changed. Increment cores from 300 trees were taken to analyse annual growth reactions in different diameter classes. Most trees reacted with increased growth from the second or third year after the cutting.This improved growth accelerated the following six or seven years with 20-80% increase. Both small and large trees reacted, including severely suppressed trees. The initial crown volume and crown vitality after cutting is essential for the increased growth since several years are necessary to build up a larger and better crown. The relationship between increased growth and reduced volume per hectare indicates less competition between trees regarding nutrients and light after the cutting.
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