Spatial and temporal variations in photosynthetic photon flux density (PPFD) at 1 m above the forest floor were measured under and between dominant pines and in 21- to 52-m2 gaps under developing Scots pine (Pinussylvestris L.) stands in southern Finland. For each stand, PPFD data were collected continuously every 10-s and then averaged for every 5-min period during 4-6 days under completely clear and overcast sky conditions. Measurements were made during 102 consecutive days within two different areas in each of 10 different Scots pine stands ranging in age from 7 to 105 years. Mean daily percent PPFD in the understory varied from 7.5% in the 20-year-old stands to 38% in the 7-year-old stands, and did not differ significantly between completely clear and overcast sky conditions. Mean daily percent PPFD did not vary significantly between the different sensor locations within closed pine stands, indicating that at high latitudes mean daily percent PPFD does not vary much spatially under closed Scots pine forests. Measurements of PPFD made at different periods during the day under clear sky conditions, one of the main approaches used in the literature, were not representative of the whole-day conditions. Two different results suggest that PPFD can be better characterized under completely overcast sky conditions. First, the 5-min percent PPFD in the understory was fairly constant between 06:00 and 20:00 for all stand ages under completely overcast sky conditions. Second, there was an almost perfect 1:1 relationship of the mean daily percent PPFD measured at 40 different locations in the 10 stands between clear and overcast sky conditions. These two results suggest that an instantaneous measure of percent PPFD obtained under completely overcast sky conditions is representative of the mean daily percent PPFD for both sky conditions. These results provide the basis for a quick and efficient method of estimating the mean daily percent PPFD in the understory of closed forests.

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This paper reviews regeneration research during the past 20 years. The focus is mainly on planting of Norway spruce and Scots pine. Research on root morphology and nursery practices has played an important role in the introduction of containerized seedlings in northern Europe. In recent years, mini-seedlings have been tested. Conventionally, seedlings are planted in spring and early autumn but recent research indicates that Norway spruce seedlings can be planted from spring until the end of September. Soil temperature strongly influences seedling establishment in the harsh northern Scandinavian climate and a good way to increase soil temperature is to plant in elevated planting spots, which can be achieved by mounding. Soil scarification also reduces competition from field vegetation and damage by pine weevils. In southern Finland, Sweden and Norway, pine weevils are by far the most serious causes of damage to both Norway spruce and Scots pine. Therefore, designing regeneration treatments for decreasing pine weevil damage without using insecticides is one of the more important current research challenges. Examples of possible future trends in forest regeneration research are an emphasis on more basic research, use of fast growing tree species, multidisciplinary approaches and site-specific regeneration regimes.

Recent progress toward the application of process-based models in forestmanagement includes the development of evaluation and parameter estimation methods suitable for models with causal structure, and the accumulation of data that can be used in model evaluation. The current state of the art of process modeling is discussed in the context of forest ecosystem management. We argue that the carbon balance approach is readily applicable for projecting forest yield and productivity, and review several carbon balance models for estimating stand productivity and individual tree growth and competition. We propose that to develop operational models, it is necessary to accept that all models may have both empirical and causal components at the system level. We present examples of hybrid carbon balance models and consider issues that currently require incorporation of empirical information at the system level. We review model calibration and validation methods that take account of the hybrid character of models. The operational implementation of process-based models to practical forest management is discussed. Methods of decision-making in forest management are gradually moving toward a more general, analytical approach, and it seems likely that models that include some process-oriented components will soon be used in forestry enterprises. This development is likely to run parallel with the further development of ecophysiologically based models.

Forest fire ignition potential was investigated in Picea abies (L.) Karst. (Norway spruce) and Pinus sylvestris L. (Scots pine) dominated stands of 0, 15, 30-45, and 40-60 years of age. A series of small-scale (<0.5 m 2 )ignition tests were carried out in experimental plots on 61 different days in June, July, and August. Ignition success percentages were analyzed in relation to stand structural properties, preclassified stand types, and the output of the Canadian Fire Weather Index system. In addition, the number of average stand-type-specific fire days was estimated based on weather data (June-August) for southern Finland for the years 1991-2002. Factors in stand structure that significantly correlated with the ignition success percentage were canopy depth and leaf area index, the correlation coefficients being -0.575 (p < 0.005) and -0.582 (p < 0.005), respectively. In Pinus sylvestris dominated stands, ignition tests produced self-sustained surface fires in 32.0%, 24.0%, and 19.3% of cases in 0-, 15-, and 30-to 45-year age classes, respectively. In Picea abies dominated sites conditions were favorable for fire in 12.0% and 4.6% of trials in the 0-and 40-to 60-year age classes, respectively. The output of the FWI-system correlated well with the ignition success in June and July but poorly in August. Based on the 12-year time series analysis, there were on average per year 27, 18, and 14 potential fire days in 0-, 15-, and 30-to 45-year-old Pinus sylvestris stands, and 10 and 4 potential fire days in 0-and 40-to 60-year-old Picea abies stands, respectively. We concluded that the dominance of Picea abies or Pinus sylvestris and stand age appear to modify the ignition conditions significantly and should be used as threshold indicators in fire occurrence predictions. Résumé: Le potentiel d'allumage d'un feu de forêt a été étudié dans des peuplements de 0, 15, 30-45 et 40-60 ans dominés par Picea abies (L.) Karst. (épicéa commun) et Pinus sylvestris L. (pin sylvestre). Une série de tests d'allumage à petite échelle (<0,5 m 2 ) a été effectuée dans des parcelles expérimentales lors de 61 journées différentes en juin, juillet et août. Les pourcentages d'allumages réussis ont été analysés en relation avec les propriétés structurales des peuplements et la valeur de la Méthode canadienne de l'Indice Forêt-Météo. De plus, le nombre moyen de jours propices au feu spécifique à chaque type de peuplement a été estimé à partir des données météorologiques (juin-août) pour le sud de la Finlande et les années 1991 à 2002. Les facteurs de la structure du peuplement qui étaient significativement corrélés avec le pourcentage de succès d'allumage sont l'épaisseur du couvert et l'indice de surface foliaire avec des coefficients de corrélation respectifs de -0,575 (p < 0,005) et -0,582 (p < 0,005). Dans les peuplements dominés par Pinus sylvestris, les tests d'allumage ont provoqué des feux de surface qui se maintenaient par eux-mêmes dans 32,0 %, 24,0 % et 19,3 % des cas dans les peuplements âgés respectivement de 0, 15 et 30-45 ans. ...