-The development and nutrition of Norway spruce seedlings growing under controlled conditions in three different seedbed types (mineral Ah horizon, organic layer, highly decayed dead wood) obtained from two protective forest sites in the Bavarian limestone Alps was investigated for one growing season. The seedlings showed clear responses to the three natural seedbed types in biomass development and nutritional status. Their biomass was significantly lower in mineral soils and organic layers as compared to decayed dead wood. Seedlings in organic and in decayed wood substrates had significantly higher contents of N, P, K, Mn, Zn (only decayed wood), and more balanced nutrient relations as compared to seedlings grown in mineral soils. It was indicated that the acid organic layers and highly decayed dead wood represents a good seedbed for spruce natural regeneration, especially in regard to the impaired nutrient availability on alkaline dolomite sites in the Bavarian limestone Alps.Picea abies / decayed woody debris / organic layer / mineral soil / seedbed Résumé -Développement précoce et nutrition de semis de Picea abies (L) Karst. élevés sur différents substrats dans les Alpes Bavaroises calcaires. Le développement et la nutrition de semis d'épicéa poussant en conditions contrôlées sur trois types de substrat (horizon minéral Ah, strate organique, bois mort très pourri), prélevés dans deux sites forestiers protégés dans les Alpes Bavaroises calcaires, ont été étudiés pendant une saison de végétation. Les semis ont présenté une réponse nette au trois types de substrats naturels au plan du développement de la biomasse et du statut nutritionnel. Leur biomasse était significativement plus faible sur sol minéral et strate organique comparativement au bois mort pourri. Les semis sur sol organique et substrat de bois mort pourri ont présenté des teneurs significativement plus élevées en N, P, K, Mn, Zn (seulement sur bois mort pourri), et des relations nutritionnelles plus équilibrées comparativement aux semis poussant sur sol minéral. Il a été montré que les strates organiques acides et le bois mort très pourri constituent un bon substrat pour la régénération naturelle de l'épicéa, en particulier au plan de la faible disponibilité des nutriments sur les sites dolomitiques alcalins dans les Alpes calcaires bavaroises.Picea abies / débris décomposés de bois / strate organique / sol minéral / substrat

Phosphorus (P) concentrations in needles and leaves of forest trees are declining in the last years in Europe. For a sustainable forest management the knowledge of site specific P nutrition/ availability in forest soils is vital, but we are lacking verified simple methods for the estimation of plant available P. Within this study, four soil P extraction methods [water (P H 2 O ), double-lactate (P lac ), citric acid (P cit ), and sodium bicarbonate (P HCO 3 )], as well as total P content of the soil (P tot ) were tested to investigate which method is best correlated with foliar P concentrations of spruce [Picea abies (L.) H. Karst.] and beech [Fagus sylvatica (L.)]. Mineral soil samples from 5 depth levels of 48 forest sites of the Bavarian sample set of the second National Forest Soil Inventory (BZE II) were stratified according to tree species (spruce and beech) and soil pH (pH < 6.2 and > 6.2), covering the whole range of P nutrition. The extractable amount of P per mass unit of soil increased in the order P H 2 O << P lac < P HCO 3 < P cit , decreased with soil depth, and was higher in soils with pH < 6.2. Citric acid extracted up to 10% of P tot in acidic soils. Whereas P cit delivers adequate regression models for P nutrition in the case of spruce (R 2 up to 0.53) and beech (R 2 up to 0.58) for acidic soils, P HCO 3 shows good results for spruce growing on acidic soils (R 2 up to 0.66) and for beech on soils with pH > 6.2 (R 2 up to 0.57). P lac produces adequate models only for beech on high pH soils (R 2 up to 0.64), while P H 2 O did not produce acceptable regression models. P tot seems suitable to explain the P nutrition status of beech on acidic (R 2 up to 0.62) and alkaline soils (R 2 up to 0.61). Highest R 2 s are obtained mostly in soil depths down to 40 cm. As P HCO 3 and P cit showed good results for both investigated tree species, they should be considered preferentially in future studies.

Process-based models are increasingly applied for simulating long-term forest developments in order to capture climate change impacts and to investigate suitable management responses. Regarding dimensional development, however, allometric relations such as the height/diameter ratio, branch and coarse root fractions or the dependency of crown dimension on stem diameter often do not account for environmental influences. While this may be appropriate for even-aged, monospecific forests, serious biases can be expected if stand density or forest structure changes rapidly. Such events occur in particular when forests experience disturbances such as intensive thinning or during early development stages of planted or naturally regenerated trees. We therefore suggest a calculation of allometric relationships that depends primarily on neighbourhood competition. Respective equations have been implemented into a physiology-based ecosystem model that considers asymmetric competition by explicit simulation of resource acquisition and depletion per canopy layer. The new implementation has been tested at two sites in Germany where beech (Fagus sylvatica) saplings have either been planted below a shelterwood of old spruces (Picea abies) or grown under clear-cut conditions. We show that the modified model is able to realistically describe tree development in response to stand density changes and is able to represent regeneration growth beneath a gradually decreasing overstorey of mature trees. In particular, the model could represent the faster crown size development in saplings until full ground coverage is established and a faster height growth afterwards. The effect enhances leaf area and thus assimilation per tree and increases carbon availability for stem growth at early development stages. Finally, the necessity to consider dynamic allometric relations with respect to climate change impacts is discussed, and further improvements are suggested.

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