To estimate the age of Norway spruce (Picea abies (L.) Karst.) logs by means of decay classes, and to assess how long it takes for downed logs to decompose, we dated logs dendrochronologically by applying 5-and 8-grade decay classification systems. Study sites were chosen in old-growth and previously selectively cut forest stands in boreal south-central Scandinavia; 113 logs were dated to the number of years since death, 120 were dated to the number of years since fall, and 61 logs were dated to both. The number of years from death to fall showed a negative exponential distribution, with a mean of 22 years and a range of 0-91 years. Decay classes of logs (8-grade scale) reflected time since fall (R 2 = 0.58) better than time since death (R 2 = 0.27) in a linear regression model. This result is due to the lower decomposition rate of standing snags. Therefore, the decomposition time of logs should be divided into two periods: time from death to fall, which varies considerably, and time after fall, which appears to follow a linear relationship with decay class. The model predicted that it takes 100 years after fall for downed logs to decompose completely (reaching decay class 8) in old-growth stands. Logs in selectively cut stands appeared to decompose faster (64 years), which is explained by a sample shortage of old logs resulting from previous cuttings. We conclude that the decomposition time of downed logs may be severely underestimated when data is retrospectively compiled from previously logged forest stands.Résumé : Pour estimer l'âge des billes d'épicéa commun (Picea abies (L.) Karst.) au moyen de classes de décomposi-tion et évaluer le temps requis pour que les billes au sol se décomposent, nous avons daté des billes dendrochronologiquement en appliquant des systèmes de classification comportant cinq et huit degrés de décomposition. Les sites d'étude ont été choisis dans des peuplements de forêt ancienne et des peuplements où avait déjà été effectuée une coupe sélective dans la zone boréale du Centre-Sud de la Scandinavie. Le nombre d'années écoulées depuis la mort de l'arbre a été déterminé chez 113 billes, le nombre d'années écoulées depuis la chute de l'arbre au sol chez 120 billes et les deux chez 61 billes. Le nombre d'années écoulées entre la mort de l'arbre et sa chute au sol suit une distribution exponentielle négative avec une moyenne de 22 ans et une étendue de 0-91 ans. Les classes de décomposition des billes (échelle à huit degrés) reflètent davantage le temps écoulé depuis la chute de l'arbre au sol (R 2 = 0,58) que depuis la mort de l'arbre (R 2 = 0,27) selon un modèle de régression linéaire. Ceci est dû au fait que le taux de décomposition des chicots encore debout est plus faible. Par conséquent, le temps de décomposition des billes devrait être divisé en deux périodes : le temps écoulé entre la mort de l'arbre et sa chute au sol qui varie considérablement et le temps écoulé depuis sa chute au sol, lequel semble relié de façon linéaire aux classes de décomposition. Dans les peuplements de for...

Abstract. Knowing the historical variation in fire regimes is instrumental in managing forests today and in predicting what may happen in the future. By cross-dating 745 fire scars in 378 samples of remnant Scots pines, we delineated 254 individual forest fires during the past 700 years in a 74-km 2 section of Trillemarka-Rollagsfjell Nature Reserve in south-central Norway. Fire sizes, numbers, burn rates, and frequencies were compared with historical climate proxies, vegetation maps, and written sources. The results revealed patterns consistent with a predominantly climate-driven fire regime up to 1625, followed by periods of strong anthropogenic influence that increased fire frequency during 1600-1700s and diminished fires during 1800-1900s. This was documented by an abrupt increase in number of small fires from the early 1600s that markedly shortened fire intervals from a median of 73 to 37 yr. This shift in fire frequency coincided with a sudden appearance of early-season fires from 1625 and onward. Whereas late-season burn rate increased with summer temperature, no such relationship was found for early-season fires. These results were corroborated by written sources that describe anthropogenic forest fires and slash-and-burn cultivation expanding with the increasing population from the late 1500s and subsequently diminishing due to increasing timber values during 1700-1800s. Whereas human activity strongly influenced the fire regime at multidecadal to centennial scales, it was the interannual variability in climate that triggered large fire events, especially during the pre-1625 period. Prior to 1625, the percentage of years with fire tripled from 7% during cold summers (10-12°C) to 21% during warm summers (14-16°C). Burn rate increased even more, from 0.01% to 1.3% for the same temperature intervals. Ecologically, the post-1625 period is remarkable in such a way that human activity, first by greatly increasing fire frequency and subsequently almost eradicating fires, possibly influenced the fire regime to such an extent that it may be unprecedented for millennia.

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To better understand the historic range of variability in the fire regime of Fennoscandian boreal forests we cross-dated 736 fire scars of remnant Scots pine (Pinus sylvestris L.) wood samples in a 3.6 km2 section of the Trillemarka-Rollagsfjell Reserve of south-central Norway. Using a kernel range application in GIS we spatially delineated 57 individual forest fires between 1350 and the present. We found a strong anthropogenic signal in the fire regime from 1600 and onwards: (i) infrequent variably sized fires prior to 1600 shifted to frequent fires gradually decreasing in size during the 1600s and 1700s, with only a few small fires after 1800; (ii) time intervals between fires and the hazard of burning showed substantial differences pre- and post-1600; (iii) fire seasonality changed from late- to early-season fires from the 1626 fire and onwards; and (iv) fire severity decreased gradually over time. Written sources corroborated our results, narrating a history where anthropogenic forest fires and slash-and-burn cultivation expanded with the increasing population from the late 1500s. Concurrently, timber resources increased in value, gradually forcing slash-and-burn cultivators to abandon fires on forest land. Our results strengthen and expand previous Fennoscandian findings on the anthropogenic influence of historic fire regimes.

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